We are continuing to investigate the molecular design, electrochemical synthesis, and characterization of functionalized polythiophene copolymers based on poly(3,4-ethylene dioxythiophene) (PEDOT) and poly(3,4-propylene dioxythiophene) (PProDOT). Our most recent efforts have focused on the use of multifunctional monomers for introducing controlled amounts of crosslinking and branching and for promoting adhesion with solid inorganic substrates and specific interactions with soft living tissue. By introducing various amounts of these comonomers, we have created families of materials with tailored electronic and ionic charge transport and mechanical properties. We will specifically discuss the use of a tri-functional EDOT monomer 1,3,5-tri[2-(3,4-ethylenedioxythienyl)]-benzene (EPh), an amine-functionalized EDOT (EDOT-amine), and a multiply ProDOT-modified polyhedral oligomeric silsequioxane (POSS). The structure and properties of these functionalized copolymers were determined using UV/Vis, FTIR, and NMR spectroscopy; optical and electron microscopy; and cyclic voltammetry and electrochemical impedance spectroscopy (EIS).

Cardiac patches are a highly promising tissue engineering solution, to treat one of the most common cardiovascular diseases, myocardial infarction [1]. Conductive polymers are of particular interest as scaffolds for cardiac tissue engineering, primarily for their potential ability to transport the electrical pulses which are vital to the contractions of the heart. However, they need further manipulation in order to impart the properties necessary of a cardiac tissue scaffold [2], [3]. Herein the need for improved mechanical and topological properties, particularly the anisotropy of stiffness, is addressed by the auxetic micropatterning of a cardiac patch. Auxetic materials are those with a negative Poisson&’s ratio, they expand laterally when stretched longitudinally. The negative Poisson&’s ratio can also explain the other distinctive properties of auxetic materials, such as shear resistance, indentation resistance and anisotropy, which make it appealing as a possible feature of biomaterials [4]. A thin layer of a cell adhesive was successfully micropatterned with a re-entrant honeycomb design by excimer laser microablation. The high precision of this pattern is maintained upon coating and crosslinking with a conductive polymer layer. The micropatterned samples, both before and after coating have been characterised by optical microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The ability to control the porosity, effective stiffness and anisotropy of the cardiac patch, by modifying pattern dimensions, has been confirmed by mechanical tests. In addition, measurements for the mode of fracture, stress, effective stiffness and anisotropy have been compared to finite element analysis models. In vitro cell studies have also been conducted to demonstrate the level of support, adhesion and alignment of cardiac cells on the patches. Hence it can be seen, that through the incorporation of an auxetic micropattern, a material&’s properties&’ can be directed to more closely match those of native cardiac tissue, which in turn could lead to improved scaffold design for cardiac tissue engineering.
References
[1] A. Silvestri, M. Boffito, S. Sartori, and G. Ciardelli, “Biomimetic materials and scaffolds for myocardial tissue regeneration.,” Macromol. Biosci., vol. 13, no. 8, pp. 984-1019, 2013.
[2] J. G. Hardy, J. Y. Lee, and C. E. Schmidt, “Biomimetic conducting polymer-based tissue scaffolds.,” Curr. Opin. Biotechnol., vol. 24, pp. 1-8, 2013.
[3] E. S. Place, N. D. Evans, and M. M. Stevens, “Complexity in biomaterials for tissue engineering.,” Nat. Mater., vol. 8, no. 6, pp. 457-70, 2009.
[4] K. E. Evans and A. Alderson, “Auxetic Materials: Functional Materials and Structures from Lateral Thinking!,” Adv. Mater., vol. 12, no. 9, pp. 617-628, 2000.

Conducting polymers are often employed as coatings on smooth metal electrodes to improve the electrode performance with respect to the signal-to-noise ratio (SNR) for neural recording, charge-injection capacity for neural stimulation, and inducement of neural growth for electrode-tissue integration. However, adhesion of conducting polymer coatings on metal electrodes is poor, making the coating less durable and the electrical property of the electrode less stable. Moreover, conventional conducting polymers have relative low conductance, preventing their direct use as the electrode and lead material; and they are brittle, making it difficult for flexible neural electrodes to incorporate conducting polymer coatings.
We have developed a new polypyrrole/polyol-borate composite film with concurrent excellent electrical and mechanical properties. We further developed a method to fabricate a stretchable multielectrode array, directly, using this new material as the sole conductor for both electrodes and leads, in contrast with the conventional approach of incorporating conducting polymers only through coating on non-stretchable metal electrodes. The resulting stretchable polymeric multielectrode array (SPMEA) was stretchable up to 22.84% uniaxial tensile strain with minimal losses in electrical conductivity. Electrochemical testing revealed the SPMEA&’s impressive advantage for recording local field neural potentials and for epimysial stimulation of denervated skeletal muscles.
As a neural interface engineer, I would also like to compare the compliant neural interfacing technology to other technologies, such as optogenetics, radiogenetics, and even a living neural interface that is currently under development in our lab.

Implantable electrodes are being used for diagnostic purposes, for brain-machine interfaces, and for delivering electrical stimulation to alleviate the symptoms of diseases such as Parkinson&’s. The field of organic electronics made available devices with a unique combination of attractive properties, including mixed ionic/electronic conduction, mechanical flexibility, enhanced biocompatibility, and capability for drug delivery. I will present examples of organic electrodes, transistors and other devices for recording and stimulation of brain activity and discuss how they can improve our understanding of brain physiology and pathology, and how they can be used to deliver new therapies.

Active and multifunctional prosthetics could provide amputees with improved quality of life by restoring the sense of touch that is so essential to many human experiences. In order to make active prosthetics a reality, fabricated sensors must be able to communicate information about stimuli to the nervous system of the wearer. Consequently, sensing systems must be developed that produce signals that can be interpreted by the nervous system. Mechanoreceptor cells in humans transduce a force signal into a frequency signal, and the frequency is reflective of the magnitude of the force. We have created an artificial mechanoreceptor system that mimics the frequency-based force sensing properties of human skin by integrating a resistive pressure sensor with a ring oscillator. The use of printed organic ring oscillators is important for future applications in large area, low cost sensor skins. The pressure sensor functions based on the contact-area dependent tunneling resistance between a counter electrode and a micropatterned composite of CNTs and insulating polymer. The pressure sensors were optimized to function in the appropriate impedance range to facilitate the frequency-dependent operation of the oscillators. The oscillators are operated at biologically-relevant frequencies in the range from 0 to ~200 Hz. By controlling the concentration of CNTs, the pressure sensing range can be widely tuned. This can be used to mimic the different sensitivities of mechanoreceptors located in different parts of the body. By using switch-like pressure sensors with a large pressure threshold, the devices can mimic pain receptors, which act as switch-like sensors that turn on at large pressure values. The frequency signal of the sensors was used to stimulate mouse neurons using an optoelectronic approach. There was a direct correspondence between the frequency of the sensor and the resulting frequency of action potentials in the neurons. These simple, neutrally-interfaced pressure sensors are a substantial step toward the development of cost effective active prosthetics.

Soft, flexible electrode arrays have been proposed for improving the chronic performance of bioelectronic devices. Conducting polymers (CP) are the technology of choice for producing soft electrode coatings [1, 2]. Despite CPs being softer than conventional metal electrodes, brittle and friable mechanical properties have limited their use. As such, researchers have studied copolymers of CPs with flexible materials such as hydrogels [3, 4]. Although approaches have been developed for fabricating such copolymer electrode coatings, the integration of CPs within hydrogels remains suboptimal [3]. Furthermore, most of these approaches require electropolymerisation of the CP from an underlying substrate, which limits them to being coatings rather than freestanding electrode arrays. This research explored the fabrication of freestanding conductive hydrogels (CHs) by seeding hydrogels with chains of pre-synthesised CP. The dispersed CP chains were hypothesised to provide nucleation sites, from which subsequent CP could be electropolymerised.
Chemically synthesised poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) was dispersed within a 20 wt% poly(vinyl alcohol) (PVA) solution at 0.01, 0.05, 0.1 and 0.5 wt%. PEDOT:PSS is a dispersion of polymer chains, and cannot meet the fast charge transfer requirements of medical electrodes [5]. The hydrogels were crosslinked by photopolymerisation and then PEDOT was electropolymerised through the PVA-PEDOT:PSS at 0.5 mA/cm². The electrical properties and physical appearance of the gels were analysed at varied time points between 10 and 160 mins of electrodeposition.
Nucleation and growth of CP through the PEDOT:PSS loaded PVA was macroscopically observed (opaque blue particulate was seen) at the highest loading after only 10 min of electropolymerisation. No CP was observed in the lower loadings even at 160 min. The 0.5 wt% PEDOT:PSS loaded PVA, showed a significant increase in CSC from 3.8 mC/cm² at 0 min to 16 mC/cm² at 160 min. There was also a statistically significant decrease in the impedance at 1Hz with the average impedance magnitude decreasing from 1990 Omega; to 715 Omega;.
These studies demonstrate that nucleation and CP growth within an insulative material can be achieved through secondary nucleation, being the use of a CP chain from which subsequent CP can grow. Loading the PVA with 0.5 wt% PEDOT:PSS enabled the fabrication of a free-standing, electroactive material, but this was time consuming and the electroactivity achieved was still relatively low compared to other CP materials. Future work will explore loading the hydrogel with a higher percentage of PEDOT:PSS.
References
[1] R.A. Green, et al, Biomaterials, 29 (2008) 3393-3399.
[2] G.G. Wallace, G.M. Spinks, Soft Matter, 3 (2007) 665-671.
[3] S. Sekine, Y. et al, Journal of the American Chemical Society, 132 (2010) 13174-13175.
[4] R.A. Green, et al, Macromol Biosci, 12 (2012).
[5] L. Basiricograve;, et al, Organic Electronics, 13 (2012) 244-248.

The recent developement of high performance organic and carbon-based materials demonstrated charge carrier mobility and conductivity over 10 cm²V^-1s^-1 and 10³-10⁴ Scm^-1, respectively, which outperform industrial standard materials such as amorphous silicon and indium-tin-oxide. Although these outstanding electrical properties from the soft matters envision them as promising building blocks for next generation flexible electronics, reliable and scalable fine-patterning technology should be also accompanied toward realization of high-density and multi-functional soft electronics. Typically, a proper isolation/patterning of the functional materials is required to suppress parasitic and off current, leading to less cross-talk between neighboring devices and minimum power consumption in the high density integrated systems. For patterning the soft materials, fluorinated photoresist using an orthogonal solvent, pre-patterned self-assembled monolayers, and various direct printing techniques have been reported, however, several drawbacks such as the process complexity, limited choice of materials, low throughput, and resolution limit have been problematic for industrial realization. Here, we report a facile and general route to achieve scalable high-resolution (sub-micron) patterning of organic and carbon-based materials for device and material integrations via a photochemically induced molecular disordering. Upon deep ultraviolet (DUV) irradiation, the soft matters underwent dissociation of specific chemical bonds within molecules as well as loss of inter-molecular ordering, transforming them into non-functional state. Spatially-selective DUV irradiation enables large arrays of patterned functional devices on a substrate. Utilizing this patterning approach, various organic and carbon-based thin-film-transistors were fabricated demonstrating well-defined active material isolation (current on/off ratio: >10⁷) and minimized parasitic current (~pA), and low-power consumption integrated circuits on both rigid and flexible substrates without compromising their individual device performance.

Ionic transistors from organic and biological materials represent an emerging class of soft and flexible devices for bioelectronics applications. Within this context, protonic transistors are exciting targets for further research and development, despite the fact that they have received relatively little attention to date. Such devices represent a natural choice for interfacing rugged traditional electronics and biological systems due to the ubiquity of proton transport and transfer phenomena in biology. Recently, we have fabricated and characterized protonic transistors from the cephalopod structural protein reflectin.¹ We have investigated these devices with standard electrical and electrochemical techniques, discovering that they exhibit performance comparable to the best protonic transistors.¹ We have also developed simple strategies for improving the performance of our protein-based transistors by altering their active layer geometry and integrating them with flexible substrates. Overall, our findings may hold significance for a broad range of conformable biomedical and bioelectrochemical devices.
1. Ordinario, D. D.; Phan, L.; Walkup IV, W. G.; Jocson, J.-M.; Karshalev, E.; Hüsken, N.; Gorodetsky, A. A. Bulk protonic conduction in a cephalopod structural protein. Nat. Chem. 2014,6, 596-602.

PEDOT is a conjugated polymer that is of considerable interest for use in neural interfaces. However, in certain cases delamination and cracking of PEDOT-based coatings has been observed after extended implantations in vivo. Therefore, more detailed information about the mechanical properties of PEDOT films is needed. However it is difficult to obtain free-standing thin films of sufficient size to do extensive mechanical testing. A variety of indirect methods have been used to estimate PEDOT film mechanics such as nanoindentation or Atomic Force Microscopy Quantitative Nanomechanical analysis (AFM QNM). Here, we describe a novel method of testing the tensile properties of individual PEDOT thin films using the manipulating probe and capacitive force sensor inside the sample chamber of a Zeiss Auriga 60 Focused Ion Beam / Scanning Electron Microscope (FIB/SEM). The typical sample cross section was 0.7 um x 1.5 um, with gage length of ~5 um. Our results reveal that tensile modulus of electrochemically deposited PEDOT thin films is ~5 GPa, and the tensile strength is ~170 MPa.

Ag nanowire networks are considered one of the strong candidates for stretchable transparent conductor which are essential components in many of the stretchable electronics. Therefore, there have been many studies to control Ag nanowire alignment, create composites, change interfaces of substrates and more to increase the stability in conductivity under applied strains. Here we present a facile fabrication process that creates networks of wavy Ag nanowires. Strains up to 30% were applied to measure changes in resistances and cyclic tests were also conducted to analyze stability of the samples both of which tests resulted enhanced performances in the samples made using our transfer process. Scanning electron microscope (SEM) and optical microscope were utilized to analyze and understand the creation of wavy structure. Images of wavy nanowires being stretched and released to its original curved shape were acquired which support their improved performances. Developed electrodes were directly applied to dielectric elastomer actuators to further analyze their structural benefits. We anticipate that the method that we have developed could potentially be used for creating wavy structures in other metal nanowires for various applications.

Reducing mechanical degradation in organic electronics is of critical importance to create robust devices which can survive roll-to-roll manufacturing, as well as repetitive strains applied during use in portable, outdoor, and wearable applications. In applications where devices undergo these repetitious strains, plastic deformation is frequently undesirable. However, the difficulties in fabricating and manipulating free-standing thin films pose a barrier to measuring the mechanical properties of these films. The buckling technique—in which the wavelength of sinusoidal buckles on elastomeric substrates is correlated to the tensile modulus of thin films—has been frequently utilized in the literature. However, the tensile modulus only describes how a material behaves under strain while it is still in the elastic regime. To gain a more complete understanding of how stretchable organic semiconductors are, we developed an extension of the buckling technique to determine the elastic limit of thin films. By iteratively straining (increasing the strain upon each iteration) and relaxing the film on an elastomeric substrate, we could determine the elastic limit by observing the onset of sinusoidal buckles in the film in the relaxed state. Utilizing this technique, we found a positive correlation in the elastic limit of poly(3-alklythiophene)s (P3ATs) with increasing alkyl side-chain length as well as increasing film thickness. This simple technique to determine the strain devices can accommodate before plastically deforming will inform the design and selection of appropriate materials for portable, wearable, and outdoor applications.

Currently, intensive researches are ongoing for developing innovative electronics that can be bent, folded and even stretched, and it is a very exciting moment for both industry and academia as the demands on these devices are rapidly increasing due to the explosive growth of wearable devices and bio-integrable electronics. The realization of stretchable electronics has been considered to be very challenging, but the investigations on the nanowires (NWs) revealed that they can exhibit outstanding stretchability and mechanical robustness at the micro-scale, while at the same time diverse configurations and structures such as conductive percolation network are introduced to boost these properties at the macro-scale to produce large-area stretchable devices.
In this study, as one of the potential applications in soft electronics, we report bio-compatible, transparent and stretchable heater based on solution processed silver (Ag) NW percolation network. Ag NW percolation network has been received a great amount of attention as it can exhibit both high optical transmittance and low sheet resistance even at significant strain. We utilize the Ag NW percolation network as a heater by inducing an electrically driven Joule heating. For the fabrication of the transparent and stretchable heater, uniform Ag NWs are firstly synthesized by a conventional polyol process and transferred to a thin Polydimethylsiloxane (PDMS) film by vacuum filtration process. The vacuum filtration transfer provides not only homogeneously dispersed Ag NW on the PDMS film, but also the effect of mechanical pressing of the Ag NW junctions as the pressure is applied between Ag NW percolation network and the target PDMS film. As a result, the Ag NW percolation network prepared by the vacuum filtration transfer exhibits superb electrical conductivity even without any thermal treatment. By applying a constant bias voltage, we confirmed that the proposed heater successfully operates at 50 % strain, which is considered to be sufficient for general wearable applications.
As PDMS shows good conformal coverage and fine biocompatibility, the proposed transparent and stretchable heater can be directly applied to wide range of applications. As a proof-of-concept design, we attach the proposed heater on vial and human wrist to confirm its operation under various deformation and physical disturbances.

