Marc Ramuz, MINES Saint-Étienne
Roozbeh Ghaffari, Northwestern University/Epicore Biosystems Inc/MC10 Inc
Pooi See Lee, Nanyang Technical University
Cunjiang Yu, University of Houston
EP04.01: Liquid-Material Embedded Soft Structures I
Tuesday AM, April 23, 2019
PCC North, 200 Level, Room 222 A
10:30 AM - *EP04.01.01
Liquid Metals Encased in Functional Elastomers for Soft and Stretchable Electronics
North Carolina State University1Show Abstract
This talk will discuss recent work in our group (and through collaborations) to utilize liquid metals as electrical conductors for soft and strethcable electronics. Gallium and its alloys have low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be patterned into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. Because it is a liquid, the metal is inherently soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomer 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. In addition to introducing the advantages of these materials for stretchable electronics, this talk will focus on recent advances with (1) patterning the metal and metal composites, (2) intergrating liquid metal into functional devices including energy harvesters and transistors (in collaboration with the Ozturk and Franklin groups, respectively), and (3) encasing the metal within optimized and functional elastomers that are less prone to fail mechanically (and warn the user colorimetrically before failure, done in collaboration with the Craig group). These advances have implications for creating electronics that are softer than skin and with stretchability limited only by the encasing elastomer.
11:00 AM - EP04.01.02
Polymerized Liquid Metal Networks for Activatable Stretchable Conductors and Sensors
Carl Thrasher1,Zachary Farrell1,2,Nicholas Morris1,2,Christopher Tabor1
Air Force Research Laboratory1,UES, Inc.2Show Abstract
Robust stretchable conductors with advanced capabilities such as stimuli-responsiveness and high power delivery over large strains are needed for advanced soft robotic, physically reconfigurable, and wearable electronic applications. Room temperature liquid metals such as eutectic gallium-indium (EGaIn) have shown potential in next generation stretchable electronic systems, but suffer from problems with containment, fabrication, and metal oxide formation. Here we synthesize phosphonic acid-functionalized reactive EGaIn core-shell particles and polymerize them on elastomer substrates to create polymerized liquid metal networks consisting of >99.9 wt% EGaIn. Under strain the polymerized particles in the network rupture and release their encapsulated liquid metal, rapidly transitioning the network from an electrically resistive to a conductive state (108 fold conductivity increase from 0-100% strain). Once activated, these network composites adopt microstructures which mitigate the deleterious effects of strain on electronic performance. As a result of these microstructures, these networks show many remarkable characteristics including increasing volumetric conductivity with strain to over 20,000 S/cm at >700% elongation, electronic memory of previous strain states, and R/R0 < 1 between 0-300% strain. The unique electromechanical properties of these activated liquid metal networks present a superior design strategy to provide ultra-stretchable (>300%) conductors with near-zero change in resistance and can find additional use for powerless active strain indication and triggered healing in stretchable conductors.
11:15 AM - EP04.01.03
Micro-Patterned Liquid Metal Based Conductors for Large-Area Stretchable Electronics
Laurent Dejace1,Stephanie Lacour1
Ecole Polytechnique Federale de Lausanne (EPFL)1Show Abstract
Gallium is a metal in a liquid state at room temperature that offers unique electromechanical properties when integrated within a soft, elastic carrier material. However, precise patterning and manipulation of the liquid metal are challenging. High surface tension prevents homogeneous spreading of liquid gallium on lyophobic silicone carriers. A thin gallium oxide skin forms immediately at the gallium surface upon exposure to ambient conditions and sets the shape of the material. We overcame these peculiar properties to pattern reliably thin gallium features with µm-scale resolution, high surface density and over centimeters side surface areas.
Our manufacturing process combines soft lithography, self-assembly and physical vapor deposition techniques, all compatible with wafer scale manufacturing. First, a PDMS layer is cast on a photoresist mold lithographically patterned with a matrix of 1.5µm thick, 6µm wide holes distributed across the entire 4in. wafer. The PDMS layer is then demolded and transferred to another carrier for further processing on the patterned side. A 160nm thick gold film is blanket-sputtered on the textured surface, so as to increase the chemical affinity of gallium to the silicone substrate. Next a tilted Ion Beam Etching (IBE) process is used to selectively remove the gold layer only above the patterned recesses. Finally, gallium is evaporated on the substrate, where it alloys with gold and fills all empty spaces in-between the PDMS micro-posts.
