Symposium F.FL03—Flexible, Wearable Electronics and Textiles

Ultraflexible OPVs are expected to be used as a flexible energy harvester for wearable devices such as wearable textiles. Wearable devices would be easily changed and deformed according to the movement of the body. Assuming such usage, angle of incident lights to ultraflexible OPV is always changing, and along with this, power conversion efficiency (PCE) of ultraflexible OPV is also changing. Thereby the suppression of angular dependence is important to get a stable power generation. One of the major reflection losses happened at the substrate surface, between air and a substrate because of the large difference of refractive index ⁽¹⁾. Therefore, controlling the surface reflection by fabricating nanostructure to substrate is one strategy to reduce the angular dependence. A previous work reported flexible OPVs with nanostructured substrate is exhibited 1.3 times less angular dependence than OPVs with flat substrate ⁽¹⁾. However, a 1-µm-thick ultrathin substrate for ultraflexible OPV takes a susceptible to damage by conventional high energy patterning methods like laser carving method ⁽²⁾ so that it is hard fabrication process of nanopatterning to its.
This research shows the developed low energy patterning method for ultrathin substrate and the effect of reducing the angular dependence of the nanopatterning ultrathin substrate. Generally, ultrathin substrate is fabricated by evaporating the polymer materials onto glass applied fluorinated polymer. The fluorinated polymer is fabricated by spin coating as a decreasing the surface energy between the substrate and delaminating it easily. Therefore, we focus on the undried fluorinated polymer layer. Nanostructures are fabricated onto the low energy surface of the fluorinated polymer by optimizing the condition for the drying process of it like temperature and drying time with placing nanopatterned polydimethylsiloxane mold on undried fluorinated polymer. These nanostructures of fluorinated polymer layer are traced onto ultrathin substrate surface by evaporating the polymer materials onto nano patterned fluorinated polymer layer. Valid point of this method does not require the extra heating process or ultraviolet curing process for the nanopatterning. Therefore, the nanopatterned ultrathin substrate is fabricated with avoiding damage to it by this method. OPV with nano line patterns ultrathin substrate which has 760 nm of pitch and 100 nm of height was exhibited low angular dependence compare with OPV with flat ultrathin substrate. The performance of OPV was measured with changing incident light angle 0 degree to 70 degrees, every 10 degrees. For all angles >10 degrees, the difference of performance appears due to nanopattern ultrathin substrate, especially, maximum reduction of the angular dependence of 5.7% being observed at 50 degrees ⁽³⁾. Furthermore, the surface reflection rate upon the ultraflexible OPV suppress due to the reduction of the light interference effect by using nanopatterned ultrathin substrates. As a result, ultraflexible OPV with nano patterned ultrathin substrates was exhibited higher short current density of 2.1% compared with ultraflexible OPV having flat ultrathin substrate ⁽³⁾.

(1) J. Kettle, et al. J.Phys.D: Appl. Phys. 46, 105102, 2013
(2) L. Müller Meskamp et al. Adv. Mater. 24, 906 910, 2012
(3) M.Takakuwa, et al. Small Methods, 4, 1900762, 2020

Thermal comfort is essential for health and productivity. However, it consumes enormous energy to maintain thermal comfort through space heating and cooling, due to energy waste in the vast empty space of the entire building. Here, we propose the personal thermal management strategy focusing on temperature regulation of only the human body and its local environment, and present a passive personal thermal management textile system via engineering the thermal radiation property of textiles to reduce building energy consumption and improve the quality of human life.

The focus of this research is to develop and apply new and existing soft responsive materials capable of responding to target biomarkers on the body emitted from the skin. One of the approaches being investigated is the use of responsive colorimetric dyes to track volatiles being emitted from the skin. Specifically, the acidity of the volatile emission is being assessed using pH-responsive dyes. Early evidence from the group has shown that the total volatile fatty acid emission (C8-C16) correlates well with skin surface pH as measured by a glass pH ion-selective electrode. This early study used a headspace-solid phase microextraction (HS-SPME) approach to sample the skin volatiles which were then analysed by GC-MS. This project aims to exploit this correlation of acidic volatile emissions with skin pH to develop a non-contact, skin pH sensor. Aldehyde emissions from the skin are also being examined.

A range of responsive colorimetric dye spots are being tested in this work to understand their response to the skin volatile emission. pH, solvatochromic and metalloporphyrin dyes being used in the sensor array are immobilised as spots to flexible cellulose substrate. A sol-gel encapsulation matrix is used to allow the soluble dyes be converted into nanoporous pigments which are insoluble. The durability and stability of the dyes is improved by using this encapsulation method.¹ The response of each dye spot was recorded by imaging the sensor array before and after exposure to skin volatiles and the net colour response quantified. A host of different skin sampling methods have been examined including gauze sampling, swab sampling, tape stripping and also direct application on the skin incorporating a wire mesh spacer between the sensor array and the skin. This presentation will highlight some of the results from this research to date.

References:
(1) Lim, S. H.; Musto, C. J.; Park, E.; Zhong, W.; Suslick, K. S. A Colorimetric Sensor Array for Detection and Identification of Sugars. Org. Lett. 2008, 10 (20), 4405–4408. https://doi.org/10.1021/ol801459k.

This work is related to the progress on the implementation of sodium phthalocyanine for flexible organic diodes. A NaPc/TCNQ structure was deposited on TCO/PET for the electronic device and silver contacts were used to determine the optoelectronic properties. HV Thermal evaporation of small molecules and spin coating processes were used for device deposition. The Characterization of the electronic device was conducted using FTIR, AFM, UV-Vis, IV characteristics for chemical, structural, morphological, opto-electrical properties. FTIR measurements confirm an efficient transfer for the device to the PET flexible substrate with a smooth and homogeneous surface topography. A direct transition bandgap of 2.5 and 2 eV were obtained for different the NaPc deposited thicknesses. Diode type characteristics were obtained with threshold voltages of ~1.5 V and ~15 microAmpers current across the device at 6 V. IV characteristics were measured with different incident led light colors showing a variation of the curves on the NaPc-TCNQ flexible device which gives evidence of the device and interfaces quality for diode applications.

Flexible polymer-based magnetoelectronics with an organized structure are most desirable in the field of memory, energy storage, sensing, and harvesting devices. In this work, we have developed flexible nickel ferrite (NiFe203)-PVDF nanocomposite films. The obtained composite films show excellent magnetoelectric coupling at room temperature, making them good candidates for improved ferroelectric devices. The nanocomposite films were prepared by the solution casting and 3D printing method, wherein the nickel ferrite nanoparticles with different weight percentage (5, 10, 15, 20, and 30%) were incorporated in the ferroelectric PVDF polymer matrices. The structural, morphological, mechanical, electrical, magnetic, and thermal properties of the nanocomposites were studied by X-ray diffraction, FTIR, scanning electron microscope, nanoindentation, DSC, TGA, and SQUID.

Macroscale flexible and stretchable pressure sensor enables to monitor pressure distribution on a variety of nonplanar objects such as infrastructure, vehicles, and humans if very thin flexible pressure sensor arrays can be realized. In fact, many types of flexible tactile pressure sensors have been studied. For the macroscale flexible sensor sheet, one important parameter is device cost. To realize economical sensor sheets, printing, transferring, and laser ablation methods have been widely proposed. As one of the potential economical process, this study proposes a simple and scalable fabrication process to form a very thin macroscale flexible tactile pressure sensor array.
Fabrication process is briefly explained. First, laser-induced graphene (LIG) was formed by exposing CO₂-based laser over polyimide film. The LIG layer was then transferred into a polydimethylsiloxane (PDMS) elastomer film. In parallel, silver electrodes were screen-printed on a polyethylene terephthalate (PET) film. After the formations, PDMS and PET films were laminated. The total thickness of the device is ~300 µm. The sensing mechanism of tactile pressure is to measure contact resistance change between the LIG embedded in the PDMS and the silver electrodes on the PET films as a function of applied pressure.
Resistance change at an applied pressure was characterized. Due to the observation of contact area change between two conductive materials, at low pressure range less than 30 kPa, resistance change is large while the resistance change gradually decreases by increasing the applied pressure. The maximum sensitivity is ~0.84 %/kPa at 28.3 kPa, and the sensitivity at 566 kPa is ~0.1 %/kPa. The response time is relative fast to be <0.1 s. Importantly, this sensitivity and response speed are independent with the device bending even under 1 cm curvature radius. After confirming the fundamental properties of very thin flexible tactile pressure sensor, uniformity, stability, and reliability were tested. The uniformity in terms of initial resistance and sensitivity at 56.6 kPa of the sensor are 1062±62Ω and ~0.6±0.3 %/kPa, respectively. The sensor can measure the pressure precisely more than 4500 cycles for a long time.
Finally, as a proof-of-concept, pressure distribution was measured by using an integrated 4×4 array sheet. We confirmed that this sensor array sheet can monitor the finger position on a pen covered by the sensor sheet. Furthermore, by covering this sensor sheet surrounding a roller, pressure distribution of the roller under rolling onto a floor is successfully monitored in real time. These demonstrations depict that this very thin flexible sensor sheet can detect the pressure distribution regardless of the surface shape without changing the object shape drastically.

Bioelectronic interfaces rely on intimate contact with tissue to establish efficient signal exchange between electrodes and the biological target structure. Mechanically soft and ion conducting materials are beneficial in this context, in particular for semi-dry interfaces such as electrodes contacting skin. Typical natural (e.g. alginate, collagen, agar) and synthetic (e.g. polyacrylamide, polyethylene glycol, polyvinyl alcohol) hydrogels used for biomedical implementation are not inherently electronically and/or ionically conductive, wherefore development of conducting hydrogel systems is highly relevant. We demonstrate here how a supercapacitive polymer hydrogel whose network is entirely comprised of conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) nanofibril interconnections can be combined with laser-induced graphene (LIG) on a flexible substrate (i.e. polyimide). This results in a bioelectronic interface which easily conform and connect to skin and can support both recording and electrical stimulation. The PEDOT:PSS hydrogel precursor can be printed (i.e. direct ink writing) or dip-coated onto laser pyrolyzed polyimide (e.g. Kapton) film. The LIG and PEDOT:PSS hydrogel electrode was shown to have a high charge storage capacity (up to 1500 mC/cm²), electrochemical stability (retains more than 85% of the original charge storage capacity over 500 cycles) and ability to deliver long term biologically-relevant electric fields (10-1000 mV/mm). In short, via affordable technology, we were able to make devices that outperform previous technology based on noble metals. Thereby we pave the way for disposable bioelectronics as well as DC stimulation of cells and tissues at levels relevant for regenerative therapy.

Soft bioelectronics are an emerging class of wearable electronics for point of care (POC) applications; however, fabrication processes for these stretchable devices have relied on time-consuming, expensive, and complicated microfabrication techniques. The required active materials (e.g., gold) present expensive deposition or complicated galvanization methods, for developing thin films of metals for stretchable electronics, which limits the transcendence of rapid, one-time devices. Of note, electronic CDs generate a large amount of electronic waste and contribute to the fast growing global problem of landfill waste. To solve critical challenges in the rapid fabrication of soft bioelectronics and environmental challenges, we have developed a fabrication method for upcycling gold compact discs into transient, fibrous, and permeable devices.

The reflect metal layer from compact discs (CDs) can be recycled for rapid, low cost development of thin gold films. Following the removal of the CD’s protective layer, it can be easily cut to a feasible size for harvesting the gold layer. Utilizing water-soluble tape, the pickup at the optimal peel velocity highlighted the transfer efficacy, due to the stronger adhesion forces produced between the gold and tape rather than the gold and adhesive layer. Intricate patterns can be realized onto the gold film by laser engraving, capable of transforming the rigid constituent into fully flexible form factors. We present feature sizes of about 700 µm that are contingent on the laser resolution with the drawback of sacrifice the gold’s durability after patterning. Electrospinning synthetic polymers were further deposited onto the patterned gold film to increase durability and stretchability. With this fabrication method, a wide range of fibrous polymeric systems, including biodegradable and natural polymers, can apply to soft, mesh type bioelectronics. The mechanical and electrical properties of the developed bioelectronics have been systematically analyzed to confirm its conformality. Further, the devices were demonstrated in application as electrocardiography (ECG) electrodes.

Recently, as the harmfulness of ultraviolet rays (UV) is widely known, protecting skin from UV rays is attracted attention. However, the current use of liquid or stick sunscreen can cause skin problems due to the toxicity of chemicals and poor water resistance, it has a short durability, so there is inconvenienced to apply regularly. This study report a new type of sunscreen, nanofiber-based sunscreen, which is chemical-free, high water-resistance, dust resistance and gas breathability, and novel portable electrospinning system to apply it. The nanofiber-based sunscreen use biocompatible polymers and can be quickly applied by the portable electrospinning system. Nanofiber-based sunscreen consist of nanofiber-matrix structures, the fiber was solidified and chemically stable. Owing to its nature, the nanofibrous sunscreen does not dissolve in sweat or water and block micro dusts while allowing air and vapor to pass through. Also, skin trouble can be prevented due to the sterilizing effect of TiO₂ nanoparticles contained in nanofiber-based sunscreen. Due to its flexibility and stretchablity, it can coated on skin conformably and be able to stretch and relaxed repeatably along the skin without falling. Finally, the nanofiber-based sunscreen can be easily detached from skin without residue unlike liquid-type sunscreens. We believe our nanofiber-based sunscreen can be used in daily life and professional medical such as patients with organ transplants or skin diseases that are vulnerable to UV rays.

There is growing interest in flexible thermoelectric cooling devices to provide personal thermal regulation to alleviate energy demands of current HVAC systems as well as to provide safety to workers in uncontrollable outdoor environments. Here we demonstrate flexible thermoelectric cooling devices, utilizing a foam substrate, that can be integrated into wearable products. Our flexible thermoelectric devices utilize commercially available materials and scalable processing techniques to improve the manufacturability. Thermoelectric legs were embedded in a foam substrate to provide flexibility, and kirigami-inspired cuts were patterned within the foam to mimic drape found in textile materials. In total, nine different configurations, three different fill factors, and three different kirigami cut patterns, were created and tested. The devices were assessed for thermal performance, mechanical durability, flexibility and drape. The results from this characterization show that adding kirigami patterns can increase the durability of the device, improve the flexibility, decrease the drape coefficient, and have <1% of impact on cooling performance at higher fill factors, reaching temperature differences up to 4.39 ± 0.17°C. These thermoelectric cooling devices show great promise for the future of wearable cooling devices considering both the manufacturing scalability and its’ familiar drape and feel, which will allow for easy integration of the devices into products.

E-textile design heavily relies on the human use case as well as factors such as price point and emphasis on sustainable material use. The e-textiles market is in its early years of commercial establishment, so there are few standards when it comes to material and design choices and product validation. This lack of standards makes designing new products difficult and when benchmarking against existing products, makes identifying areas for innovation challenging. This research examines the use of reverse engineering principles based on the disassembly, characterization, and analysis, of popular commercial e-textile products, allowing for both an analysis of the products disassembled and providing a framework of best practices for future designs. Using a reverse engineering framework, commercial products were evaluated based on their system block diagram and materials characterization, relating each to understanding the design criteria employed for their construction. Products with similar use cases are compared and evaluated for functionality, performance, and sustainability. This research ultimately provides a framework to better understand design criteria and material selection in order to improve future product designs.

Textile-based structures exhibit inherent properties such as flexibility, stretchability, breathability, and high durability under severe mechanical deformations. These properties make textiles an ideal platform to create conformable wearable electronics that rely on flexible circuitry. However, current wearable power supplies are not suitable for textile-based wearable platforms or smart textiles since they utilize functional materials that are rigid, heavy, and exhibit negative environmental impacts. Therefore, researchers have turned to harvesting energy resources from the human body that would have otherwise been wasted, as an alternative power supply solution for smart textiles.
In this work, we review the possible forms of harvestable mechanical energy from various human motions (i.e. Running, Bending Arms, etc) using piezoelectric polymers integrated into textile-based structures. Piezoelectric polymers such as polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-co-trifluoroethylene) (PVDF–TrFE) are ideal for smart textiles due to their high throughput, low-temperature processing capabilities and larger range of deformations compared to piezoelectric ceramics. We introduce the state of the art in textile-based piezoelectric energy harvesters using PVDF and PVDF-TrFE with regards to the hierarchical levels of textile structures (i.e. fiber -> yarn -> fabric). We categorize these textile energy harvesting devices as either in-situ or ex-situ based on their fabrication method and structural design of their electrodes. Then, we suggest standardized testing methods based on this in-situ/ex-situ classification system to accurately characterize energy output for various mechanical stimuli and working harvesting areas. Lastly, we create a framework based on the challenges and exciting opportunities for practical use case scenarios of these textile-based piezoelectric energy harvesters.

Flexible conducting materials has been in the forefront of a rapidly transforming electronic industry focusing on wearable devices for a variety of applications in our world today. Over the past few decades, bulky, rigid devices have been replaced with a surging demand for thin, flexible, light weight, ultra-portable yet high performance electronics. But the inter-connects available in the market today only satisfy a few of the desirable characteristics, making it necessary to compromise one feature over another. Materials like thin copper foils provide metallic conductivity but cannot be stretched or bent repeatedly over the lifetime of the product owing to bending fatigue and ultimately mechanical deformation leading to electrical failure. On the other hand, stretchable metal polymer composite structures previously explored by researchers in this industry failed to achieve high conductivity to compete with metal counterparts.
We present a method to prepare a thin, flexible, and stretchable inter-connect with much improved conductivity compared to what has been achieved before. It also satisfies most mechanical and electrical conditions desired in the wearable electronics industry today. The composite prepared with the widely available, low cost silicon-based organic polymer - Polydimethylsiloxane (PDMS) and Silver (Ag) is sandwiched between two cured PDMS layers. These two protective layers improve the mechanical stability of the inter-connect. The stretchability of the material is enhanced by over 100% by laser cutting it into a serpentine structure without compromising its electrical stability. The structure, less than 500 um thick, can be integrated into thin electronic packaging. The synthesis process of the composite material, along with their electrical, mechanical and corrosion resistive properties will be presented in detail.

Fusion of physical and chemical sensing onto a single platform cannot only provide information for physical parameter such as Electrocardiogram (ECG) but also offer more insightful and comprehensive chemical parameters from biofluids such as sweat [1]. From monitoring glucose and lactate level with ECG, biosensor could prevent the exerciser from hypoglycemic shock during intense exercise.
In this work, intrinsically stretchable electrode is fabricated to monitor biochemical and biophysical information using elastomeric composite comprised of Ag flake and photo-patterned Ecoflex [2,3]. Direct pattening of the Ecoflex benefits from convenient fabrication process. Furthemore the encapsulation of Ecoflex improves the conductivity of Ag nanocomposite when it is stretched. That is induced by an efficient strain energy dissipation at an inteface between the conductor and the encapsulation layer.
Since Ag electrode could be readily oxidized in biological circumstance and leach out the Ag ions to human tissue, the stretchable conductor consisting of Ag nanocomposite is limited to bioelectronics. To enchance the biocompatibility of Ag, the electrode is modified to form Pt and Au nanoparticles onto counter electrode and working electrode respectivly by galvanic replacement process [4]. Various functional materials for multisensor are electrodeposited on to Au modified working electrodes. A pH sensor is perfomed via Polyanilne showing the 100mV/pH sensitivity stretched up to 50%. Glucose and Lactate sensors are opreated in vitro showing 3nA μM^-1, 260nA μM^-1 sensitivity respectively. Finally intrinsically stretchable multi biosensor in vivo are successfully monitoring a variation of analytes concentration of glucose and lactate in sweat before and after meal and exercise simultaneously with electrical signal.
[1] H. Lee, T. K. Choi, Y. B. Lee, H. R. Cho, R. Ghaffari, L. Wang, H. J. Choi, T. D. Chung, N. Lu, T. Hyeon, S. H. Choi and D.H. Kim, Nature Nanotech. 2016, 11, 566-572
[2] S.H. Kim, S. Jung, I. S. Yoon, C. Lee, Y. Oh and J. M. Hong, Adv.Mater. 2018, 30, 1800109
[3] A S. Bhagat, P. Jothimuthu and I. Papautsky, Lap on a chip, 2007, 7, 1192-1197
[4] P.Won, J.J Park, T. Lee, I. Ha, S. Han, M. Choi, J. Lee, S. Hong, K.J. Cho, S.H. Ko, Nano Lett. 2019, 19, 6087-6096

Development of sensors for early warning and protection against chemical and biological agents is a priority for front line first responders as well as military personnel. We have developed inkjet-printed surface enhanced Raman spectroscopy (SERS) sensors that is specifically designed to work with field portable Raman analyzers for the detection of chemical and biological agents. These SERS sensors are inkjet-printed on filter paper or fabric substrates. Ink formulation are carefully optimized colloidal silver or gold nanoparticle solutions synthesized in house. They are flexible, lightweight and provides point-of-sampling capabilities that other rigid planar SERS substrates lacks. These sensors can be integrated with the wearable gear of the first responders. These flexible and wearable sensors are designed for qualitative detection such as binary detection of either positive or negative presence of certain chemical/biological agent. Conventionally, performance of a SERS substrate is characterized by its enhancement factor which is defined as the ratio of the normalized SERS to normal Raman intensity. This is an inadequate measurand for a chemical/biological sensor. The performance of a chemical/biological sensor is best characterized by its sensitivity, probability of true positive detection, false positive rate and its response time. The sensors receiver operating characteristics (ROC) captures the performance trade-off between all these factors. It is meaningless to specify sensitivity without the confidence level of detection expressed as the probability of detection and the false positive rate. Although ROC has been used commonly for assessing chemical/biological sensors, it is not trivial to adapt it for SERS detection. In this presentation, we will present a new methodology developed specifically for the binary analysis of the SERS sensors using the receiver operating characteristic concept. This can be applied for all SERS sensors but we will focus its use with the printed SERS sensor.