In this study, we fabricated stretchable Silver nanowries (Ag NWs)/PDMS composite strain sensor with arbitrary micro-pattern electrodes using dispensing nozzle printing. In order to find mechanically stable electrode design, we proposed two types of electrodes such as a series of rings and diamonds shape pattern. Also, we investigated that the electrical resistance of patterned line could be modified according to the printing condition such as speed of nozzle. Through the static simulation about the two geometries to compare proper pattern when strain sensor was stretched. As a results, we obtained highly stretchable strain sensor (up to 60 % strain) with suitable electrode design. Based on experimental results, the authors are expect that directly drawn electronic skin (E-skin) via printing method can be fabricated with multifunctional sensing abilities in the near future.

The ability to directly print a 3D microfluidic device and sensor using the equivalent of a home printer is a very attractive technology for rapidly deploying chemical and biological sensors. One of the challenges of printing a 3D microfluidic device is removing the “filler” material that is used to define the future microfluidic channel. In this work we examine the use of common Poly(Ethylene Glycol) (PEG) ink as a phase-change filler material with Polydimethylsiloxane (PDMS) ink acting as a printed matrix material. The printed PEG is solid near room temperature and undergoes a phase-change at mild temperatures enabling the filler to be simply “pushed” out of the microfluidic device. This ink combination allows us to print a fully functioning microfluidic device from a 3D Computer Aided Design (CAD) file using drop-on-demand printing techniques.
This work examines how PEG and PDMS molecular weight, ink composition, solvent type, print viscosity, jetting parameters, and substrate temperature impact printed feature resolution. Computational models and experiments show how PEG melt viscosity impacts both directly driven flow channel clearing along with pocket clearing where diffusion is required to completely remove the filler material from a “dead-end” reservoir. A simple chemical detector is exampled by printing metal electrodes using metal-reactive inks. Overall, this work demonstrates a simple method to fabricate 3D microfluidic devices using readily available drop-on-demand printing techniques and helps bring the concept of at-home device fabrication one step closer to reality.

We propose a new structure of the substrate for fabricating stretchable electronic circuits, which provides well-controlled wavy and stretchable regions for interconnects and flat and rigid areas for active devices, respectively. Although many approaches have been demonstrated to be very successful in realizing highly stretchable electronic circuits, it is still difficult to achieve high reliability and reproducibility for more practical, especially large-area applications. For overcoming the inherent drawbacks of previous technologies based on pre-stretching and transfer processes, a new strategy is required for obtaining highly stretchable interconnects with no pre-stretching step, while completely suppressing the deformation within active device areas. In the present work, we demonstrate a new approach employing a hybrid structure for polyimide substrates, where thick-and-flat islands are located within a thin-and-wavy layer. For achieving the hybrid structure, we first prepared a wavy Si substrate with a water-soluble sacrificial layer on it, and coated and cured a thin (~600nm thick) polyimide layer so that the wavy surface profile could be maintained. After additionally coating and pattering a thick (~9mu;m thick) polyimide layer by photolithography and wet etching processes, we could obtain a hybrid substrate with thick-and-flat islands with tapered sidewalls on the thin-and-wavy layer. After forming various types of active devices (organic or oxide thin film transistors) and metal interconnects (Al or Au) within the island and wavy region, respectively, the whole circuit was transferred to a stretchable sheet by casting a silicone elastomer onto the substrate and dissolving the sacrificial layer. At this state, even when the whole circuit was stretched up to ~40%, the strain within the thick island region remained nearly 0%, while most strain was compensated by the deformation of the wavy interconnecting area. Such observation can be attributed to large difference in the local stiffness of the thick and thin polyimide areas. It was also observed that the transistors and interconnects still maintained proper functions under highly stretched conditions, which implies that the simultaneous control of local stiffness and surface profile can enhance the circuit stretchability effectively. As the whole circuit can be fabricated by conventional processes which are completely compatible with existing flexible circuit technologies, the present technique is also desired to be readily applicable for practical systems.
This research was funded by the MSIP(Ministry of Science, ICT & Future Planning), Korea in the ICT R&D Program (The core technology development of light and space adaptable energy-saving I/O platform for future advertising service).

Considering the dramatic need of various wearable and body-attached devices, it will be very useful to develop stretchable acoustic devices applicable to them. Until now, however, the research and the fabrication of the acoustic devices remained flexible, mainly because of the limited materials of the operating component, such as rigid metal wire of voice coil.
In this work, we report on the fabrication of the stretchable dynamic acoustic devices of actuator/sensor using elastic flat polymer matrix with embedded spiral microchannel filled with liquid metal that exhibits stable electric and mechanical properties under applied uniaxial strain of up to 50%.
The stretchable acoustic device is composed of the deformable liquid metal voice coil on which neodymium magnet is attached. To fabricate the liquid metal voice coil, we prepared SU-8 mold with embossed spiral pattern via photolithography and poured the mixtures of elastic polymers (PDMS & Ecoflex) on the SU-8 mold. The use of the liquid metal Galinstan makes the stretching of the voice coil possible by providing electrical stability under mechanical deformation.
Because of the principles of the dynamic acoustic devices, it can perform as both acoustic actuator and sensor simultaneously. If the device is once fed by audio frequency electric currents, liquid metal voice coil generates mechanical vibration by creating alternate magnetic field that interacts with neodymium magnet, and emits acoustic wave. On the other hand, if the device is fed by external acoustic vibration, the vibrating interaction between neodymium magnet and liquid metal voice coil generates audio frequency electric currents which can be recorded by commercial computer programs.
To demonstrate the acoustic performance of the device, we carried out frequency-response test by applying audio frequency sine wave sweep signal from 100 Hz to 22 kHz. As an acoustic actuator, it generates audio frequency acoustic wave and shows stable performance under uniaxial strain of up to 50% and biaxial strain of up to 30%. As an acoustic sensor, it detects external audio frequency vibration and generates acoustic frequency electric current, also shows stable performance under the 50% uniaxial strain. To show the performance of the device more eidetically, we played and recorded pop song through the device, and both demonstration is also conducted under uniaxial strain respectively. Such acoustic device invented in this study has an enormous potential for their application as a stretchable, wearable and bio-implantable acoustic sensor and actuator electronics.

Measurement of galvanic skin response (GSR) is critical for the evaluation of tissue dehydration as well as sweating as an indicator to assess the autonomic nervous system (ANS). Therefore, the monitoring of the electrodermal response could have many clinical applications in different fields including neurology psychiatry, emergency medicine and also in pharmaceutical research. The main obstacle for using this well-established physiological fact for the real clinical application is the lack of stretchable and wearable sensing systems that could be used for long-time monitoring purposes. Although the GSR measurement has been widely used for decades, the commercial electrodes suffer from several drawbacks such as lack of comfortability and stability for prolonged measurements. This work presents a novel nanofibrous electronic textile suitable for monitoring of skin conductance that can be used for wearable wireless health monitoring applications. Several moisture sensitive and skin conductance sensors are integrated in the form of a stretchable wearable textile that can enable intimate and conformal contact with the skin of patients and be used for accurate monitoring of skin moisture and conductance. The stability of sensors are studied by prolonged GSR measurements for up to four hours. The effect of mechanical deformation on the performance of the sensors are examined by testing the sensors subjected to successive bending up to 1000 cycles. The repeatability and reliability of the sensors were also investigated. The fabricated sensors provide a fast response, stable and accurate performance compared to the commercial sensors. The nanofibrous electrodes are promising candidates for GSR measurement sensors due to their unique mechanical and electrical characteristics owing to their particular structural and material properties which results in high conductivity, high surface area and high flexibility.

Soft electronic materials that enhance compatibility with living tissues have attracted much attention in the field of electronics. Conventional electronics that are manufactured on rigid substrates with high controllability exhibit electric performances; however, when they are pressed against biological tissues, they cause biological reaction or discomfort. It is desirable that the contact surfaces of bio information sensing are to be made of soft materials. From this view point, building integrated circuits or sensors on plastic or rubber substrates, called flexible and stretchable electronics, have been intensively investigated. Especially, by reducing the total thickness of a bio information sensing system, bending strain is reduced, allowing the system to achieve incredibly small minimum bending radius. In fact, we have fabricated, on one micron thick plastic film, an organic thin film transistor (OTFT), an organic photovoltaics (OPV), and an organic light emitting diode (OLED), and have confirmed their excellent mechanical durability. Furthermore, besides the above mentioned organic devices, pressure and temperature sensors are also integrated on ultrathin plastic foils (total thickness of these devices are as thin as a few tens micrometers) achieving amazing conformability. In this talk, we will report recent progress of flexible organic thin-film devices and sensors. Those electronic devices are utilized for measuring biological signals such as body temperature, electromyogram (EMG) and/or electrocardiogram (ECG). We also address remarkable stability of these devices in physiological conditions. Finally, their outlook and future prospect in the field of biomedical applications will also be described.

New applications of hydrogels such as soft robotics and cartilage tissue scaffolds require hydrogels with enhanced mechanical performance, which has stimulated an investigation into how hydrogels may be made electrically conducting, tougher and more enduring. This will open up application of these tough conducting materials as pressure and strain sensors for electronic skin devices for monitoring biological function.
In this presentation, I will describe our approach to the fabrication and characteristics of sensor devices based on electronically active soft materials (consisting of the edible biopolymers gellan gum and gelatin). Gellan gum and gelatin are versatile ingredients in well-known food products such as the commercially available product Aeroplane Jelly. These polymer are combined into ionic-covalent entanglement (ICE) gels consisting of ionically cross-linked gellan gum and covalently cross-linked gelatin networks. These ICE gels exhibit suitable mechanically and electrical characteristics for device applications. In addition, I will demonstrate that ICE gels can be prepared in a “one-pot” synthesis approach which facilitates their processing into devices through additive manufacturing (3D printing). Finally, I will present our understanding of the mechanical robust and electrical behaviour, and discuss the properties of the strain and pressure sensors in detail.

Electronics becomes highly conformable, with the prospect to fit to randomly shaped 3D-objects like skin, an area of research that is booming, evolving from laboratory curiosity to practical implementations. Here we describe a skin-spired electronic platform for highly stretchable interconnects with stretch independent electrical conductivity. With a total thickness below 2µm our electronic foil is practically imperceptible. In conjunction with a stretchable carrier, gold, and copper conductors with a resistivity close to that of the bulk metals are unimpaired by mechanical cycling up to 50% strain for up to 1000 cycles. These stretchable conductors can be readily combined with off-the-shelf electronic items to form stretchable LED stripes that remain functional when stretching up to 240% strain and twisting by 720°. In addition we demonstrate a hybrid 2D biaxial stretchable LED network that is readily stretched to 2.5 times its initial area repeatedly. Conducting polymer electrodes extend the concept of imperceptible electronics to transparent devices. The developed technology platform marks first steps towards imperceptible electronic devices that camouflage their presence with potential applications in mobile appliances, sports, health care, food and environmental quality monitoring.

We demonstrate a wearable tactile skin fitted to a human or robotic hand. Strain, proprioceptive-like, sensors cover the dorsal side of the fingers while pressure sensors are distributed along the inner side of the fingers. We manufacture the electronic skin combining conventional microfabrication techniques and rapid prototyping tools. Stretchable gold thin films on silicone are used to form parallel plate capacitive sensors with soft, compressible foam as dielectric layer. The same metal film technology is used to define stretchable resistive flexion sensors on a silicone substrate, which are then interconnected with highly conductive, strain insensitive liquid metal printed wires. An array of six pressure sensors is mounted on the palmar side of a textile glove, covering the entire length of the finger. Two flexion sensors are mounted on the dorsal side of the glove, one on top of the metacarpophalangeal joint and one on top of the proximal interphalangeal joint. Total thickness of the pressure-sensing layer (including sensor nodes, shielding, interconnects, and encapsulation) is less than 1.4 mm while the flexion-sensing layer is 0.5 mm thin. The sensors are read out in real time at 20 Hz using commercial microelectronic components.
Sensitivity of the foam-based pressure sensors is between 0.1 and 0.001 kPa^-1 over a 0 to 405 kPa range, which corresponds to standard forces exerted by a human hand, and can withstand uniaxial tensile strains to 30%. Flexion sensors exhibit a linear response to joint&’s rotation with a sensitivity of more than 0.75 rad^-1. The sensors were mounted on a textile glove and were used to monitor the manipulation of hard and soft objects. We also included our system in a closed sensory feedback loop to reach and maintain a defined pressure when grasping an object. These sensors work toward enabling generalized tactile sensing in robotic and prosthetic applications.

Flexibility, compliance and weight will turn out to be key metrics for future electronic appliances and power supplies. Imperceptible plastic electronic wraps integrate nanometer thin film active components on sub-2-mu;m polymer foils and create devices unmatched in mechanical flexibility, stretchability and weight.
Organometallic halide perovskites are capable of delivering very high power per weight when fabricated on ultrathin substrates, an important metric for wearable and ultraportable electronics, for remote sensing, powering electronic skins or for space applications.
Here we demonstrate methods to fabricate perovskite solar cells on 1.4mu;m thick PET substrates with 12% power conversion efficiency and a record high solar cell specific weight of 30W/g. The solar cells are less than 2mu;m in total thickness and can be bent into radii smaller than 50mu;m. Our devices are fabricated from solution in ambient air at temperatures below 120°C to ensure process compatibility with ultrathin polymer foil substrates. Their unique mechanical properties are achieved with an all ITO/FTO free device architecture that does not require titanium oxide interlayers and avoids high sintering temperatures typically employed for rigid devices on glass substrates. These potentially low cost power sources conform to arbitrary shapes and are expected to provide electrical energy wherever high specific power is critical, as in next generation ultra light portables, wearables, small-scale autonomous robots, electronic and robotic sensor skins or space technologies.
The authors acknowledge funding from the Wittgenstein award and the ERC advanced investigators grant “Soft Map”.

In this talk, I will present our recent progress in developing skin-inspired electronics in terms of materials and applications.

This talk will describe our group&’s efforts to use intrinsically—“molecularly”—stretchable electronic materials in wearable applications. Organic semiconductors have a wide range of mechanical behavior, which will significantly affect the ability of these materials to form conformal chemical interfaces with biological structures. There is also an apparent competition between good electronic properties and favorable mechanical properties. That is, state-of-the-art organic semiconductors tend to be stiff and brittle. We have developed several approaches based on molecular design, processing, and the use of plasticizers that can maximize semiconducting performance and mechanical softness. Mechanical measurements, combined with spectroscopic characterization of solid-state microstructure, inform our design of materials, while finite-element modeling is used to design device layouts and measure mechanical forces when deformed in operational environments. Applications include the first skin-mounted organic solar cell, all-elastomeric transistors, and a sensing “glove” that can be used to translate manual motions—e.g., sign language—into electronic signals for individuals with sensory impairment or for consumer electronic devices.