This process enables a range of channel designs and geometries leading to soft conductors displaying highly stable electromechanical properties when reversibly and repeatedly stretched to 50% elongation for hundred cycles. This work represents a significant advance towards the miniaturization of gallium-based technology for robust and reproducible, soft and mechanically compliant electronic circuits.
11:30 AM - EP04.01.04
Stretchable Elastic Shape Memory Fibers with Electrical Conductivity
Sungjune Park1,Hardil Shah2,Neil Baugh2,Dishit Parekh2,Ishan Joshipura2,Yubo Ouyang2,Michael Dickey2
Chonbuk National University1,North Carolina State University2Show Abstract
This paper describes shape memory polymer fibers consisting of an elastic shell and a gallium liquid metal core. These fibers are interesting because they have metallic electrical and thermal conductivity, elastic mechanical properties, and excellent shape memory properties due to the ability to change the phase of the core from a low viscosity liquid to a solid metal. It is straightforward to form the fibers by via melt processing followed by injection of the liquid metal into the core. The elastic energy stored in the fiber allows it to relax back to its original geometry upon melting the solid gallium. The ability to change the core of the fiber from liquid to solid allows an enormous modulus change in the range of from 4.5 and to 1618.3 MPa and the ability to have shape-memory effect; that is, the fiber can preserve a deformed shape and then return to its original shape upon heating. The shape memory behavior of these fibers is quantitatively characterized in terms of fixity (i.e. the ability to retain deformed temporal shape) and recovery (i.e. the ability to return to its original geometry). The use of a rigid metallic core provides perfect fixity. Recovery rate time is effectively minimized due to the elastomeric shell with having minimal viscous dissipation as well as rapid conversion of the metallic core to a liquid with a low viscosity. Notably, the use of gallium—with a melting point above room temperature but below body temperature –allows the user to melt and deform local regions of the fiber by hand. These stretchable elastic shape memory fibers may enable new applications in soft robotics, stretchable and wearable sensors, and bioinspired electronic skin.
11:45 AM - EP04.01.05
The Freeze/Thaw Properties of the Conformable Conductor Eutectic Gallium-Indium-Tin
Amanda Koh1,2,Randy Mrozek2,Geoffrey Slipher2
University of Alabama1,U.S. Army Research Laboratory2Show Abstract
Room-temperature eutectic liquid metals have become an exciting basis of research into soft materials for sensors, actuators, and robotics. This work relies on the low melting point of the liquid metal, which allows for a material which is both conformable and conductive. While the melting point of metals such as eutectic gallium-indium (EGaIn) and eutectic gallium-indium-tin (Galinstan) are often cited, their mechanical and electrical properties around the melting point are not well-studied. These properties, particularly the mechanical transition between a liquid and solid phase, are crucial to understanding the use of EGaIn and Galinstan as conformable conductive elements in a wide variety of environments. This work focuses on Galinstan, which is reported to have a melting point <-10oC and a low room-temperature modulus. By measuring the rheology of Galinstan between 50oC and -20oC, the range at which Galinstan can truly be called “soft” with respect to common elastomer matrices was determined. Mechanical changes caused by the effects of the freeze/thaw cycle on Galinstan were also observed. The transition between the solid and liquid state were also observed using powder x-ray diffraction to determine if a crystalline solid was formed, and what changes the freeze/thaw cycles may cause in the material. The mechanical properties of Galinstan-in-PDMS dispersions were also measured through freeze/thaw to better understand the use of Galinstan as a conformable conductor in a stretchable, electronic device. Through this work, we were able to add important mechanical performance detail to the description of Galinstan as a conformable conductor, as well as point out important considerations when manufacturing a device intended for hot and cold environments.
EP04.02/EP02.02/EP03.02: Joint Session: Soft, Biointegrated Electronics and Photonics
Tuesday PM, April 23, 2019
PCC North, 200 Level, Room 222 A
1:30 PM - *EP04.02.01/EP02.02.01/EP03.02.01
Skin-Inspired Organic Electronics
Stanford University1Show Abstract
Flexible organic electronics have attracted considerable attention over the past decade. Stretchable electronics represent another type of optoelectronic devices that are intrinsically elastic, that is they are foldable, twistable, and stretchable while maintaining performance, integrity and durability. Incorporated into devices, properly designed stretchable materials may result in more robust devices under bending and strain compared to flexible but not stretchable materials. For intrinsically stretchable electronics, it is desirable to have intrinsically stretchable materials, ranging from stretchable conductors, stretchable dielectric to stretchable semiconductors. In this talk, I will present various molecular design concepts for realizing stretchable electronic polymers without compromising electronic properties. Their applications in bioelectronics will also be presented.