Diabetes is one of the significant causes of fatality throughout the word in the present day. The deficiency of insulin and hyperglycemia is caused by the metabolic disorder of diabetic patients. This disorder promotes high risks of kidney failure, heart attack, chronic liver diseases, and blindness. Thus, constricted day-to-day monitoring of glucose concentration is a demand for the diagnosis and treatment of diabetes. Also, fast, accurate and continuous monitoring of glucose concentration is of great importance. As a successful consumer product of wearable biosensors, it is important to demonstrate reliability, multiplexed functionality and ease-to-use noninvasive monitoring of glucose from the body fluids, such as sweat. Sweat is easy to analyze and is considered as one of the most widely used body fluids having significant medical information. However, the introduction of nanomaterials allows many new signal-transduction technologies results improved the sensitivity of different biosensors. ZnO thin film-based biosensors are promising to determine the glucose levels in sweat with better sensitivity, reliability, continuously, and in a noninvasive manner. ZnO itself has exclusive chemical and physical characteristics to enhance chemical stability in physiological environments. Moreover, it has highly catalytic activity, biocompatibility, and higher isoelectric point (IEP). ZnO nanoflakes (ZnO-NFs) exhibit several unique advantages compared to bulk materials. For example, polarized (0001) plane orientation enhances enzyme immobilization which improves sensing performance. A high isoelectric point (9.5) of ZnO NFs enhances the immobilization of any biomolecules which eliminates the necessity of an additional binding layer. This elimination of additional layer and immobilization of enzyme offer a steady and shorter pathway for quick electron transportation increasing the electron transfer rate. A single step sonochemical approach was developed to synthesize a thin layer of ZnO-NFs virtually on any substrate. This technique is fast, catalyst-free, less expensive, and ecologically benign. The rate of reaction of the sonochemical approach is 100 times faster than conventional hydrothermal procedures, which enables a well-oriented growth on polyethylene terephthalate (PET) over an extensive range. The temperature for the process is low enough to permit the application of different materials, for example, fabrics, paper, and polymers.
In this study, an electrochemical glucose biosensor was fabricated by immobilization of glucose oxidase on ZnO nanoflakes with a thickness of 20nm, synthesized on Au-coated stretchable PET. The results demonstrated a fast response within 5s. The sensor was tested in the range of 10 pM – 20 mM, which covers the entire physiological range and capable of detecting as low as 1 μM of the glucose concentration. The sensitivity of the sensor was determined by 4.53 μA/decade/cm². Moreover, the sensor performed repeatable characteristics having with average standard deviation (S.D.) of 0.14 μA. Furthermore, in our experiment, the fabrication process is simple, and the thin film-based nano-biosensor provides excellent performance in stability, selectivity, sensitivity, and reproducibility.

Expected to become mainstream in the electronics industry, flexible electronics still face major challenging issues. For polymeric-based flexible electronic substrates in particular, these challenges include a lack of electromagnetic shielding capability and poor heat dissipation. Here, we report a highly flexible and thermally conductive macroscopic polydimethylsiloxane (PDMS) polymer film embedded with a copper-coated reduced graphene oxide (rGO) fiber mesh. The rGO fibers are assembled into 3D fiber meshes and electroplated with micrometer-thick copper coatings, displaying excellent electrical and thermal conductivities. Oriented in the horizontal and perpendicular directions within the PDMS polymeric matrix, the fiber mesh serves as a highly electrically and thermally conductive backbone through the in-plane direction. Meanwhile, the fiber mesh also effectively shields electromagnetic interference in the X-band without causing thermal damage. The macroscopic film remains electrically insulated in the through-plane direction. Utilizing both the favorable thermal and electrical properties of the graphene fiber-based mesh and the flexibility of the PDMS matrix, our film may exhibit potential for flexible electronics applications such as wearable electronic thermal management and flexible microwave identification devices.

Thermoelectrics are materials consisting for an electron conductor (n-type) and a hole conductor (p-type) that are connected electrically in series and thermally in parallel. They work through both the Seebeck effect (power generation) and the Peltier effect (refrigeration). Through the Seebeck effect, a heat gradient causes current to flow generating electricity. Through the Peltier effect, a current flow generates a heat gradient leading to one side of the material being actively cooled. Traditionally inorganic materials are sued that have a low natural abundance, high costs, and are rigid and brittle. Polymeric thermoelectrics or organic thermoelectrics (OTEs) overcome the limitations of conventional materials as they are low cost and flexible; though they typically have lower conductivity so they require enhanced designs such as high surface fibers. OTEs cannot readily be made into fibers as they are low molecular weight and have strong molecular interactions and high conductivity that alter fiber formation. In this work, we use electrospinning to prepare composite ultrafine fiber mats of organic based n- and p-type conductors enabling close contact between fibers. Electrospinning is a solution processing technology that creates nonwoven ultrafine fibers (diameter ~100nm-2um). Ultrafine fibers have very high surface area allowing for many close contact points between fibers. PEDOT:PSS, a p-type conductor, is able to be electrospun with an insulating polymer at amounts up to 56wt%. NiETT, a n-type conductor, is made by forming a PVA-nickel salt complex through electrospinning then polymerizing the final fibers. These materials can then be layered to create a soft, flexible, wearable, electronic device.

Recently ^[1-3], polyethylene (PE) has gained significant interest by the textile fiber/film industry and opened up new advanced textiles for personnel thermal management with simple structures and unique optical properties. Controlling these optical properties in the long wave infrared (LWIR) spectral range could have a strong effect on cooling and heating the body and per consequence it could be a solution to obtain a textile with enhanced radiative cooling/heating for passive releasing/trapping of human body heat and this is not found in conventional textiles. Moreover, PE is a durable polymer, scalable, cost-effective and lightweight materials.
In this study, we have demonstrated both the effect of polymer chain alignment and integration of inorganic particles into ultrahigh molecular weight polyethylene (UHMWPE) polymer on the optical properties of UHMWPE films. We have seen that the total transmittance of the films increases with higher the draw ratios while the scattered light can be controlled over LWIR by introducing a small percentage of inorganic particles where the forward scattering was found to be preferred over backward scattering. The high transparency of the films could be explained by the film thickness reduction on one side and the surface morphology of the film on the other side. In addition, the stretched films become thinner which allow more light comes through the film whereas the surface morphology of the films become one hand smoother and aligned and on other hand the films crystallinity increase significantly. Most importantly, we have seen that the intensity of the scattered light in the forward direction can be controlled by addition of additives into the pure UHMWPE film. The integration of inorganic nanoparticles into the pure UHMWPE polymer allows both the properties from inorganic nanoparticles (higher refractive index and higher forward scattering) and UHMWPE polymer (higher transparency) to be combined and enhanced the optical properties of films, particularly, in the LWIR regime. By preferentially scattering in the forward direction, these novel textile materials, in thin films, could release heat from an overheated body (eg. the human or warm equipment) better than if there is significant backscattering.
In this talk, the following will be presented and discussed : the fabrication process of highly oriented and transparent PE films, the heat-releasing data of PE films collected by a simulated device for skin temperature that operates at constant power output to produce the human skin temperature and lastly the controlled light scattering measurements in PE films over LWIR spectral range.

Keywords:
Polyethylene film, forward scattered light, backward scattered light, LWIR, UHMWPE, thermal management, thermal radiation, textiles.
References:
^[1] Svetlana V. Boriskina,” An ode to polyethylene”, MRS Energy & Sustainability: A Review Journal, volume 6, E14, (2019).
^[2] Yi Cui et al. “Nanoporous polyethylene microfibres for large-scale radiative cooling fabric”, Nature Sustainability, volume 1, 105–112, (2018)
^[3] Yi Cui et al. “A dual-mode textile for human body radiative heating and cooling”, Science Advances, Vol. 3, no. 11 (2017)

Over the past decades, foldable electronics have become a major area of research in the electronics industry. Our devices are getting smaller, more portable and the need for flexible electronics is booming for various applications. While a major part of the research is being performed on high performance flexible and foldable batteries, the longevity of the materials used as the interconnects has remained under-explored. The thin metal sheets available in the market today is an ideal choice for such applications owing to their high conductivity. Researchers have also explored conducting composite polymer interconnects for improved stretchability and flexibility despite their reduced conductivity. The lifetime and mechanical stability of these materials are most important before considering them for integration into flexible electronic packages.
In this paper, we propose testing methods and results for mechanical and electrical stability over extensive flexing and stretching cycles. The materials put into test are copper foils, copper coated polyimide (PI), aluminium coated Polyethylene terephthalate (PET) and conductive Silver (Ag) - Polydimethylsiloxane (PDMS) composite in a sandwich structure.

Nano-engineering is one of the most important technologies for development of the state-of-art electronics, healthcare device, and eco-friendly systems that provide the methods to control the extremely small size and properties of materials. In addition, nanostructured materials can serve the unique mechanical, electrical, and optical properties leading the advance of technology. Recently, there is an increasing demand for nano-engineering in various industry such as IoT sensor, smart grid, next-generation energy system, and self-diagnosis system to improve the performance and extend the functionality of that. However, nano-engineering process has not reached at a sufficient level to satisfy the design and suggestion. In this study, nanoimprint-assisted electroplating is suggested to fabricate metallic 3D nano/micro- structure applicable to various electronic and optoelectronic devices.Nano-engineering is one of the most important technologies for development of the state-of-art electronics, healthcare device, and eco-friendly systems that provide the methods to control the extremely small size and properties of materials. In addition, nanostructured materials can serve the unique mechanical, electrical, and optical properties leading the advance of technology. Recently, there is an increasing demand for nano-engineering in various industry such as IoT sensor, smart grid, next-generation energy system, and self-diagnosis system to improve the performance and extend the functionality of that. However, nano-engineering process has not reached at a sufficient level to satisfy the design and suggestion. In this study, nanoimprint-assisted electrodepositon and electroless deposition is suggested to fabricate Ni/Au core/shell micro-heater applicable to various sensor applications. In briefly, sacrificial template was fabricated by nanoimprint lithography. Then, Ni core was selectively deposited on conducting substrate. After that, template was removed and Au shell was coated by electroless plating to prevent oxidation problem during heating of the micro-heater. Finally, as-fabricated Ni/Au micro-heater was transferred on arbitrary substrate such as flexible polyimide film, PET film, and so on and characterized the heating performance.

Laser-assisted production has increased in popularity due to their low cost and ease of use for general prototyping. In the context of materials science, laser-produced composites have the potential to increase the research throughput and enable unique complex phases. Recently, graphene-based conductive materials have been obtained in the process of laser ablation from the polyimide substrate and were efficiently applied for electrochemical biosensing. We sought to expand this synthetic framework with a platform that allows for the production of semiconductor-based structures and harness a plethora of electronic, electrochemical, photoelectrochemical, and photothermal properties for their application in bioelectronics.

Here, we show that laser writing can be used to synthesize semiconductor materials. We used polydimethylsiloxane (PDMS) as a precursor to silicon carbide (SiC) based materials. The doping and internal structure of the materials can be controlled with varying the processing parameters, which allowed us to synthesize both oxygen- and nitrogen-doped, glassy and crystalline materials, including rare SiC polytypes. A variety of doping levels and patterns create materials with tunable electronic properties. Synthesis of the material directly in the insulating elastomer matrix yields ready-to-use bioelectronic devices in which electrical and electrochemical properties can be used for cardiac pacing and muscle cell stimulation. Our results show the great potential for direct fabrication of semiconductor-based devices for tissue- and organ-modulation as well as building platforms for organoid research.

Cochlear implants are widely recognised as a solution for patients with profound hearing loss. The implant restores auditory sensation by electrically stimulating nerve, remarkably improving patients’ quality of life. Despite overall successful outcomes, the main problem with the present cochlear implants are their imprecise stimulation causing frequency dispersion and ‘blurring’ of the perceived sound. Traditional testing platforms, such as animals and cadavers, are impractical for studying the intra-cochlear potential distribution evoked by electrical stimuli. Animal models have different anatomy and electrical properties compared to human cochleae, and the limited availability of cadavers greatly restricts its popularity in cochlear research. It is also difficult to instrument them to measure electrical spread. Therefore, this study has developed a novel methodology that exploits 3D embedded printing and a neural network approach to create an artificial stimulation model of human cochleae for replicating the intra-cochlear potential distribution evoked by electrical stimuli.

We fabricated the model by embedded 3D printing a sacrificial cochlea-shaped structure inside a support bath. Our fabrication method offers two key innovations – tuneable geometry and tuneable resistivity. With 3D embedded printing, our model can replicate the general geometric features of human cochleae and the large variation in the shape of human cochleae. By structuring a conductive network in the support bath, the resistivity of our model can be tuned over 2 orders of magnitude from 0.19 kΩcm to 22 KΩcm. This wide range of resistivity, which covers the reported resistivity of human cortical bone, cannot be replicated easily with the existing printable materials, such as thermoplastics and hydrogels.

To evaluate the ability of our model to replicate the physiological intra-cochlear potential distribution evoked by cochlear stimulation, we compared the electric field imaging (EFI) profile of our model and the patients’ EFI profiles. EFI is a clinical method that measures how the intra-cochlear potential is distributed across the electrode array of a cochlear implant in patients’ cochleae. Our results show that our model can replicate the mean intra-cochlear potential distribution in patients’ cochleae. By fabricating a wide variety of models with different matrix resistivity and cochlear shapes, a dataset of EFIs is obtained. We then deployed a neural network (nn) approach to build a computational model that predicts the trend in the EFI resulting from different matrix resistivity and different cochlear shapes. Our nn model correctly predicts the EFIs within the dataset with over 85% accuracy. It can determine how fast the voltage decays in a particular cochlear shape and resistivity.

This study presents a simple workflow in modelling the intra-cochlear potential distribution evoked by electrical stimuli using 3D printing and neural network. Our model could potentially serve as a screening model for developing new cochlear implants, or a tool to predict patient-specific clinical outcome after cochlear implantation. Our methodology is general and can be readily adapted to other anatomical sites of electronic implants.

Resistive-type pressure sensor, which are mainly utilized in industry, are easy to manufacture and are not significantly affected by external electromagnetic fields, unlike capacitive type. However, the produced signal is not linear, and it is also difficult to measure a wide range of pressures using such a sensor. Therefore, before being utilized, the extracted nonlinear data from them need to be modified and re-processed for a specific target use. Here, we presented resistive-type pressure sensors whose response characteristic can be freely adjusted without the need of any additional computer processing. Five different types of introducing sensors performed different functions by the sensors themselves and they were manufactured by only modifying their electrodes: normal pressure sensor, linear pressure sensor, on/off switch sensor, high-pressure band sensor, and low-pressure band sensor. The operating mechanism of the sensor is the electron shortcut pathway in a designated electrode circuit from the contact area change of the deformed conductive elastomeric lenticular pattern. Sensors fundamental properties such as sensitivity and measurable range(~4 MPa) could be also controlled by selecting the polymer or controlling the electrode slope.

C. Jeong, J.S. Lee, B. Park, C.S. Hong, J.U. Kim and T.-i. Kim, Adv. Mater. 31,(36), 1902689

Conjugated polymers have been studied for developments of novel lighting devices, electronic components, and energy conversion and storage systems. The electronic energy levels and bandgaps of the conjugated polymers can be studied by photoelectron spectroscopies, electrochemical redox potentials, and UV-Vis-NIR absorption spectroscopies. In this work, we prepared thin films of polythiophene derivatives and investigated their electrochemical doping processes and UV-Vis-NIR absorption spectra. Polymer films were laminated with a gel electrolyte and electrodes and electrical potential was applied at the electrodes. Because the molecular structures of polymers govern their electronic and macroscale properties, we observed distinct spectral changes of the polymers caused by redox of the materials. The polymer films in a neutral state showed absorption in a visible range. When electrochemically activated, the absorption bands of pristine polymers decreased and strong NIR absorption bands evolved. We tried to link the observed spectral phenomena with performance of the electrochemical devices.
This work was supported by the research programs (2019M3F3A1A03079821 and 2019R1A2C1087234) through the National Research Foundation of Korea (NRF). This research was also funded in part and conducted under "the Competency Development Program for Industry Specialists" of the Korean Ministry of Trade, Industry and Energy (MOTIE), operated by Korea Institute for Advancement of Technology (KIAT), with the project No. P0012453, Next-generation Display Expert Training Project for Innovation Process and Equipment, Materials Engineers.

Recently, polymer-based pressure sensors have drawn much attention due to the importance of sensing device for soft robotics, wearable or/and health monitoring device etc. However, electrospun polyurethane (PU) nanofiber webs having very high amount of nano-sized open cell can be used as a piezo-capacitive sensor for monitoring both static and dynamic pressures due to excellent electrospinnability and very good elastic properties that has been reported in our previous study ^[1]. In contrast, electrospun Polyvinylidene fluoride (PVDF) exhibits high β-crystalline phase content and superior piezoelectricity ^[2]. Consequently, in this study, the mixture solution of PU and PVDF was electrospun under controlled compositions to obtain PU-PVDF hybrid nanofiber webs. Their physico-mechanical and piezo-responsive behaviors were investigated. When the dynamically changing force was applied to this hybrid sensor, dynamic pressure could be measured through the electric current generated by the orientational change of C-F dipoles in the PVDF part of the composite nanoweb as a function of time. When the static force was applied to this hybrid sensor, static pressure could be measured through the increasing capacitance value caused by the decreasing thickness of the composite nanoweb rather than the piezoelectric current. The piezoelectric and piezocapacitive properties of this hybrid sensor could be measured simultaneously using a piezoelectric amplifier and an LCR meter, respectively. Overall, 25~50 wt.% addition of PVDF in PU greatly enhanced the piezoelectric output signal as well as reduced pressure-capacitance hysteresis and higher capacitance change with the applied pressure due to the rubbery PU of this hybrid sensor. Moreover, AgNO₃, which acts as a precursor of silver nano-particle (AgNP) formed in the electrospun PVDF nanoweb, was added into the PU-PVDF electrospun solution to get enhanced piezoelectric performance of the hybrid sensor caused by the preferential formation of β-phase of PVDF

Both polymer flexible substrates and two-dimensional materials (2DMs) have highly strict nanomanufacturing requirements that they should avoid using strong chemicals and minimize exposure to radiation damage. The current fabrication method for flexible 2DMS based devices mainly relies on transferring the devices prefabricated on rigid substrates by electron beam lithography or photolithography onto polymer substrates. This fabrication process is time-consuming and expensive, which restricts this method to a limited number of applications. Here, a water based technique, atomic force microscope mechanical bilayer lithography was proposed to improve this fabrication process by directly building devices on flexible substrates without introducing any hazardous chemicals during fabrication. It has been demonstrated to be suitable for both SiO₂/Si and flexible polyethylene terephthalate (PET) substrates. The mechanical force applied on the AFM tip can be controlled flexibly without risk of substrate damage or pattern loss. It has also been proved to be effective to build MoS₂ photodetector on flexible PET substrates, which brings more opportunities to directly construct flexible 2DMs based devices.

Printable resistive materials are as important for printed electronic devices as highly conductive materials, but they are not investigated as widely, nor are they as commercially available. Of the few demonstrations of printed resistors, most require a variable component geometry to achieve different resistances. This requirement presents challenges for circuit layout as spare board space is often scarce and valuable.

Here we present ink synthesis and printing results for variable resistance patterns that are inkjet-printed with a novel and sustainable carbon nanomaterial—multilayer graphene nanoshells, the byproduct of a biofuel synthesis process from biomass¹. The dispersed multilayer graphene nanoshells are stabilized with a surfactant (Triton X100). The optimal surfactant dosage achieves sufficient particle coverage without leading to agglomeration. Here, the optimal dosage is determined with a surface tension-based adsorption analysis technique, whose results suggest adsorption is driven primarily by hydrophobicity of the anchoring group in the surfactant molecule. The conformation of adsorbed surfactant molecules is explained by a geometric model. The difference in efficacy between sterically and electrostatically stabilized particles is demonstrated by comparing the energetic interparticle potentials between approaching particles stabilized by either mechanism.

Resistive patterns are inkjet-printed and electrically characterized. We find the resistivity of printed patterns can be varied by adjusting the solid-loading and the number of printed layers. We also discover that for few-printed layers, the presence of residual steric stabilizing material enhances the electrical conductivity of resistive patterns. This finding challenges the paradigm around the necessity of stabilizing material removal post-deposition and suggests some potential nascent and unique functionality for stabilizing materials. The multilayer graphene nanoshell inks demonstrated here present a promising new pathway toward practical and sustainable printed resistors that can achieve variable resistances within a constant areal footprint without a post-process.
¹ M. J. Wagner, J. Cox, T. McKinnon, K. Gneshin (George Washington University), “Hollow carbon nanosphere based secondary cell electrodes” U.S. 8262942B2, 2012.

With the recent advances in the field of electronic information technology, optoelectronic devices have become an indispensable part of the society and are widely used for various applications. Among them, photodetectors are extensively studied. Commercial photodetectors with active materials such as Germanium, Silicon, GaAs are rigid, which hinders their application in wearable electronics technology and further, these photodetectors are not responsive to a wide extend of electromagnetic range [1]. Hence, novel semiconductor materials which can easily be integrated with flexible substrates need to be researched for future flexible broadband photodetectors. In recent years, the research on two-dimensional (2D) materials and their heterostructures based photodetectors has tremendously increased, due to their tunable bandgap, high carrier mobility and high specific area for sufficient light harvesting [1-3]. Large number of reports published on MoS₂ based photodetectors [4], which shows the tremendous application prospects of MoS₂ in the next generation photodetectors. Pristine MoS₂ appears to show strong absorption in the NIR and visible region of the electromagnetic spectrum but not in UV locale [2,4]. The range of absorption of pristine MoS₂ in the electromagnetic spectra needs to be further increased which limits its applicability for far reaching applications. One of the possible ways to overcome this problem is by fabricating heterostructures with higher bandgap semiconducting materials such as quantum dots (QDs) [5]. QDs engineering the 2D materials by constructing 0D-2D heterostructures is an effective strategy to assist the dissociation of photogenerated electron-hole pairs by forming heterojunctions, along with increasing the range of absorption.
In this work, flexible broadband photodetector based on novel MoS₂QDs/MoS₂ on a cellulose paper substrate was fabricated, wherein MoS₂ QDs and MoS₂ flakes were synthesized by one-pot hydrothermal method. The resultant MoS₂ flakes were deposited on paper substrate using facile and scalable vacuum filtration method and thereafter, MoS₂ QDs solution was drop-casted on MoS₂/paper to form the unipolar heterostructures. Physico-chemical characterizations such as X-ray diffraction analysis, UV-vis spectroscopy, X-ray photoelectron spectroscopy, Transmission electron microscopy, Raman spectroscopy and photoluminescence spectroscopy reveals the formation, morphology and optical properties of MoS₂ and MoS₂ QDs with good stoichiometry.
The formation of heterostructures at the interface of MoS₂ QDs and MoS₂ flakes resulted in the improved responsivity and broadband photoresponse range from UV to NIR. The discretely decorated MoS₂QDs on MoS₂ flakes not only enhances the sensitivity in the UV region but also create localized electric fields that help in the efficacious separation of the generated electrons and holes upon illumination of light. Further, the metal contacts for the device were taken from MoS₂ flakes, so as to reinforce the collection of the photogenerated carriers. The fabricated device exhibited good photodetection performance with a responsivity of 3.94 mA/W, 12.82 mA/W, and 7.55 mA/W and under NIR, visible, and UV light illumination, respectively. This work provides guidance for the development of large-area and flexible paper based broadband photodetectors using mixed dimensional 0D/2D heterojunctions that finds potential applications in the field of wearable electronics.