In this work we report on the fabrication of highly flexible, ultra-conformable free-standing organic thin film transistors (OTFTs) operating at low voltages. We have tested several sub-micrometer, free-standing plastic substrates, on which low voltage organic TFTs and inverters have been fabricated. At first, an aluminum gate electrode is patterned on the ultrathin substrate by standard photolithography, and afterwards the sample is baked overnight in order to obtain an ultrathin Al₂O₃ layer acting as first gate dielectric. In order to obtain a high performing nanometric gate dielectric, a 80 nm thick Parylene C film is deposited over the whole structure. Finally, on top of this structure gold source and drain are fabricated by a self-alignment process [1] in order to dramatically reduce the overlapping between source and drain electrodes with the underneath gate electrode, thus lowering the parasitic capacitances. Two different organic semiconductors have been employed, namely p-type TIPS-Pentacene and n-type N1400, both deposited from liquid phase.
Thanks to the high capacitance coupling induced by the ultrathin hybrid double-layer insulating film, such devices can be operated at ultra-low voltages, as low as 2V, showing hole mobility up to 0.4 cm²/Vs (electron mobility up to 1x10^-2 cm²/Vs), Ion/Ioff up to 10⁵ and remarkably low leakage currents (100 pA).
Full swing complementary inverters have been also fabricated, showing low noise margins and gains up to 20. Thanks to the properly engineered self-aligned structure, these devices are also characterized by a very good frequency response, with a cut-off frequency usually ranging around 100 kHz.
The fabricated structure is highly robust to mechanical stress: devices can be bent down to bending radii as small as 150 um without giving evidence of any significant variation of their electrical properties. In fact, since the electrical response of OTFTs to mechanical deformation is mainly related to the surface strain induced by the device substrate in the active layer, these effects are dramatically reduced (by orders of magnitude) by using ultra-thin free-standing substrates. Thanks to the extreme flexibility of the proposed structure and their ultra-conformability, such patterned films can be easily transferred on whatever kind of structure like paper, fabric, or 3D structures representing a step forward to the realization of highly flexible, compliant electronics particularly suited for smart wearable systems.
[1] S. Lai, P. Cosseddu, G.C. Gazzadi, M. Barbaro e A. Bonfiglio, Org. Electr. 14, 754-761 (2013)

Spontaneously coated epidermal electronics systems (EES) have demonstrated their unique advantages in healthcare and human-machine interface. These EES largely rely on external power supplies, which limit their convenience and mobility. Here, we introduce an epidermal radio frequency (RF) energy scavenger system based on ultrathin passive and active electronic components. This flexible and stretchable system offers tunable system parameters and stable performance under mechanical deformation. It can harvest environmental RF power directly and support the operation of microscale light-emitting diodes ( mu;-LEDs) and simple radio circuits, eliminating the need for external power sources that present in conventional medical sensors and most EES. A modularization process divides the system into several components based on their functions and fabrication requirements. Series of impedance matching LC circuits provides tunability to overall circuit efficiency. Each component is separately fabricated and tested before transfer-printing on a soft elastomer to enhance yield and the overall system performance. Here we report systematic characterization of individual components and overall system performance in various dielectric environments (air, skin, and wet sponge). In addition, we demonstrate long distance, wireless powering of mu;-LED in air and on skin. Infra-red thermal analysis is utilized to evaluate impedance matching efficiency and heat distribution on the device. The system is verified to be fully operational under IEEE regulation&’s maximum permissible exposure for RF energy. The cold-welded metal interface between interconnects is investigated by focus ion beaming milling and scanning electron microscopy. The results suggest a robust wireless epidermal system for RF power harvesting and represent important progress in developing self-supported mobile healthcare systems.

Electrocardiagraphy (ECG) is widely used to diagnose abnormal rhythms of the heart and other cardiovascular diseases non-invasively. Electrical impulses go through the heart with heartbeats and an ECG device detects small changes in electric field on the skin above the heart by the electrical impulses during heartbeats. ECG electrodes' tight attachment to the skin is a key function for reliable transmission of electrical signals from the skin to the electrodes with low impedance between skin and the electrodes. Thus, an electrically conductive gel is usually used on the ECG electrodes to remove gaps between skin and the electrodes. However, it is inconvenient for patients to apply and remove the conductive gel every time.
In this study, we develop stretchable and attachable ECG electrodes by utilizing printed metal nano-ink-based stretchable electrodes, which enables conformal contact to the skin without any conductive gel. The stretchable electrode is fabricated as a strain-relief pattern of thin silver electrodes shaped with crossing horseshoes is printed on a stretchable substrate via direct stamping of silver nano-ink. Conformal contact of the fabricated stretchable ECG electrodes to the chest allows for clear electrical signal from the heart. For a future study, wireless connection to the stretchable ECG device could be established, and then, even a simple self-diagnosis by an electrocardiogram obtained through the stretchable ECG system would be possible for smart phone applications.
[Ref] Jiseok Kim and Woo Soo Kim, " Stretching Silver: Printed Metallic Nano Inks in Stretchable Conductor Applications", IEEE Nanotechnology Magazine, 8[4] (2014).

The ability to pattern functional materials in planar and three-dimensional forms is of critical importance for several emerging applications. We have developed a multimaterial 3D printing platform that enables the rapid design and fabrication of soft functional devices in arbitrary shapes without the need for expensive tooling, dies, or lithographic masks. In this talk, I will describe our recent efforts to create soft, stretchable electronics and sensors for applications ranging from wearable sensors to intelligent biochips.

Nanofibrillated cellulose (NFC) is a fibrillar nanomaterial that has drawn significant attention due to its excellent mechanical properties and transparency among other interesting properties. Highly transparent NFC films were prepared by solvent casting from an aqueous suspension from which aggregates and poorly fibrillated material were removed by centrifugation, leading to excellent optical transparency and clarity. The NFC films were used as transparent substrates for high performance floating catalyst CVD synthesized single-walled carbon nanotube (SWCNT) networks. The NFC-SWCNT hybrid material fabricated by simple room-temperature press-transfer process [1], exhibits excellent transparent conductive film performance, with with sheet resistance of 100 Ohm/sq. at 77 % without any chemical doping, while offering superior flexibility compared to commercially available transparent conductors such as ITO-on-polymer. The mechanical characteristics of the NFC-SWCNT hybrid, especially its high ultimate tensile strength, compare favorably with conventional polymers such as PET. Additionally, biocompatibility and intrinsic porosity of the NFC-SWCNT hybrid material can enable unique applications of this novel hybrid material in bioelectronics.
[1] A. Kaskela, A. G. Nasibulin, M. Y. Timmermans, B. Aitchison, A. Papadimitratos, Y. Tian, Z. Zhu, H. Jiang, D. P. Brown, A. Zakhidov, and E. I. Kauppinen, “Aerosol-Synthesized SWCNT Networks with Tunable Conductivity and Transparency by a Dry Transfer Technique,” Nano Lett., vol. 10, no. 11, pp. 4349-4355, Nov. 2010.

Traditional electronics systems are designed for long term stability. In contrast, transient electronics are designed to operate for user-defined life times and then physically degrade. This new paradigm in the design of electronic systems is predicated on the degradation behaviors of the materials used in device fabrication. The development of a new class of degradable substrates that can be triggered to degrade at controlled rates by exposure to a variety of environmental stimuli (e.g. mechanical stress, UV light, pH) would enable new applications in a variety of industries.
In this work, we present a photodegradable transient substrate made of cyclic poly(phthalaldehyde) (PPA) doped with a photo-acid generator (PAG). Exposing the substrate to UV light generates acid through reaction of the PAG which promotes the cleavage of the acetal backbone of PPA, leading to rapid film degradation. We monitored the degradation of the film&’s storage modulus using dynamic mechanical analysis (DMA) and the depolymerization of the film using Fourier transform infrared spectroscopy (FTIR). Results demonstrate that the polymer degrades into monomer and that the degradation rate is controlled by varying the concentration of PAG and the intensity of the UV source.
Additionally, electronic transistors, diodes, and resistors were fabricated from magnesium (Mg) and silicon nano-membranes using our newly designed substrate. A combination of transfer-printing and electron-beam evaporation were used to demonstrate lithographic compatibility. We demonstrate electronic transience of a Mg resistor in as fast as 20 minutes with substantial physical degradation of the electronic package in 72 hours.

Microcracked gold films on elastomeric substrates can function as stretchable, conformal interconnects and electrodes. In these stretchable interconnects and electrodes, the gold film is sandwiched between two layers of an elastomeric substrate, such as silicone. Experimental evidence shows that the resistance upon bending changes depending on three factors: the direction of the bending, the distance of the film from the neutral plane, and the bending radius. The combination of the first two factors determines whether gold film is (i) compressed, (ii) stretched, or (iii) neither compressed nor stretched. Specifically, the resistance of the gold interconnects decreases upon bending when the film is on the compressed side of the neutral plane, and increases when the film on the stretched side of the neutral plane. In both cases, the magnitude of the change in resistance increases with distance from the neutral plane, but the decrease in resistance in compression is much smaller than the increase in resistance in tension. When the film is on the neutral plane (i.e., the film is neither stretched nor compressed), its resistance does not change upon bending. Furthermore, the resistance increase is significantly higher for smaller bending radii when the film is in tension, but the resistance decrease is nearly independent of the bending radius when the film is in compression. A model of the mechanics of the propagation of microcracks in the gold film is presented to offer additional understanding of the changes in resistance upon bending. Together, these results can provide guidance on the most suitable placement of the gold film within the stretchable interconnect/electrode to meet the requirements of a specific application.

There is a high potential of reversible mechano-responsive and flexible nanocomposite materials based on plasmonic nanoparticles for various applications such as flexible sensors,[1] flexible electronics[2] and optical filters[3].
However, homogenous incorporation of noble metal nanoparticles in elastomers preserving their unique optical properties remains a challenge. Some efforts have been done to transfer gold nanorods into thermoplastic polymers such as PVA to induce uniaxial orientation by thermoplastic deformation resulting in optically anisotropy.[4, 5] These kinds of nanocomposites are however ultimately fixed to one alignment constitution in which the alignment degree cannot be altered by external stimuli.
Here, we present a thermoplastic elastomer, incorporated with anisotropic gold nanoparticles (Au NRs) with high AuNR filling fractions (2 wt%). In an easy and scalable ligand exchange process, CTAB-stabilized gold nanorods (AuNR@CTAB)[6] were first functionalized with a telechelic thiolated ABA tri-block co-oligomer precursor containing isoprene and styrene.[7] The resulting highly filled viscous nanocomposite material is either directly physically cross-linked[7] or embedded in a matrix consisting of a thermoplastic elastomer. The final plasmonic elastomeric material was studied with respect to its strain responsive reversible structure-property relationships using UV-Vis-NIR spectroscopy with polarized incident light and small angle x-ray scattering (SAXS). Indeed, the nanocomposites show reversible strain induced alignment of gold nanorods along with high optical anisotropy characterized by a strong polarization dependent decrease in extinction.
As a consequence from its anisotropic optical properties, the resulting flexible nanocomposite material can be used as a simple strain sensor or as a mechano-responsive filter where elongation renders the material dichroic.
1. Segev-Bar, M. and H. Haick, ACS Nano, 2013. 7(10): p. 8366-8378.
2. Kim, Y., et al., Nature, 2013. 500(7460): p. 59-63.
3. Wilson, O., G.J. Wilson, and P. Mulvaney, Advanced Materials, 2002. 14(13-14): p. 1000-1004.
4. Pérez-Juste, J., et al., Advanced Functional Materials, 2005. 15(7): p. 1065-1071.
5. van der Zande, B.M.I., et al., The Journal of Physical Chemistry B, 1999. 103(28): p. 5761-5767.
6. Vigderman, L. and E.R. Zubarev, Chemistry of Materials, 2013. 25(8): p. 1450-1457.
7. Pletsch, H., M.J. Schnepf, and S. Agarwal, Chemistry of Materials, 2014

Deformable optical elements (lenses, mirrors and gratings) are fundamental ingredients for the fabrication of compact, inexpensive and portable devices based on adaptive optics. In particular the use of tunable gratings based on stretchable reflective substrates could revolutionize the design of optical miniature spectrometers and widen significantly their field of applications.
Here we present and discuss an effective approach to the fabrication of nanocomposite-based deformable and stretchable optical systems by means of Supersonic Cluster Beam Implantation (SCBI) of metal nanoparticles in PDMS [1]. A beam of electrically neutral nanoparticles, accelerated by a pressure difference and focused with a very low divergence by aerodynamical effects, is directed towards the polymeric substrate and gain sufficient kinetic energy to get implanted. The extremely good resilience of the nanoparticles layer embedded in the polymer upon deformation of the so obtained nanocomposite allows to maintain extremely good optical performances upon substantial deformation of the material and a large number of deformation cycles [1]. Stretchable reflective optical gratings can be easily fabricated by supersonic cluster beam implantation in PDMS [2].
Optical power can be imposed on stretchable gratings by sticking them onto curved surfaces, provided that the diffraction properties are preserved. Experimental results confirm the effectiveness of this solution, which eventually enables to get gratings with strong optical power in an extremely simple and very cheap way.
In force of this solution it is straightforward to realize very small optical devices for spectral analysis. This opens completely new perspectives for hyperspectral imaging [3], a field which is currently limited in widespread use by the need of the integration of many optical elements in small packages [4]. Using stretchable elements with optical power, a single grating could be exploited together with any low-cost imaging device and a medium computation capability for the fabrication and integration of portable and user-friendly hyperspectral devices.
[1] C. Ghisleri et al., Laser & Photon. Rev. 7, 1020 (2013)
[2] C. Ghisleri et al., Appl. Phys. Lett. 104, 061910 (2014)
[3] D. Bannon, Nature Photonics 3, 627 (2009)
[4] C.P. Warren, et al., Opt. Eng. 51, 111720 (2012)

Construction of transparent, conductive films on bent and non-planar substrates is a fundamental objective in the emerging field of "flexible electronics". We present a new strategy for creating conductive and transparent films in 2-D and 3-D, based upon incubation of water-soluble gold thiocyanate with amine-displaying surfaces, producing spontaneous of crystalline, metallic Au coating. Notably, no external reducing agents were needed to initiate or promote formation of the metallic Au films; in essence, the thiocyanate ligands provide the means for surface targeting of the complex, guide the Au crystallization process, and, importantly, donate the reducing electrons. The Au films exhibit unique “nano-ribbon” morphology which allow significant light transmittance. We show that the generic technology can be employed for generating transparent conductive coatings on a variety of substrate materials in both two dimensional (flat) as well as three dimensional surfaces.
References:
- “Patterned transparent conductive Au films through direct reduction of gold thiocyanate”, Ahiud Morag, Natalya Froumin, Dimitry Mogiliansky, Vladimir Ezersky, Edith Beilis, Shachar Richter, Raz Jelinek, Advanced Functional Materials, 2013, 23, 5663-5668.
- “Flexible conductive surfaces via "bottom-up" gold nanotechnology”, T.P. Vinod and Raz Jelinek, ACS Applied Materials and Interfaces2014, 6, 3341-3346.

Although 1D carbon nanotubes (CNTs) show semiconducting and metallic properties depending on the chirality and recent achievement of separation of semiconducting nanotubes, small portion of metallic nanotubes and short length of the separated semiconducting nanotubes still limit the use of CNTs for thin film transistors. The newly developed 2D large-area graphene by chemical vapor deposition opens the new possibility of their use to switching devices but still limited by the gapless Dirac cone. Motivated by this, semiconducting 2D layered structures, for example, transition metal dichalcogenides (TMD), have been successfully synthesized recently and demonstrate unusual electronic and optical properties. We will show a recent progress on how these materials including their hybrids have been evolved for switching devices with strong emphasis on transparency, flexibility and stretchability for future soft electronics.
Firstly, we report the fabrication of highly stretchable and transparent field-effect transistors combining graphene/single-walled carbon nanotube (SWCNT) electrodes and a SWCNT-network channel with a geometrically wrinkled inorganic dielectric layer. The resulting 1D/2D hybrid devices exhibited an excellent on/off ratio of ~10⁵, a high mobility of ~40thinsp;cm²thinsp;V^minus;1thinsp;s^minus;1 and a low operating voltage of less than 1thinsp;V. Importantly, because of the wrinkled dielectric layer, the transistors retained performance under strains as high as 20% without appreciable leakage current increases or physical degradation.
Secondly, we report the fabrication of new TFTs using only 2D materials. All components of TFTs are consist of 2D materials (graphene, TMD, boron nitride (BN)), and fabricated TFTs are also highly stretchable and transparent. Aside from carbon, other classes of 2D materials and their stacked structure are promising in the field of soft electronics when a superior electrical/mechanical property is required.

Applications of liquid metal microfluidic devices include soft robotics, biomedical devices, and flexible electronics. Currently two types of liquid metal based strain sensors exist: single microchannel resistive and two microchannel capacitive sensors. To achieve a sensible output, both of these types of sensors currently require microchannels with length on the order of a few centimeters. With winding channel geometry, these sensors typically have a footprint of about a square centimeter. To address this issue, we developed a compact two liquid single straight channel capacitive strain sensor. The device consists of a dielectric liquid sandwiched between two liquid metal electrodes within a single channel. Fabricating of this capacitor with a liquid dielectric instead of PDMS enables capacitance increase through selection of high permittivity liquids. We demonstrate that use of water and glycerol instead of silicone as the dielectric material can increase the device capacitance by factor of five. Furthermore, this device only has a footprint of a few square millimeters. We discuss the effect of channel diameter, dielectric spacing, interfacial meniscus shape, and oxide shell on device capacitance as well as response to strain.