2:00 PM - *EP04.02.02/EP02.02.02/EP03.02.02
Flexible Bioelectronics—Enzyme-Based Body-Worn Electronic Devices
University of California, San Diego1Show Abstract
Wearable bioelectronic devices rely on oxidoreductase enzymes and have already demonstrated considerable promise for on-body applications ranging from highly selective non-invasive biomarker monitoring to epidermal energy harvesting. Critical to such progress is the judicious design of the enzyme-electronic interface, along with flexible platforms with mechanical properties similar to those of biological tissues. Such devices require special attention to the enzyme-electronic interface and to several considerations related to wearable applications, such as mechanical properties (flexibility and stretchability), operational stability in different biofluids and under changing conditions (e.g., pH, temperature), biofouling, selectivity, and low target concentrations. Keeping these requirements in mind, our group has pioneered a variety of wearable biocatalytic sensors and biofuel cells devices. By leveraging the advantages of biocatalysis, electrochemistry, and flexible electronics, and addressing key challenges, wearable bioelectronic devices could have a tremendous impact on diverse biomedical, fitness and defense fields.
2:30 PM - EP04.02.03/EP02.02.03/EP03.02.03
Human Skin Interactive Bio-e-skin for Self-Powered Health Care Monitoring
Dipankar Mandal1,2,Sujoy Ghosh2
Institute of Nano Science and Technology1,Jadavpur University2Show Abstract
Among various pressure sensors, transduction mechanisms, piezoresistive, capacitive, triboelectric, and piezoelectric effects are generally considered for converting tactile stimuli into electrical signals.1,2 Owing to the fast response time and higher sensitivity, piezoelectric pressure sensors (PEPS) are the most useful for the detection of full-range human activities. It has been found that pressure sensor based artificial electronic skin (e-skin) can mimic the human skin and detects subtle pressure (1 Pa–1 kPa). A broad range of applications is expected to be implemented such as wearable health care systems, human–machine interfacing devices, artificial intelligence and prosthetic skin. Nevertheless, piezoelectric materials with adequate flexibility, light weight, ease of large-area processing, low cost and environmental safety are attractive but several issues are remaining for next-generation pressure/force sensors and mechanical energy harvesters. Recent advances on PEPS development is primarily focused on varies types of inorganic and semiconducting piezoelectric materials such as, barium titanate (BaTiO3), zinc oxide (ZnO), zinc sulfide (ZnS) and lead zirconate titanate (PZT).2 In spite of their superior electromechanical responses, the brittleness and toxic properties limit their implementation in wide range of biomedical and flexible electronics applications. On the other hand, natural piezoelectric materials are not yet extensively investigated as an e-skin for measuring and quantifying human physiological signal.2
In this context, we have developed human skin interactive self-powered piezoelectric biomaterials based e-skin (Bio-e-skin) which can detect and discriminate acute human physiological signals such as low frequency as subtle as wrist pulse and even intra-body pressures such as intraocular and intracranial pressure, to relatively high frequency dynamic human motions such as movements of synovial joints of wrist, elbow, knee and finger bending. Natural bio-polymers such as, collagen, chitin, gelatin and cellulose were found to possesses superior mechanosensitivity which was sufficient to fabricate self-powered wearable bio-e-skin that can mimic spatiotemporal human perception and monitors real-time human physiological signals in non-invasive rational strategy. In addition, the bio-skins are found to generate superior output power density which is sufficient to operate the commercial consumer electronics. These observations suggest that fabricated bio-e-skins could eventually find a wide range of applications in autonomous epidermal electronics, implantable medical device, surgery, e-healthcare monitoring, in vitro and in vivo diagnostics apart from its broad range applications in the field of self-powered personal portable electronic devices.3-5