References:
1. M. Long, P. Wang, H. Fang, and W. Hu, Adv. Funct. Mater., 2019, 29, 1803807.
2. P.T. Gomathi, P. Sahatiya, S. Badhulika, Adv. Funct. Mater., 2017,27, 1701611.
3. V. Selamneni, N. Nerurkar, P. Sahatiya, IEEE Sens. Lett., 2020, 4, 1-4.
4. H. S. Nalwa, RSC Adv., 2020,10, 30529-30602.
5. H. Qiao, Z. Li, Z. Huang, X. Ren, J. Kang, M. Qiu, Y. Liu, X. Qi, J. Zhong, and H. Zhang, Appl. Mater. Today, 2020, 20, 100765.

Following their promising demonstration in various fields such as real-time healthcare monitoring, tactile information systems, gesture recognition and so on, there is an ever-increasing demand for flexible, lightweight and wearable sensors. To address this demand, various strain sensing mechanisms have been introduced and investigated. In this work, we fabricate a wearable tactile pressure sensor using a commercially available sponge decorated with conducting 1T phase of MoS₂ nanosheets and a very thin layer of elastomer. Solution based preparation of 1T phase of MoS₂ allowed scalable and cost-effective fabrication of the wearable tactile pressure sensors. MoS₂ nanosheets were decorated onto the sponge using a simple dip and dry method. Thanks to highly porous structure of the sponge, fabricated pressure sensors possessed high flexibility, favorable sensitivity (~ 0.5 kPa^-1) and a wide working pressure range from 10 Pa to 100 kPa. A very thin layer of elastomer overcoat improved the stability of the sensors under cyclic compression and bending tests, while preserving its electromechanical properties. Finally, we demonstrated the use of tactile pressure sensors for real-time body motion monitoring and human-machine interface device. Results provided herein proves that the fabricated sensors hold strong potential as wearable sensors.

Significant research efforts have been spent on capacitive pressure sensors due to recent developments in wearable electronics. However, the fabricated sensors still have some problems yet to be solved. Due to the additional electrode layers, most of the reported parallel plate capacitive pressure sensors suffer from excessive weight, lack of air permeability and washing stability, all of which are directly related with the user comfort. To overcome these issues, a parallel plate capacitive sensor with monolithic structure was demonstrated. In this work the monolithic capacitive pressure sensor was fabricated using 3-D masking technique, using a commercially available sponge with silver nanowire (Ag NW) electrode layers and a very thin protective layer of polydimethylsiloxane (PDMS). Finite-element analysis revealed the differences in deformation mechanisms between the classical parallel plate design and the monolithic design. More homogeneous stress distribution was obtained for the investigated monolithic system. Benefitting from these unique features, the developed monolithic capacitive sensors are light, have high sensitivity (up to 1 kPa^-1), fast response/recovery times, very low-pressure sensing ability (< 1 Pa) and high mechanical stability. Finally, fabricated sensors were used to demonstrate real-time body motion monitoring.

There is a rising interest in the development of wearable sensors for monitoring human performance and health. One type of promising sensors is breath sensor that detects breath volatile organic compound (VOC) as biomarkers of certain diseases such as cancer and diabetes. However, a major challenge is the limited sensitivity and poor selectivity for many of the current breath sensors. This presentation describes recent results of an investigation of nanocomposite-structured materials as chemiresistor array sensing interfaces to enhance the sensitivity and selectivity of breath sensors. Results from both experimental testing and theoretical simulation of sensor arrays for detecting the VOCs will be discussed, revealing excellent agreement of the experimental data with the theoretical modeling. The fabrication of nanoparticle-printed chemiresistor devices on paper substrates and the formulation of the nanocomposites as printable sensing films will also be discussed. Implications of the findings for the design of advanced sensing materials and breath sensors will also be discussed.

Fibrous materials serve as an intriguing class of 3D materials to meet the growing demands for flexible, foldable, biocompatible, biodegradable, disposable, inexpensive, and wearable sensors and the rising desires for higher sensitivity, greater miniaturization, lower cost, and better wearability. The use of such materials for the creation of a fibrous sensor substrate that interfaces with a sensing film in 3D with the transducing electronics is however difficult by conventional photolithographic methods. This presentation will describe a highly effective pathway featuring surface-mediated interconnection of metal nanoclusters (NCs) and nanoparticles (NPs) in fibrous materials at ambient conditions for fabricating fibrous sensor substrates or platforms. Bimetallic gold-copper and silver-copper alloy NCs and NPs are used as a model system to demonstrate the semiconductive-to-metallic conductivity transition, quantized capacitive charging, and anisotropic conductivity characteristics. Upon coupling SMI of NCs/NPs as electrically conductive microelectrodes and surface-mediated assembly of the NCs/NPs as chemically sensitive interfaces, the fibrous chemiresistors function as sensitive and selective sensors for gaseous and vaporous analytes. This strategy has significant implications for manufacturing high-performance fibrous platforms to meet the growing demands of the advanced multifunctional sensors and biosensors.

Nanomaterials in the form of nanomembranes are thin, flexible, transferable and can be shaped into unique 3D micro and nanoarchitectures. This makes them attractive for various scientific disciplines ranging from «Electronic Soft–Wear» to «Microrobotic Hard–Ware». This talk explores the underlying science of nanomembranes and discusses the fascinating application potential of this particular class of nanomaterials ranging from integrated microelectronic skin [1] and self–assembled 3D microelectronics [2] to medical microbots [3] and the world’s smallest self–propelled microelectronic systems [4].
[1] M. Kondo, M. Melzer, D. Karnaushenko, T. Uemura, S. Yoshimoto, M. Akiyama, Y. Noda, T. Araki, O. G. Schmidt, T. Sekitani, Sci. Adv. 6, eaay6094 (2020)
[2] D. Karnaushenko, T. Kang, V. K. Bandari, F. Zhu, O. G. Schmidt, Adv. Mater. 32, 1902994 (2020)
[3] M. Medina–Sánchez, O. G. Schmidt, Nature 545, 406 (2017)
[4] V. K. Bandari, Y. Nan, D.Karnaushenko, Y. Hong, B. Sun, F. Striggow, D. D. Karnaushenko, C. Becker, M.Faghih, M. Medina–Sanchez, F.Zhu, O. G. Schmidt, Nature Electron. 3, 172 (2020)

Inkjet printing is highly promising fabrication technology for electronic devices, including energy storage systems, due to the fact that is an industrially mature, readily scalable, low-cost, digital, contactless and mask-free method with roll-to-roll capability, minimal processing steps and minute quantity of material waste. Solution-processing of nanomaterials in printed electronics via ink-jet allows for very efficient integration of different types of nanoparticles in electrochemical energy storage devices. However, it is important to address the challenges associated with the variation in printed layers properties and to investigate the effects of printing including substrates, materials, printing and processing on the electrochemical performance and response of the devices.
In this work, we have fabricated and characterised flexible, fully solution processed, co-planar, NiO-nanoparticle-based, micro-supercapacitors through inkjet printing. Cyclic voltammetry studies were performed to investigate the effect of substrate and printing resolution, different types of current collector silver inks, sintering temperatures and carbon residues in electrodes, different types of electrolytes, including liquid and gel electrolytes, binders and other organic compounds in electrolytes, and the effect of electrode gaps on the electrochemical response and capacitance of supercapacitors. The devices were prepared on flexible polyethylene terephthalate (PET), polyimide Kapton substrates and photographic paper. Two types of silver nanoparticle inks have been used as the current collectors in the devices. A NiO nanoparticle ink was developed and has been used in different concentrations as the active electrode material in the devices. Aqueous solutions of sodium hydroxide (NaOH), sodium nitrate (NaNO₃), lithium hydroxide (LiOH), potassium hydroxide (KOH) and saturated magnesium perchlorate aqueous solution (SMPAS) have been used as electrolytes, either in liquid or gel form at different concentrations. The devices were scanned at voltage a window from 0 to 1 V, at scan rates from 10 to 1,500 mV/s. Capacitance has varied significantly depending on the substrate, printing resolution and the use of electrolyte and scan rate. Some of the highest capacitance values of 117 F/cm³ at 10 mV/s were obtained on PET substrates. XPS measurements of carbon residues in active layers have been correlated with sintering temperatures and supercapacitors performance.
Highly flexible, inkjet-printed supercapacitors, with a particular focus on wearable applications, are finally demonstrated as epidermal printable supercapacitor, water-transferred on the skin of a human subject, to potentially serve as the power source for wearable health systems.
We will discuss the summary of results on the effects of substrates, printing, sintering, choice of materials on microsupercapacitors performance and outline how these results can be extended to flexible electronics, realised through printing fabrication processes, including wearable devices, batteries and energy generation-and-storage systems.

With today’s fast-paced research environment, a need is being established to bring new form-factors to new and existing technologies. One such technology RF antennas, systems and components. For this reason, it is beneficial to examine communication components and adapt their design paradigms for advanced manufacturing techniques; in this case to develop 5G antennas and waveguides in a flexible or 3D printed form factor. Our work at PERC has extended state-of-the-art printed electronic technologies beyond simple circuits, and into RF antennas and sub-systems; while also allowing IC integration with conformal/flexible designs. RF devices such as Bluetooth boards, conformal RF antennas, and planar phased array systems, have been fabricated using printed electronic technologies to allow everything from wireless external communications to phased array tracking systems [1,2].
This project investigates additively fabricated conformal 5G (6 GHz) antenna designs and their performance when printed on the conformal surfaces. Design factors such as antenna type, orientation, and RF properties are considered. Here we analyze waveguide designs such as grounded co-planar, co-planar, and microstrip for conformal single-layer and multi-layer designs. As such, we explore multiple printable material systems including: silver nano-inks, thin-flexible laminate boards, and Kapton polyimide films, in order to determine potential systems that satisfy both functionality and manufacturability requirements. Our initial designs utilize direct-write micro-pen dispensing systems in order to develop a tool-box for printed material systems which can then be applied to the Optomec 5X 5-axis direct-write system. All antenna designs (planar and conformal) are simulated in ANSYS HFSS to ensure printed, conformal layer thickness and feature sizes produce an optimal antenna response. Additionally, this work investigates potential antenna performance characteristics in relation to system longevity and durability. For example, the adhesion between printed layers to potential end-use conformal material systems, as well as the effectiveness of RF-transparent polymeric layers for ruggedization. As such, all designs are modeled to incorporate spray-coatable fluoropolymer coatings. This work describes problems that reside in conformal RF applications such as retaining printing accuracy and compensating for the rounded shape and functional properties of the surface.
[1] C. Shemelya, L. Banuelos-Chacon, A. Melendez, C. Kief, D. Espalin, R. Wicker, G. Krijnen, E. MacDonald, “Multi-functional 3D printed and embedded sensors for satellite qualification structures,” IEEE, SENSORS, Nov. 2015. 1-4.
[2] M. Haghzadeh, C. Armiento, A. Akyurtlu, "All-Printed Flexible Microwave Varactors and Phase Shifters Based on a Tunable BST/Polymer," IEEE Transactions on Microwave Theory and Techniques, 65 (6), 2030-2042, 2017.

We demonstrate a bending-insensitive pressure sensor with extremely high sensitivity (2×10⁵ kPa^-1) by the simple blade coating of a composite containing carbon flowers. To date, highly sensitive pressure sensors have been fabricated by molding microstructures such as pyramids and domes. The printing of conductive particle:elasotmer composites does not require any molds, and can simplify the fabrication. However, composite based pressure sensors have suffered from poor flexibility and large-area uniformity. This is because the percolation-based sensing mechanism can be easily influenced by the composite thickness, requiring a considerable thickness for high sensitivity. Our composite is made of an SEBS elastomer and monodisperse carbon particles, which have flower-like superstructures and can be synthesized at very low cost (carbon flowers) [1]. The unique structural properties of carbon flowers enable a contact-based pressure sensing mechanism to achieve extremely high sensitivity which is insensitive to the printed thickness. The active layer can be as thin as 13 µm thickness, which to our knowledge is the thinnest among printed pressure sensors. The capability of pressure sensing was investigated under several bending radius down to 5.5 mm and showed consistent characteristics, although most previously reported pressure sensors have only been tested in a flat state after bending. Furthermore, we printed a pressure sensor over a substantial area (1 m × 2 cm), over which the sensors showed significantly uniform pressure sensing characteristics. This is achieved by the contact-based sensing mechanism that is robust against thickness variation across the large area printed composites. Our sensor demonstrates both high pressure sensing performance and high scale-ability, which is extremely suitable for applications in human-machine interfaces, healthcare, and Internet of Things (IoTs).

[1] S. Chen, D. M. Koshy, Y. Tsao, R. Pfattner, X. Yan, D. Feng, Z. Bao, J. Am. Chem. Soc. 140, 10297 (2018).

The ultimate autonomous smart wearable device is one in which all parts are seamlessly integrated. One of main benefactors of these devices is the health sector. As currently the tools available for the evaluation of a patient’s recovery are limited to either bulky stationary equipment, or non-customized wearable devices. Therefore, with the recent strong progress in the field of 3D printing it is now more than ever possible to 3D print completely customized objects on the fly. Especially due to the digital nature of the fabrication tools used, wherein the overall process is called digital manufacturing, it is possible to adapt a general device layout for a patient's needs. Especially of interest here is that these 3D printed, and custom-tailored devices could be worn to continuously monitor the gait and joint motions of a patient during recovery. Currently this is something that often can only be measured in a specialized room within an hospital. Furthermore, the usage of 3D printing tools allows for the fabrication of more complex geometries, and functional layers not possible to produce with more conventional methods. These advantages can be exploited to introduce smart features into other 3D printed objects as well.

One of the key challenges for the full exploitation of these technologies is the integration of the printing of functional layers within a repeatable process chain. The main bottleneck being currently that the range of 3D printable materials is often limited to either hard or non-flexible materials, let alone functional ones. Therefore, we have developed and are continuously improving an elastomeric material system that is fully printable using Direct Ink Writing (DIW). Our material system is created by making use of natural cellulose based fibers to be able print self-supporting soft structures. This system thus makes it an ideal candidate to produce truly three dimensional and fully soft wearable devices with a high degree of comfort. In parallel, we are exploiting the use of a custom made 5-axis multi-tool printing platform to be able to directly print multiple materials on curved surfaces. This platform is driven by software that allows for on the fly modifications to the input model, which has resulted in a early stage approach towards personalization of our sensing devices. With the multi-tool capability, it allows us to perform all production step within the platform.

Accompanying this, we have so far investigated the fabrication of piezoresistive, capacitive and triboelectric mechanical sensors made from our materials system. These sensors have been tested in both ideal circumstances, using mechanical test benches to compress and bend the sensors, as well as by actual human motions. We will present their evaluation by means of simulated and non-simulated human motions. The simulated evaluations were performed using static compression tests, up to 1000 kPa, and dynamic compression tests, up to 2 Hz. The hysteresis characteristics of the sensors were also found using the mechanical test benches.

At this stage, we are exploiting these results to integrate the sensors into a wearable demonstrator. By using our 3D printing technologies, we also show that we can print structures in three dimensions to create geometries which are not possible or unfeasible with conventional technologies. This approach, which uses our material’s inherent self-supporting ability, allows for the miniaturization of 3D printed structures. Using this we can create more compact sensors with improved responses, as well as enable multi-modal sensing. All these factors combined allow for the fabrication of truly novel fully 3D printed sensors. Since these sensors are produced in a completely digital fashion, they can be fully personalized for each person, while using the same printing infrastructure.

Inkjet processing is a promising method for printed electronics for flexible and wearable electronics, allowing for nano- to micron-scale films with low temperature processing. In this work, we explore process engineering of inkjet printing electronic films and textiles (e-textiles) with the application of multi-layered conductive, dielectric, and semiconductor materials. Considering fluid characteristics and jetting parameters, electronic inks were formulated based on common inkjet processing conditions of low viscosity (<25cP) and 25-50 dyn/cm of surface tension. Particle-free reactive silver inks based on silver carboxylates (Ag⁺−O₂C−R₁) and aqueous ammonia ligand (−NH₂/−NR₂H) was processed both on films and textiles. The quality of the patterns was able to be controlled by the drop spacing variation (5-200 µm) with consideration of a single drop size (~50 µm). After low-temperature annealing process (150°C), the low sheet resistance of <0.5 Ω/sq was achieved by print resolution of 1693 dpi (15 µm drop spacing) - 2540 dpi (10 µm drop spacing) with only one print pass. The dielectric ink was formulated by the equal mass ratio of poly(4-vinylphenol) (PVP) and poly(melamine-co-formaldehyde) (PMF) in propylene glycol methyl ether acetate (PGMEA) solvent. Cross-linking process with UV and thermal annealing (150°C) after the inkjet process of the PVP/PMF dielectric ink as an insulator of the conductive silver layer. The inkjet processed reactive silver ink on the dielectric layer kept the dielectric layer compatible, showing potentials as the insulating materials for capacitors and transistors. In addition, p-type semiconductor ink with 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pentacene) and polystyrene blend in tetralin was able to be inkjet printed on the flexible substrates to demonstrate the compatibility with the conductive and dielectric inks within the multi-layer device architecture via inkjet processing. Our study highlights viability of fully-inkjet processed flexible electronics in wearable technology applications.

Printing methods are rapidly being developed for the improvement of fabrication efficiency in electronic devices, particularly in the field of electronic textiles (E-Textiles) of particular interest has been the formation of high efficiency antenna devices that provide a means for body area networking and data transfer. This work details a novel fabrication process for flexible textile antenna by a combination of the direct writing printing to enhance the dielectric properties of the textile. Using a direct write process, an ink consisting of a UV curable monomer with 4 wt.% 200 nm barium titanate nanoparticle loading is embedded within fabrics to enhance the dielectric constant of the naturally air porous textile architecture. This method is demonstrated to be universal for all kinds of the fabric, include nonwoven, woven, and knit fabric types. By controlling the spray parameters (time and translation speed) of the ink, the dielectric constant can be increases to 1.4 - 1.6 without significant blockage of the air gaps in the fabric. In this way, the flexibility and air permittivity of the fabric has minimal change. As a demonstration, the dielectric ink is applied to enhance the permittivity of a jersey knit fabric panel, allowing for a simplified manufacturing process for these traditionally complex e-textile systems. Combinations of nonwoven and knit layering through heat lamination is shown to provide the necessary structure for the antenna device with a conductive Ag screen print active and ground layers with a conductivity of 1.3 *10⁶ S/m. The methods explored are extended to a novel dual-port textile antenna aimed at 2.45 GHz with a low height profile and enhanced bandwidth for wearable applications, exhibiting a working bandwidth of 2.32 -2.45 GHz.

Fabric-based circuits are an important component of developing wearables. Thermistors have proven to be a difficult factor to measure accurately using a fabric. Temperature sensing is an important part of health monitoring, as abnormalities could signal conditions such as fevers, infections, or heatstroke. Early detection of these temperature deviations can lead to more effective treatments. Currently, methods of monitoring body temperature are either invasive or inaccurate, since accurately measuring the temperature of the body’s core is difficult to do from the surface of the skin. Additionally, temperature sensing could prove useful in other forms of health monitoring when paired with other types of sensors, such as humidity sensors. Herein, we demonstrate a technique to produce an all fabric, wearable thermistor. Using reactive vapor deposition (RVD) we produce a thermoresistive coating of PEDOT-Cl on a variety of commercially available fabrics. We show that the thermistive properties and sensitivities of the coatings can be tuned by using different weave porosities, fabric thicknesses, and fabric materials. We further explore the response of the PEDOT-Cl fabrics when subjected to varying humidity, or the hygro-resistivity, and how this can influence the thermo-resistivity of the coated fabrics. Additionally, we utilize these coatings to create a wearable device, capable of sensing body temperature. By measuring the temperature of several key locations on the body, the temperature of the core can be monitored. This work provides a platform to monitor body temperature in a non-invasive and comfortable way. We also use the PEDOT-Cl coated fabric to demonstrate how the thermo-resistive and hygro-resistive responses can be combined to measure breathing when incorporated into a face mask.

The growing demand of wearable technology has caused a resurgence of paper-based flexible electronics, offering a biodegradable and low-cost alternative to petrochemical substrates. In this research, “smart” papers were produced from renewable pulp fibers using a pilot-scale web former operated at 4 m/min. The surface of bleached softwood fibers was modified in situ by electrically conductive carbon nanotubes pre-adsorbed with cellulose nanofibrils during the papermaking process. The interfacial properties were tailored based on a layer-by-layer nanoassembly method, enabling good retention and uniform distribution of nanofillers in the sheet even at high content (up to 15 wt%). The as-produced multifunctional smart papers were employed as an inexpensive and sustainable alternative for various chemical and physical resistive sensing applications. An Internet of Things (IoT) device was fabricated utilizing an affordable microcontroller and simple circuit to rapidly and reliably detect variations (temperature, pH, chemical presence) in aqueous media for over thirty response cycles, with a maximum relative resistance increase of ~12,000%. This device was able to detect the presence of small volumes of water, as low as 0.5 μL, and the presence of water in organic solvents as low as 0.10 wt%, allowing for immediate signaling in circumstances where high water contents are intolerable. Electrical resistance changes remained stable and reversible in impure aqueous solutions, including tap water, and across a range of saline water concentrations which varied from 0.1 to 1 wt% NaCl. As-produced smart papers were successfully implemented as a piezoresistive device which demonstrated a wide sensitivity range (6 Pa~500 kPa) and low detection limit, with applications in gait and respiratory sensing. The versatility and efficacy of as-produced smart papers allows for a low-cost and biodegradable replacement for petrochemical substrates in multifunctional sensing applications.

Skin-compatible microfluidic valving systems with on demand sweat capture are necessary to understand the temporal variation of chemical biomarkers. Here, we demonstrate solution-based electrowetting valves with rapid actuation integrated into a flexible microfluidic sweat collection patch. The valve is produced by inkjet-printing a pair of silver electrodes with spacings of 0.2 to 2 mm and modifying the downstream electrode with a hydrophobic self-assembled monolayer. To complete the valve, a microfluidic channel is fabricated from laser ablation of adhesive layers and pressed over the silver electrodes. Artificial perspiration is driven by capillary action within the channel until stopped by the electrowetting valve. A low voltage is applied to the electrodes, altering the hydrophobic monolayer, and allowing the fluid front to continue through the microchannel. Statistical analysis demonstrates that applied voltage and not electrode spacing influences valve actuation time, with 17±8 s for 4 V and 40±16 s for 1 V. Inkjet-printing conditions were also optimized to achieve a valve fluid retention time of 9 hours. Using four electrowetting valves, an integrated wearable device is designed for artificial perspiration collection through valve actuation at distinct time points over 40 min. Finally, these inexpensive, user-friendly, and disposable electrowetting valves offer exciting opportunities for non-invasive point-of-care sweat monitoring.