ABSTRACT:
Electronic devices and sensors which exhibit large amounts of mechanical deformability have many applications such as in smart wallpapers and human-machine interfaces for prosthetics. In this regard, tremendous advancements have been made in engineering solid-state electronic materials and devices on elastic substrates. Recently, sensors based on using liquid active components embedded within soft elastomeric substrates have shown much promise for such applications as liquids present the ultimate limit in deformability.
One limiting factor in the fabrication of more sophisticated liquid based sensors is the inability to combine multiple liquids into one device due to intermixing. Through proper design of the liquid-liquid junction, we enabled heterojunction confinement without intermixing. As a proof of concept, we utilize multiple ionic liquids as the active sensing liquid to sense different stimuli including temperature, humidity, and oxygen gas sensing, with excellent performance characteristics even when under deformation^[1].
The device was fabricated by photolithography techniques using PDMS as the substrate, eutectic GaInSn as electrodes and an ionic liquid as a sensing material. To prevent intermixing of the liquids, they were connected via “junction channels” 30 mu;m width and 150 mu;m length. The maximum width of the junction channels without intermixing is limited to 250 mu;m due to capillary forces. The sensor showed highly structural stability to mechanical deformation. In order to conduct electrical measurements, a constant phase element-based equivalent circuit of the device was constructed from the result of a Nyquist plot.
The sensor showed stable sensitivity to temperature without hysteresis as shown with a 0.039/°C increase in conductivity, which is quite high compared to other reports. The conductance and capacitance increase by 0.13 and 0.08 respectively as the device undergoes 30% uniaxial stretch perpendicular to the axis of the junction channels, which is low compared to the temperature sensitivity. We also show proof of concept for humidity and oxygen gas sensing using three kinds of ionic liquids, [EMIM][Otf], [BMIM][PF6], and [BMPYR][NTf2]. The sensitivity of each ionic liquid to humidity and oxygen differed depending on the ionic liquid. For each type of stimuli, the sensing can be optimized by choosing the proper ionic liquid. In the future, multiple liquid heterojunction sensors, each with a different ionic liquid may be multiplexed into a fully integrated system to sense different stimuli with calibrated response.
REFERENCE:
[1] H. Ota, K. Chen, Y. Lin, H. Shiraki, D. Kiriya, Z. Yu, T.-J. Ha, and A. Javey,: “Highly-deformable liquid-state heterojunction sensors.” Nature Communications, 5(5032), 2014

The most commonly used electrophysiological probes for neural signal recording are based on materials with elastic moduli significantly exceeding those of neural tissues. This mismatch in mechanical properties prevents such devices from probing arbitrary locations in the brain without causing significant damage to the tissues. In addition, shear-induced inflammations limit the ability of using such probes for long-term experiments. Flexible electronics devices also exist, but currently these devices are mostly limited to probing the surface of the brain. Scalable fabrication methods of flexible electrodes that would mimic the brain compliance and be capable of deep brain recording are still largely unexplored. Here, we present a new pathway for the fabrication of electrophysiological probes, based on custom-made composite polymer electrodes drawn into thin and flexible fibers. We first produce electrically conductive bulk composite electrodes by mixing either carbon black or graphite in a polymer binder, through conventional methods such as solution-casting or melt-processing. Then we incorporate such electrodes in a polycarbonate cladding, and draw the material into a fiber through the preform-to-fiber thermal drawing method. This process enables us to produce flexible fibers with thicknesses lower than 300 µm, containing conductive polymer composite electrode with sizes down to 10 µm. The use of polymer composites enables additional control over the materials properties. We investigate the effects of the filler material and volume fraction over the DC conductivity and the Young&’s modulus of the resulting fiber. We additionally study the effects of the same parameters on the AC impedance of our devices, a critical parameter in neural recording. Lastly, we demonstrate that we can use our fibers to measure neural activity. Our ability to optimize the electrical and mechanical properties of our electrodes offers a great advantage over silicon or metal-based probes, and paves the way towards more complex flexible fiber neural probes.

Developing multifunctional devices for stimulating and probing neural tissue is important to the study of information processing as well as pathologies of the nervous system. With the invention of optogenetics it became possible to uniquely optically identify specific cells types via expression of light-sensitive proteins, opsins, of bacterial and archeal origin. Since optogenetic experiments typically rely on viral delivery of opsin genes and require visible light, an invasive two-step surgery is often employed to inject the viral vector solution and then implant an optical fiber and a neural recording device.
Here we report an approach that enables optogenetic study of neural circuits with minimal mechanical invasiveness. Our approach relies on all-polymer fiber-based neural probes, which incorporate six recording electrodes, optical waveguide core, and two microfluidic channels for viral delivery. These devices are produced via thermal drawing process, which is compatible with a broad palette of polymer materials allowing for combination of a transparent optical core and cladding with conductive polymer electrodes within an integrated structure. Our multimodal fiber probes based on polycarbonates, cyclic olefins and conductive polyethylene composites exhibit outer dimensions of <300 µm, while individual recording electrodes have dimensions 20-40 µm thus reducing the damage to the surrounding tissue. Furthermore, all-polymer architecture yields low flexural moduli and permits operation of all three modalities under 90° bending angles.
We show that our flexible multimodal fiber probes allow for simultaneous recording and optical stimulation of neural activity. Furthermore, the same device is used for injection of viral vectors, such as adeno-associated viruses (AAVs) commonly used for delivery of opsin genes in vivo. This technology offers a flexible integrated alternative to multi-step surgeries required for optogenetic mapping of brain function.

The ability to chronically record electrical activity from single neurons enables mapping of the brain, spinal cord and peripheral nerve circuits. This is essential to develop effective therapies for debilitating disorders of the nervous system. Neural tissues, however, impose stringent design constraints on the electronic interfaces with neurons, which, if not followed, lead to glial scarring. This irreversible foreign body response is thought to isolate electrodes from neurons, resulting in recordings with low signal-to-noise ratio (SNR). Based on the results from biocompatibility studies, we hypothesize that small, flexible and chemically inert implants will minimize the tissue response to the implanted probe. The improved biocompatibility should lead to recordings with high SNR over long periods of time.
To address the neural probe design challenges, we use thermal drawing process (TDP), which allows us to reduce macroscopic multimaterial templates into hundreds of meters of fibers with microscale features. Consecutive TDP steps can be used to draw down polymer-metal composite fibers to produce dense arrays of recording microelectrodes, each with a diameter of approximately 5 mu;m. While TDP allows for fabrication of flexible devices with micron features, the final geometry is limited by the fundamental axial symmetry of the fiber as well as dimensions required for stable thermal drawing.
To overcome both of these limitations, we have developed a combinatorial approach that uses multi-layer fiber design and incorporates polymers with orthogonal solubilities. In particular, sacrificial claddings of polyphenylsulfone (PPSU) or polycarbonate (PC) can be introduced into fibers with a polyetherimide (PEI) or cyclic olefin copolymer (COC) matrix, respectively. Selective etching of these claddings yields neural probes with diameters of less than 100 mu;m inaccessible with TDP alone. This process can be extended to other polymers, such as polymethylmethacrylate (PMMA), that are soluble in more common solvents, such as acetone. The use of these polymers gives additional flexibility in the final probe geometries, allowing for less invasive implants. Selective etching can also be applied to metals and combined with electrodeposition. This could improve biocompatibility of the electrode interface with neurons by electroplating individual electrodes with materials such as gold or iridium oxide.

When a body is physically injured, specifically when a wound on the skin is formed, medical treatment is needed to prevent the condition from worsening. Wound management aims at managing and accelerating the healing of an acute, chronic or surgical wound. The bandage is an old though the most common used medical material in both slight and severe wound management. It is consisted of several functional layers, usually including an absorbent layer for wound exudate, antimicrobial barrier dressing, and a low adhesive layer to keep the bandage from adhering to the wound. In addition, cryotherapy and thermotherapy are often used to ease aches from the wound, although current medical devices cannot deliver accurate temperature control in a small and specific area.
Here we firstly demonstrate a “band-aids” like flexible electronic bandage in which multiple wound healing techniques including micro-area temperature management, wound exudate absorption and electrical field assisted cell migration healing are integrated.
The smart bandage is built on the bendable substrate of polyimide, spin-coated with a layer of patterned PEGDA hydrogel for wound exudate absorption. The micro-area temperature management and electrical field assisted healing are achieved in arrays with each pixel of 1mm² by micro-fabrication techniques. The temperature management array is composed of thermoelectric micro-coolers and resistive micro-heaters. Each pixel in the array has eight thermoelectric junctions connected in series, fabricated by sputtering Te and Bi thin films, as well as a micro-heater consisting of the Cr-Au thin films. Several device structures are designed and compared to obtain better thermal conduction on one side and thermal isolation on the other side. The AC electrical field is generated by separate micro-electrodes to perform the cell migration healing treatment.
The temperature of the bandage could be tuned from 18#8451; to 50#8451; at room temperature of 23#8451;. And the temperature distribution could be adjusted to conform to the shape of the wound by independently turning on or off each cooler and heater in the array to inhibit the bacteria growth and to host an optimal temperature for wound healing. The hydrogel absorbent layer ensured the suitable humidity around the microenvironment of the wound. The practical protecting effect was shown by detailed biological characterizations. The wound healing time was accelerated by micro-electrodes arrays as well, which promoted the cell migration across the wound. It could attach to human skin easily due to the patterned high surface area on the substrate.
In this work, smart bandage with adjustable recovery patterns designed for precise wound management is proposed. Compared to traditional bandage materials, this electronics-integrated medical material shows great potentials in many occasions requiring intensive professional wound cares such as the battlefield, sport field and so on.

It is crucial to establish stable, soft, and nonallergic contacts between the biological tissue and the electrode from the devices as well as the good performance to precisely measure bio signal. It has been difficult to form a stable interface, especially when the surface of the biological tissue is wet and/or the tissue exhibits motion. So we suggest one simple way to solve the difficulty by designing and fabricating stress-absorbing electronic devices that can adhere to wet and complex surfaces and allow for reliable, relatively long-term electronic measurements of vital signals. A multielectrode array (MEA) is manufactured on an ultrathin polymeric substrate show how this design can absorb the stress under extreme deformation. The surface of each electrode is coated with a photo patternable adhesive gel. The MEA could be attached to the surface of a moving body, resulting in good conformal contact for more than 3 h. Furthermore, arrays of highly sensitive, stretchable strain sensors are fabricated using a similar electronic design. These could be attached on human skin to detect the strain induced by the motion of the joint.

Artificial electronics are of great interests in diverse applications such as robotics. In fact, there are a variety of demonstrations including artificial electronic eye (e-eye), e-skin, and e-nose. Due to animal/human mimicking devices, these devices need to be on a macroscale flexible substrate. Considering the device cost, macroscale, and economical fabrication technique such as a printing method is another key factor in addition to improving the functionality and performance. Here, we report fully-printed strain and temperature sensors on a user-defined flexible substrate for the application of ultralight and compact artificial electronic whisker (e-whisker) for robots as an example.
For strain sensor, silver nanoparticle (AgNP) ink and carbon nanotube (CNT) ink were mixed (8:10 wt% ratio) and printed on a flexible substrate via a screen printer. For temperature sensor, conductive polymer (PEDOT:PSS) and CNT ink were used with 10:1 wt% ratio. To show different patterning method, the ink was first spin-coated, and the film was then patterned by using a laser cutter tool. Both strain and temperature sensor films were cured at 70°C for 1 hour.
Printed strain sensor is first characterized. By reading the electrical resistance change of AgNP-CNT film, strength and directions of strain (i.e. tensile and compressive) can be determined. The maximum sensitivity was ~59 %/Pa, and the device in terms of mechanical reliability was stable at least 200 bending cycles. Importantly, the composition ratio of inks can readily tune the sensitivity depending on the applications. Next, for temperature sensor, linear resistance change was observed up to 50 °C with the sensitivity of ~0.6 %/°C. For the integration of both sensors, we confirmed the selectivity of each sensor that is high enough to distinguish strain and temperature (strain sensitivity of temperature sensor ~0.02 %/Pa and temperature sensitivity of strain sensor ~0.03 %/°C).
Finally, we integrated strain and temperature sensors and formed 5 whisker array to map spatial and temperature distributions by scanning an object like an animal whiskers. As the results, three-dimensional spatial and temperature mapping can be successfully conducted with a scan speed of 0.6 mm/s. Based on the results, the strain sensor on a flexible substrate can monitor at least 200 µm height difference. By changing the whisker design such as length and thickness of flexible substrate (here we used 100 µm thickness and 10 mm length), we experimentally confirmed that the sensitivity can be tuned, resulting in that height resolution can be improved if needed.
In summary, we developed the fully-printed high sensitive strain and temperature sensor and demonstrated e-whisker applications. The technique to fabricate highly sensitive sensors on a user-defined substrate using only printing methods should lead the future flexible and wearable electronics.

Various soft electronics using flexible polymers and carbon based materials have been demonstrated recently. Many of those devices are eventually expected to be in the form of clothing and to operate along the human body. Above all, strain sensors which can detect various human motions have gained attraction in the realization of wearable electronics.
Here, we report on the fabrication of wearable and highly stretchable strain sensors by using fragmentized graphene networks (graphene foams) and PDMS elastomer composite. Graphene foams (GFs) with interconnected porous microstructure have attained lots of attention because of their high specific surface area combined with excellent mechanical and electrical properties of graphene. GF/PDMS composite was proved to have good flexibility and stretchability, however it has limitations as materials for a strain sensor since the change in the resistance with the applied strain is not significantly outstanding. In this work, we fragmentized GF grown via chemical vapor deposition into few hundred micro-sized branches using a vortex mixer with isopropyl alcohol in order to fully utilize the specific area of the porous microstructure. Although the whole interconnection was broken, the three-dimensional structure of the GF was conserved. When the GF fragments/PDMS composite was stretched, the contact resistance of the fragments changed and the sensitivity of the sensor could be enhanced since the reorganized fractured graphene branches densely contact with each other, as a result. Our strain sensors are capable of sensing minimal strain down-to 0.1% as well as high strain up-to100% with high durability over 1000 cycles and their gauge factors are tunable in the range of 2.4 to 13 depending on the amount of the GF fragments. Also, the devices can be fabricated in various sizes with simple processes.
We could detect various human motions such as pulse, elbow and finger movements by attaching our sensor onto the human skin. The gloves integrated with the strain sensors and LEDs could visualize finger motions through the change of the light intensity. Moreover, we demonstrated stretchable touch sensor array embedded with LEDs which can be used as electronic skin.

Recently, the research on the artificial skin has been actively performed due to its potential applications in sensing physical properties such as tactility, distortion, and temperature. Natural skin receives tactile sensation via detecting the pressure distribution with excellent pressure sensitivity. In order to mimic such tactile sensation of natural skin, artificial skin needs to be equipped with stretchable pressure sensors of high sensitivity over a wide range of pressure: Stretchable sensor enables a conformal detection along with the movement of human body.
In this work, we demonstrate the stretchable and highly sensitive pressure sensor array that employs electrodes of Au-coated polydimethylsiloxane (PDMS) micropillars and polyaniline (PANI) nanofibers on deformable polymer substrate, in which their contact resistance changes with applied pressure. PDMS micropillars were fabricated for increasing the contact area by using the patterned mold of SU-8 photoresist and then coated with e-beam evaporated Ti/Au thin film. PANI nanofibers were grown on polyethylene terephthalate (PET) film by potentiodynamic deposition.
The fabricated pressure sensor showed high sensitivity of 1.79 kPa^-1 in low pressure region of < 0.3 kPa and 0.04 kPa^-1 in high pressure range of >2kPa, respectively, where the response was very fast to have a response time of 50 ms. Of particular interest, it could also detect a very low pressure applied by a rice grain of 20 Pa. With this pressure sensor attached directly above the artery of the neck, the change in pulse before and after exercise could be reliably distinguished.
In order to be used as artificial tactile sensor, it is required to have stable sensing of pressure under applied strain such as bending, twisting, and stretching. We have fabricated the 5x5 array of the pressure sensors on a specially designed deformable substrate, where the strain applied to each pressure sensor was minimized: The deformable substrate consists of rigid PDMS islands where the pressure sensors are attached and soft thin mixed film of Ecoflex and PDMS between the PDMS islands. Liquid metal interconnects of Galinstan were embedded in the thin polymer film for electrical connection between the pressure sensors. Under biaxial stretching up-to 30%, the distribution of pressure was successfully measured to confirm its possible application as tactile sensors for artificial skin, wearable electronic device and robotics.

The composites industry is becoming a valid alternative to many traditional heavy metal industries because of the high specific stiffness of composite materials over the more classical construction metals. Because of the continuous growth of the composites industry with new materials and complex shapes, a method to optimize the production process is imposed. As such the different manufacturing stages (e.g. the injection of resin, the curing and post curing stage) of a composite component can be understood to increase the quality of the produced part while reducing manufacturing costs (less scrap, energy and material reduction).
In this work, the current progress of the Self Sensing Composite project on the in situ cure degree monitoring of glass fibre reinforced plastic is shown. By controlling the curing phase one can obtain a superior quality component and, as a consequence, will increase the reliability and optimize the design of the part, leading to a decrease of the lifecycle cost (i.e. maintenance costs, material costs, design costs). Two strategies are followed, firstly, the embedding of a capacitive-sensor-based deformable microsystem in the composite material, and secondly, the installation of the deformable microsystem in the production mould of the composite part. In the former case, the embedded microsystem can later on be used to sense the ageing effects of composites in operation as well.
The research leading to these results has received funding from the Flemish Agency for Innovation by Science and Technology (IWT) - through the program for Strategic Basic Research (SBO) under grant agreement n° 120024 (Self Sensing Composites).