1. Z. L. Wang, J. Song, Science 2006, 312, 242.
2. F. R. Fan, W. Tang, Z. Lin Wang, Adv. Mater., 2016, 28, 4283–4305.
3. S. K.Ghosh, D.Mandal, Appl. Phys. Lett., 2016, 109, 103701.
4. S. K. Ghosh, D. Mandal, Nano Energy, 2016, 28, 356–365.
2:45 PM - EP04.02.04/EP02.02.04/EP03.02.04
Fully Implantable Wireless Battery-Free Optoelectronic Systems for Multimodal Optogenetic Neuromodulation
Philipp Gutruf1,Vaishnavi Krishnamurthi2,Abraham Vázquez-Guardado3,Zhaoqian Xie4,Anthony Banks5,Chun-Ju Su4,Yeshou Xu4,Chad Haney4,Emily Waters4,Irawati Kandela4,Siddharth Krishnan4,Tyler Ray4,John Leshock4,Yonggang Huang4,Debashis Chanda3,John Rogers4
University of Arizona1,RMIT University2,University of Central Florida3,Northwestern University4,Neurolux5Show Abstract
Recently emerging classes of battery free, ultrasmall1, fully implantable devices for optogenetic neuromodulation2 eliminate physical tethers associated with bulky head-stages and batteries in alternative wireless technologies and conventional setups by leveraging cellular scale light emitting diodes on flexible injectable filaments as light sources3. These highly miniaturized systems enable untethered, operation for behavioral studies that eliminate motion constraints and enable new experimental paradigms in a range of complex 3D environments and contexts (e.g. social interactions) that cannot be explored with conventional technologies. These devices are, however, purely passive in their design, thereby precluding any form of active control or programmability, resulting in limitations when investigating circuit dynamics where independent operation of multiple light sources with precise active control is needed. Here we present a series of important concepts that enable controlled device operation, independent of position and angle relative to the experimental arena, with advanced wireless power harvesting capabilities and full user-programmability over multiple devices. This level of functionality is demonstrated in integrated platforms that are compatible with noninvasive imaging technologies such as computed tomography and magnetic resonance imaging and have sizes and weights not significantly larger than those of previous, passive systems. The resulting devices qualitatively expand options in brain tissue illumination for optogenetic neuromodulation and multimodal operation, with broad potential applications in neuroscience research, with specific advances in precise dissection of neural circuit function during unconstrained behavioral studies.
1 V. K. Samineni, J. Yoon, K. E. Crawford, Y. R. Jeong, K. C. McKenzie, G. Shin, Z. Xie, S. S. Sundaram, Y. Li, M. Y. Yang, J. Kim, D. Wu, Y. Xue, X. Feng, Y. Huang, A. D. Mickle, A. Banks, J. S. Ha, J. P. Golden, J. A. Rogers, and R. W. I. Gereau, PAIN 158,2017).
2 G. Shin, A. M. Gomez, R. Al-Hasani, Y. R. Jeong, J. Kim, Z. Q. Xie, A. Banks, S. M. Lee, S. Y. Han, C. J. Yoo, J. L. Lee, S. H. Lee, J. Kurniawan, J. Tureb, Z. Z. Guo, J. Yoon, S. I. Park, S. Y. Bang, Y. Nam, M. C. Walicki, V. K. Samineni, A. D. Mickle, K. Lee, S. Y. Heo, J. G. McCall, T. S. Pan, L. Wang, X. Feng, T. I. Kim, J. K. Kim, Y. H. Li, Y. G. Huang, R. W. Gereau, J. S. Ha, M. R. Bruchas, and J. A. Rogers, Neuron 93,2017).
3 T. I. Kim, J. G. McCall, Y. H. Jung, X. Huang, E. R. Siuda, Y. Li, J. Song, Y. M. Song, H. A. Pao, R. H. Kim, C. Lu, S. D. Lee, I. S. Song, G. Shin, R. Al-Hasani, S. Kim, M. P. Tan, Y. Huang, F. G. Omenetto, J. A. Rogers, and M. R. Bruchas, Science 340,2013).
3:30 PM - *EP04.02.05/EP02.02.05/EP03.02.05
Self-Powered Ultra-Flexible Organic Electronics for Health Monitoring
Takao Someya1,2,Kenjiro Fukuda2,Tomoyuki Yokota1
University of Tokyo1,RIKEN Center for Emergent Matter Science2Show Abstract
On-skin sensors have attractive much attention as the next-generation wearable devices, because the conformal contact on human skin enables accurate and continuous detection of physiological signals. One of the most important technologies to improve usability of on-skin sensors is a power source to continuously supply electricity to health-monitoring systems. In this talk, we will report on recent progresses of ultraflexible organic photovoltaic cells for applications to wearable sensors. First, ultraflexible organic power sources that can be wrapped around an object have been developed with mechanical and thermal stability in long-term operation. Then, the integration of these power sources with functional electric devices including sensors has been achieved.