Printed electronic technology has become an important tool for the production of low-cost flexible electronic devices. However, due to the limited thickness of printed materials, they have a limited electric current capacity and cannot be used for applications that need high power.
This work aims to fabricate a 3D conductivity patterned paper from nanocellulose and PEDOT:PSS based materials using syringe injection and 3D printing methods. Unlike the usual concept of electronic papers, comprising conductive ink on a paper substrate, these 3D patterned papers integrate the conductor patterns in the bulk of paper, while non-conductive sections are used as support to form free-standing papers. Two different inks, non-conductive and conductive, were derived. The non-conductive ink consists of cellulose nanofiber (CNF), microfibrillated cellulose (MFC), glycerol, and alginate in different concentrations for syringe injection and 3D printing, respectively. CNF, PEDOT:PSS, glycerol, and ethylene glycol were used to prepare the conductive ink. Both inks were deposited on a substrate using syringe injection or 3D printing, creating 3D patterned papers that can be peeled off from the substrate. The patterned papers are flexible, conformable, wearable, and strong. The conductive parts show electrical conductivity of up to 202 S/cm, current capacity above 0.7 A, and the full patterned papers have a tensile strength of 25 MPa. The non-conductive sections have some ionic conductivity, making it possible to use the conductor patterns as electrodes, and the non-conductive patterns of the same paper as electrolyte for simple supercapacitor devices. This successful 3D patterning of electronic paper will pave the way for manufacture of paper-based power transistors, supercapacitors, wearable devices or smart packaging.

Research interest in new materials for X-ray detection is rapidly growing, driven by demand from diverse applications such as medical imaging, radiotherapy, space exploration, security and defence, and personal wearable dosimeters.^[1–2] Current detectors use inorganic semiconductors such as silicon, selenium or cadmium zinc telluride. However, X-ray detectors fabricated from these materials are expensive to manufacture, and cannot be easily fabricated into flexible sensors or large-area pixelated arrays.^[4] Furthermore, these semiconductors are composed of heavier elements, which exhibit much stronger X-ray attenuation than the lighter elements in human tissue. Consequently, use of these materials for medically relevant dosimetry faces two major challenges:

1. Allowing X-rays through the detector without altering carefully calibrated beam intensity.
2. A requirement for complex and often unreliable calibrations to produce a response equivalent to the human body.

Here we report the development and characterisation of printable, flexible and fully tissue equivalent X-ray detectors fabricated from solution-processable organic conductors. The photodetectors are composed of donor polymer P3HT and two different acceptors: fullerene derivative PCBM and non-fullerene acceptor o-IDTBR. We show these organic electroactive inks are uniquely capable of both tissue equivalent X-ray attenuation and electrical conductivity. The non-fullerene acceptor material shows both higher charge carrier mobility and a greater radiation hardness, which we establish is due to greater crystallinity and improved nanoscale morphology using synchrotron-based scanning X-ray transmission microscopy. We systematically characterise the optical-to-electrical conversion efficiency of the photodetectors with a range of steady state and transient optoelectronic techniques, demonstrating that the photodetector performance of these nanoscale systems can be tuned to exhibit higher values than any other reported printable organic systems by varying the fabrication parameters.

X-ray nanosensors fabricated by coupling the photodetectors with a plastic scintillator can operate without the need for external bias, a key feature of wearable electronics. The X-ray performance is shown to be energy independent between 50 keV and 6 MeV by testing with clinical orthovoltage or medical linear accelerators, providing versatile materials for use across a wide range of applications without additional calibrations. We show the X-ray detection sensitivity can be tuned to match that of state-of-the-art inorganic semiconductors or slightly reduced to provide almost completely radiolucent devices for wearable radiation dosimetry. The X-ray nanosensors were fabricated into pixelated arrays using inkjet printing to provide spatial resolution that matches state-of-the-art medical detectors. The organic nanosensors fabricated with o-IDTBR also exhibited remarkable temporal responses, detecting a pulsed X-ray source with microsecond resolution that was indistinguishable from a silicon PIN photodiode. Furthermore, the device exhibited excellent radiation hardness, withstanding a radiation dose that represents a 5-year working lifetime.

The new materials science and flexible devices we report here are an exciting breakthrough, providing the first stable, printable, flexible, and fully tissue equivalent X-ray detectors with functionality that is tuneable for optimized X-ray attenuation, or high radiolucency for applications such as radiotherapy, where simultaneous monitoring and transmission of the X-ray absorbed dose to the human body is a required.

References:
[1] M. J. Griffith, S. Cottam, J. Stamenkovich, J. A. Posar, M. Petasecca, Front. Phys. 2020, 8, DOI: 10.3389/fphy.2020.00022.
[2] B. Fraboni, A. Fraleoni-Morgera, N. Zaitseva, Adv. Funct. Mater. 2016, 26, 2276.
[3] L. Basiricò, A. Ciavatti, T. Cramer, P. Cosseddu, A. Bonfiglio, B. Fraboni, Nat. Commun. 2016, 7, 13063.

Carbon nanotubes (CNTs) possess tremendous potential applications owing to their excellent mechanical, electrical and thermal properties. Although isolated CNTs possess excellent anisotropic physical properties, their bulk ones are, in general, significantly restricted by the highly random networks. Therefore, bulk fabrication of aligned CNT films is expected to be important for many applications to get anisotropic and high thermal/electrical conductivity along the CNT orientation direction. Various attempts have been made in the past to increase the degree of CNT alignment to realize their theoretical properties using different methods such as cutting, electrophoresis, melt processing, using crowding effect, chemical vapor deposition, templated growth, solid spinning of CNT forest, magnetic field to induce alignment, flow coating, and mechanical stretching. However, few techniques are suitable for the fabrication of highly conducting and flexible strings.
In this presentation, we report a novel method to fabricate ribbon-like films of CNT/polymer composite containing highly aligned CNT networks using a robotic dispenser [1]. This method can easily produce a uniform ribbon-like film with oriented CNTs along the drawing direction of the ribbon. We prepared 400–2000 μm wide oriented ribbon of CNT/polymer films that can be used for roll-to-roll processing in length scale. CNTs were found to be aligned along the drawing direction as confirmed by polarized Raman mapping and scanning electron microscope images. Dependence of the electrical and thermal conductivities on the degree of CNT orientation will be also discussed.
[1] M. Pandey, R. Abe, N. Okamoto, Y. Sekimoto, N. Kotaro, Y. Okajima, M. Nakamura, Appl. Phys. Express 13, 065503 (2020).

Self-healing semiconducting materials have had limited successes due to the tradeoff between excellent electronic properties and suitable rheology properties. This is because high performance inorganic semiconductor materials have a crystalline structure, unlike fluidic and amorphous polymers which use secondary bonding forces to heal. However, hybrid inorganic-organic halide perovskites are high performing semiconducting materials with crystal-liquid-like behaviors. Their fluidic crystal lattice features a large combination of secondary bonding interactions, giving perovskites their remarkable electronic properties, defect tolerance, and low formation energy. Here, we discuss our investigations in creating the first mechanically self-healing hybrid inorganic-organic halide perovskite semiconductor. A molecularly tailored self-healing polymer is coupled with fluidic methylammonium lead iodide perovskite to form bi-continuous composites capable of healing mechanical damage through synergistic grain growth and solid diffusion processes at slightly elevated temperatures. These self-healing composites are demonstrated in solar cell devices with power conversion efficiencies reaching ~10% with improved thermal and mechanical stability. This pioneer research showcases the concept of joining perovskite and organic materials to create a unique composite with first-of-its-kind functionalities.

Polymer nanocomposites with two-dimensional (2D) nanofillers like graphene (Gr) and hexagonal boron nitride (hBN) have found widescale acceptance in healthcare, automotive, aerospace, energy storage, filtration, and military applications due to their exceptional mechanical, electrical, chemical, optical and thermal properties. However, the commercial use of this class of material is hindered due to problems with the nanomaterial fabrication and composite formation process-namely impurity incorporation, agglomeration, use of chemicals harmful to the environment, multi-step processes and cost. Here, we present one step innovative method to directly incorporate Gr and hBN to thermoplastic (polysulfone) and silicone elastomer (Ecoflex®, Dragon skin®) by in-situ exfoliation of bulk precursors (e.g. graphite) within the polymer. Unlike other conventional methods, this technique does not require use of solvents or harsh chemicals for preparation of nanomaterials, which additionally minimizes the probability of inadvertent impurity introduction during processing. Additionally, nanomaterials are exfoliated directly inside the polymer, providing pristine surface to form strong covalent bonds with the polymer matrix, resulting in significant material property enhancement^{1, 2}. These flexible smart materials have high conductivity of ~1.8 S/cm and were used as wearable health monitoring sensors (pulse, breathing, speech, muscle movement), mass sensors that can detect mass as small as ~3 mg, chemical sensors that can differentiate between droplets of solvents (acetone, hexane, ethanol, water), temperature sensors that can detect temperature in the range from 10-80 C (sensitivity factor of 12), and as strain sensors for structural health monitoring (gauge factor of 50 at 0.5% strain). Due to external stimuli (temperature, chemical infiltration-based swelling, strain), the distance between graphene flakes and the dimension of the polymer matrix changes, leading to change in electrical resistance path, which is utilized for sensing. The hBN nanomaterial was added as a second filler, which acts as an effective dispersion aid for Gr in the nanocomposite. The composite structure, bonding with nanomaterials and influence of fabrication parameters were investigated with scanning electron microscopy, X-ray photoelectron spectroscopy, differential scanning calorimetry, X-ray diffraction, tensile, flexural and impact testing, Raman spectroscopy and Rheometry. Moreover, mid infrared based thermography technique with micro scale imaging capability was utilized to determine the dispersion of nanomaterials within these composites³. Our one-step, in-situ, low cost process is compatible with manufacturing techniques like injection molding, 3D printing, screen printing, soft lithography, and therefore, has the potential to be a commercially viable production technique for flexible multifunctional sensors with high sensitivity.
1. Lynch-Branzoi, J. K.; Ashraf, A.; Tewatia, A.; Taghon, M.; Wooding, J.; Hendrix, J.; Kear, B. H.; Nosker, T. J., Shear exfoliation of graphite into graphene nanoflakes directly within polyetheretherketone and a spectroscopic study of this high modulus, lightweight nanocomposite. Composites Part B: Engineering 2020, 107842.
2. Nosker, T.; Lynch, J.; Hendrix, J.; Kear, B.; Chiu, G.; Tse, S. In situ exfoliation method to fabricate a graphene-reinforced polymer matrix composite (G-PMC). Patent US9896565B2, 2018.
3. Ashraf, A.; Jani, N.; Farmer, F.; Lynch-Branzoi, J. K., Non-Destructive Investigation of Dispersion, Bonding, and Thermal Properties of Emerging Polymer Nanocomposites Using Close-Up Lens Assisted Infrared Thermography. MRS Advances, 1-8.

Recent progress in optoelectronic devices for wearable electronics, bioelectronics, and soft robotics demands their outstanding physical deformability for versatile systems in daily life. However, the reliability of devices is often severely restricted by concomitant failure, especially when excessive strains are applied beyond mechanical withstanding limits. Therefore the formation of stretchable interconnects with both high conductivity and high resolution is essential for providing conventional electronic systems an extra degree of deformability with high device integrity. This talk presents a high-resolution, reconfigurable 3D printing using liquid metal alloys and its application for wearable electronic devices with stretchable integrations. A minimum line-width of 2 μm can be reliably formed using direct printing through a nozzle, and these printed patterns can be successively reconfigured into diverse free-standing 3D structures, while maintaining pristine resolutions. Furthermore, the high-resolution printing of 3D coil antennas and interconnects with wearable biosensors and displays represents substantial progress toward next-generation electronics.

Low melting point metals and alloys such as those utilizing gallium and bismuth have garnered significant attention in flexible electronics due to their ability to provide highly repeatable electrical performance under elongation, compression, and other harsh mechanical conditions. While these properties have been demonstrated in the lab to impressive degrees, the ability to process these materials and package them in order take advantage of their highly attractive mechano-electronic properties within textiles has been limited due to difficulties of processing the bulk material and the need to retain the liquid in pre-defined areas. Through taking advantage of the increased implementation of additive manufacturing to develop wearable electronics, a printable liquid metal ink has been developed in our lab which fully sinters without the need for high temperatures, providing flexible and strain tolerant liquid electronics. The key enabler of this ink is our ability to chemically tether each particle to its neighbor, creating a polymerized Liquid Metal Network (poly-LMN). The polymerized network forms a self-supporting framework that retains the liquid ink as deposited until full encapsulation is performed, allowing for high definition features (<30 um linewidth) and the autonomous formation of an internal tortuous conductive path that provides strain insensitivity to the circuits up to at least 700% strain. Applications of poly-LMNs range from wearable power distribution, physically deformable antennas, and stretchable resistive heaters.

The sensing of diverse deformations is an important functionality in soft electronics and wearable devices intended for use in health care, Internet-of-Things, or soft robotics. Current approaches usually rely on intricate arrays of sensors, which have no spatial resolution and cannot distinguish between different types of deformation. An alternative strategy toward distributed and multimodal sensing is electrical reflectometry in transmission lines. However, to create sensitive mechanical probes, the transmission lines must be built entirely with soft materials and feature a controlled microstructure over extended lengths. Here, we report the scalable fabrication of soft and stretchable transmission lines, composed of tens of liquid metal microchannel conductors meticulously arranged within an elastomer dielectric.[1] The interrogation with time-domain reflectometry converts the soft transmission lines into effective probes of deformation, enabling the detection of the mode, intensity, and location of multiple pressing and stretching events that are simultaneously applied. Additionally, the dynamically responsive line conductors yield a pressure sensitivity that is increased by a factor of 200 compared to conventional hard lines. Integrated within a larger fabric, an individual soft transmission line with a single interface port can decipher convoluted mechanical stimulation, bringing a new paradigm to sensing in functional textiles and robotic skins.

Reference:
[1] Leber, A. et al. Soft and stretchable liquid metal transmission lines as distributed probes of multimodal deformations. Nat. Electron. (2020). doi:10.1038/s41928-020-0415-y

In the last few decades, the evolution of electronic devices has shown unprecedented progress, due to various discoveries of advanced materials, and improvements in fabrication technologies. Much more complicated tasks can be dealt with by miniaturized devices, such as smart wearable systems, which seemed impossible in the past. Electronic devices will become smaller and smaller in the future for multifunctionality, high performance, and power saving purposes, and will be densely integrated and interconnected on limited areas. In particular, wearable devices that are deformable, and mountable on skin, need to be monolithically integrated on a single substrate for multifunctionality and miniaturization of devices. However, as electronic components, such as light emitting diodes (LEDs), transistors, and various sensors become smaller at microscale, there are some limitations on integrating them on the proper substrate by a general assembly method. Metal wiring, metal soldering, and anisotropic conductive film (ACF) are the most common integration method for electronics. However, metal wiring and metal soldering suffer from size limitation, significant reduction of the overall throughput yield, and poor mechanical stability. The ACF bonding requires high pressure of 2 – 5 MPa (20 -50 bar) and high temperature of 200 °C simultaneously for successful bonding. This process could be expected that the integration of a large number of microscale electronics needs significantly higher force to form electrical interconnect, which potentially raises assembly failure.
Recently, research has been conducted to improve the fine pitch capability of interconnection by applying various methods, such as metal-coated polymer ball, multi-layered ACF and non-conductive film (NCF), sidewall insulation, and silver nanoparticle ink with eutectic gallium indium (EGaIn) liquid metal. But they still display multiple limitations such as size limitation and poor mechanical stability.
Here, we introduce a facile electrical interconnection method driven by selective dewetting of polymer adhesive that is applicable to a deterministic microelectronics assembly for flexible electronics. The interconnection system consists of polymer adhesive with nano-sized metal particles, or on protruding structured electrodes, so it can be used by a simple coating process, like spin-coating and jetting. As the stability and interfacial potential of thin polymer adhesive depend on the materials of substrate and adhesive, and the thickness of adhesive, the wettability and dewetting phenomenon of polymer adhesive could be controlled by modulating the condition of the coated polymer adhesive, and the size of nanoparticles (or morphology of the structured electrode). The minimum feature size for assembling microdevices is 30 μm × 60 μm for microscale inorganic LED with 20 μm × 15 μm electrode size and pitch. The transfer printing yield in terms of devices operation was ~99.9 %, so that thousands of micro LEDs can be printed at high throughput on a substrate with gold circuits. This interconnection of the microelectronics was strictly confirmed by harsh industrial levels under dramatic temperature change (-40 – 85 °C for 5 min, ramping rate: 25 °C/min) over 300 cycles, and high humidity and temperature environment (85 °C, 85 % relative humidity) for 300 hours. In addition, the theoretical calculation for the dewetting phenomenon is revealed.

An extensive range of nanocarbon, such as 0D fullerene, 1D carbon nanotubes and nanofibers, 2D graphene and graphene oxide, and 3D carbon aerogels have attracted significant attention from the scientific society. Because of its excellent properties and outstanding performance, nanocarbon has been demonstrated successfully in the application of fabricating electrodes towards wearable electrochemical energy storage and sensing devices. Graphene, being a well-studied and outstanding electrode material has already been evidenced by its performance for manufacturing supercapacitors and skin electronics. The direct laser writing was reported to be an innovative and efficient methodology that enabled to fabricate graphene electrode patterns within a single-step. Inspired by the dual-functions of piezoresistive and electrochemical properties offered by laser-scribed graphene (LSG), the LSG can be fabricated as the integrated flexible device with both functions of strain sensor and supercapacitor. In this work, we have demonstrated the LSG utilized in the integrated supercapacitor-sensor system for the first time. When two LSG electrodes were used as a supercapacitor, its current density was improved from 0.02 to 0.6 mA/cm² with the rising scan rate from 10 to 200 mV/s. The highest capacitance was calculated as 18 mF/cm² at the consistent current density at 0.02 mA/cm². Also, the LSG supercapacitor showed excellent cycling stability after 5000 cycles. When the single electrode from the LSG supercapacitor was adopted as a strain sensor, the LSG electrode enabled to detect the phalangeal joint’s motion by its resistance changes while bending up and down. The single LSG electrode could detect the strain signals simultaneously when charging and discharging the LSG supercapacitor. The integration strategy has permitted the possible promises for developing self-powered intelligent sensory, flexible, wearable, and healthcare electronics.

Organic electronic devices are regarded as promising for wearables because the deposition of organic semiconductors is compatible with various substrates with a low thermal budget. This key benefit can allow for the realization of devices with versatile form factors such as flexible, foldable, and conformable devices that are considered desirable particularly for wearable applications. However, form factors are not the only advantage of organic electronic devices from the perspectives of wearable devices. Many of the organic semiconductors have rich optical properties, the strength of which has already been well recognized by the success of displays based on organic light-emitting diodes (OLEDs) and by the impressive progress recently made for organic solar cells. In this talk, we discuss what we can and what we need to do to unlock the full potential of organic optoelectronic technologies for wearable devices. The first part of the talk is dedicated to introducing some of our key efforts to make OLEDs ultra-flexible, stretchable, and more efficient. The second part then presents a case study where some of the inherent benefits of organic optoelectronic technologies are employed to realize organic wearable health-monitoring sensors with far less power consumption than that of conventional commercialized sensors with maximized photon utilization.

The power conversion efficiency of organic solar cells has been significantly improved over the past decade. The certified power conversion efficiency (PCE) of single-junction organic solar cells exceeded 17%. In addition to a high PCE and excellent operational stability, good mechanical durability is also important to wearable/textile applications to ensure continuous operation under various moving conditions. It is possible to reduce the thickness of organic solar cells by using an extremely-thin (<10 um) polymer substrate. Owing to their extreme light-weight, improved bending stability, good conformability, and improved stretchability, these types of ultrathin solar cells cause minimal discomfort when integrated with textiles. Such ultrathin organic solar cells expand the applicability of flexible organic solar cells as power solutions for wearable electronics or for soft robots in the society of internet-of-things (IoT) in which numerous sensor devices need to be used. Recently, some integrated devices with these cells—such as conformable sensor systems—have been reported and used as power sources.
In this talk, we show our recent progress of flexible organic solar cells and integrated systems for wearable/textile electronics. First, we introduce how we improve the PCE and stability of ultra-thin organic solar cells [1]. Then the strategies to achieve good stability including air[2], high-temperature [3], water [4], and light illumination conditions [5][6] are discussed. Finally, we show some examples of integrated systems with charging devices [7], actuators [8], and sensors [9].

[1] W. Huang et al., Joule 2020, 4, 128.
[2] Z. Jiang et al., Proc. Natl. Acad. Sci. 2020, 117, 6391.
[3] X. Xu et al., Proc. Natl. Acad. Sci. 2018, 115, 4589.
[4] H. Jinno et al., Nat. Energy 2017, 2, 780.
[5] H. Kimura et al., Adv. Mater. 2019, 31, 1808033.
[6] M. Takakuwa et al., Small Methods 2020, 4, 1900762.
[7] R. Liu et al., Adv. Energy Mater. 2020, 2000523, 1.
[8] K. Yu et al., Proc. IEEE 2019, 107, 2137.
[9] S. Park et al., Nature 2018, 561, 516.

Wearable strain sensors are essential for the realization of applications in the broad fields of remote healthcare monitoring, soft robots, immersive gaming, among many others. These flexible sensors should be comfortably adhered to skin and capable of monitoring human motions with high accuracy, as well as exhibiting excellent durability. However, it is challenging to develop electronic materials that possess the properties of skin. The presented skin-like electronic material exhibits ultrahigh stretchability, repeatable autonomous self-healing ability, quadratic response to strain, and linear response to flexion bending. This conductive polymer system, under ambient conditions, synergistically constructs a regenerative dynamic polymer complex crosslinked by hydrogen bonds and electrostatic interactions, which enables these unique properties. Sensitive strain-responsive mechanisms owing to the homogenous and viscoelastic nature provide omnidirectional tensile strain and bending deformations. Furthermore, this material is scalable and simple to process in an environmentally-friendly manner, paving the way for the next generation wearable sensors.

Several strategies have been developed to impart stretchability to polymer semiconductors, which can be divided into two categories: 1) chemical structure modification, and 2) post-modification. However, altering chemical structures often results in reduced mobility, whereas post-modification of semiconducting polymers easily suffers from batch-to-batch variation. Here, a conjugated donor-acceptor terpolymer design is used to control microstructures of diketopyrrolopyrrole-based polymeric chains, which significantly improves stretchability of the polymers without any mobility and elasticity compromise. Analysis on the microstructures of the terpolymers confirms that they show well-maintained short-range ordered aggregates, while their crystallinity is significantly reduced. More importantly, the terpolymer films retain most of their ordered domains even under strain, which leads to record high mobility under strain. Fully stretchable transistors fabricated with our newly established semiconductor films exhibit one of the highest mobilities after repeated strain. All of the above results indicate that our simple molecular design is an effective way to achieve high-performance and stretchable semiconductors.