Pressure sensors or touch sensors are popular components to detect the changes of forces (per unit area) applying on objects for the applications such as human touch/pressure detection, or touch screen pads. The pressure sensor material needs to have proper electrical properties as well as being transparent and mechanically durable for fore-mentioned applications. Graphene has been used as piezoresistive sensing materials with really superior electrical and mechanical properties. Considering the unique electro-mechanical properties of graphene; thin film sensors based on graphene could be conductive and transparent with acceptable sensitivity.
Here, we report a highly sensitive pressure sensor array fabricated with photo-thermally reduced graphene oxide (r-GO). Non-conductive GO thin film is converted into conductive (~ 104S/m) thin layer by fast and precisely designed laser scribing method to generate pressure sensors with sensitivity of 19mV/kPa. The fabricated piezoresistive pressure sensor shows the pressure-sensitivity of 19mV/kPa which is superior to the previously reported MEMS-based pressure sensors' sensitivity with capacitive-type MEMS-based pressure sensor:0.001 mV/kPa. And it is similar with the other highly sensitive diaphragm type piezoresistive MEMS-based sensor with sensitivity 37.5 mV/kPa, or carbon nano tube (CNT)-based sensor's sensitivity as 10 mV/kPa. The fabricated array is easily attachable on any surface for monitoring applied forces or pressure and maintains excellent electrical conductivity under high mechanical stress and thus holds promise for durable sensors.
[Ref]
R. Kazemzadeh, Kimbal Andersen, Lazarus Motha and W.S. Kim “Highly Sensitive Pressure Sensor Array with Photo-thermally Reduced Graphene Oxide” under review in IEEE Electron Device Letters (2015)

Measurement and analysis of plantar pressure caused by contact between human body and external environment is of importance because this pressure directly affects human body. Accordingly, analysis of plantar pressure has been used for many applications including footwear design, performance improvement and injury prevention of sports players, clinical gait analysis and diagnosis of foot-related diseases such as diabetic ulceration, plantar fasciitis and arthritis. Among plantar pressure sensing systems, the in-shoe system can conveniently provide real-life value of plantar pressure because it is meant to measure the pressure inside the shoes.
We demonstrate in this study, an instrumented soft insole embedded with printed inter-digitated capacitive pressure sensors for measurement and analysis of plantar pressure. Unlike the conventional mode of pressure sensing of interdigitated capacitors in which change in dimension of electrodes by external pressure leads to variation of capacitance, for this study, the change in capacitance is entirely led by variation of relative permittivity of the surrounding dielectric medium with applied pressure. For reliable measurement, capacitance is changed into voltage with the assistance of a simple capacitance-to-voltage converter. The measured sensitivity of the sensor is 4-5 V/MPa with high linearity in the pressure exerted by human weight. The sensors are placed at three high-pressure regions, hind-foot, mid-foot, and fore-foot, and plantar pressure is successfully registered for various foot postures.
[Ref] J. Kim, K. Wubs, B. S. Bae and W.S. Kim*, “Direct Stamping of Silver Nanoparticles toward Residue-free Thick Electrode ” Sci. Technol. Adv. Mater.13, pp. 035004 (2012).

Nowadays, the need of human-friendly devices is increased because the performance of devices has been already saturated. Therefore, a lot of researchers have started the study associated with stretchable electronics or wearable electronics. However, ordinary electronics are brittle, so that they cannot be used as wearable ones. Thus, stretchability has to been applied into conventional devices in order to realize human-friendly electronics in improving the convenience for human. To accomplish this goal, many research results or methods have been presented during the last decade. The most typical one is the transfer method. While this transfer method is very easy, it has to be accompanied the transfer of devices onto pre-strained substrate which has problems relevant low yield and size limitation.
Here, we demonstrate a fabrication process of stretchable oxide semiconductor TFT backplane which does not include any transfer process. For this aim, we applied two systems. To explain, the first system is a hybrid substrate. This substrate contains both rigid and soft areas to limit strain at specific regions. Thanks to this hybrid substrate, materials used in conventional electronics can be also exploited. The hybrid substrate exhibits not only substantial mechanical property (stable until 100%), but also high transparency (~ 97%) simultaneously. The second system is a stretchable conductor that plays a role as interconnects. This stretchable conductor is employed to tolerate mechanical strain on the soft regions. In detail, the produced one also shows superb mechanical and electrical property at the same time. In addition, its conductivity maintains during stretching test.
By using these systems and methodology, we can demonstrate stretchable active matrix based on oxide semiconductor which shows general electrical properties (mobilities, on/off ratio, etc.). According to these properties, this stretchable TFT backplane can be adapted on existing devices like the backplane of displays immediately. As a result, we believe that this fabrication process presents a promising strategy toward stretchable and wearable electronics.

Human-adaptive, stretchable artificial skin has recently attracted tremendous interest because of its unique capability of detecting subtle stimuli changes, which may come out its potential applications in wearable health monitoring, sensitive tactile information display, prosthetics, and multifunctional robot skin. In particular, the development of an artificial skin with multimodal sensing capabilities as well as high sensitivity is desired to more plausibly emulate human skin. Moreover, effectively combining the chemical/biological detection with tactile sensing into a single pixel remains a challenging task to augment a sensor skin beyond the capabilities of human skin, whereas this has not been explored.
In this talk, we describe a highly sensitive, wearable, and multimodal sensor skin fabric based on hierarchically assembled elastic and highly conductive carbon nanotube (CNT) microfibers, which is capable of simultaneously detecting versatile external subtle stimuli into a single pixel. We utilized piezocapacitive-type device architecture that consists of highly stretchable CNT microfiber circuitry incorporated with stretchable elastomer dielectric of Ecoflex, where CNT microfibers were aligned to allow point-to-point overlap for implementing high sensitivity as well as high spatial resolution of the sensors. Based on this kind of capacitive sensory system, we could successfully demonstrate the highly sensitive and reliable tactile sensor arrays to allow position detection under even ultralow pressure of 0.4Pa and exhibit fast response time of 63ms. In particular, multimodal output electric signals can be effectively manipulated as a change of resistance or capacitance under mechanical deformations (including pressure and flexion), touch, temperature change, and even chemical variables with different dipole moments.
We believe that elastic CNT microfiber fabric skin will be an effective way to implement human-interactive smart robots capable of recognizing the robot-human-environment interface and in-situ human monitoring capable of biologically detecting some fluidic markers as well as tactile stimuli.

Human skin is the largest organ of our body. It provides a remarkable network of highly sensitive diverse sensors, such as those for tactile sensing, health monitoring, and temperature sensing, that are able to convert mechanical stimuli into physiological signals, which are then interpreted by the brain. Because skin has numerous potential applications, scientists are now exploring the field of electronic skin (e-skin), dreaming to mimic the properties of skin and create novel applications. Besides human skin, the animal and insect skins exhibit many amazing functions. For example, chameleon skin is famous for its color-changing abilities. A chameleon can control its pigment cells and change its coloration for purposes of camouflage, temperature maintenance, and communication, which has inspired scientists to develop biomimetic applications through various approaches. However, previous studies only demonstrated this color changing ability without integrating the most crucial function of e-skin#8213;tactile sensing.
Herein, we present a user-interactive and color-tunable e-skin that enables not only detecting but also distinguishing various pressure forces through real-time absorption change. In addition, the different pressure forces accompanied with corresponding absorption bands in the system demonstrate that skin color can be controlled through applying various pressure forces; on the other hand, the controllable skin color change can be utilized to identify various pressures. Stretchable and ultrathin carbon-based electrochromic devices, and the tunable resistance and tunable switching range of pressure sensors were compliantly integrated in this study. This is the first demonstration of camouflaging e-skin with tactile sensing control. The e-skin integrated color-tunable properties and a tactile sensing ability that incorporates diverse functionalities, and we believe that it will meet diverse applications in health monitoring, interactive and wearable devices, military applications, artificial prosthetics, and smart robots.

In the last decade, significant progress has been made towards stretchable electronics. Conductive elastomers have interesting properties for use in diverse conformal devices for monitoring, diagnosing and therapeutic purposes in medicine, e.g. human motion sensors [1,2] and stretchable multielectrode arrays (MEAs) for electrical stimulation [3]. Until now our focus have been on developing stretchable MEAs which can withstand a rat&’s movement when the MEA is implanted in the spinal canal for epidural spinal cord stimulation. This requires that the conductive material can maintain impedances below 10k#8486; (< 20 #8486;/sq) for strains as high as 50%. Furthermore, the implanted material composite needs to be biocompatible, mechanically robust and stable over several months in vivo. To match the mechanical properties of biological tissue we use polydimethylsiloxane (PDMS) that has a Young&’s modulus around 1 MPa and is biocompatible. Stretchable MEAs based on conductive PDMS have been successfully and chronically implanted in spinalized rats for epidural spinal cord stimulation [4]. However, the electrode density of the implant is limited by the fabrication method and the electromechanical properties of the material.
Novel nano-composites as silver nanowire (AgNW) networks embedded in PDMS have been shown to be promising for use as highly conductive elastomers as well as in strain sensing applications [2,5]. Our presented work focuses on photolithographically micro patterned AgNW networks embedded in PDMS for miniaturizing stretchable MEAs as well as producing highly sensitive strain sensors.
We investigate the electrical response of micropatterned AgNW structures under mechanical stress by conducting uniaxial tensile and fatigue tests. We can tailor the electromechanical properties by adapting our microfabrication process to tackle both electrode and strain sensing applications. We explore possibilities and limitations of this approach by controlling track dimensions in all three dimensions as well as other material property parameters to tailor the electromechanical response of the nano-composite material.
Our approach of photolithographically micro patterned AgNW networks embedded in PDMS can improve limitations in current implants both in terms of its electromechanical material property as well as in terms of electrode dimensions and potentially enable fabrication of miniaturized stretchable MEAs. Furthermore, by adapting our fabrication method, we can tailor the electromechanical properties of the nanocomposite for use in strain sensing applications with high sensitivity over high deformations.
References
[1] Yamada, T. et al., Nat. Nanotechnol., 6, 296-301 (2011).
[2] Amjadi, M. et al., ACS nano, 8(5), 5154-63 (2014).
[3] Van den Brand, R. et al., Science, 336(6085), 1182-5 (2012).
[4] Larmagnac, A. et al., IFMBE Proceedings, 37, 1180-8 (2012).
[5] Xu, F. & Zhu, Y., Advanced materials, 24(37), 5117-22 (2012).

Sensors integrating to clothing system provide many advantages as they are personal, comfortable, close to the body and used almost anywhere and anytime, therefore, are now increasing demand for health care and human-robot interface. Resistance type strain sensor is a simple sensor by detecting correlation between electrical resistance and deformation and obtaining corresponding information of human motions. The sensor fabricated with metal fillers and stretchable binder has mechanical flexibility for wear comfort and shows low electrical resistivity with reduce power consumption. However, the electrical resistance of the sensor was not stable as the sensor held at same strain due to stress relaxation of the binder. In order to remove the effect of binder, high aspect ratio of metal filler, i.e. nanowire, was required to make electrical network without binder on clothes composed by similar yarn with large openings and roughness. Here, we developed long silver nanowires up to 400 micrometer in a length by one step polyol method and demonstrated resistance type strain sensor on clothes. Long silver nanowires were synthesized by the reduction of silver nitrate in the presence of polyvinylpyrrolidone and reduction agent in ethylene glycol. After the reaction, the solutions were washed by water/ethanol and mounted as conductive tracks on a several kinds of clothes by thermal transfer method. The conductive tracks performed as the strain sensor correlated with electrical resistance during elongation and contraction. It is noted that less change of electrical resistance was found during keeping at same strain due to the matching of long metal filler and clothing yarn. The detailed performances of the resistance type sensor will be presented in the meeting.

A stretchable organic digital information storage device has been demonstrated, which enables to advance the development of future smart and digital stretchable electronic systems. The stretchable organic memory with a buckled structure was configured by the mechanical flexible and elastic graphene bottom electrode and polymer compound. The current-voltage curve of the wrinkled memory showed the electrical bistability with typical write-once-read-many-times (WORM) memory features and a high ON/OFF current ratio of ~10⁵. Even under repetitive stretching, the stretchable organic memory exhibited excellent electrical switching functions and memory effects. We believe this the first proof-of-concept presentation of the stretchable organic nonvolatile memory may progress the development for information storage device in various stretchable electronic applications,such as stretchable display, wearable computer and artificial skin.

Stretchable transparent antenna is an important part for wireless system of wearable electronics. Flexible antennas are fabricated by printing or etching conductive materials but these antennas are not stretchable. Recently, there are several attempts to fabricate stretchable antennas using carbon based materials such as CNT, graphene and metal nanowires. Among these materials, Ag nanowires are promising candidates with their electrical and mechanical properties. Although there have been efforts in enhancing the stretching performance of antenna, attempts to create stretchable architectures are rare. In here, we present the facile fabrication of wavy nanowire network as stretchable electrode material for antenna. Fabricated wavy nanowire network electrodes are applied to stretchable transparent antenna. To construct wavy Ag nanowire electrode, filtered Ag nanowires are floated on water and then subsequently compressing the floated Ag nanowire network. After transferring the compressed Ag nanowire network to flexible substrate, we have successfully produced stretchable transparent micro strip antenna patch. Radiating properties of the wavy nanowire antenna are superior to straight nanowire antenna under tensile strain because the wavy structures can be reversibly stretched and compressed large extent without nanowires fracture. We believe the proposed novel fabrication technique that produced wavy nanowire network antenna is applicable to other types of stretchable antennas with different patterns and is possible to be integrated into previous studies to create synergistic effects.

Wearable biosensors could reduce hospital and clinic visits while lowering costs associated with staffing and equipment. These sensors require means of continuous skin interfacing and sampling. We use hydrogels as a biomimetic interface for early prototypes of wearable health monitoring devices. The goals include constructing new electrocardiogram (EKG) electrodes utilizing our hydrogels and EGaIn, a eutectic liquid metal alloy, to create a truly flexible electrode. A low impedance electrode is needed to obtain a strong heart&’s electrical signal from the body for an EKG. A potentiostatic electrochemical impedance test method has been created. On this basis, we can measure both the hydrogel resistivity as well as the impedance of the hydrogel-EGaIn interface. The inclusion of ionizable groups into the hydrogel matrix directly reduces the electrical resistance. The hydrogel-EGaIn interface has a large impedance at low frequencies, where the EKG signal exists. It was believed that this is due to the presence of an oxide skin on the EGaIn. The oxide skin on EGaIn can be removed by adding acid or base. Creating hydrogels that are acidic or basic enable the construction of lower interface impedances by removing this oxide skin. Although both acid and base remove the oxide skin, a high ionic strength is still required to decrease the low frequency impedance. This hydrogel-EGaIn system was successfully modeled with an RRC circuit. Low impedance hydrogels were utilized to create a prototype soft electrode which successfully sampled an EKG signal. These hydrogel based flexible EKG electrodes will later be implemented into a sweat capture device operating under the principles of osmotic and capillary pressure. This provides the multifunctionality needed for future biosensors.

Synthesis of conductive mesoscale structures is essential for developing low-cost, energy efficient, flexible and lightweight devices for next-generation micro and nano- electronics. Despite the wide variety of electronic materials available including soft organic materials such as conducting polymers, carbon nanotubes, as well as inorganic materials such as metals and semiconductors, the creation of high-conductivity and high-resolution microcircuits in high aspect ratio layouts is a major challenge. Conventional electronics is based on solid inorganic materials with very high conductivities (sim;10³-10⁵ S/cm) but with limited mechanical robustness and flexibility. Moreover, the substrates used for flexible electronics are also not compatible with the high temperature processing generally needed for forming metal circuits. Recent attention has therefore focused on using organic materials such as polymers and carbon-based materials as the conduction medium. In the field of organic electronics, however, typical conductivities of soft organic materials are sim;10minus;6 S/cm, limiting their practical implementation in electronics. We report the directed, colloidal synthesis of conductive organic-inorganic hybrid mesoscale structures. The technique is simple but allows hierarchical assembly and patterning of hybrid materials. We use a focused laser spot to direct colloidal assembly of nanoparticles (NPs) into electrically conductive organic-inorganic hybrid mesoscale filaments with arbitrary permanent patterns on a glass surface. High aspect ratio structures are fabricated with high deposition rate from colloids containing carbon and gold NPs, as well as other NP types. Growth mechanisms elucidated through finite element and kinetic Monte Carlo calculations suggest that this optically directed mesoscale assembly and patterning (ODMAP) operates through optical trapping, convective fluid flow, and chemical interactions forcing NPs to fuse near the laser focus and to continuously grow as the spot is moved along the glass-colloid interface. The synthesized filaments exhibit high ohmic con- ductivities, sim;430 S/cm, which is supported by non-equilibrium Green&’s function based density functional theory calculations. As ODMAP is applicable to other types of NPs and can be inte- grated with existing fabrication processes, it represents a simple and economical route to realize a variety of large-area electronic devices.