4:00 PM - *EP04.02.06/EP02.02.06/EP03.02.06
Physical Biology and Material Dynamics at the Semiconductor-Based Biointerfaces
The University of Chicago1Show Abstract
Recent studies have demonstrated that in addition to biochemical and genetic interactions, cellular systems also respond to biophysical cues, such as electrical, thermal, and mechanical signals. However, we only have limited tools that can introduce localized physical stimuli and/or sense cellular responses with high spatiotemporal resolution. Inorganic semiconductors display a spectrum of physical properties and offer the possibility of numerous device applications. My group integrates material science with biophysics to study several semiconductor-based biointerfaces. In this talk, I will first pinpoint domains where semiconductor properties can be leveraged for biointerface studies, providing a sample of numbers in semiconductor-based biointerfaces. Next, I will present a few recent studies from our lab and highlight key biophysical mechanisms underlying the non-genetic optical modulation interfaces. In particular, I will present a biology-guided two-step design principle for establishing tight intra-, inter-, and extracellular silicon-based interfaces in which silicon and the biological targets have matched mechanical properties and efficient signal transduction. Finally, I will discuss new materials and biological targets that could catalyze future advances.
4:30 PM - EP04.02.07/EP02.02.07/EP03.02.07
Autonomic Self-Healing and Intrinsical Stretchability of PEDOT:PSS Films
Polytechnique Montréal1Show Abstract
Organic electronic devices, apart from consumer applications, are presently paving the path for key applications at the interface between electronics and biology. In such applications, organic polymers are very attractive candidates, due to their distinct properties of mechanical flexibility, self-healing and mixed conduction.
My group investigated the processing conditions leading to high electrical conductivity, long-term stability in aqueous media as well as robust mechanical properties of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS) [1-3].We have demonstrated that stretchable PEDOT:PSS films can be achieved by adding a fluorosurfactant to the film processing mixture and by pre-stretching the substrate during film deposition. We have achieved patterning of organic materials on a wide range of substrates, using orthogonal lithography and pattern transfer [4-5]. Recently we have discovered that PEDOT:PSS films can be rapidly healed with water drops after being damaged with a sharp blade  or show autonomous self-healing if processed in presence of certain additives.
My talk will deal with processing, characterization and patterning of conducting polymer films and devices for flexible, stretchable and healable electronics. I will particularly focus on the strategies to achieve films with optimized electrical conductivity and mechanical properties, on unconventional micro patterning on flexible and stretchable substrates, on the different routes to achieve films stretchability and self-healing.
F. Cicoira et al. APL Mat.3, 014911, 2015.
F. Cicoira et al.Appl. Phys. Lett. 107,053303, 2015.
3. F. Cicoira, et al. Appl. Phys. Lett. 111, 093701, 2017
F. Cicoira et al. Chem. Mater. 29, 3126-3132, 2017.
F. Cicoira et al. J. Mater. Chem.C 4, 1382–85, 2016.
F. Cicoira et al. Adv. Mater.29, 1703098, 2017.
EP04.03: Poster Session I: Soft and Stretchable Electronics—From Fundamentals to Applications
Pooi See Lee
Tuesday PM, April 23, 2019
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - EP04.03.01
Vapor-Phase Synthesis of Organic-Inorganic Hybrid Gate Dielectric for Flexible Electronics
Min Ju Kim1,Kwanyong Pak1,Wan Sik Hwang2,Sung Gap Im1,Byung Jin Cho1
Korea Advanced Institute of Science and Technology1,Korea Aerospace University2Show Abstract
Organic-based thin film transistors (OTFTs) are beneficial for wearable architecture because of their lightweight, flexibility and low cost. One of the critical issues in flexible OTFTs is gate dielectric for high performance flexible elecronics. The hybrid materials consisting of organic and inorganic materials have emerged as a feasible gate dielectric for high-performance flexible electronics. Therefore, it is necessary to develop ultrathin hybrid organic-inorganic dielectrics and correlate their material properties with device performance at different bending strains. In this work, we propose a one-step synthesis method to form a high-k, ultrathin and homogeneous organic-inorganic hybrid dielectric via initiated chemical vapor deposition (iCVD). iCVD is a well-established process to deposit various functional polymer films based on a radical polymerization via a vapor-phase deposition. In this study, trimethylaluminum (TMA) and 2-hydroxyethyl methacrylate (HEMA) were used as the inorganic precursor and monomer, respectively, leading to the formation of an AlOx-embedded polymer matrix hereafter called a hybrid