Stretchable electronics are emerging technologies that develop devices with the ability to conform to nonplanar and dynamic surfaces such as a human body. Especially, stretchable light emitting devices (s-LEDs) could be potentially applied to tactile displays and biomedical devices. Several approaches based on geometrical designs such as mechanical buckling and island interconnect have been used to fabricate s-LEDs. However, geometrical designs possess several limitations. For instance, s-LEDs based on mechanical buckling approaches have high surface roughness, which prevents conformal contact with human body. Also, s-LEDs based on mechanical buckling were only uni-axially stretchable, not bi-axially stretchable. Very complicating fabrication processes were required for s-LEDs based on island interconnect, also an areal density of s-LEDs was highly limited to be stretchable. Hence, a demand for s-LEDs consisting intrinsically stretchable (is-) constituent layers continues to increase. However, the researches of is-LEDs are still largely in its infancy. Most works related to is-LEDs were mainly focused on the development of the stretchable and transparent electrodes. Hence, development and in-depth investigation of other is-constituent layers have not been performed yet. In this work, we reported is-organic light emitting diodes (is-OLEDs) which constituent materials are all highly stretchable. To accomplish this objective, all constituent layers including anode, hole transport layer (HTL), emission layer (EML), electron transport layer (ETL), and cathode were carefully modified to be highly stretchable. For anode and cathode, hybrid materials between organic and low-dimensional inorganic nano-materials were used. To make HTL and EML mechanically stretchable, surfactant and polymer blending induced nano-structure modification method was applied. This unique method enabled realization of is-functional layers with superior mechanical and electrical properties. The resulting is-OLEDs can emit light when exposed to strains as large as 80%. The turn-on voltage is as low as 8 V, and the maximum luminance is 4500 cd×m^-2 on both the anode and cathode sides. It can also survive repeated stretching cycles up to 200 times, and small stretching to 50% is shown to significantly enhance its light-emitting efficiency. These results showed that the is-OLEDs superior mechanical stretchability compared to previously reported is-LEDs. Also, owing to the versatility of our approaches, is-OLEDs with various colors including red, green, and blue were successfully fabricated, which is essential in the realization of the stretchable displays. We expect that our works would be a milestone for development of is-optoelectronic devices by offering a general platform for is-optoelectronic devices.

Polymer semiconductors have gathered much interest for their potential in next-generation deformable and wearable electronics applications. The optimum conjugated polymer film morphology for efficient charge transport consists of crystalline domains and effective inter-domain connections. Unfortunately, such a film morphology usually suffers from device failure under small strain deformation. Thus, it remains challenging to design polymer semiconductors that can maintain good electronic properties under mechanical strain.

The field of stretchable polymer semiconductor has witnessed rapid growth in recent years. Various strategies have been reported, including: (1) adding a secondary component and (2) molecular structural modification. The general design principle is to decrease the overall crystallinity of polymer semiconductor film, providing energy dissipation mechanisms from polymer conformation change during stretching. More design principles need to be investigated, to expand the tunability of polymer semiconductor mechanical properties and even imparting more functionalities.

Dynamic bonding has been widely incorporated into elastomers and hydrogels to enhance their mechanical properties. However, this strategy has not been used extensively to develop stretchable semiconductors. Our group reported that introducing 2,6-pyridine dicarboxamide (PDCA) H-bonding unit into a conjugated polymer backbone helped maintain its electrical performance under strain. The breakage of H-bonding between PDCA units was one of the mechanisms responsible. However, the structure-property relationship regarding H-bonding interactions and strain energy dissipation in polymer semiconductors needs to be further understood.

In this study, conjugation breakers (CBs) with different H-bonding chemistry and linker flexibility are designed. Two structural parameters need to be considered: (1) chemistry of H-bonding unit, and (2) linker between π-conjugated backbone and H-bonding unit. First, we choose urea, amide and urethane. They have structural similarities and can serve as fair comparisons in terms of H-bonding strength. Second, we use alkyl-chain or ether-chain as the linker. Choosing different linkers will influence the packing order of these H-bonding units, thus providing the possibility to tune H-bonding domain size and distribution.

After these H-bonding CBs are incorporated into a Diketopyrrolopyrrole (DPP)-based conjugated polymer backbone, the effects of H-bonding interactions on polymer semiconductor morphology, mechanical properties and electrical performance are systematically investigated. We observe that CBs with H-bonding self-association constant >0.7 or denser packing tendency are able to induce higher polymer chain aggregation and crystallinity in as-casted thin film, resulting in higher modulus and crack on-set strain. Additionally, the rDoC (relative degree of crystallinity) of the stretched thin film with the highest crack on-set strain only suffers a small decrease, suggesting the main energy dissipation mechanism is the breakage of H-bonding interactions. By contrast, other less stretchable polymer films dissipate strain energy through the breakage of crystalline domains, indicated by drastic decrease in rDoC. Furthermore, we evaluate their electrical performance under mechanical strain in fully stretchable field-effect transistors. The polymer with the highest crack on-set strain has the least degradation in mobility as a function of strain. Overall, these observations suggest that we can aptly tune the mechanical properties in polymer semiconductors by modulating intermolecular interactions, such as H-bonding chemistry and linker flexibility. Such understanding provides molecular design guidelines for future stretchable semiconductors.

Two-dimensional metasurfaces comprised of ordered nanoparticle arrays can be tailored to obtain desired electromagnetic properties, which depend on the size, shape, composition, and spacing of the nanoparticles. Potential applications of these materials include active and lightweight textiles, e-textiles, smart fibers, signature management, power/energy harvesting, encryption, and identification capabilities. These ‘real world’ applications typically require magnetic nanoparticle (MNP) arrays on flexible substrates; therefore, there is a need for research that addresses the fabrication and performance of these materials in the context of flexible, wearable platforms.

Traditional fabrications strategies for MNP arrays require costly equipment and highly trained personnel. For example, e-beam lithography has been used to fabricate such arrays; however, this technique is not feasible for large-area samples. In addition, the majority of MNP arrays are fabricated on Si wafers, which provide an ideal, flat, surface, but do not readily translate to flexible platforms or roll-to-roll processes. In this presentation, we will discuss fabrication strategies for large-area MNP arrays on flexible substrates, along with the resultant optical and magnetic properties. The fabrication strategies are based on nanosphere lithography (NSL) and nanoimprint lithography (NIL). In the NSL strategy, polystyrene spheres (PS) are packed in a monolayer on the selected substrate and used as a template to produce ordered particle arrays. The size and spacing of the MNP array can be controlled by the size of the PS beads and other fabrication parameters. Several NSL methods can be used to produce the MNP array—the method depends on the desired MNP array dimensions and the substrate material. In the NIL strategy, a 3D mold is used to form the desired pattern in a UV curable polymer on the selected substrate. Working NIL molds are made from a single master template using e-beam lithography, which enables formation of precise structures in a wide range of geometries with dimensions down to tens of nanometers. There patterns can be used to either contain MNPs or the patterns can be formed from MNPs. NIL techniques lend themselves to large-area, cost-effective manufacturing using roll-to-roll technology.

Understanding the optical and magnetic properties of MNP arrays enables future design strategies for flexible, wearable, multifunctional optical and electronic devices. For example, total integrated transmission and spectral scattering were measured using a spectrophotometer with an integrating sphere accessory to account for scattering characteristics of the nanostructured arrays on flexible substrates. Nanometric Co films and MNP arrays on Kapton were subjected to Superconducting Quantum Interference Device (SQUID) magnetometer measurements. Finally, obtained hysteresis loops (magnetic moment vs. applied magnetic field) were used to determine the saturation magnetization, M_s, and magnetic coercivity, H_c, of Co films and MNP arrays.

The tremendous development of flexible electronics seems to pave a way to develop the three-dimensional optoelectronics, including hemispherical and polyhedral image sensors, and curved display system, which offers low optical aberration and wide field of view, from planar membrane-type electronics. Regarding optoelectronics, there are critical issues for 2D to 3D transformation. First, the devices require a transition of rigidity, soft for transformation, but rigid in the final shape. In the case of using membrane-type electronics, its flexibility allows the former but does not allow the latter, which naturally requires a supportive framework. Second, in order to minimize the optical path from or to the devices, the device layer should be placed on the top surface of the supportive framework; this effort naturally causes out of the neutral mechanical plane. In this case, during the deformation into either inward or outward curvature, the stress on the device naturally suffers from compressive or tensile tension. Third, In order to allow the pixel matrix design with common column and row connection lines, the Kirigami approach, which cuts off some areas, should be avoided. Fourth, removing the blind area is also important to enrich the visual information. The strategy uses the acrylonitrile-butadiene-styrene (ABS) framework that can support a membrane-type electronic layer and can undergo plasticized by using small molecules such as acetone and N,N-dimethylformamide (DMF). Sorption and desorption of the plasticizer in a controlled manner allows a dramatic decrease of elastic/hardening moduli of ABS by a thousand times of magnitude, with increased fracture strain up to 240%, which prevents mechanical failure from the deformation. Notably, a viscoplastic reflow of the ABS substrate can reduce stress/strain on the device layer, even in the case of placing the device layer on the top surface of the ABS substrate. Moreover, confining the plasticizing region within a microfluidic channel allows extreme folding of the ABS framework, and thereby a nondisruptive tucking-based origami to form a polyhedral optoelectronic device without cutting the connection lines. As an application, we demonstrate a tetrahedral image sensor for an omnidirectional camera^[1] and double-sided/holographic/ hexahedral LED arrays.^[2]

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (No. 2018R1A2B2005067).

References
[1] H. S. Jang, G.-G. Kim, S. H. Kang, Y. Kim, J. I. Yoo, S. Yoo, K.-K. Kim, C. Jung, and H. C. Ko, Adv. Mater., 30, 1907384 (2018).
[2] G.-G. Kim, Y. Kim, S. Yoo, H. S. Jang, and H. C. Ko, Adv. Mater. Technol., 5, 2000010 (2020).

With the increasing demand for wearable devices, realization of high-performance flexible electronic circuits becomes an important topic. Some oxide-semiconductor-based thin-film transistors (TFTs) have exhibited good operational stability and can be fabricated with a mass production compatible technology at low substrate temperatures. Therefore, they have been considered as potential candidates for flexible on-plastic circuits. Three-dimensional integrated circuit technology has been widely used in memories, image sensors, and logic circuits. The vertical integration of electrical components can enhance the device density. So far, only a few flexible oxide-TFT-based complementary inverters have been reported. In this work, we successfully demonstrate vertically-stacked complementary inverters by monolithically integrating n-type bottom-gated zinc oxide (ZnO) TFTs and p-type top-gated tin monoxide (SnO) TFTs on polyimide foil substrates. The ZnO TFT shows a linear field-effect mobility of 0.6 cm²V^-1s^-1, subthreshold swing of 0.31 Vdec^-1, and on/off current ratio of >10⁷. The corresponding values for the SnO TFT are 0.7 cm²V^-1s^-1, 1.5 Vdec^-1, and 10⁴, respectively. The vertically-stacked complementary inverter exhibits a static voltage gain of 36 V/V at a supply voltage of 10 V. To investigate the influence of mechanical strains on the electrical performance, the devices are evaluated when bent to various curvatures outwardly and inwardly. There is no significant change in the voltage gain and noise margins when the inverter is subjected to a tensile or a compressive strain. However, the stability against gate-bias stress for the SnO TFT deteriorates when a mechanical strain is applied, which may be mitigated by introducing an encapsulation layer to reduce the distance between the device and mechanical neutral plane.

High quality piezoelectric thin films are typically crystallized at temperatures above 500°C, so integration in a flexible form factor can be challenging. In this work, two approaches will be described: films on metal foils for energy harvesting applications, and transferred films on polyimide substrates for ultrasound applications. For energy harvesters, Pb(Zr,Ti)O₃ (PZT) and (K,Na)NbO₃ (KNN) based harvesters using the piezoelectric compliant mechanism design, with resonances at 20, 40 and 70 Hz. Energy harvesting with the compliant design is more than twice as much efficient as for the simple cantilever beam. At 70Hz, the measured power densities corresponding to PZT and KNN are 113.9 and 3.1 µW/cm² respectively. The power density ratio between PZT and KNN is of the same magnitude as the ratio of their respective Figures of Merit. For ultrasound applications, PZT films were released from a high-temperature-compatible substrate by dissolution of a ZnO release layer. This enabled the design and fabrication of flexible ultrasonic transducers using 1 µm thick PZT on 5 µm thick polyimide substrates. The transducers were patterned single elements of PZT bar-resonators operating in width extension mode with center frequencies in the 8-80 MHz range. Pitch-catch was demonstrated with two 100 µm × 1000 µm neighboring elements respectively transmitting and receiving against an aluminum reflector at 1.5 cm distance. The element in receive detected a 0.2 mV pre-amplified signal for a driving frequency of 9.5 MHz.

Skin-conformable and mechanically imperceptible wearable electronics, a.k.a. epidermal electronics, is one of the trending fields in personal and biomedical device research. Although ultrathin, ultrasoft biometric sensors have been widely developed, most of them are dependent on rigid and bulky batteries for power, which is still limited in energy capacity. Wearable energy harvesters hence emerged as a non-depletable power source. However, ultrathin generators with ultrahigh power output are yet available. We introduce a stretchable and wearable piezoelectric generator made of 28-µm-thick polyvinylidene fluoride (PVDF) serpentine ribbons. Through analytical and numerical modeling, the serpentine shape and electrode coverage has been optimized to yield a power density of 0.24 μW/cm² at 50% applied strain under a frequency of 0.5 Hz. The combined thinness, high stretchability, and high power density qualify our generator as one of the top wearable energy harvesters. We have applied this stretchable generator at the crook of the human arm as a wearable energy harvester. We demonstrate that it was able to support the reading and storage of temperature every 10 minutes without battery. All the stored data can be read through an NFC-enabled cell phone whenever the cell phone approaches the e-tattoo.

Despite the fast development of various energy harvesting and storage devices, their judicious integration into efficient, autonomous and sustainable wearable systems has not been widely explored. Here, we introduce the concept and design principles of microgrids to the world of wearable electronics by demonstrating the operation of a multi-module bioenergy microgrid system. Unlike earlier hybrid wearable energy systems,a bioenergy microgrid relies solely on human activities to work synergistically and autonomously, harvesting biochemical and biomechanical energy using energy harvesters such as sweat-based biofuel cells and triboelectric generators, and regulating the harvested energy via wearable energy storage modules such as batteries and supercapacitors. Through careful energy budgeting and performance pairing, the system can offer features like fast-booting, extended-harvesting, and smart energy management to efficiently power various wearable electronics. Implementing the “compatible form factors, commensurate performance and complementary functionality” design principles, the flexible, wearable bioenergy microgrid offers attractive prospects for the design and operation of efficient, sustainable and autonomous wearable systems.

Next-generation photovoltaics provide important energy sources for flexible devices. Among them, perovskite solar cells (PSCs) stand out for their high performance and low cost that is associated with solution processability. However, the critical bottleneck in commercializing PSCs is the processing speed. Thermal annealing is widely used in research labs as the post-deposition processing method for transport layers and active layers of PSCs. These minutes or even hours of processing time are incompatible with high-throughput manufacturing methods, e.g. roll-to-roll processes. At web speeds of 30 m/min, a typical 20 min annealing time would translate to an impractical 600-m-long oven. Additionally, thermal annealing is an equilibrium heating method, i.e. substrates and the surrounding environment are heated up together with the films, incurring large energy usage and inducing mechanical failure due to coefficient of thermal expansion mismatch between different layers. Consequently, eliminating thermal annealing steps is the key to achieve high-throughput, flexible PSCs. In this work, we have successfully utilized a pulsed light irradiation approach, photonic curing, to anneal both NiO and mixed cation perovskite (CsFAMA) films on Willow^® Glass substrates. We substantially reduced the total processing time of p-i-n PSCs. The processing time for sol-gel NiO is below 0.5 ms, which is 10⁶ times shorter than thermal annealing (0.5 ms vs. 45 min). The processing time for CsFAMA is 20 ms, which is a 10⁵ reduction as well. The materials properties, including optical absorption, crystallinity, surface morphology, and work function, as well as device performance are compared between thermally annealed and photonically cured films. Successful fabrication of PSCs using photonic curing will establish it as a viable path forward to realizing high-throughput manufacturing of flexible PSCs.

Triboelectric generators and sensors have great potential as self-powered wearable devices for energy harvesting, biomedical monitoring, and recording human activity. Here, we report a process for 3D printing highly flexible functional fibers, stretchable membranes, meshes, and hollow 3D structures using elastomeric (silicone) metal (Cu)-core extrudates to produce triboelectric nanogenerators (TENGs). The triboelectric performance of single 3D-printed functional fibers and 3D-printed membranes is quantified by cyclic loading tests, which showed maximum power densities of 31.39 and 23.94 mW m^-2, respectively. The utility of the flexible silicone-Cu TENG fibers and the 3D printing process is highlighted through various applications to wearable mechanosensors for organ and human activity monitoring. The use of machine learning algorithms for classification of sensor response is also discussed. Overall, this work expands the conductive and functional materials palette for 3D printing and encourages the use of 3D-printed triboelectric devices for self-powered sensing applications in biomanufacturing, medicine, and defense.

With the rapid development of the multi-sensing electronic skin (E-skin) imitating human skin by converting the external stimuli into electrical signals, the needs for maintenance-free operation of E-skin have been triggered. To bridge the E-skin of the users and their surrounding environment, the sensors are core components with their functions, such as detecting the physiological signals of a person or noticing the hazardous surroundings. As the number of the sensors and the functions increases, the needs for the external power supply like batteries are still unsolved with the problems of recycling, health and environmental hazards, and miniaturization. Recently, to fabricate the power sources operating the many sensors light-weighted and miniaturized, various kinds of nanogenerators converting mechanical, chemical, thermal, or solar energy into electricity have been developed. Among those power generators, the triboelectric nanogenerators (TENGs) have attracted massive attention for scavenging the energy from the mechanical motions. By contact electrification and electrostatic induction at the interface of the TENG and the touching object, TENG has been used as a touch sensor. Using the impedance matching property of the TENG to the sensor (passive electric element), we could expect the self-powered sensing without any other power supply. In this study, we successfully fabricated the multifunctional tactile sensor using the gel polymer electrolyte capacitor. Additionally, we utilized electrospun P(VDF-TrFE) nanofibers for supplying enough energy to operate the multifunctional tactile sensor with recognizing the initiation of the touch. Embedding the nanofibers in the PDMS elastomer, we can fabricate an efficient, flexible, and transparent TENG. For sensing temperature and pressure in the single sensing component, we exploited P(VDF-TrFE-CTFE) relaxor nanofiber and temperature-responsive [EMIM][TFSI]/P(VDF-HFP) gel polymer electrolyte. Porous structure made by the relaxor nanofibers can induce the polarization and be the channels for the ion transfer. Those transferred ions are the factors for the capacitance increases by pressure and temperature stimulus. Our multifunctional tactile sensor showed the characteristic pressure response of the wide sensing range (0~100 kPa) with ~0.05 /kPa sensitivity and the temperature response of the high sensitivity up to 0.283 /^oC compared with other wearable temperature sensors. The multifunctional sensor is integrated with the TENG expecting that sensor can transduce the generated signals at the outer touching surface depending on the pressure and the temperature. Using a self-powered multifunctional sensor, we could successfully map the pressure and the temperature at the same time without any extra power supplier.

Fiber-based organic photovoltaics (OPV) offer the advantages of low-cost, rapid fabrication, ease-of scale up, high flexibility and versatility of tailoring properties through molecular design of the active materials. We explore a solution-coating process to fabricate fiber-based OPV comprising of a working primary electrode (polished 316LVM stainless steel wire) coated with a titanium dioxide electron transport layer, followed by a nanostructured phenyl-C61/71-butyric acid methyl ester: Poly(3-hexylthiophene) (PC61BM:PC71BM:P3HT) active layer and a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole transport layer, which is wrapped around by a secondary counter electrode (silver-plated copper wire), for extraction of dissociated charge carriers. The device is encapsulated in an exfoliated mica (e-mica) reinforced modified epoxy based UV-curable cladding for imparting oxygen and moisture barrier properties. Individual wire specimens yielded a power conversion efficiency (PCE) of 3.4 ± 0.4% and an output power density of ~ 1.5 × 10-4 W/cm2. Multiple weaving configurations of the energy harvesting textiles based on fiber-based OPV was explored on a thread controller 2 (TC2) digital Jacquard loom to yield 10 × 10 in2 OPV modules. The PCE, effective series resistance (Rser),), fill factor (FF), open-circuit voltage (Voc), short-circuit current density (Jsc) and maximum power output of this energy harvesting textiles will be reported.

P(VDF-TrFE) poly(vinylidene fluoride-trifluoroethylene) is a well-known piezoelectric electroactive polymer (EAP) with high crystallinity. If the third monomer (CTFE) is introduced to form P(VDF-TrFE-CTFE) poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene), the ferroelectric domains will be fragmented into a relaxor ferroelectric domains which is nano polar region with high dielectric constant. In this study, we fabricated electrospun nanofibers of P(VDF-TrFE) and P(VDF-TrFE-CTFE). We conducted wide angle x-ray diffraction (WAXD) to study the impact of lattice structure and crystallinity on the mechanical and piezoelectric properties.The mechanical and electrical properties of the copolymers and terpolymers were compared.

Soft wearable robotics is an emerging field which looks at methods of enabling functional, powered support to assist human movements using lightweight, low-profile, and flexible materials. Textiles hold great promise in establishing a platform to develop assistive devices that fit and feel like regular clothing. A diverse team of researchers in the Biodesign Lab at the Harvard School of Engineering and Applied Sciences are exploring the use of inflatable soft wearable robotics that leverage the functional properties inherent in textiles to provide mobility assistance for people with hand weakness through a soft robotic glove.

Hand weakness limits a person’s ability to perform common activities of daily living such as eating, drinking and writing that can be the result of many conditions including stroke, ALS and spinal cord injury. For stroke survivors, hand function rehabilitation is one of the most important aspects to improve their quality of life and independence. Access to therapy is a key factor in a person’s ability to perform rehabilitation exercises and recover motor function, however most stroke survivors do not receive the amount of therapy they need due to budget, time, and mobility constraints. Furthermore, achieving the high amount of repetitive hand movements required for successful rehabilitation is a major challenge in conventional methods of therapy.