The nervous system is a complex mechanical system. Neural tissues are mechanically heterogeneous and viscoelastic with typical elastic modulus E in the 1-100kPa range. Furthermore neural tissues reversibly deform during daily living activities: changes in posture and free movement may reversibly extend the spinal cord, by tens of percent; breathing induces physiologic motion of the brain. In contrast, most electrode implants are rigid.
We developed a neuroprosthesis with mechanical properties mimicking those of dura mater, one of the components of the nervous system. Dura mater (E ~ 100kPa) acts as a protective membrane for the brain and spinal cord and plays a major functional role in their stability and mechanical integrity. The neuroprosthesis is soft, manufactured with silicone, stretchable thin metal film and elastic platinum-silicone composite. It integrates an electrode array for stimulation and recording of brain and/or spinal tissue, and microfluidic channels for local drug delivery.
The implant is soft enough to be surgically inserted in the sub-dural space (below the natural dura mater) of the spinal cord. After 4 to 6 weeks in vivo, we observed unprecedented biointegration of the implant with the tissue. We further used the soft technology to pioneer a chronic electrochemical neuroprosthesis implanted in the spinal subdural space, deliver locally electrochemical neuromodulation that restored walking in rats with paralyzing spinal cord injury.

In the past decades, significant progress has been made in the neural stimulation and recording technologies. Nevertheless, the majority of the devices available to date have been developed to primarily interface with brain circuits, and there is a technological gap for neural recording and modulation in the spinal cord. This is potentially due to the complex neurophysiology, and inhomogeneous and flexible structure of the spinal cord that impede the development of neural probes and future spinal neuroprosthetics. The latter may, in principle, allow for restoration of motor and sensory functions in paralyzed patients. Advances in optical neural interrogation tools have recently enabled cell-specific neural stimulation compatible with concomitant recording of neural activity. Thus it is highly advantageous to create flexible multifunctional neural probes that can conform to the spinal cord geometry and mechanical properties, while providing optical stimulation and neural recording.
We mimic the fibrous and flexible structure of the spinal cord and fabricate all-polymer fiber probes that consist of a polycarbonate waveguide and carbon nanoparticle polymer composite electrodes. The polymer fiber probes exhibit low-loss light transmission 0.5-2 dB/cm even under repeated deformation at bending radii < 1 mm. The conductive polymer composite electrodes exhibit tip impedance 1-3 MOmega; suitable to record single neuron activity. The mechanical properties of the integrated devices match those of biological tissues, which is difficult to achieve with traditional electrode materials such as metals or doped silicon. We demonstrate the utility our devices for recording and optical stimulation in the spinal cord of transgenic mice expressing the light sensitive protein channelrhodopsin 2 (ChR2). Furthermore, we find that optical stimulation of the spinal cord with the polymer fiber probes induces on-demand limb movements. Finally, we illustrate that the modest dimensions 50-100 µm and high flexibility of our devices permit chronic implantation into mouse spinal cords with minimal damage to the neural tissue.

A variety of sophisticated neural interfaces have been developed to communicate electrically with the central and peripheral nervous systems to restore function after disease or injury. Micro- and nanoscaled devices are of particular interest to allow specific targeting of small areas, allowing communication with small groups of cells. To reach cells deep beneath the surface of the tissue, penetrating electrodes may be required. Despite the outstanding clinical potential of a variety of neural interfaces, the applicability of these devices remains limited by the failure of the neural interface over time. Failure is due in part to the foreign body inflammatory response to implanted devices (glial scarring), which creates an insulating barrier between electrodes and neural tissue. The mechanical mismatch between stiff devices and soft tissue contributes to this response, particularly during motion. Soft devices, mimicking the mechanical properties of neural tissue, are therefore desired, provided that they are also both suitably stiff to enable insertion, and conductive for stimulation or recording. In study, we describe flexible electrode arrays consisting of penetrating Pt-Ir microwire electrodes connected by a soft, biocompatible silicone elastomer base. Individual Pt-Ir microwires (10s of µm in diameter) have been shown previously to carry large current densities (~10s of µC/cm²) while causing minimal glial scar formation. Using a surrogate tissue model, we demonstrate that our arrays can deform with the tissue as it is elongated, undergoing behavior similar to individual microwires. Stiff-base arrays, by comparison, impede the deformation of the surrogate tissue. Our finite element modeling shows that under deformation, higher levels of stress developed at the electrode-tissue interface when the electrodes are connected by a stiffer rather than less rigid base. We explore different methods for fabricating arrays of penetrating electrodes connected by flexible bases. In one method, individual microwires, which comprise a continuous lead wire and electrode, are first arranged in a rapid prototyped mold, and the elastomer is then set in the mold to form the base. This approach reduces the number of points at which the device can undergo mechanical failure due to separation of the conducting wires. In another approach, micropatterning gold or conductive ink traces are formed and encapsulated on flexible elastomer substrates, and penetrating microwires and lead wires are then connected to the patterned surfaces. This design allows superior mechanical isolation of the lead wires from the penetrating electrodes. Advantages and challenges associated with each of these methods will be reviewed in this presentation. Our work focuses on devices for use in the spinal cord (which undergoes significant deformation during daily motion) but our designs could easily be adapted for use in the brain or peripheral nervous system.

There are an increasing number of evidences indicating the mechanical mismatch between neural tissues and implantable electrodes contribute to their poor long-term biocompatibility. In an effort to improve this heterogeneous interface, flexible and conformable bioelectronics implement electrodes on ultra-thin polymers and elastomers rather than silicon. While matching the geometry and surface contour of the neural tissues is important, one should also consider the mechanical dynamic response of the tissue. For example, the spinal cord stretches and flexes during daily activities.
We have developed a mechanical model of the spinal cord in order to define the most appropriate geometrical and mechanical parameters of implants designed to conform the surface of the delicate tissue. The spinal cord surrogate is composed of an artificial dura mater and spinal tissues, fabricated from silicone and gelatin hydrogel, respectively. The model can flex to controlled bending radii, and stretch reversibly to cover the entire range of physiologically relevant spinal movement. We inserted two types of implants in the subdural space of the spinal cord model: a plastic (polyimide based) implant and an elastic (silicone based) implant.
We quantified the spinal cord deformation upon bending and stretching of the model. Upon bending, the polyimide implant wrinkles thus induces local compressions along the hydrogel core (i.e. the spinal cord). In contrast, the silicone implant does not affect the smoothness of simulated spinal tissues. When the model is stretched, the stiff implant slides relative to the hydrogel core, whereas the soft implant elongates together with the entire spinal cord.
These results suggest soft, elastic materials are better suited to interface with the delicate neural tissues.

The multiplicity of brain signaling modalities and their high spatial resolution impose a challenging barrier to high-density mapping neural circuits in vivo. Existing neural probes are limited in their long-term utility due to decreasing signal-to-noise ratio (SNR) and the number of separable neuronal units, caused by glial scarring and cell death surrounding the device. It has been hypothesized that this neural probe failure stems from a mismatch in elastic moduli between the neural tissue and the neural probes. Incorporation of additional modalities such as optical waveguides or drug-delivery channels is expected to further aggravate the foreign body response to neural probes.
We developed minimally invasive neural probes by employing a thermal drawing process, commonly used in the photonics industry, which allows straight forward integration of multiple materials and geometries into a single probe. In this process we apply heat under controlled stress to a macroscopic template of the final probe to reduce its size (30-200 times) until the desired diameter is reached. By performing multiple thermal drawing steps, the reduction factor is increased exponentially. By combining polymers and low-melting temperature metals and alloys in defined arrangements we fabricated flexible probes that facilitate high-density mapping of brain circuits. Our probes incorporate up to several tens of tin or tin-indium alloy electrodes, each as low as 5 µm in diameter, with defined inter-electrode spacing. Hollow channels and polymer waveguides can be incorporated within the same structure to enable pharmacological or optogenetic neural interrogation.
We fabricated flexible, high-density fiber probes as thin as 80 µm in diameter (comparable to size and flexibility of a human hair). These devices enabled chronic recording of well-isolated action potentials in freely moving mice with average SNR=13. In a comprehensive three-months long histological study we found that these flexible fiber probes produce minimal foreign body response in the mouse brain as indicated by negligible accumulation of glia, astrocytes and macrophages and minimal blood-brain barrier breach. We show that the high-density fiber probes exhibit higher biocompatibility than the best-performing existing neural electrodes.

In this study, we report the in situ polymerization of PEDOT into agarose hydrogels coated on cortical neural probes, as well as their electrical performance and associated biological response when implanted into rat hippocampus for 10 weeks. EDOT monomer was premixed with an agarose solution before thermal gelation. A dip-coating method was used to create a ~500 micron thick film of EDOT-hydrogel on the probe. The precise thickness of gel coating could be controlled by the total number of dips and the speed of withdrawing the probe from the solution. A potentiostatic (constant voltage) method was used to electrochemically polymerize PEDOT into the hydrogel matrix. After polymerization, the PEDOT-gel was dehydrated to a condensed morphology that was suitable for implantations into rat hippocampus. After implantation, the dehydrated PEDOT-gel was rehydrated in the wet tissue. The thickness of the PEDOT-gel structure was controlled by the total amount of charge delivered during electrochemical polymerization. Local field potential measurements (LFP) shows cleaner signals with PEDOT-gel coated probes even after 10 weeks of implantation. In vivo impedance showed that the bare electrodes have high impedance after 10 weeks of implantation, at both high and low frequencies. Histological results show fewer macrophages and astrocytes around the hydrogel-coated probes after 10 weeks of implantation.

In the U.S., nearly 1.7 million traumatic brain injuries (TBIs) occur each year resulting in 52,000 deaths and 275,000 hospitalizations with an estimated cost of $76 billion. The majority of TBIs are considered ‘mild&’ yet can impair neurological function, including learning and memory, both acutely and chronically. TBI is caused by deformation of the brain, and we have taken an in vitro approach, which allows for the precise control over injury biomechanics, to better understand the pathobiology of TBI. In conjunction with organotypic brain slice cultures and an in vitro injury device, we studied changes in electrophysiological function of the hippocampus, which is central to learning and memory. Our studies were greatly enhanced with our stretchable microelectrode array (SMEA), which was incorporated into the culture substrate such that the culture substrate was stretched simultaneously with the SMEA to injure the hippocampus (22±2% equibiaxial Langrangian strain). The SMEA consisted of 28 recording electrodes and was fabricated from a sandwich of thin-film conductors (Ti, Au, Ti) on polydimethylsiloxane with a feature size < 100mu;m. Because the brain tissue was cultured directly on the SMEA, post-injury recordings were normalized to pre-injury activity. As an in vitro correlate of learning, we tested the ability of cultures to remodel in response to a short challenge with the chemical bicuculline. In both uninjured and injured cultures, bicuculline acutely increased the firing rate and the degree of synchronization of neuronal activity recorded by the SMEA. However, 24h after injury and wash-out of bicuculline, the synchronization of neuronal activity in injured cultures was significantly lower despite a greater neural activity (i.e. event rate) compared to uninjured controls. These results suggest a number of future experiments to better understand the mechanisms of neuronal dysfunction after TBI. The incorporation of the SMEA allowed for multiple recordings both before and after injury for normalizing results to pre-injury values while keeping the cultures sterile. This new technology has the potential to help us better understand the pathobiology of TBI and to speed discovery of new therapeutics.
Supported by the National Highway Traffic Safety Administration (DTNH22-08-C-00088), the New Jersey Commission on Brain Injury Research (08-3209-BIR-E-1), and by a Multidisciplinary University Research Initiative from the Army Research Office (W911MF-10-1-0526). The authors gratefully acknowledge the pioneering efforts of Stephanie P. Lacour (EPFL).

PURPOSE:
Recent advances have led to the development of sophisticated prosthetic limbs designed to improve amputees&’ quality of life. The University of Michigan Neuromuscular Laboratory is developing a robust, long-term, Regenerative Peripheral Nerve Interface (RPNI) between nerves and such advanced prostheses. In the future, the RPNI will require high fidelity transducers for volitional control of multi-degree-of-freedom prostheses. Sensor fidelity and prosthetic control may improve with multi-channel electrodes on flexible polymer substrates like polydimethylsiloxane (PDMS). However, substrate porosity may affect RPNI revascularization and regeneration. Therefore, we evaluated the viability of RPNIs encapsulated in PDMS of different porosities.
METHODS:
Sixteen RPNIs consisting of an extensor digitorum longus muscle free graft neurotized by the transected common peroneal nerve (CPN) were constructed in the rat hindlimb and divided into four PDMS substrate groups: 1) none (Control); 2) 50% porosity (PDMS-50%); 3) 25% porosity (PDMS-25%); 4) 0% porosity (PDMS-0%). At three months, peak-to-peak voltage (V_PP) and compound muscle action potential peak rectified area (CMAP_A) were measured from each RPNI after CPN stimulation. Sections of RPNIs were stained with hematoxylin and eosin (H&E), Masson&’s trichrome, and von Willebrand Factor antibody (vWF) to analyze overall muscle health, scar tissue, and vasculature, respectively.
RESULTS:
V_PP of PDMS-0% (0.297±0.434 mV; mean±SD) was significantly less than Control (2.79±1.65 mV) and PDMS-50% (2.93±1.46 mV) (p<0.05). V_PP of PDMS-0% was less than PDMS-25% (2.21±1.30 mV) (p=0.05), trending toward significance. CMAP_A of PDMS-0% (0.356±0.541 mV-ms) was significantly less than PDMS-50% (3.29±1.69 mV-ms) (p<0.05). CMAP_A of PDMS-0% was less than Control (2.79±1.62 mV-ms) (p=0.05) and PDMS-25% (2.15±1.36 mV-ms) (p=0.07), trending toward significance. Qualitatively, from H&E and Masson&’s trichrome, Control RPNIs displayed the largest cross-sectional area and overall health, as evidenced by fiber diameter, circular fiber shapes, and peripherally located nuclei. The next healthiest RPNIs were PDMS-50% and PDMS-25%, with smaller RPNI cross-sections and more scar tissue. The PDMS-0% RPNIs were greatly atrophied, consisting mainly of scar tissue. In the PDMS RPNIs, vWF illustrated a predominance of blood vessels surrounding the PDMS; vessels were more evenly distributed in Control RPNIs.
CONCLUSIONS:
Based on histological and electrophysiological outcomes, porous PDMS substrates remained viable, despite the observation that RPNIs in the porous PDMS groups were atrophied and less vascularized compared to Control RPNIs. PDMS-0% RPNIs were severely atrophied, thus illustrating that pores are likely necessary for RPNI viability. This indicates that porous PDMS RPNIs might be suitable for multi-channel RPNI electrodes; however further studies are necessary to evaluate specific pore geometries and long-term viability.

Polymer nanofiber scaffolds for use in neural tissue engineering have been fabricated via electrospinning of poly-L-lactic acid (PLLA) directly onto a 3D printed support. Previously, the investigators have shown success in promoting the directed growth of neural axons on highly aligned PLLA substrates both in vitro and in vivo. However, one criticism of the earlier in vitro studies is that by spinning fibers on a flat, two-dimensional surface, the growth of the axons is restricted to one plane. Thus the axon-to-fiber attachment may not be the sole mechanism for aligning the growth of the axons along the fibers, and the channels between the fibers and the substrate could contribute to the results. Using 3D-printing, elevated or “bridge” spinning stages were made with supports at varying heights, allowing the fibers to be suspending 2 to 5 mm above the substrate surface in many different configurations. This 3D structure promotes better access of in vitro cell cultures on the fibers to the growth media during incubation, reduces substrate effects, allows more degrees of freedom for axonal growth, and more closely simulates the growth environment found in vivo. Using these 3D stages, we have electrospun highly aligned fiber scaffolds of two types: pure PLLA fibers, and coaxial fibers with a PLLA sheath and a second core polymer. These coaxial fiber scaffold structures offer additional opportunities for in situ delivery of growth agents and/or electrical stimulation for improved axonal growth results.

We demonstrate an easily fabricable, robust, and versatile platform for in-vitro neural studies. Specifically, the platform is based on an ultra-thin Si nanomembrane (NM) combined with a conventional soft material, such as Polydimethylsiloxane (PDMS) or Polyacrilimide (PAAM). The mechanical properties of this composite nanomaterial can be engineered to match those of neural tissue as well as single neurite processes. We fabricate ordered arrays of 3D compliant scaffolds taking advantage of guided self-assembly of the NM on the soft substrate. We evaluate NM channels arranged either a checker-board-like or a linear geometry as structures for mice neurons culture. Both configurations demonstrate a strong guidance effect on neurite outgrowth, even in the absence of adhesion factors. Specifically NM scaffolds with openings larger than the cross-sectional area of a single neurite yield strong physical confinement and guidance of single neuronal processes through the channels. On the other hand, in channels with openings smaller than the dimension of a single axon, cells send processes on top of the NM, following its topography along the axis of the channel. Finally we demonstrate that electronic and photonic functionality can be integrated into the 3D scaffolds, indicating a path forward for real-time electronic and photonic investigations of cell function.
The fabrication of the nano- and microstructures used in this work was supported by DOE, Grant # DE-FG02-03ER46028. The cell culture experiments were supported by the Wisconsin Alumni Research Foundation (WARF).