The aim of our research is to show the viability of using a soft robotic glove to perform home-based hand rehabilitation for stroke survivors. The soft robotic glove facilitates high dosage repetitive stretching, active movement, and functional task training exercises through the use of comfortable, textile-based inflatable chambers that gently straighten and bend the fingers while an engaging digital interface guides the user through their exercises with motivating games.

Soft wearable robotic devices present a promising approach to hand rehabilitation due to their lightweight, compliant, and low-cost design. This talk will explore the development of the soft robotic glove through the lenses of functional apparel design and will present the human-centered design methods used in the collaborative development process.

The future of textiles is electronic, with new wearable devices integrated into “smart clothing” systems that will incorporate sensors to detect biometric data, light-emitting devices to display data, and integrated wiring and power sources. At present, e-textiles can be made by sewing discrete, rigid elements like light-emitting diodes (LEDs), optical fibers, or bulky “black box” battery packs into clothing, which reduces softness, stretchability, and wearability. The next generation of e-textiles are being made from functional materials seamlessly integrated into the textile itself, while maintaining softness and stretchability. The challenge with this integration is the porous, 3D structures of textiles that are obviously different from the flat and rigid surfaces conventionally used for device fabrication. Textiles present a non-planar surface for fabrication and readily absorb and wick solutions of functional inks. Approaches that planarize the textile surface using polymer layers lose the softness and stretchability of the original textile. In this presentation, we show that instead of masking textiles with planar polymer layers, the textile structure itself can be exploited to form the basis for a new, textile-centric design approach. As an example of this approach, we discuss how the open structure of a low-denier nylon and spandex ultrasheer fabric can be used as a framework for highly stretchable transparent electrodes in wearable and stretchable light-emitting devices. We furthermore discuss how the tufted structure of a velour fabric forms the basis for an architectural engineering approach to protect brittle functional materials from strain and demonstrate this approach by fabricating stretchable textile-based lithium-ion batteries.

This talk will focus on textile-integrated wearable technologies. I will discuss current best practices related to textile and garment integration of sensors, actuators, and circuitry, providing examples of work by the University of Minnesota Wearable Technology (WTL) Laboratory and other leading research institutions. I will highlight the specific advantages, challenges, and applications of garment-integration of these types of technologies, and will discuss future directions in this space where innovation can have significant impact, ranging from aerospace/military scenarios to medical, athletic, and commercial applications.

Wearable technology is poised to explode in the next decade. As flexible electronic devices have begun to be incorporated into clothing, there are pressing needs to turn responsive materials into fibers and yarns to create smart textiles that will response to environmental changes and report personal health information.

Here, I will present our work on fabrication of polymer fibers and meter-long yarns with embedded chemical receptors and selective wettability. The yarns can be knitted, embroidered or weaved into various wearable fabric prototypes. By control of surface chemistry of the fibers, we demonstrate selective wettability of the yarns, which wicks aqueous solutions, and change colors in response to humidity, pH, and electrolytes. By selective embedding of functional yarns into different geometries, we create multi-sensors and “blooming” flowers that can response to UV light intensity, temperatures, and electrical fields.

Abstract:
There is nothing more personal than healthcare. Health care must move from its current reactive and disease-centric system to a personalized, predictive, preventative and participatory model with a focus on disease prevention and health promotion. As the world marches into the era of Internet of Things (IoT) and 5G wireless, technology renovation enables industry to offer a more individually tailored approach to healthcare with more successful health outcomes, higher quality and lower costs. However, empowering the utility of IoT enabled technology in personalized health care is still significantly challenged by the shortage of cost-effective and wearable biomedical devices to continuously provide real-time, patient-generated health data. Textiles have been concomitant and playing a vital role in the long history of human civilization. In this talk, I will introduce our current research on smart textiles for biomedical monitoring and personalized diagnosis, textile for therapy, and textile power generation as an energy solution for the future wearable medical devices.
References
1. G. Chen, Y. Li, M. Bick, J. Chen*. “Smart Textiles for Electricity Generation”, Chemical Reviews, 120, 8, 3668-3720 (2020)
Front Main Cover.
2. Z. Zhou, K. Chen, X. Li, S. Zhang, Y. Wu, Y. Zhou, K. Meng, C. Sun, Q. He, W. Fan, E. Fan, Z. Lin, X. Tan, W. Deng, J. Yang*, J. Chen*. “Sign-to-Speech Translation Using Machine-Learning-Assisted Stretchable Sensor Arrays”. Nature Electronics, in press (2020)
3. K. Meng, S. Zhao, Y. Zhou, Y. Wu, S. Zhang, Q. He, X. Wang, Z. Zhou, W. Fan, X. Tan, J. Yang, J. Chen*. “A Wireless Textile Based Sensor System for Self-Powered Personalized Health Care”. Matter, 2, 896-907 (2020) Research Highlight by Prof. Trisha L. Andrew: “The Future of Smart Textiles: User Interfaces and Health Monitors”. Matter, 2, 794-795(2020)
4. N. Zhang, F. Huang, S. Zhao, X. Lv, Y. Zhou, S. Xiang, S. Xu, Y. Li, G. Chen, C. Tao, Y. Nie, J. Chen*, and X. Fan*. “Photo-Rechargeable Fabrics as Sustainable and Robust Power Sources for Wearable Bioelectronics”. Matter, 2, 1260-1269 (2020) Research Highlight by Prof. Xiaodong Chen: “Powering Body Area Sensor Networks”. Matter, 2, 1079-1090(2020)
5. Z. Zhou, S. Padgett, Z. Cai, G. Conta, Y. Wu, Q. He, S. Zhang, C. Sun, J. Liu, E. Fan, K. Meng, Z. Lin, C. Uy, J. Yang, J. Chen*. “Single-Layered Ultra-Soft Washable Smart Textiles for All-Around Ballistocardiograph, Respiration, and Posture Monitoring during Sleep”. Biosensors and Bioelectronics, 155,112064 (2020)
6. J. Chen, Y. Huang, N. Zhang, H. Zou, R. Liu, C. Tao, X. Fan and Z. L. Wang. “Micro-Cable Structured Textile for Simultaneously Harvesting Solar and Mechanical Energy”, Nature Energy, 1, 16138 (2016)
7. Y. Peng, J. Chen, A. Y. Song, P. B. Catrysee, P.-C. Hsu, L. Cai, B. Liu, Y. Zhu, G. Zhou, D. S. Wu, H. R. Lee, S. Fan, and Y. Cui. “Nanoporous Polyethylene Microfibres for Large Scale Radiative Cooling Fabric”, Nature Sustainability, 1, 105 (2018) (Co-first author)

Wearable technologies exist in a variety of form factors to accommodate a broad range of health sensing applications. Textile-based form factors are among the most enticing because they enable the creation of large-area sensing networks with sensing modalities such as ECG, EMG, and EEG. But, multiple form factor design considerations need to be accounted for to ensure the full wearable system exhibits exceptional sensing ability. In this talk, we explore the key form factor design parameters affecting ECG sensing performance of an Electronic textile (E-textile) armband worn on the upper left arm. The E-textile armband is integrated with screen-printed dry electrodes arranged in a three-lead sensing configuration. We identify the optimal electrode location needed to achieve high-quality ECG signals at low measurement frequencies on the upper left arm, without sacrificing the practical assembly of the armband. Our analysis reveals that the armband diameter plays a crucial role in dictating contact pressure against the upper arm and the resulting ECG signal quality. We also develop a predictive contact pressure model based on the mechanics of thin-walled cylindrical pressure vessels. Our model provides a framework to predict the contact pressure and ECG signal quality as a function of armband diameter. The fundamental design principles discussed in this presentation lay the groundwork for creating future E-textile form factors that enable efficient sensing capabilities.

Most architected materials to date have enabled properties such as extreme stiffness and strength, at levels unattainable independently by their constituent materials, with few works focusing on the compliant, extensible regime pertinent to flexible electronics. While exceptional mechanical properties such as extreme resilience and high deformability have been realized in many three-dimensional (3D) architected materials using beam-and-junction-based architectures, stress concentrations and constraints induced by the junctions limit their mechanical performance. Compliant architected materials that overcome these limitations, in combination with conductive components or constituent materials, could help enhance the performance of wearable electronics.

We present a new hierarchical design concept for flexible 3D architected materials in which fibers are interwoven to construct effective beams, such as the ones in classical monolithic-beam lattice architectures. Via in situ tension and compression experiments of additively manufactured polymeric woven and monolithic lattices at the microscale, we demonstrate the superior ability of woven architectures to achieve high tensile and compressive strains (>50%)—without failure events—via smooth reconfiguration of woven microfibers in the effective beams and junctions. Cyclic compression experiments reveal that woven lattices accrue less damage compared to lattices with monolithic beams. We employ numerical studies of woven beams with varying geometric parameters to present new design spaces for the development of architected materials with tailored compliance that is unachievable by similarly configured monolithic-beam architectures. This woven hierarchical design offers a pathway to make traditionally stiff and brittle materials more deformable by introducing a new building block for 3D architected materials, with complex nonlinear mechanics, applicable to flexible electronics applications.

Development of stimuli-interactive sensing display capable of spontaneously visualizing various external human sensible inputs has been of great interest and tremendous efforts are devoted to the visualization of nonvisible human senses such as touch, smell, taste, and sound. Field-induced electroluminescence under alternating current (AC) has been extensively studied and its unique device architecture in which an emitting layer is separated with an insulator from electrode offers a new platform for designing and developing emerging stimuli-interactive displays. In this presentation, we present 3 different types of interactive sensing displays. First, we demonstrate that simultaneous sensing and visualization of the conductive substance is achieved when the conductive object is coupled with the light emissive material layer on our novel parallel-type device. Second, a novel interactive skin display with epidermal stimuli electrode (ISDEE) is demonstrated, allowing for the simultaneous sensing and display of multiple epidermal stimuli on a single device. The ISDEE is directly mounted on human skin, which by itself serves as a field-responsive floating electrode of the display, allowing for displaying a variety of physiological skin factors, such as the temperature, sweat gland activity, and pressure. Finally, an interactive sensing memory platform is demonstrated, capable of sensing, monitoring, and storing the information of various liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid, allowing for facile liquid sensing and identification. Moreover, our device successfully monitored the flow of a human serum liquid passing through channels and determined its velocity. Mechanically flexible and optically transparent tube-type LIFS AC devices allow for in situ analysis of the flow of a liquid in terms of sound.

The transformative emergence of smart electronics, human-friendly robotics and supplemented or virtual reality will revolutionize the interplay with our surrounding. The complexity that is involved in the manipulation of objects in these emerging technologies is dramatically increased, which calls for electronic skins (e-skin) that can conduct tactile and touchless sensing events in a simultaneous and unambiguous way. Integrating multiple functions in a single sensing unit offers the most promising path towards simple, scalable and intuitive-to-use e-skin architectures. However, by now, this path has always been hindered by the confusing overlap of signals from different stimuli.
Here, we put forward the field of soft, flexible electronics by developing a compliant magnetic microelectromechanical platform (m-MEMS), which is able to transduce both tactile (via mechanical pressure) and touchless (via magnetic field) stimulations simultaneously and discriminate them in real time [1]. For the first time, the electric signals from tactile and touchless interactions are intrinsically separated into two different regions, allowing the m-MEMS, a single sensor unit, to unambiguously distinguish the two modes without knowing the signal history.
Owing to its intrinsic magnetic functionality, our complaint m-MEMS platform is able to discriminate magnetic vs. non-magnetic objects already upon touchless interaction. With this intrinsic selectivity, we address the long-standing problem in the field of touchless interaction – namely, the issue of interference with objects, which are irrelevant or even disturbing the interaction process. In addition, the interaction process is programmable. The sensitivity of the two interaction modes could be tuned by adjusting the magnetic field of the objects able to meet the requirements of different interaction tasks.
By using tactile and touchless sensing functions simultaneously, our m-MEMS e-skins enable complex interactions with a magnetically functionalized physical object that is supplemented with content data appearing in the virtual reality. We demonstrated data selection and manipulation with our m-MEMS e-skins leading to the realization of a multi-choice for augmented reality through three dimensional (3D) touch. Beyond the field of augmented reality, our m-MEMS will bring great benefits for healthcare, e.g. to ease surgery operations and manipulation of medical equipment, as well as for humanoid robots to overcome the challenging task of grasping.

[1] J. Ge, X. Wang, M. Drack, O. Volkov, M. Liang, G. S. Cañón Bermúdez, R. Illing, C. Wang, S. Zhou, J. Fassbender, M. Kaltenbrunner, and D. Makarov. A bimodal soft electronic skin for tactile and touchless interaction in real time. Nature Communications 10, 4405 (2019).

Closed-loop control system is critical in sophisticated object manipulations such as grasping and cracking a raw egg or flipping a note page, to avoid drop and slip. Human hands can perform such actions naturally even without watching the objects, because mechanoreceptors on hands can provide wide dynamic range tactile feedback on a variety of physical parameters such as shear forces to the brain for closed-loop control of hands. For robots to operate autonomously, similar tactile perception is needed to provide versatile manipulation of our everyday objects and fixtures. Furthermore, in majority of applications, a tactile sensitivity on flexible substrates is required for interaction with arbitrary geometries. Multiple modalities have been suggested so far to indirectly infer tactile information, but direct tactile sensation with shear force sensitivity is important for high fidelity haptic feedback. The challenges for achieving high spatiotemporal resolution with shear sensing ability can be addressed by employing tactile sensors which can perform both transducing forces and multiplexing signals [1]. This approach circumvents the tradeoffs for density and performance, since no additional driving element is required in construction of the array. Additional structural engineering can allow them to sense shear forces with high spatiotemporal resolution, which can lead to achieve human-level tactile perceptions for robotics. Still, manufacturing challenges remain in the realization of flexible multi-functional tactile sensors with desired density and shear-force sensation, because only scalable techniques are are suitable for industrial applications.
Here, we report scalable flexible tactile sensor array with ultra-high spatiotemporal resolution including shear force sensation and real-time closed-loop grip adjustment for robotic android fingers and beyond [2]. We leveraged the flexible display technology and fabricated advanced dual-gated piezoelectric zinc oxide (ZnO) thin-film transistors (TFTs) on thin and flexible polyimide substrate. We attached a 3D Polydimethylsiloxane (PDMS) pillar array on the fabricated TFT arrays which exhibited excellent normal and shear force sensing over a broad range of forces, in both dry and wet environments. An individual ZnO TFT can intrinsically transduce the force and multiplex the signal, which allows integrating hundreds of elements with uniform sensitivity. The array composed of 256 TFTs achieved maximal spatial resolution of 100 μm and maximal frame refresh rate of 100 Hz, which to the best of our knowledge is the highest spatiotemporal resolution reported to date. The TFTs were reliably operational under curved environments with bending radius of 5 mm, or after 10000 bending cycles. Closed-loop control of robotic fingers showed rapid slip-detection and grip adjustment using feedback exclusively from the tactile sensor array, enabling autonomous robotic tactile perception. Other novel applications such as water-proof operation and flow detection are also demonstrated.

[1] Vishniakou, Siarhei, Renjie Chen, Yun Goo Ro, Christopher J. Brennan, Cooper Levy, Edward T. Yu, and Shadi A. Dayeh. "Improved performance of zinc oxide thin film transistor pressure sensors and a demonstration of a commercial chip compatibility with the new force sensing technology." Advanced Materials Technologies 3, no. 3 (2018): 1700279.
[2] Hongseok Oh, Gyuchul Yi, Michael Yip and Shadi A. Dayeh, “Scalable Tactile Sensor Arrays on Flexible Substrates with High Spatiotemporal Resolution Enabling Slip and Grip for Closed-Loop Robotics”, submitted.

Fitness activity trackers and health-monitoring devices are becoming increasingly prevalent in everyday life. Motion trackers constitute a class of wearable devices, with numerous applications, including physical therapy (PT) and analysis of sports performance, in which monitoring of the angular positions of joints can accelerate rehabilitation and improve performance. The shoulder joint is notoriously difficult to track, as it exhibits more degrees of freedom and greater articulation than most joints in the human body. Many traditional methods, as well as other wearable sensors used to record limb and shoulder movement remain quite limited in versatility or scalability.

We created a wearable patch from a flexible kirigami substrate, enabling a planar sheet with embedded circuits to conform to the three-dimensional shape of the shoulder. Upon deformation, the localization of stress near saddle-points in the kirigami structure is related to the degree of curvature, determined through finite-element modeling and experimental measurements, and assessed as a function of the sheet material and cut parameters. Strain gauges are strategically placed on the platform based on the stress distribution, creating a sensing device that corresponds to a standardized 19-camera motion tracking system, and can differentiate between many types of motion, including running, shrugging, arm circles, etc., in real time. The combination of electrical sensing elements and closed-shape kirigami structures demonstrated by this work enable a highly wearable, garment-integrable, inexpensive joint motion monitoring device that can be used for rehabilitation, athletic training, feedback control for augmented mobility, robotics, and other applications.

In recent years, wearable sensors have attracted considerable interest among the general public due to increased awareness in personal health. Technologies as such can be particularly beneficial to industries related to virtual reality, sports performance, military, and healthcare. However, most wearable sensors that are available today are still based on bulky, rigid, inconvenient, electronic devices. Furthermore, they are limited to discrete sensing at their instrumented locations. On the contrary, sensors in the form of patches are more conformable to the human body and are capable of distributed sensing. For this reason, the purpose of this study is to develop a flexible, skin-like sensor capable of continuous, real-time, human physiological monitoring. The approach was to integrate strain-sensitive graphene-based thin films with commercially available kinesiology tape (K-Tape) as the substrate. The strain and motion sensing properties of these Smart K-Tape were characterized using a load frame. Upon verifying their strain sensing properties, the wearable sensors were tested on individuals performing different activities and undergoing different types of movements. Data acquired from the wearable sensors were compared against post-processed recorded video of subjects performing different controlled motions. In addition, a network of these graphene-enhanced K-Tape could form a network of sensors for distributed motion monitoring. In general, repeatable and consistent sensing responses were obtained and confirmed, which demonstrated their potential applications for the development of next-generation human motion and physiological monitoring systems.

In this talk, we propose a novel method to make conformable devices on non-zero Gaussian surfaces, i.e. flexible devices that can be transformed into any complex 3-dimensional shape. We used computational polyhedral edge unfolding methods to obtain planar figures of arbitrary complex 3-dimensional shapes. It is well-known that 2-dimensional substrates cannot be attached on the non-zero surfaces without stretching, however, by computational approximation of 3-D surface and making it flat by algorithmic method, we could convert any 3-D shapes into a 2-D sheet. We can make devices having the programmed unfolded figures and return it to the original 3D shape. In this way, we could make electroluminescent lighting and primary battery which can conformably wrap diverse 3-D surfaces without failure. Shape programmable devices with computational wrapping design can cover anywhere on the 3-dimensional shapes including human skin and are able to be applied not only personalized wearable or skin attachable devices but also various industries, such as automotive design, clothing, and fashion accessories, medical services and so on.

The advancement of smart apparels capable of tracking human physiological signals and body locomotion has a great potential to revolutionize human performance sensing and personalized health monitoring through transforming daily life clothing into sensors. Quantitative evaluation of kinetic parameters of individual gait along with physiological signals can be employed in games and sports, as well as in the diagnosis of many diseases such as Parkinson’s, Multiple Sclerosis, and sleep disorders. Among different methodologies in developing wearable sensors such as inertial measurement units, textile-based electromechanical sensors encompass the majority of widely adopted applications. Electromechanical sensors fall into three major categories based on their active mechanisms: piezoelectric sensors, triboelectric sensors, and piezoresistive sensors. Recently, different combinations of materials and designs have been reported to develop wearable sensors, each of which provides a unique window into a slightly different range of motion sensitivity. For example, some human signals and motions lie in the small-scale range of pressures such as those of a subtle touch, arterial pulses, and sound vibrations, while the other body movements lie in medium to a large range of pressures, such as joint movements, and locomotion during sleep and intense activities. The ability to design an unobtrusive wearable sensor being highly responsive in the required range of signals calls for getting an insight into the difference between the three mechanisms of electromechanical sensing and their corresponding responses under certain conditions.
Here, we introduced a set of materials selection principles which gives researchers an in-depth insight into how to design a wearable electromechanical sensor when it comes to acquiring data from a specific source of motion. In order to achieve this goal, we performed a set of purposefully designed experiments on three types of wash-stable fabric-based electromechanical sensors that had already been introduced by our lab, i.e., triboelectric sensor, piezoelectric sensor, and piezoionic sensor as a subset of piezoresistive ones. These experiments explored the effect of impact pressure, bending angle and speed, frequency, presence of a base pressure, response time, breathability, and having a multi-layer structure on the performance and sensitivity of each type of sensors. For an instance, it turned out that the triboelectric and piezoelectric sensors are a more reliable sensing element for dynamic pressures, such as joint movements, with the former being failed in the presence of a base pressure. Piezoresistive sensors are the ones with the ability to sense both static and dynamic pressures, as well as being responsive under a base pressure. However, piezoresistive one would not be a choice when it comes to bending applications. Upon this comprehensive comparison, we demonstrated a conclusive map that can provide the researchers with distinguishing features of these three types of sensors to be used in corresponding niche applications.

Capacitive pressure devices have been used for different applications such as wearable health sensors, soft-robotics, and prosthesis. These devices can be designed as parallel plate capacitors with an elastomer dielectric film like polydimethylsiloxane (PDMS). When different microstructures are added to these elastomer films then the pressure range and sensitivity can be tuned. Here, we conducted simulation for pyramidal microstructures as they spread the force evenly and can be repeatably fabricated. We introduce a simple finite-element-method simulation model for determining the capacitance and sensitivity of these microstructured films. The model sees the PDMS films as consisting of parallel and series capacitors, and volumetrically solves for the total capacitance. COMSOL Multiphysics is used to generate various geometries. Key parameters, such as pyramid spacing, width and height are seen to influence the performance of sensors. We studied these parameters from the 10 to 100 μm range. We can modify the initial capacitance from 12.74 to 82.75 pF/cm2, and the sensitivity from 0.008 to 0.18 per kPa. As well, penetration into a lamination layer, and layer thicknesses are also studied. These are factors come into play during the moulding and bonding processes. Penetration was studied for 1 to 6 μm range, and layer thicknesses from 7 to 105 μm. These allow the control of capacitance from 12.15 to 27.9 pF/cm2, and sensitivity from 0.01 to 0.03 per kPa. Pyramidal structures can be fabricated via a moulding process of PDMS and bonding it to metalized plastic sheets. Moulds consist of square pyramidal arrays obtained by KOH etching of silicon wafers, and bonding can be performed by lamination to a thin PDMS layer. We will present the performance of devices in relation to these parameters and compare them with other mathematical models. Furthermore, we use the simulation to predict performance trends, and compare them with experimentally gathered data from fabricated devices. This simulation model may be used to predict the performance of new devices too by modifying the geometry. Doing so, devices can be tuned to obtain different ranges of pressure sensing and sensitivity for specialized applications. Ranging from monitoring pressure sores, with a necessary range up to 100kPa, to artificial skin with sensing below 1 kPa.