Recent advances in materials and mechanics concepts allow integration of high performance electronic and optoelectronic systems into sizes and shapes that create new opportunities in neural interfaces. This talk describes recent progress in (1) ‘epidermal&’ electronics for electroencephalography and electrocorticography, with demonstrations of capabilities in long term monitoring and spatiotemporal resolution that would be difficult to achieve with conventional systems and (2) ‘cellular-scale&’ injectable optoelectronics, with examples in tether-free optogenetics studies of both the central and peripheral nervous systems.

Human limb has both sensory and motor function enabled by its skins, muscles, and joints. However, most studies of prosthetic limbs have been focused on cosmetic uses and motor functions. Although there have been many advances in understanding the sensory function of skin based on its mechanoreceptors and thermo-receptors, replicating such functions on prosthetic limbs with the artificial skin still needs further developments. Recently flexible and stretchable electronics opened new possibilities of artificial skin containing various electronic sensors, similar with human sensory functions. To achieve a complete replacement for lost limbs, however, the electronic skin still needs further improvements, such as the better stretchability, the larger detection range, and the higher spatio-temporal resolution. The optimizations/improvements of high performance stretchable sensor arrays, therefore, in consideration of site-dependently diverse degree of stretched states in human skin are needed. Here, we describe a stretchable artificial skin equipped with mechanical and temperature sensor arrays using site-specifically designed high-performance single crystalline silicon nanoribbon (SSNR) electronics. The ultrathin SSNR sensor arrays enable superior flexibility, high electrical sensitivity of mechanical sensors, and reliable performances of temperature sensors, while achieving optimized sensing capability and minimized interferences between sensors. Humidity sensors and electroresistive heaters are integrated as well in the artificial skin, which make the electronic skin more similar with the real human skin. We also demonstrated transmission of sensor signals from integrated sensors to the brain through peripheral nerve stimulations using stretchable multi-electrode arrays (MEAs). This system enables the human-like sensing through artificial one, providing the possible solution for prostheses technologies.

Nanoscale electronic materials that can sense and control biological functions have been attracting increasing interest in the fields of neuroscience, regenerative medicine, wearable or implantable biomedical devices. Biological information conveyed by ions or chemicals can be converted into an electrical signal at the electrode surfaces, and vice versa. Therefore, nanostructured electronic materials assembled on flexible substrates with large surface area can provide an efficient coupling with the biological systems. Single-walled carbon nanotubes (SWNTs), rolled-up sheets of graphene, are attractive low-dimensional electronic materials for nanostructured electrodes for flexible and biocompatible electronic devices because of their excellent electrical and thermal conductivities, mechanical flexibility, chemical stability, and biocompatibility, as well as a one-dimensional shape with an extremely high aspect ratio. However, the strong tendency of SWNTs to form bundles and the difficulty of patterning SWNTs onto flexible substrates with precise control of nanostructures and good adhesion imposes significant challenges for utilizing SWNTs for highly efficient bio-interface electrodes. Here, we report a biological material-based approach to assembling and delivering patterns of SWNTs onto flexible substrates with controlled nanostructures for the preparation of highly efficient flexible bio-integrated electronics. We hydrodynamically assembled two-dimensional free-standing network of SWNTs in solution by utilizing a genetically engineered filamentous M13 phage with a strong binding affinity toward SWNTs. This unique biological material-based in-solution assembly process enabled the realization of conductive nanomesh of SWNTs, independent of the substrate, as well as the delivery of intact nanostructures onto large-scale flexible devices. We demonstrate that the assembled SWNT-nanomesh, integrated into a 40-channel flexible microarray and used for high-density electroencephalography (EEG) on a mouse skull, greatly reduced the in vivo contact impedance and significantly increased the detection rate of high frequency brain signals (HFBS) that are low in amplitude and have been studied mainly by using invasive electrodes. We believe that our controllable and scalable approach holds great promise for practical flexible electronic applications that require highly efficient biological interfaces.

Stretchable electronic devices can conform to arbitrary, complex or moving, expanding surfaces due to their softness and deformability. Such devices hold great promise for bio-medical diagnostics tools as wearable or even implantable sensors [1]. The realization of a highly conducting, stretchable wiring is one of the limiting factors in the realization of such stretchable devices. Printing techniques such as printable elastic conductors are key enabling technologies for large-scale stretchable devices that are required to cover large surfaces.
Here we report printing process of stretchable conductor on silicone elastomer and stretchable textile. The printable elastic conductor is comprising silver flakes, fluorine rubber and fluorine surfactant. The printed conductors exhibit record-high electrical conductivity exceeding 100 S/cm at large strains exceeding 150% tensile. Moreover, this material can be printed with stencil printing and dispenser. The minimum resolution is 50 µm. Then, we have developed a stretchable organic transistor active matrix array and a textile electromyogram (EMG) measurement system. The intrinsically stretchable organic transistor active matrix array combines islands of flexible organic transistors embedded in a silicone elastomer with a modulus gradient and printed elastic conductor interconnects. The modulus gradient around the transistor islands is facilitated due to the diffusion of silicone rubber cross-linking agent [2] and plays a crucial role for device robustness [3]. Multilayer wiring with our printable, stretchable conductor was realized by print-patterning a soft dielectric elastomer. Low operating voltage transistors (2V) were multiplexed in a 2×2 stretchable active matrix and exhibit stable performance even under 110% tensile strain. A textile EMG measurement system was realized by printing our elastic conductor on both sides of a fabric. Here, the printed, soft elastic conductor functions as wiring, EMG sensor and connector simultaneously. The printed textile EMG measurement system realized the measurement of myogram signals. The here developed high performance stretchable printed conductors and stretchable transistor active matrix are key components and building blocks for large area stretchable multipoint indicators or sensors and EMG measurement systems, and open up new avenues for next-generation wearable electronics.
The Authors are grateful to Daikin Inc. for providing fluorine rubbers and N. Matsuhisa appreciates the support from ALPS.
[1] D.-H. Kim, et al., Science 333, 838-843 (2011).
[2] D.-J. Guo, et al., Langmuir21, 10487-91 (2005).
[3] R. Libanori, et al., Nat. Commun.3, 1265 (2012).

Devices that are fabricated onto flexible substrates allows for electronic systems to be conformally laminated onto nonplanar surfaces, with potential applications in flexible displays, wearable electronics, and interactive surfaces. In particular, the fabrication of large area electronics beyond the range of that feasible by conventional batch processing methods at low cost is of interest.
Here, we report the fabrication of a fully printed carbon nanotube active matrix backplane, configured with pressure sensors to enable mapping of tactile pressure across the surface of the array. The device consists of a 20×20 array of carbon nanotube thin film transistors (TFTs) fabricated via a multi-step gravure printing method. First, carbon nanotubes are deposited onto a polyethylene terephthalate (PET) substrate and then silver nanoparticle ink is printed on for the source and drain contacts. Using an alignment camera, a barium titanate oxide (BaTiO₃) gate dielectric is printed on followed by etching of excess nanotubes using oxygen plasma and a final third printing step for the gate metal electrodes. Using this process flow, a yield of up to 97% can be obtained in ambient printing conditions outside of a clean room.
Pressure sensors are integrated into this fully printed active matrix backplane with sensitivities of <1 kPa and the fully integrated system is capable of spacially mapping the applied tactile pressure over the array. The fabrication process for the active matrix backplane is fully scalable to a roll-to-roll gravure printing process that can enable high throughput, large area flexible electronics at low cost.

Octopus, squid, cuttlefish and other cephalopods exhibit exceptional capabilities for visually adapting to or differentiating from the coloration and texture of their surroundings, for the purpose of concealment, communication, predation and reproduction. Long-standing interest in and emerging understanding of the underlying ultrastructure, physiological control and photonic interactions has recently led to efforts in the construction of artificial systems that have key attributes found in the skins of these organisms. In spite of several promising options in active materials for mimicking biological color tuning, existing routes to integrated systems do not include critical capabilities in distributed sensing and actuation. Here we achieved progress in this direction, demonstrated through the construction, experimental study and computational modeling of materials, device elements, and integration schemes for cephalopod-inspired flexible sheets that can autonomously sense and adapt to the coloration of their surroundings without user input or external measurement. These skin systems combine high performance, multiplexed arrays of thermal actuators and photodetectors in laminated, multilayer configurations on flexible substrates, with overlaid arrangements of pixelated, color-changing elements. The concepts provide realistic routes to thin sheets that can be conformally wrapped onto solid objects to modulate their visual appearance, with potential relevance to consumer, industrial and military applications.

In past decade, wearable sensors for monitoring physical parameters, like heart-rate, temperature etc. have been successfully demonstrated. However, obtaining complete health information requires continuous chemical monitoring of the body in addition to measuring the physical parameters. At present only blood analyzers are available to measure these chemicals. But the intrusive nature of these analyzers, due to blood sampling step, renders them impractical for continuous, everyday use. Wearable sensors that can perform continuous non-invasive monitoring of vital chemical biomarkers are thus highly desired, yet missing. Furthermore, an important criterion for any wearable device is the ability to withstand extreme mechanical deformations usually encountered during daily bodily movements. Majority of the work performed in the field of body-compliant stretchable devices either deals with fabricating the device using expensive techniques like lithography or involves techniques like spin/spray coating that are incompatible for large scale production. We have thus developed low-cost, highly stretchable temporary tattoo-based electrochemical sensors with the aim of filling these technological gaps currently present in the wearable sensors field. The sensors can be applied to the skin just like any commercial rub-on temporary tattoo. By leveraging screen printing technology we have been successful in demonstrating, for the first time, the large-scale inexpensive fabrication of high-fidelity tattoo-based electrochemical sensors. The stretchable tattoo-based sensors represent the first example of a class of electrochemical sensors that can withstand stretching deformations up to 100% with minimal effect on their sensing ability. The body-compliant electrochemical sensors have been realized by synthesizing specially formulated screen printable conducting polymer and silver/silver chloride inks. The sensors have been extensively characterized using various electrochemical, optical and microscopic techniques to study the effect of various repeated mechanical deformations like linear stretching, radial stretching and bending. We also demonstrate the ability of these ergonomic sensors towards detection of glucose, an important analyte for diabetes management. Additionally, the sensors can be easily modified to measure other vital chemicals on the skin. Our work thus holds great promise in the wearable health monitors domain.

Conformable electronics could allow unprecedented applications ranging from smart objects to unperceivable personal monitoring systems. Progresses in this emerging technological area are related to the reliable integration of key electronic components on top of ultra-thin polymeric substrates.[1] Indeed, ultra-conformability, as an intrinsic property of sub-micrometric films, allows for the intimate contact between such films and surfaces with arbitrarily complex shape and topography, including biological tissues, as the skin.[2]
As regards the development of human-device interfaces, in particular for skin-contact applications (e.g. physiological monitoring for healthcare or during exercise), a seamless interaction is needed to improve portability and comfort. In this vision the goal is to manufacture ultra-conformable (UC) devices that are unnoticeable to the user.[3] Despite the impressive advancements, still release, manipulation and transfer of UC devices impose challenges. From an end-user standpoint, it is crucial to have a robust, low cost, safe and fully-integrated solution to these problems.
Recently we proposed UC circuits composed of free-standing PEDOT:PSS nanosheets.[4-5] By extending such approach, we developed a new technique that uses temporary tattoo paper as an unconventional substrate for fabrication and release of nanosheets.
Consequently, in this work UC conducting tattoos are tested as surface dry electromyography (EMG) electrodes for recording muscle activity.
The basic tattoo is composed of an ethylcellulose (EC) layer and a PEDOT:PSS layer (overall thickness 350 < t < 700 nm). Noteworthy, the ultra-thin EC layer is contained in the paper used as substrate and available in commercial temporary tattoo kits. Simple circuit patterns were drawn on tattoos by means of combined spin-coating and ink-jet printing. Gold electrodes for providing external connections are embedded on the same tattoo within an acrylic glue layer, available in the same tattoo kit. Such layer is laminated on the tattoo prior to release and simultaneously provides electrical insulation of metal with respect to skin. Transfer of conductive tattoos is simple as in standard temporary tattoos for children: the transfer paper is pressed against the skin and wet with water for a few seconds to permit release and adhesion of the tattoo.
Tattoos are tested as dry electrodes for EMG and their performance is assessed and compared with conventional pregelled electrodes through experimental validation on able bodied subjects. In addition, control of a robotic hand through EMG signals recorded with tattoo nanosheets on skin is successfully demonstrated.
References
1. M. Kaltenbrunner et al., Nature499, 458 (2013).
2. D. H. Kim et al., Science333, 838 (2011).
3. J.-W. Jeong et al., Adv. Mater.25, 6839 (2013).
4. F. Greco et al., Soft Matter7, 10642 (2011).
5. F. Greco et al.ACS Appl. Mater. & Interf.5, 9461 (2013).

In recent years a lot of research has been spent on the development of dynamically deformable electronics and sensor circuits. Such circuits are based on the use of extensible or compressible electrical interconnections, embedded in an elastic polymer like PDMS (silicone rubber) of PU (polyurethane). The circuits, shown in literature, also very often have the property, that they can take different shapes, even when no force other than gravity is executed on it. The same circuit can e.g. be in a flat or a folded state. There is however also a lot of interest in electronic circuits which are still soft and dynamically deformable, but return to a fixed shape when external forces are removed. An example of such a circuit is a smart lens with embedded electronic circuits. Such a smart lens should be deformable and soft, but should always have a spherical shape, when no forces are executed on it. Even further, such circuits with a fixed shape can even be completely rigid and not meant to be deformed in any way. Applications for such 2.5D free-form rigid circuits include new types of light sources, automotive interior parts (e.g. ceilings with LED illumination), non-flat man-machine interfaces with touch sensors in e.g. household appliances, etc. In this contribution a technology will be presented for the fabrication of such 2.5D free form rigid or soft fixed-shape type of circuits. In the current state of the art such circuits are often built using 3D-MID technology (Moulded Interconnect Device), but this technology is time consuming and expensive due to the fact that the circuits are fabricated and the components assembled on a 2.5D non-flat surface, while in industry all electronics, even flexible circuits, are produced on flat substrates, which is far more fast and cost effective. Therefore in order to be compatible with these current industrial standards we first produce our circuits on a flat temporary carrier. Susequently in a 2-step process the circuit is embedded in thermoplastic materials like PU (polyurethane), PC (polycarbonate), PET (polyethylene terephthalate), ABS (acrylonitrile butadiene styrene), etc. The last step in the production process is a thermoforming operation during which the thermoplastic carrier with the embedded circuit is deformed from its original flat shape to the final 2.5D free form, while heating up the polymer in order to make it deformable with relatively small forces. For a final rigid free form circuit this means that the electrical interconnections should be 1-time stretchable only, in contrast to the large amount of stretch and release cycles an elastic, dynamically stretchable circuit has to withstand. This single stretch in thermoplastic circuits occurs during the thermoforming step. In our contribution we will describe the complete production process in detail, show some examples of applications, and indicate the ways to industrialisation, as they are pursued in the frame of the EC funded FP7 project TERASEL.

Recent studies on electronic textile (E-textile) where various electronic elements are fabricated into fabrics have attracted considerable attention for the advanced wearable and flexible devices. Especially, textile-based pressure sensor have been widely explored for a lot of applications such as detecting vital signals of patients, diagnostic and motion detection by embedding them in clothes. For the realization of the highly sensitive textile-based pressure sensors, various types of pressure sensors such as capacitive, piezoelectric, piezoresistive and optical types have been investigated. Among these various types of sensors, capacitive pressure sensors have advantages in terms of simple design and analysis of the devices, high sensitivity, excellent stability and low power consumption. However, since fabrication of the sensors with high performances is difficult due to limitations of techniques and materials, it is very challenging to apply these capacitive fabric pressure sensors to advanced wearable devices.
In this research, we describe a novel method of fabricating highly sensitive textile-based pressure sensor that can be pixelated through weaving techniques. Highly conductive fibers were first fabricated using metal nanoparticles and elastomeric polymer composites through chemical reduction process, followed by uniform coating of polymeric dielectric layers on the surfaces of the conductive fibers. By using the dielectric polymer-coated conductive fibers, capacitive type textile-based pressure sensor was successfully fabricated. The textile-based pressure sensor has unprecedented sensitivity and can be pixelated to arrays in fabrics via the weaving method. The textile-based pressure sensors were successfully applied to monitor human radial artery pulse waves and operate robots by detecting motion of hand using the fabric pressure sensor-inserted gloves.

While human tissues are mostly soft, wet and bioactive; machines are commonly hard, dry and biologically inert. Bridging human-machine interfaces is of imminent importance in addressing grand challenges in health, security, sustainability and joy of living faced by our society in the 21^st century. For example, seamless human-machine interfaces can allow us to more effectively control existing and future machines as natural extensions of our bodies; such interfaces will also allow close and real-time monitoring of various physiological parameters of human bodies, and taking timely actions to significantly improve our health and wellbeing.
However, interfacing humans and machines is challenging, mainly due to the fundamentally contradictory properties of human and machine. Here, we propose to use a bioactive hydrogel system to bridge human-machine interfaces. On one side, hydrogels with similar physiological properties as tissues can be naturally integrated with human body, playing functions such as scaffolds, catheters, drug reservoirs, and wearable devices. On the other side, the hydrogels embedded with electronic and/or magnetic components can control and/or response to machines they are connected. In the talk, I will discuss a general framework to design bioactive and robust hydrogels as the matrices for human-machine interfacing. Then I will focus on various strategies to integrate electrical circuits and other active components with the hydrogels. A number of applications of the bioactive gel system on human-machine interfaces will be demonstrated with future directions discussed.

This talk will discuss work in our group to pattern and characterize liquid metals as device materials for stretchable, soft, and reconfigurable electronics. We focus on alloys of gallium. These alloys are noted for their low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be molded into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. The metal can be patterned by injection into microchannels or by direct-write techniques including 3D printing. Because it is a liquid, the metal is extremely soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomeric or gel substrates, it is possible to form soft, flexible, and conformal electrical components, stretchable antennas, and ultra-stretchable wires that maintain metallic conductivity up to ~800% strain. The ability of the oxide to reform instantaneously also allows the metal to self-heal in response to damage. In addition, the ability to remove the oxide electrochemically provides a new means to control the shape of the metal for reconfigurable electronics. Finally, we combine the metal with hydrogels to create electrodes, diodes, and memristor memory devices that are composed entirely out of soft, liquid-like materials. These materials create comfortable interfaces with the skin for non-invasive sensing.

Microfluidic electronics have been recently highlighted as an approach to enabling highly reconfigurable RF electronics and RF antennas by physically mobilizing metallic interconnects and altering dielectric environments. This paradigm allows electronic components to be extremely robust, whereby the flexibility and stretchability of an electronic material are dictated by the choice of microvascular substrate and not the electronic material, since a conductive fluid adapts and is confined within the changing channel dimensions. This allows highly soft electronics through careful selection of the host material, such as an elastomer. Our group has utilized this approach to explore gallium liquid metal alloys (GaLMAs) as soft electronic materials. A variety of work is being done in our research group to enable these fluids to be used in various applications, from flexible electronics to reconfigurable RF apertures. The native oxide skin on the surface of the GaLMA fluids has historically been a cause for concern (1). We have demonstrated the use of novel interface materials such as polymeric proton exchange membranes (PEMs) containing acidic moieties, which can result in the facile removal of the passivating gallium oxide skin. We have characterized the material chemistry at the interface between the PEMs and shown the utility as extruded sheets or tubes of various respective thicknesses and diameters, as well as a coating material for traditional elastomer microfluidic channels. We have also demonstrated control over the surface chemistry to enable additive manufacturing of the soft electronics through various ink jet and aerosol jet printing of novel GaLMA electronic inks. This technique allows us to print sub-100um feature sizes with liquid metal alloys at room temperature, which retain their shapes and geometries until post-printing treatment because of the native oxide skin. Several proof-of-concept devices have been reported by our group, showing the applicability of the material interface science towards real world capabilities such as pneumatically controlled electronic switches and reconfigurable radiative antenna elements (2).
1. M. D. Dickey et al., Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature. Advanced Functional Materials18, 1097 (2008).
2. B. L. Cumby et al., Reconfigurable liquid metal circuits by Laplace pressure shaping. Applied Physics Letters101, (2012).

Stretchable electronics has received significant attention in recent years due to the prospects of new and exciting applications. Conformal, soft and stretchable electronic devices are especially attractive for medical applications where the matching of the mechanical properties of tissue is essential. To date a variety of applications have been developed, ranging from electronic skin, in vivo temperature sensors to multielectrode arrays (MEAs) for stimulation and recording. A major focus of our group has been to develop stretchable MEAs for epidural spinal cord stimulation in rats.[1] These MEAs have been chronically implanted for months and have managed to restore mobility in paralyzed spinal rats. This application illustrates how tough the demands can be on a device, as it should be biocompatible, implanted for months and withstand repeated strains up to 50% without significant degradation in electrical performance. In order to improve the mentioned applications and to enable new ones there is a general need for high performance conductors which can be patterned at high resolution. Many different stretchable conductors, such as CNTs, Ag nanowires (AgNWs), graphene and liquid metals have been developed to meet these needs. The AgNW-PDMS composite may be especially promising due to its high conductivity and stretchability. [2]
Here we report on the patterning and properties of AgNW-PDMS conductors. We employ a novel additive patterning approach which allows for thick AgNW patterns without wasted residual material. This allows for the fabrication of conductor lines with sheet resistance below 0.5 #8486;/#9633; unstretched and 2 #8486;/#9633; at 50% strain. Further, we vary the AgNW-PDMS composition and perform fatigue tests to evaluate how the different compositions affect the performance of the conductors.
References
[1] Larmagnac, A. et al., 2012. Skin-Like PDMS-Based Multi-electrode Array for Epidural Electrical Stimulation to Promote Locomotion in Paralyzed Rats, in IFMBE Proceedings: 5th European Conference of the International Federation for Medical and Biological Engineering, ed, Ákos Jobbágy, vol. 37, pp. 1180-81
[2] Xu, F. & Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 24, 5117-5122, doi:10.1002/adma.201201886 (2012).

In this presentation, we will introduce a new concept called “fractal cuts”, which was applied for highly expandable electrode and batteries. A 2D material can be differentiated into various forms with a wide range of desired shapes and patterns by introducing fractal cuts: a set of simple cuts that can be arranged in a multi-level hierarchy and/or with different motifs. By putting simple cuts in the right geometry, we demonstrate that we can engineer a variety of expanded material morphologies with strain-isolation of base units. Upon stretching our 2D materials, a completely new set of structures can be produced compared with the original material depending on encoded fractal cuts. The concept was experimentally confirmed and applied to the stretchable electrode and batteries by utilizing highly flexible and expandable nature (> ~800% in area) without deforming the basic units during stretching. Since the approach is general, this universal design strategy of fractal cuts can be applied to tune expandable structures for various applications.

Future electronic skin aims to mimic nature's original in functionalities like the sensation of touch and temperature¹ or self-healing². However, electronics also allows going beyond imitation and equip us with unfamiliar cognition. Magnetoception is a sense which allows bacteria, insects and even some vertebrates like sharks to detect magnetic fields for orientation and navigation. Humans are however unable to perceive magnetic fields naturally.
Here we go well beyond our original works on stretchable magnetoelectronics^3,4 and introduce e-skins with a magneto-sensory system that equips the recipient with a “sixth sense” able to perceive the presence of static or dynamic magnetic fields. The demonstrated ultra-thin giant magnetoresistive (GMR) sensor foils are less than 2 µm thick, extremely lightweight (asymp;3 g/m²) and feature, unmatched flexibility (bending radii <3 µm) and mechanical endurance. At the same time, they reveal high sensitivities (up to 0.25%/Oe) that are identical to their counterparts on rigid Si/SiO₂ wafer substrates. Due to their unique mechanical properties they conform to arbitrary soft and curved surfaces and seamlessly follow deformations or distortions without performance degradation. Attached to the human skin, the magneto-sensitive elements are haptically imperceptible and enable wearable proximity detection, navigation and touchless control.
Biological skin is soft and flexible but also stretchable, a feature that is most desirable for the artificial equivalent. Imperceptible electronics¹ offers an elegant route to facilitate very high levels of strain without sacrifices in device performance by a facile transfer onto a pre-strained elastomer. Our elastically stretchable magneto-sensitive elements can withstand uniaxial or biaxial tensile deformations up to 270%, nearly a 10-fold increase over previously reported concepts⁴. Upon stretching, both, the GMR response and sensor resistance are unchanged, which makes them truly strain invariant sensors that need no additional strain calibration. They furthermore feature excellent long term reliability, as they endure over 1,000 stretching cycles without fatigue.
Our ready-to-use sensing elements extend the cognition of electronic skins to a medium that by no means can be detected naturally by human beings and offer magnetic functionalities as well as motion and displacement sensorics for soft robotics⁵ and medical implants⁶. The integration with other imperceptible electronic elements^1,7 will enable autonomous and versatile smart systems with a multitude of sensing and actuation features.
References:
1 Kaltenbrunner, M. et al.Nature,499, 458 (2013).
2 Tee, B. C.-K. et al. Nat. Nanotechnol.7, 825 (2012).
3 Melzer, M. et al. Nano Lett.11, 2522 (2011).
4 Melzer, M. et al.Adv. Mater.24, 6468 (2012).
5 Kwok, S. W. et al.Adv. Funct. Matter.24, 2180 (2014).
6 Kim, D. H. et al.Proc. Natl. Acad. Sci. USA.109, 19910 (2012).
7 Salvatore, G. A. et al.Nat. Commun.5, 2982 (2014).

The flourishing and eagerness of portable consumer electronics necessitates functional elements to be lightweight, flexible, and even wearable [1,2]. Next generation flexible appliances aim to become fully autonomous and will require ultra-thin and flexible navigation modules, body tracking and relative position monitoring systems. Such devices fulfill the needs of soft robotics [3], functional medical implants [4] as well as epidermal [5], imperceptible [6] and transient [7] electronics. Key building blocks of navigation and position tracking devices are the magnetic field sensors.
We developed the technology platform allowing us to fabricate high-performance shapeable, namely, flexible [8,9], printable [10,11] and even stretchable [12,13] magnetic sensorics. The technology relies on smart combination of thin inorganic functional elements prepared directly on flexible or elastomeric supports. These novel magnetoelectronics can be stretched up to 270% without degrading in performance. The unique mechanical properties open up new application potentials for smart skins, allowing to equip the recipient with a “sixth sense” providing new experiences in sensing and manipulating the objects of the surrounding us physical as well as digital world.
Combining large-area printable and flexible electronics paves the way towards commercializing the active intelligent packaging, post cards, books or promotional materials that communicate with the environment and provide the respond to the customer. Realization of this vision requires fabrication of printable electronic components that are flexible and can change their properties in the field of a permanent magnet. For this concept, we fabricated high performance magnetic field sensors relying on the giant magnetoresistive (GMR) effect, which are printed at pre-defined locations on flexible circuitry and remain fully operational over a temperature range from -10°C up to +95°C, well beyond the requirements for consumer electronics. Our work potentially enables commercial use of high performance magneto-sensitive elements in conventional printable electronic industry, which, although highly demanded, had not yet been possible.
In this talk, I will review the recent advances in the field of shapeable magnetic sensorics and emergent applications of this novel technology.
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Electroluminescent (EL) devices with good mechanical compliance can benefit and inspire a plethora of new applications such as deformable and wearable displays, visual readout on artificial skins, biomedical imaging and monitoring devices etc. Stretchable EL devices have been demonstrated either by employing intrinsically stretchable materials or stretchable device structures. Challenges in the intrinsically stretchable devices persist in that their emission intensity significantly reduced under stretching strains and the devices could not survive large strain cycles, while the devices employing stretchable structures encountered the difficulties of complicated fabrication procedures and unstretchable emissive components. Furthermore, all these pioneering works were geared toward maintaining the device functionality when they were passively deformed by external force.
In this work, we develop a novel approach to fabricate intrinsically stretchable inorganic light-emitting devices with sustained functionality under stretching strains and excellent stability under large strain cycles. The stretchable and transparent electrodes were fabricated with AgNW networks embedded in Polydimethylsiloxane (PDMS) matrix. The light-emitting layer was made of an inorganic material of ZnS:Cu with imparted stretchability by the elastomer matrix. The resultant device exhibited high stretchability, withstanding strain up to 100% with good cycling stability at 80% stretching strain. The good mechanical compliant property of the devices is competitive to the stretchable polymer light-emitting devices while advantages of inorganic materials in their long life-time and fast response time might exceed their organic counterparts. Taking full advantages of the simple device fabrication procedure, we demonstrate that the stretchable EL devices could be driven to dynamic shapes upon integration with dielectric elastomer actuators (DEAs). Compared to conventional stretchable EL devices, the actively deformable EL device offers the special feature of dynamic shape display. The fabrication procedure and devices developed in this report will meet wide applications in light-weight and miniaturized EL elements for volumetric display and other applications.

Dielectric elastomers for soft transducers (actuators, sensors, and energy harvesters) should exhibit low elastic moduli together with high dielectric permittivities and high electrical breakdown strengths. In a systematic approach, soft silicones were modified in order to arrive at the desired combination of elastic, dielectric, and electro-mechanical properties. Several ways to achieve enhanced dielectric performance have been reported recently [1, 2, 3], and appropriate materials-design strategies at the molecular scale enable homogeneous high-performance dielectrics and prevent early electrical breakdown. In this contribution, the dielectric and elastic responses of a silicone grafted with strong organic dipoles are studied. The electro-mechanical properties of the enhanced silicone elastomer are characterized in constant-force and constant-strain experiments on free-standing clamped membranes. The results are compared to theoretical predictions e.g. from the theory of electro-mechanical instabilities [4, 5]. In order to evaluate the enhanced performance and the high durability, long-term tests were performed and mechanical and electrical aging effects were identified. It is found that the desired permittivity increase is accompanied by a decrease in the breakdown strength, while the breakdown strength increases with stretch. Our results allow for further improvements of the materials-design strategies to achieve more sensitive and reliable actuators as required e.g. for specialized transducer applications in optics.
[1] H. Stoyanov, et al. J. Mater. Chem., 2010, 20, 7558 - 7564.
[2] F. B. Madsen et al. Smart Mater. Struct., 2013, 22, 104002.
[3] B. Kussmaul, et al. Adv. Funct. Mater., 2011, 21, 4589 - 4594.
[4] K. H.Stark, K. C. G. Garton, Nature, 1955, 176, 1225 - 1226.
[5] M. Kollosche, G. Kofod, APL, 2010, 96, 071904.

In this work we address two critical issues of stretchable electronics at the same time - the poor adhesion of films on elastomeric substrates and the difficult patterning with standard photolithography. The surface of elastomeric polydimethylsiloxane (PDMS) is first exposed to ultraviolet (UV) radiation (172nm) in order to increase the local adhesion. In a subsequent electroless deposition (ELD) solid silver films are created from the liquid phase only at the exposed regions. This way facile deposition of interconnects from the liquid phase is realized at the same time as their photoresist- and solvent-free patterning.
With the novel method the PDMS is only exposed to an aqueous solution and kept at room temperature at all time. Hence undesired swelling or shrinkage of the substrate due to thermal heating and cooling or exposure to organic solvents is avoided. This enables the application of a well-defined strain during deposition providing the fabrication of stretchable electrodes with a new degree of control.
We demonstrate feature sizes of 5µm patterned with the mentioned technique and fabricate various stretchable structures including meanders and wrinkled stripes and combinations of both. Some structures enable reversible stretching of up to 40% while the sheet resistance is increased from 3 Ohm/sq. to about 3.5 Ohm/sq., only. Our method is in principle roll-to-roll compatible which paves the way for mass production of sophisticated elastically stretchable electrodes and interconnects.

Nanotransfer printing (nTP) technology offers outstanding simplicity and throughput in the fabrication of transistors, metamaterials, epidermal sensors, and other emerging devices. Nevertheless, the development of a large-area sub-50 nm nTP process has been hindered by fundamental reliability issues in the replication of high-resolution templates and in the release of generated nanostructures. Herein, we present a solvent-assisted nanotransfer printing (S-nTP) technique based on high-fidelity replication of ultrahigh-resolution (8 - 20 nm scale) patterns using a dual-functional bilayer polymer thin film. For uniform and fast release of nanostructures on diverse receiver surfaces, interface-specific adhesion control was realized by employing a polydimethylsiloxane (PDMS) gel pad as a solvent-emitting transfer medium, providing unusual printing capability even on biological surfaces such as human skin and fruit peels. The adhesion switching phenomenon induced by injected solvent molecules can be explained by the several orders of magnitude change of frictional energy. Based on this principle, we also present facile printing of plasmonic nanostructures for non-destructive and rapid surface-enhanced Raman spectroscopy (SERS) analyses. Fabrication of Pd-nanowire-based hydrogen detection sensors on flexible plastic substrates and cylindrical glass substrates is also reported as a demonstration of the feasibility of S-nTP for application to various printed devices equipped with sub-20 nm nanostructures.