Abstract：It is critically important to develop miniature electronic devices for a variety of emerging new fields, such as wearable facilities, smart textiles and internet of things. Here a novel family of energy and electronic devices with various functionalities, e.g., energy harvesting, energy storing, lighting, and sensing, in 1D fiber configuration are carefully discussed with unique and promising advantages such as lightweight and weaveable compared with the conventional planar architecture. The main efforts will be made to highlight the recent advance in the electrode material, device structure and property extension.

This talk will explore the new realm of using textile threads as an ultimate platform for flexible and stretchable bioelectronics. Threads offer unique advantages of universal availability, low cost, material diversity and simple textile-based processing. Interestingly, threads also provide an ideal platform for passive microfluidic sampling and delivery of analytes. In this talk, I will report reel-to-reel fabrication of functional smart threads for variety of sensing and electronics application. I will report on nanomaterial-infused smart threads for sensing physical activity and temperature. Nano-infused threads will be presented for sensing pH, glucose, lactate, ammonium and other chemical and biological biomarkers directly in biological fluids such as sweat or wound exudate. Coloimetric washable threads that can sense both ambient and dissolved gases will be shown. Beyond sensing and microfluidics, I will present our recent work on making super-thin transistors and electronics directly on threads. This new toolkit of highly flexible thread-based microfluidics, sensors, transistors and electronics makes it possible to realize smart surgical sutures and flexible smart bandages for chronic wounds. Our recent work on using threads for closed loop spatiotemporal dosage controlled drug delivery will also be presented.

Soft polymer-based wearable sensors as an integral part of textile structures have attracted considerable scientific and commercial interest recently because of their potential use in healthcare, security, and other areas. Electronic sensing capabilities can be integrated into textiles at fiber, yarn, or fabric level but unobtrusive integration of sensing capabilities while preserving all desirable textile qualities like comfort and flexibility can be better achieved by introducing the desired functions at the fiber level. In this research, we demonstrate a transformative sensing technology through the design of a multimodal and multifunctional woven sensor array by employing bicomponent fibers consisting of insulating and electrically conducting segments of unique cross-sectional shape, wherein each cross-over point in the woven fabric structure acts as a sensing pixel. To demonstrate the concept the fibers are initially fabricated using a benchtop coextrusion method. The insulating segments are formed using an ultraviolet curable poly(dimethylsiloxane) (PDMS) while the conducting segments are made from a conducting polymer composite containing PDMS and carbon black. The multimodal characteristic of the sensors is demonstrated through the measurement of capacitive and resistive responses while multifunctional capabilities are explored by measuring tactile (normal force), tensile, and shear deformations, as well as wetness and biopotential. As a potential biomedical application, these fiber-based sensing arrays are integrated into a prosthesis socket to monitor pressure within the inner socket environment of lower limb amputees. Also, a large channel sensor array is fabricated and its potential for contactless sensing of gestures is demonstrated as a potential textile-based human-machine interface. In order to demonstrate the scalability and commercial relevance of the concept, we have also fabricated the same sensory fiber using the tricomponent melt extrusion (TME) method. It is a process that involves the simultaneous extrusion of three distinct polymers to produce unique fiber shapes. Due to the complex nature of this process and the specialty equipment required, TME has rarely been utilized in research efforts and has not been explored for producing fiber-based sensors.
So far in this ongoing research, we have extruded the sensory fibers of the desired shapes using the TME process and explored the influence of the shape control parameters such as flow-rate and fractional throughput. We have characterized the fibers for their morphological, mechanical, and electromechanical properties. Initial results indicate that the micron-scale fibers can be produced at speeds of >100 m/min thus providing a scalable and commercially relevant method of producing fiber-based sensors for wearable technology.

Individual carbon nanotubes (CNTs) have remarkable properties that are often hard to scale directly to macroscopic materials and devices. A tremendous effort has been made to assemble CNTs at the micro and macro scale in controlled methods to utilize their anisotropic material properties. Here we report a single-step method of assembling CNTs wrapped textile yarns for wearable electronic textile applications. The dry spinning method was used to fabricate CNT yarns from millimeter tall vertically aligned CNT arrays produced by the chemical vapor deposition method. Both pristine CNT and CNT wrapped textile yarns were produced and their structure-properties relationship were compared. This unique method of assembling CNTs with traditional textile yarns provides greater freedom to tailor the strength and conductivity of the yarns for desired applications. Surface morphology examination showed that CNTs were wrapped uniformly over the core textile yarn and have a similar diameter to those yarns suitable for producing highly flexible textile fabrics for body-worn applications. The CNT yarn showed very good mechanical properties and could withstand 1000 loading cycles at a 2% strain. The use of polyurethane binder enhanced the mechanical stability of the CNT yarn and prevented the delamination and loss of CNTs from the core yarns during mechanical strain. The CNT yarn was knitted to produce a seamless 3D knitted structure for strain sensing and Joule heating applications. The produced knitted fabric could sense applied strain and successfully track the movement of the finger. When voltage was applied to the fabric, a highly stable heat was generated in seconds and the fabric can produce heat above skin temperature with as low as 3 volts applied. To realize the true potential of the CNT yarns for use in next-generation wearable electronics, the CNT yarn was subjected to an extensive wash durability study. It was found that the CNT yarn was extremely resistant to washing showing no significant changes in conductivity after 30 washing cycles. This yarn may also find uses in electromagnetic shielding and energy storage textile applications.

The integration of conducting architectures within thermally drawn thin and flexible fibers is emerging as a versatile platform for smart sensors and imaging systems, robotic surgery, biological and neural probes, and advanced textiles^1-3. The fabrication of conductors with nanoscale feature sizes, large aspect ratios, complex transverse geometries and excellent conductivity within thermally-drawn fiber electronics, however, remain unresolved. Here we, for the first time, address this challenge via the thermal drawing of metallic glasses (MGs), a material system that is metallic yet with a disordered atomic structure compatible with controlled thermal deformation⁴. The high viscosity during drawing, combined with a viscous interface provided by a thermoplastic matrix surrounding MGs domains allows us to control fluid instabilities that are normally detrimental for molten crystalline metals. This fabrication strategy enables the production of ultra-long, highly uniform and well-ordered nanoscale MG domains with arbitrary cross-sectional geometries within flexible polymer fibers. In terms of integrating metallic electrodes within flexible fibers, catheters, capillaries or textiles, the figure of merit we can achieve, that can be defined as the ratio of the conductivity to feature sizes, significantly surpasses existing strategies. Beyond using MG components as electrodes, we further show that they can be monolithically integrated with other functional materials, forming novel all-in-fiber flexible devices and smart textiles. Specifically, optoelectronic fibers that integrate MG ribbons serving as highly conductive electrodes and a semiconducting selenium core serving as a photodetector are fabricated. The device exhibits a unique combination of excellent responsivity and sensitivity that compares favorably with other reported nanoscale planar devices. Moreover, the flexibility and form factor of the polymer cladding enable to weave the fiber into different types of textiles. Our work brings, for the first time, extreme miniaturization of highly conducting metal-based electrodes, paving the way towards the integration of more complex functionalities and devices in flexible electronics, fibers and fabrics.
W.Y and I.R contributed equally.
1. Wei Yan, Alexis Page, Tung Nguyen, Yunpeng Qu, Federica Sordo, Lei Wei, Fabien Sorin*. Advanced Multimaterial Electronic and Optoelectronic Fibers and Textiles. Advanced Materials 31 (2019) 1802348.
2. Gabriel Loke#, Wei Yan#, Tural Khudiyev, Grace Noel, Yoel Fink*. Recent progress and perspectives of thermally-drawn multimaterial fiber electronics. Advanced Materials 32 (2020) 1904911.
3. Wei Yan, Chaoqun Dong, Yuanzhuo Xiang, Shan Jiang, Andreas Leber, Gabriel Loke, Wenxin Xu, Chong Hou, Shifeng Zhou, Min Chen, Run Hu, Perry Shum Ping, Lei Wei, Xiaoting Jia, Fabien Sorin, Xiaoming Tao, Guangming Tao* Thermally drawn advanced functional fibers: new frontier of flexible electronic devices. Materials Today (2020) https://doi.org/10.1016/j.mattod.2019.11.006
4. Wei Yan, Inès Richard, Güven Kurtuldu, Niclolas James, Giuseppe Schiavone, Tung Nguyen-Dang, Tapajyoti Das Gupta, Yunpeng Qu, Jake Cao, Reinis Ignatans, Stéphanie P. Lacour, Vasiliki Tileli, Grégoire Courtine, Jörg F. Löffler, Fabien Sorin*. Structured nanoscale metallic glasses fibers with extreme aspect ratios. Nature Nanotechnology. (In press, 2020).

As an emerging format of wearable electronics, the electronic textile (e-textile) display devices consisting on emitting fibers have been actively studied for substantiating ‘in-cloth’ information displays. Several technical issues have to be resolved to create fiber-based e-textile displays, such as high electroluminescence performance of luminescent fibers, solid passivation systems, and interconnectable configuration of pixel elements in fibers for the two-dimensional matrix-addressing. Recently, we successfully demonstrated a class of the e-textile displays by weaving interconnectable organic light-emitting diode (OLED) fibers with conductive fibers. This OLED e-textile display could be operated by the passive matrix addressing scheme due to perpendicularly aligned woven fiber network with interconnectable OLED fibers and conductive fibers that were served as scan and data lines, respectively. Moreover, it was reliably operated even in the water owing to the chemically/mechanically sturdy but electrically conductive passivation system formed on each OLED pixel. Despite the noticeable proof-of-concept, the reported devices could exploit a half area over the available entire textile area at the most due to the nature of woven network that OLED pixels placed alternately along the conductive fiber, which limited the imaging resolution. In this regard, here we present a strategy to achieve matrix-addressable e-textile OLED displays throughout the entire textile area by weaving only OLED fibers that can support the scan and data lines at the same time. To create the united functional fibers, phosphorescence OLED pixels are regularly deposited on the common indium tin oxide (ITO) electrode coated on one side of the rectangular-shaped polyimide (PI) fiber. The conductive metal bus electrode that can work as a data line in the matrix addressing scheme is thermally deposited on the opposite side of the PI fiber except the defined emitting spots. As the OLED pixels exist at every cross points of interlaced perpendicularly arranged OLED fibers, full-area OLED e-textile displays can be achieved, where high-resolution images are produced by the passive matrix addressing process.

In the past few decades, applications such as wearable electronics, biosensing, and flexible displays have led to advances in flexible electronics.^1,2 Textile threads are a highly suitable substrate for truly flexible and biocompatible electronics. Recent works have demonstrated thread-based transistors (TBTs), but their methods were not scalable to manufacturing more than a few transistors.^3,4 To realize complex large-scale thread-based circuits for wearables, diagnostics, and flexible displays, a high throughput fabrication method of TBT is required.

In this work, we present a low-cost, high-throughput, and cleanroom-free fabrication process for ionogel gated, poly(3-hexylthiophene)-based organic TBTs. This lithography-inspired approach involves the use of a flexible Ecoflex “mask” that threads are pierced into, to fabricate the active channel area gap. Then, conductive carbon ink is painted or drop cast on to fully coat the exposed threads, and when dried, the threads are removed to reveal the active channel area of the transistor. Notably, the flexibility of the mask is paramount for easy insertion of threads when stretched and provides a tight seal around each thread when relaxed to inhibit electrode ink seepage.

In our preliminary experiments, we achieved a density of 1 TBT fabricated per cm² of Ecoflex mask; we demonstrate fabrication of close to 50 TBTs in a mask of approx. 50 cm². After correcting for some geometrical variations from manual effort, we show consistent device performance among all TBTs with a threshold voltage of (1.48 ± 0.11) V, an operating region of 0 to -3V, and an ON/OFF ratio of the order of 10². These results prove the feasibility of this new approach for batch fabrication of several TBTs which with automation, holds potential for even higher areal densities of fabricated TBTs and much tighter tolerances. What we have proposed will address the key bottleneck in scaling up threads as a substrate for very large-scale electronics on thread with applications in wearable devices, flexible displays, and diagnostic toolkits.

¹ A. Nathan, A. Ahnood, M.T. Cole, S. Lee, Y. Suzuki, P. Hiralal, F. Bonaccorso, T. Hasan, L. Garcia-Gancedo, A. Dyadyusha, et al. in Proc. IEEE (Institute of Electrical and Electronics Engineers Inc., 2012), pp. 1486–1517.
² J. Sheng, H.-J. Jeong, K.-L. Han, T. Hong, and J.-S. Park, J. Inf. Disp. 18, 159 (2017).
³ M. Hamedi, R. Forchheimer, and O. Inganäs, Nat. Mater. 6, 357 (2007).
⁴ R.E. Owyeung, T. Terse-Thakoor, H. Rezaei Nejad, M.J. Panzer, and S.R. Sonkusale, ACS Appl. Mater. Interfaces 11, 31096 (2019).

Polymers such as poly(vinylidene fluoride) (PVDF), and its copolymers with hexafluoropropylene (HFP), trifluoroethylene (TrFE) and chlorotrifluoroethylene (CTFE) are known to possess electroactive properties and have been studied for decades, enabling them one of the most promising candidates for wearable and smart textile applications. Nonetheless, the practical applications are still limited and are in the early stage of development because of poor mechanical integrity. Most of researches have focused on the form of film or electrospun mat (nanofiber) which showed weak mechanical properties to practically use. Since the high mechanical properties are the critical requirement to smart textiles because the resistance ability to external damage is related with the durability of textiles, the film and mat should be replaced with a stronger fibers.
In the current study, PVDF and PVDF/cellulose nanocrystal (CNC) nanocomposite fibers were dry-jet wet spun at CNC concentrations of 0, 1, and 3 wt% with respect to the polymer. The high draw ratio (DR) of 8 and the interaction between PVDF matrix and CNC lead to superior mechanical properties and β phase fraction (F_β), which is beneficial for practical application in smart textiles. The behavior of various conformation of PVDF were also analyzed according to DR and CNC contents. All fibers showed increase in tensile properties and F_β as the polymeric chains become extended with increasing DR. Among them, the highest 298.0 MPa of tensile strength, 3.3 GPa of tensile modulus, and 69.1% of F_β were exhibited in fibers with the addition of 1 wt% of CNCs at DR 8 based on their active interaction with PVDF. To demonstrate the use of these PVDF-based fibers in wearable devices, the triboelectric textile sensors were fabricated through a conventional weaving method based on their high mechanical integrity. They can detect a broad pressure range of 980 Pa to 98 kPa, enabling diverse pressure-sensor applications.

The rising research interest in personalized medicine promises to revolutionize traditional medical practices. This presents a tremendous opportunity for developing wearable devices toward predictive analytics and treatment. In this talk, I will introduce our recent advances in developing fully-integrated skin-interfaced flexible biosensors for non-invasive molecular analysis. Such wearable biosensors can continuously, selectively, and accurately measure a wide spectrum of sweat analytes including metabolites, electrolytes, hormones, drugs, and other small molecules. These devices also allow us to gain real-time insight into the sweat secretion and gland physiology. The clinical value of our wearable sensing platforms is evaluated through multiple human studies involving both healthy and patient populations toward physiological monitoring, disease diagnosis, and drug monitoring. This talk will also feature our very recent works on laser-engraved lab on the skin and biofuel-powered battery-free electronic skin toward metabolic/nutritional management as well as dynamic stress monitoring. These wearable and flexible devices could open the door to a wide range of personalized monitoring, diagnostic, and therapeutic applications.

The highly-compliant and skin-conformal magnetic field sensors have been of great interest for the human-interactive on-skin electronics capable of position recognition and tracking gesture. To design such on-skin electronics, solution-processable printing technologies are attractive way due to simple, cost-efficient, and highly adaptable to various materials. Here we report printable giant magnetoresistive (GMR) sensors on ultra-thin films that can naturally comply with the skin. Thanks to the strong interfacial bonding of triblock copolymer in GMR paste, the printed GMR microflakes could be firmly adhered to the any substrates with excellent mechanical stability under 16 µm bending radius without any damage. The excellent percolation contacts among GMR microflakes coupled with 2^nd antiferromagnetic maximum attribute the high sensitivity of 3 T^-1 in low magnetic field ranges of 0.88 mT on the bending curvature of 16 µm, resulting in 3409 times higher figure of merits than current state-of-the-art printable magnetic field sensors. Since the fast quenching of binding matrix induced random orientation of GMR microflakes rather than 2D stacks, the printed GMR sensors are omnidirectional magnetic field sensing capability regardless of angular change from 0 to 360^o for freely locating GMR sensors on skin. With this performance, our printed GMR sensors on skin demonstrate a touchless control of virtual objects for the practical application in human-interactive wearable devices, artificial prosthetics, robotics, and internet of things.

A lot of people around the world have suffered from alopecia, which can lead to aesthetic issues, low selfesteem, and social anxiety. Therapeutic technology including thermal, electrical, pharmacological, and photo stimulation have been suggested to treat hair loss. Among them, laser stimulation to hair is a strong treatment, activating the proliferation of hair follicles without side effects. Particularly, periodically irradiating red light with a wavelength of 650nm in the hairless area can help efficient stimulation of hair follicles under the skin, because this light can penetrate deeper into the skin tissue than blue and green light with short wavelengths. However, this laser stimulation treatment has difficulty in high power consumption, large size, and spatiotemporal restriction. Recently, flexible micro-light-emitting diodes (μLEDs) have been a great attention for future biomedical devices, such as biosensors, pulse oximetry (SpO2) sensors, and optogenetic stimulators due to its high performance and outstanding stability. However, flexible μLEDs still have significant drawbacks in application to wearable phototherapeutic devices, such as high heat, low optical efficiency, and high energy consumption.
Herein, we report high-performance 30 × 30 AlGaInP fVLEDs fabricated with transfer-free monolithic method for a wearable phototherapeutic device. Ultrathin flexible μLEDs were fabricated by vertically interconnecting top and bottom electrodes. The red f-VLED arrays were conformally attached on a human wrist with high irradiance of ∼30 mW mm−2 and low forward voltage of ∼2.8 V. The red light with 650 nm wavelength deeply penetrate the skin and stimulate the hair follicles, which is located 2 mm under the skin. The thermal stability of the monolithic f-VLED was theoretically investigated by using finite element method (FEM) simulation and experimentally optimized by operating f-VLEDs on the skin. Mechanical stability of monolithic f-VLEDs was confirmed by harsh periodic bending/unbending motions, which is necessary for skin-attachable phototherapeutic devices. Finally, photostimulation in hairless region was performed to promote hair growth in a living depilated mouse via f-VLED array with thermal stability. Hair regrowth in the mice via f-VLED was verified by analyzing immunofluorescent and histological effects.

Although many delivery systems have been proposed to improve drug permeation through the skin, all of them suffer from several limitations. This drives a continuous search for innovative systems that can provide safe and effective transdermal drug delivery solutions. In line with this, microneedle arrays (MNs), a hybrid combination of hypodermic injections and transdermal patches, have been proposed. MNs consist of micro-scale needles that can pierce the skin by a simple, minimally invasive and painless route, enabling the transport of drugs and macromolecules into the human body. This study reports, for the first time, the use of a bio-based, biodegradable and biocompatible polymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), P(3HB-co-3HV), for the fabrication of a biopolymer-based MNs patch. Molds of poly(dimethylsiloxane) (PDMS) were prepared by direct laser writing technology, a low-cost and mask-less technology, and used to produce biodegradable P(3HB-co-3HV) MNs by a thermosetting process. The best results were obtained with a laser power of 30 W at 0.15 m/s, with a spiral model as a pattern. The obtained MNs had a length of 0.69 mm and a diameter of 0.33 mm, ideal for painless penetration of the skin. Additionally, the produced MNs had good mechanical properties and the ability to successfully impregnate fluorescent dye (Rhodamine 6G). These features render P(3HB-co-3HV) a promising material for the development of novel MNs with improved functionality

The monitoring of microvolt-level biosignals such as electroencephalograms (EEG) requires the application of low-noise signal amplifier circuits, which, for single-use cases, must be fabricated for disposability using low-cost manufacturing techniques. In this study, we propose a scalable electrode printing process for organic transistors that can carry out precision measurement of EEG using a low-noise organic amplifier. In the transistor fabrication process, Ag electrodes are processed via flat stamp parallel printing (FSPP), and fine-process optimization to minimize the charge-trapping effect enables a three-order reduction in transistor noise. The low-noise transistors were used to produce an organic pseudo-CMOS amplifier with a small noise level of 2.2 µVp-p at 10 Hz. The ultra-low-noise organic amplifier enables precision EEG monitoring in a close correlation with signals obtained using a commercialized measurement system.
Wearable biosensing equipment is a highly anticipated next-generation medical device technology for obtaining advanced personal healthcare and self-medication. A critical requirement for a wearable device is that it does not interfere with daily human activity, which means that it must be mechanically flexible and lightweight. Furthermore, to avoid motion-induced artificial noise and obtain higher-quality biosignals, the sensor device must be lightweight and remain in close contact with the surface of the human body. To meet these requirements, a number of sheet-type flexible biosensors have been recently developed [1-5]. A further requirement for wearable biosensor devices is disposability for single-use cases, which in turn requires the use of low-cost manufacturing processes. Also, the most critical circuit parameter in the development of biosignal detection circuits is transistor noise, which limits the minimum detectable signal level and quality of measurement. In this study, we demonstrate a low-noise printing process that minimizes transistor noise by optimizing the source-drain electrode stacking process. The device employs a bottom-contact, top-gate transistor structure in which source-drain silver electrodes are printed using the flat stamp parallel printing (FSPP) method. The FSPP is a type of reverse offset printing method that enables both the patterning of fine source-drain electrodes and a large-area biopotential input electrode. In the transistor stacking process, several improvements such as thinning the contact electrodes and insulator film and optimizing the chemical treatment process for the contact electrodes were conducted to evaluate their respective impacts on transistor noise. This process optimization resulted in improved mobility, reduced contact resistance, and reduced subthreshold slope and enabled the fabrication of a low-noise organic transistor, 2.3 × 10^–11/Hz, with improved transistor characteristics. Using the low-noise transistors, a pseudo-CMOS amplifier with an ultra-low output noise of 2.2 µVp-p at 10 Hz was successfully fabricated and enabled precision EEG monitoring. The scalable printing method for realizing low-noise organic amplifier is a useful fabrication method for a single-use daily personal healthcare system.

References
[1] T. Someya et al., Nature 540, 379 (2016).
[2] J. Heikenfeld et al., Lab Chip 18, 217 (2018).
[3] W. Gao et al., Acc. Chem. Res. 52, 523 (2019).
[4] M. L. Hammock et al., Adv. Mater. 25, 5997 (2013).
[5] J. J. S. Norton et al., Proc. Natl. Acad. Sci. USA 112, 3920 (2015).

Electronic devices with stretchable and wearable features have been attracting a huge interest because of their potential applications in wearable electronics, electronic skins, biomedical engineering, and in-vivo bioelectronics. Among various types of soft and wearable electronic devices, textile electronics which combine conventional textiles and electronic devices is one of promising fields because clothes are essential at all times for all humans regardless of age or gender. However, conventional planar electronic devices have been limited to being woven into flexible textiles or integrated onto complex nonplanar substrates. Such limitation has hindered their use in textiles electronics or advanced wearable electronics.¹ In this regard, stretchable and wearable electronic devices in fiber (1D) form, which can be directly integrated into daily clothes or textiles without any inconsistency are greatly promising for future wearable electronics. In addition, the fiber-based electronic devices can successfully overcome practical limitations (e.g. structural mismatching, suturability. etc.) of previous planar soft electronics in in-vivo applications.²
In this talk, fiber-based (1D) soft pressure and strain sensors based on metallic nanomaterials, which can overcome the existing limitations of previous 2D electronic devices, are presented. The research starts with the development of stretchable conductive fibers with outstanding conductivity (>20,000 S/m), stretchability (>450%) and stability over 10,000 stretching cycles using metallic nanoparticles.³ Based on the conductive fibers, various fiber-based mechanical sensors such as pressure, strain, and multimodal sensors are fabricated for smart textile and wearable applications.^3,4,5 In addition to their successful demonstrations in wearable applications, this research is also focusing on considering practical issues in current implantable electronics which is important for clinical applications but have been barely considered so far.²

1. Recent advances in 1D stretchable electrodes and devices for textile and wearable electronics: materials, fabrications, and applications, Lee et al., Advanced Materials, 2020.
2. Suturable and solderless one dimensional strain sensors with wireless readout for biomedical applications, Lee et al., Nature Electronics, 2020, under revision
3. Conductive Fiber Based Ultrasensitive Textile Pressure Sensor for Wearable Electronics, Lee et al., Advanced Materials, 2015.
4. Highly Sensitive Multifilament Fiber Strain Sensors with Ultrabroad Sensing Range for Textile Electronics, Lee et al., ACS NANO, 2018.
5. Conductive Hierarchical Hairy Fibers for Highly Sensitive, Stretchable, and Water Resistant Multimodal Gesture Distinguishable Sensor, VR Applications, Choi et al., Advanced Functional Materials, 2019.

Creating artificial sensory skins that shows the tactile-sensing capability of human skin has been a big challenge in wearable skin electronics. In particular, biomimetics has emerged as a burgeoning area in the field of deformable tactile sensor skins that has led innovations in material designing and device structure manipulation with the aim of imitating sensing mechanism of human skin, intelligently.
In this talk, inspired by the sophisticated physiological ion dynamics of living cells, we describe a uniquely designed deformable ion channel consisting of artificial ions confined into well-designed hybrid interface. Novel bottom-up strategy employed here resulted into supramolecular polymer networks through non-covalent interactions between poroelastic artificial ions and viscoelastic polymer chains or through hydrogen-bonding triggered ion dynamics, which endows effective ion drift under mechanical stimuli, simultaneously. This design allows for high-performance synaptic plasticity as well as ultrasensitive mechano-transduction over a wide spectrum of pressure, which can serve next neuromorphic tactile platform for highly interactive human-machine interface.

The development of electronic devices that offer biointegration capabilities is of interest in various clinical and biological applications, where they could be used for monitoring, diagnosis and therapy. In general, an intimate interface is required between the device and the biological material, so the devices need to be built in a flexible form. At the same time, the increasing demand for portability restricts the allowable power dissipation of any device. Here we show several examples of wearable smart sensor systems for human body monitoring, including triboelectric nanogenerator-based self-powered sensors for respiratory and pulse monitoring, ultrasensitive flexible pressure sensors based on wrinkled microstructures for vocalization recognition, general 3D adjustable sensor platforms conformable to curved biological surface by utilizing kirigami art to extend 2D structures into 3D ones, and integrated smart sensor systems on ultrathin plastic foil with in-situ signal processing capability based on high-performance carbon nanotube complementary electronics. These cases show the great possibility of next-generation wearables for personal health care.

References:
L. Xiang, F. Xia, H. Zhang, Y. D. Liu, F. Liu, X. L. Liang, Y. F. Hu*, Advanced Functional Materials, 29, 1905518 (2019)
X. W. Zeng, Z. X. Wang, H. Zhang, W. Yang, L. Xiang, Z. Z. Zhao, L.-M. Peng, Y. F. Hu*, ACS Applied Materials & Interfaces, 11, 21218 (2019)
Z. X. Liu, Z. Z. Zhao, X. W. Zeng, X. L. Fu*, Y. F. Hu*, Nano Energy, 59, 295 (2019)
C. Yang, H. Zhang, Y. D. Liu, Z. L. Yu, X. D. Wei, Y. F. Hu*, Advanced Science, 5, 1801070 (2018)
H. Zhang, Y. D. Liu, C. Yang, L. Xiang, Y. F. Hu*, L.-M. Peng*, Advanced Materials, 30, 1805408 (2018)
L. Xiang, H. Zhang, G. D. Dong, D. L. Zhong, J. Han, X. L. Liang, Z. Y. Zhang, L.-M. Peng, Y. F. Hu*, Nature Electronics, 1, 237 (2018).
H. Zhang, L. Xiang, Y. J. Yang, M. M. Xiao, J. Han, L. Ding, Z. Y. Zhang, Y. F. Hu*, L.-M. Peng*, ACS Nano, 12, 2773 (2018).
Z. Z. Zhao, C. Yan, Z. X. Liu, X. L. Fu, L.-M. Peng, Y. F. Hu*, Z. J. Zheng*, Advanced Materials, 28, 10267 (2016).

In order to diagnose and monitor the human disease, it is essential to continuously and periodically monitor the biomarker related to the disease. In a human body, eyes can act as sites that have the potential to provide various information about one’s health condition including chemical and physical biomarkers. In particular, smart contact lenses have been studied as the wearable medical device to diagnose and monitor diseases noninvasively, detecting biomarkers from the eyes. However, there has been no clear study regarding the correlation between the signals obtained from the eyes and the actual state of the disease. Smart contact lenses can have actual effects only when these two factors are established whether they are correlated.
To address this problem, we studied the correlation of diseases using our smart contact lenses. One of the biomarkers that can be monitored by smart contact lenses is tear glucose level, which is related to diabetes. We fabricated the smart contact lens which includes electrochemical glucose sensor, antenna, and NFC chip, enabling its operation only with the smartphone. By applying this smart contact lens to the in-vivo test using live animals, we demonstrated that the tear glucose level is correlated with the blood glucose level. Accordingly, we verified that the smart contact lenses detecting tear glucose concentration can be used to diagnose and monitor diabetes. Another bio-signal that can be obtained from the eyes is the IOP(intraocular pressure), which can act as the indicator of glaucoma. To detect the IOP, we integrated the IOP sensor, transparent and stretchable antenna, and NFC chip onto the smart contact lens. We successfully measured human IOP with a smartphone by wearing this smart contact lens onto a human eye, presenting the feasibility of smart contact lens monitoring the state and progress of glaucoma.

Epidermal electronics that conform to the irregular topography of the skin enable non-invasive monitoring and measurement of biomechanical, physiological, and biochemical parameters relevant to monitoring human health and performance. Materials such as polydimethyl siloxane (PDMS), polyethylene terephthalate (PET), and polyimide (PI) are routinely used as substrate materials; however, such materials are not absorbent thereby limiting their utility for applications that necessitate the wicking of large volume bodily fluids when placed on the open skin, such as for wound healing applications. Polyvinyl alcohol (PVA) has been routinely used as a sacrificial layer in fabricating epidermal electronics; however, its utility as a standalone material for substrates or flexible electrodes has not been extensively studied. The biocompatibility, relative ease of fabrication, cost, absorbing capabilities, and scalability render it an ideal material for wearable bioelectronic devices. The objective of this study was to develop an absorbent, flexible, transparent, and inexpensive substrate (herein referred to as AFTIDerm) for use in wearable bioelectronic applications that necessitate the aforementioned material properties. PVA solutions were created with varied glycerol concentrations (1 wt%, 3 wt%, 5 wt%, 7 wt%, and 10 wt%) and drop cast in 9 mm plastic dishes and cured overnight at room temperature for subsequent use. The absorbent capabilities of the substrate were assessed in phosphate buffer saline solution (PBS), pH 7.4 (mimicking a chronic wound environment), over a one-week period with samples assessed daily. An inverse relationship between the varied glycerol concentrations and percent mass increase was observed. The high variability among the 1% glycerol samples over the one-week period compared to the other glycerol concentrations (~ greater than 300% difference in mass increase over one week) suggest a potential percolation threshold at or below this level. The 5% glycerol samples demonstrated greatest stability (~120% mass increase) over the one-week period. Previous work by our group found that ~10% of individuals are sensitized to current commercial adhesives that are included in substrates today often resulting in erythema. Incorporation of silicone-based adhesives (compared to currently utilized acrylate-based adhesives) deposited in pre-defined geometries on the AFTIDerm substrate enable long-term adhesion to the skin without deleterious reaction. When placed over the wound in a porcine chronic wound model following a 4 mm biopsy, the 5% glycerol AFTIDerm substrate with the silicone adhesive absorbed exudate from the wound without swelling over a one-hour period. Mechanical stability of the AFTIDerm substrates was evaluated under cyclic loading over a strain range of 0 to 2.5% for 50 cycles. Data from each glycerol concentration suggest that the AFTIDerm substrates undergo a short burn-in period for less than 10 cycles before stabilizing. The 5% glycerol samples demonstrated the largest tensile stress of ~800 KPa over this duration with an increase in tensile stress noted over the initial burn in period, which was opposite to the decrease observed for the other PVA-glycerol samples. Overall, the measured tensile stress of the AFTIDerm substrate was significantly greater than that of Tegaderm™ (~50 KPa), which served as controls for all benchtop studies. Further elucidation of the mechanical properties of the PVA-glycerol substrate through pre-clinical and clinical testing will provide first-hand assessment of the utility of the AFTIDerm substrate for use in bioelectronic devices. Doping of the AFTIDerm substrate with conductive elements will assess the utility of the substrate as a platform for flexible electrode technologies.

Recently introduced classes of thin, soft, skin-mounted microfluidic systems offer powerful capabilities for continuous, real-time monitoring of total sweat loss, sweat rate and sweat biomarkers. Although these technologies operate without the cost, complexity, size, and weight associated with active components or power sources, rehydration events can render previous measurements irrelevant and detection of anomalous physiological events, such as high sweat loss, requires user engagement to observe colorimetric responses. Here we address these limitations through monolithic systems of pinch valves and suction pumps for purging of sweat as a reset mechanism to coincide with hydration events, microstructural optics for reversible readout of sweat loss, and effervescent pumps and chemesthetic agents for automated delivery of sensory warnings of excessive sweat loss. Human subject trials demonstrate the ability of these systems to alert users to the potential for dehydration via skin sensations initiated by sweat-triggered ejection of menthol and capsaicin.

Flexible and wearable electronics have become a rapidly emerging technology and are continuously finding new applications in the commercial world. The use of wearables has expanded far beyond being just a fashionable gadget and is now being recognized as a critically important and essential technology. In particular, the potential benefit it offers in the healthcare industry could revolutionize the field as it enables real-time continuous monitoring of biomarkers in a non-invasive manner without interrupting the user’s daily activities. The use of nanomaterials in biosensors has shown to be very effective because of their high surface-to-volume ratio, enhanced reactivity, and in some cases, biocompatibility (Geim & Novoselov, 2007). Especially, 2D nanomaterials such as graphene show promising results in this regard. It not only offers highly sensitive detection of biomarkers due to its monolayer structure, exposing each carbon atom of the lattice to the sensing environment, but also provides flexibility in terms of its mechanical bendability which make them an unparalleled candidate for flexible wearable biosensors. As a result, its application as a channel material in field-effect transistors (FETs) offers numerous advantages in terms of mass-scale production, integration, power consumption as well as miniaturization. Graphene-based FETs (GFETs) have been shown by our group (Ghosh et al., 2018; Khan et al., 2020) and others (Wang et al., 2018; Zuccaro et al., 2015) to be effective in detecting biomarkers with high sensitivity. However, these devices developed for rigid substrates are not well-suited for the wearable platform requiring flexibility and stretchability. Recently, a flexible and stretchable GFET-based biosensor was developed for its potential use in wearable and implantable applications (Wang et al., 2019). However, the electrodes were deposited with metal evaporation and patterned with conventional photolithography and thus making the fabrication process expensive and complex. A viable alternative approach to manufacturing a wearable GFET device is the printing of conductive inks. The advantages of this method include rapid prototyping, low cost, and flexibility in the printed patterns among others. In this work, we investigate the feasibility of using a printed circuit board (PCB) printer with a polymer-based flexible substrate such as Kapton or polyethylene terephthalate (PET) film to develop a GFET biosensor. A Commercially available Voltera V-One PCB printer was used to pattern an electrode with conductive silver ink. A minimum feature size of 200 µm was achieved. Afterwards, the fabricated flexible GFET device was used to monitor biomarkers commonly present in human sweat sample (such as Dermcidin, a breast cancer biomarker) to test its feasibility as a wearable sweat-based biosensor.

References
Geim, A.K., Novoselov, K.S., 2007. Nature Materials. 6, 183–191.
Ghosh, S., Khan, N.I., Tsavalas, J.G., Song, E., 2018. Frontiers in Bioengineering and Biotechnology. 6.
Khan, N.I., Mousazadehkasin, M., Ghosh, S., Tsavalas, J.G., Song, E., 2020. Analyst.
Wang, X., Zhu, Y., Olsen, T.R., Sun, N., Zhang, W., Pei, R., Lin, Q., 2018. Electrochimica Acta 290, 356–363.
Wang, Z., Hao, Z., Yu, S., De Moraes, C.G., Suh, L.H., Zhao, X., Lin, Q., 2019. Advanced Functional Materials. 29, 1905202.
Zuccaro, L., Tesauro, C., Kurkina, T., Fiorani, P., Yu, H.K., Knudsen, B.R., Kern, K., Desideri, A., Balasubramanian, K., 2015. ACS Nano. 9, 11166–11176.

The ongoing improvement of human health and increased accessibility to wearable electronics has created new garment-based platforms for continuous health monitoring. Additional innovation in sensor design for long-term use is necessary for these smart textiles to be comfortable and accurate. Wet electrodes traditionally used in biopotential monitoring cannot be used in these platforms due to a limited shelf life and the potential to cause skin irritation. Dry electrodes can be used in smart textiles, but also face considerable comfort and sensing challenges such as breathability, application pressure, noisy bioelectronic signals, and motion artifact. This study investigates the effect of dry electrode design on bioelectronic signal quality. Dry electrodes were designed with varying surface areas and vacancies in the conductive ink and polyurethane layers to test moisture vapor transmission rate and application pressure and to correlate those effects to bioelectronic signal quality. Results showed that electrode design influenced its conformability where an increased number of holes increased the pressure exhibited. The increased number of vacancies also improved electrode breathability. Electrodes were sewn into an armband to evaluate electrocardiogram (ECG) sensing performance where the ECG signal to noise ratio was compared to the dry electrode surface area and vacancy designs. There is a notable trade-off between breathability, skin-electrode impedance, and ECG signal quality attributed to the electrode design. These results demonstrate that an optimal balance between electrode design and contact area can be achieved to realize increased user comfort while retaining desired biopotential electrode properties for potential long-term wearability of e-textiles.

Fibrous structures and materials widely exist in nature and have played an important role in our daily life and industry. Thanks to the high aspect ratio, low bending stiffness, flexibility and transparency, fibres possess unique physical and optical properties that are usually inaccessible with conventional planer structures. Combining with functional materials and polymers, such as piezoelectric, conducting polymers and biocompatible materials, novel fibre-based devices and textiles could open up possibilities for emerging applications. Such prospects could range from transparent textile-based sensors to biointerfacing architectures. However, challenges largely remain with the current fibre fabrication techniques, in which the synthesised fibres are usually deposited to a surface substrate. Thus, the optomechanical properties of the planer substrates would largely compromise the advantageous properties of the fibres. Moreover, conventional fibre printing techniques usually suffer from inefficient functional material usage (e.g. blending functional material with passive polymer), and this could lead to an overall reduced performance in the final products.

Herein, we present two original fibre printing techniques, which are especially developed to efficiently print substrate-free functional fibres with different fibre dimensions and applications. First, the dynamic near field electrospinning is developed to fabricate in situ poled piezoelectric nanofiber mesh, with high visible light transparency (> 97%) and air permissiveness. Such suspended nanofibre mesh harnesses the physical merits of spider web in its high acoustic sensing ability and broad active bandwidth. Combined with piezoelectric polymers, such spider-web inspired acoustic sensor has a broad sensitivity bandwidth covering 200–5000 Hz at hearing safe sound pressure levels. Second, inflight fluidic fibre printing, which integrates conducting fibre production and fibre to circuit connection in a single step is developed to produce metallic (silver) or organic (PEDOT:PSS) fibres with 1-3 μm diameter. Using PEDOT:PSS fibres as a cell-interfaced impedimetric sensor and a moisture sensor, we show that even a single fibre component can achieve complex functions or outperform conventional film-based devices. The capability to design suspended fibres and networks of homo-, hetero- cross-junctions, paves the way to applications including flow-permissive devices, and 3D optoelectronic and sensor architectures. Overall, we demonstrate the versatility and scalability of novel fibre printing methods that could pave way for the next-generation fibre-based devices that could fully harness the potential of the physical merits of fibres and the functional performances of advanced materials.

For emerging skin-integrated electronics/optoelectronics, thermal management should be achieved for reliable signal acquisition as well as thermal comfort. However, the current layout of skin-integrated devices has limited their use to indoor settings, because of solar energy gain outdoors, particularly strongly in oximeters. Various approaches have been reported to release accumulated heat in the skin-integrated device, such as a thin metallic heat sink to dissipate the concentrated heat and a copper/paraffin bi-layer acting as a heat sink based on phase variation. However, metallic layers hinder wireless communication between the receiver and transmitter, and phase shift materials function irreversibly at high temperatures of ~ 50 °C. In particular, in conventional wearable oximeter reported in the literature, since black encapsulation layer is adopted to block ambient light, using the conventional layout in outdoor environment considerably exacerbates device heating owing to solar energy absorption. Previous works could not provide an obvious solution for these challenges.
Here, we introduce a new platform of radiatively cooled wearable optoelectronics that is free from heat and light concerns, which enables assessing athletic performance in outdoor environments under direct sunlight. To maximize the radiative cooling performance, a nano-/micro-voids polymer (NMVP), a nonmetallic and flexible radiative cooler, is realized by applying bi-layer layout of two perforated polymers (i.e., polymethylmetacrylate (PMMA) and styrene-ethylene-butylene-styrene (SEBS)), which are optically and mechanically optimized for wearable optoelectronics. The NMVP boasts near-unity reflectivity and emissivity in the solar spectrum and atmospheric window (i.e., 8-13 μm-wavelength) with flexible feature. As a result, the optimized NMVP exhibits sub-ambient cooling of 6 °C in daytime under various conditions such as scattered/overcast clouds, high humidity, and clear weather. Additionally, the integration of the optimum NMVP with the device enables maintaining temperature of ~ 34 °C on the skin under sunlight, whereas the normally used, black encapsulated device shows over 40 °C owing to solar absorption. The heated device exhibits an inaccurate tissue oxygen saturation (StO₂) value of ~ 60 % compared with StO₂ in a normal state (i.e., ~ 80 %). However, our thermally protected device presents reliable StO₂ of ~ 80%. This successful demonstration provides a potential of thermal management in wearable devices for outdoor applications.

Due to the immediate requirement of accurate control of humidity in various industries such as agricultural development, life and health, etc. the humidity sensors based on two-dimensional materials have provoked much attention because of their exceptional properties (i.e. high surface to volume ratio, large exposed surface area, tunable semiconducting properties, etc.)[1]. Rhenium disulfide (ReS₂), being less explored transition metal di-chalcogenide (TMD) has unusual but exciting physicochemical properties[2]. Herein, ReS₂ film was deposited on a flexible cellulose paper substrate using a vacuum filtration technique and was further applied as a humidity sensor. The 2D ReS₂ humidity sensor demonstrated elevated sensitivity (~115%), faster response time (~15 seconds), and enhanced long-term stability[3]. In particular, the electropositivity of the transition metal plays a key role in humidity sensing. Compared to tungsten, molybdenum, and other well explored transition metals of TMDs rhenium has higher electropositivity, which indicates that ReS₂ has more surface energy to capture water molecules and attach to the substrate[4]. The structural and compositional characterization of pristine ReS₂ showed a hexagonal phase with the P63/mmc space group. Further, to demonstrate a real time application, the fabricated ReS₂ humidity sensor was utilized as a respiration sensor to sense normal, slow, and fast breath patterns. The frequency and amplitude of the sensor current peaks were utilized to differentiate between various breath patterns which hold potential applications in non-invasive healthcare. The successful demonstration of scalable, cost-effective ReS₂ based humidity sensor on flexible and decomposable cellulose paper offers exciting applications in flexible and wearable electronics.

Keywords: Breath patterns, Cellulose paper, Flexible electronics, Humidity sensor, ReS₂.

References:
[1] Y. Zhang, W. Zhang, Q. Li, C. Chen, and Z. Zhang, “Design and fabrication of a novel humidity sensor based on ionic covalent organic framework,” Sensors Actuators B Chem., vol. 324, p. 128733, 2020.
[2] M. Rahman, K. Davey, and S.-Z. Qiao, “Advent of 2D Rhenium Disulfide (ReS₂): Fundamentals to Applications,” Adv. Funct. Mater., vol. 27, no. 10, p. 1606129, 2017.
[3] R. Bakiya Lakshmi and A. Vimala Juliet, “Effect of annealing on humidity sensing properties of Sm-doped SnO₂ thin films,” J. Mater. Res. Technol., vol. 8, no. 6, pp. 5862–5866, 2019.
[4] Y. Feng, S. Gong, E. Du, K. Yu, J. Ren, Z. Wang, and Z. Zhu, “TaS₂ nanosheet-based ultrafast response and flexible humidity sensor for multifunctional applications,” J. Mater. Chem. C, vol. 7, no. 30, pp. 9284–9292, 2019.