dielectric. The chemical bonds of the synthesized hybrid dielectric were characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) which clearly demonstrated that the hybrid dielectric was successfully synthesized and deposited via iCVD process. The X-ray diffraction (XRD) spectra shows that the amorphous AlOx moiety is dispersed in the polymer matrix to achieve high dielectric strength and mechanical flexibility. No apparent grain-like or pinhole-like morphologies are observed from the atomic force microscopy (AFM) images, which indicated that the hybrid dielectrics were homogeneously synthesized without any phase separation. To characterize the electrical properties of the hybrid dielectric with various Al contents, crossbar-type metal-insulator-metal (MIM) devices were fabricated. The dielectric constant was extracted from the capacitance (Ci) measured at a frequency (f) of 10 kHz and film thickness measured via scanning transmission electron spectroscopy (STEM). The dielectric constant (k values) increased from 3 to 7 as the Al concentration in the hybrid dielectric increased, which was attributed to a greater number of dipoles in the polymer matrix. Unlike the higher k values with higher Al concentrations, the electrical breakdown field (Ebreak) decreased linearly from 6 to 3 MV/cm, and the gate leakage current (J) at 2 MV/cm increased exponentially from 1.0 × 10-9 to 3.0 × 10-7 A/cm2. The electrical results revealed that there was a trade-off between performance and power consumption. The electrical energy bandgap (Eg) was extracted from the electron energy loss spectroscopy (EELS) with low loss spectra resulting in a wide Eg of 7.0 ~ 7.5 eV. Based on these preliminary experiments, the hybrid dielectric with an Al concentration of around 18% exhibited a decent k value of 6 and J value of lower than 10-7 A/cm2 at 2 MV/cm. The MIM device with the hybrid dielectric exhibited negligible J changes of up to 2.6 % applied strain, where the soft polymer matrix effectively released the applied strain. Finally, the iCVD hybrid dielectrics were applied to the bottom-gated OTFTs with p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13). Two typical linear and saturation operating regions emerged, revealing well-behaved transistor characteristics. The mechanical flexibility of the p-type pentacene OTFTs with the hybrid dielectric was investigated at different bending strains. The transfer characteristics of the OTFTs were shown as a function of mechanical bending strain, showing negligible changes while maintaining a low gate leakage current. The results suggest that the hybrid dielectric synthesized via the iCVD process is a promising candidate for high-performance low-power consumption flexible electronics.
5:00 PM - EP04.03.02
Highly Conducting MXene Composite Fibers with Conductive Polymer Binder for Fiber-Shaped Supercapacitors
Jizhen Zhang1,Shayan Seyedin1,Zhiyu Wang1,Si (Alex) Qin1,Joselito Razal1
Deakin University1Show Abstract
Fiber-shaped supercapacitors (FSCs) are lightweight and conformal forms of energy storage devices that are being developed for powering portable and wearable electronics. Recently, 2D transition metal carbide Ti3C2Tx (Tx stands for surface termination), the most widely studied MXene, has be demonstrated as one of promising electrode materials for fiber-shaped supercapacitors. Among various fiber fabrication methods, wet-spinning is an industrially viable approach to fiber production and has therefore attracted considerable attention for the fabrication of various FSC electrodes, such as graphene fibers, CNT fibers and conducting polymer fibers. The wet-spun composite fibers for use in FSCs must ensure that the active material and the binder are homogeneously dispersed but neither of the two components must limit the function of the other component particularly at high active material loading, i.e. the high conductivity and electrochemical performance of the active material must be maintained and the binder must provide durability and flexibility. However, the poor interaction between MXene flakes and spinnable templates resulted in low electrical conductivity and compromised energy storage performance.
In our work, MXene fibers with high conductivity and enhanced energy storage properties were prepared using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a highly conductive and flexible binder. We demonstrated the continuous production of PEDOT:PSS/MXene hybrid fibers (over decades meters) with conductivity of ~1489 S cm-1 and tensile strength can be tailed from ~58 MPa to ~143 MPa, which are about 5-time higher than previously reported MXene composite fibers. When MXene loading increased to 70 wt. %, composite fiber shows enhanced volumetric capacitance of ~614.5 F cm-3 at 5 mV s-1 and excellent performance of 375.2 F cm-3 at high scan rate (1000 mV s-1). We also demonstrate elastic wire-shaped supercapacitors using a coiled design to yield 96 % capacitance retention after 200 stretch-release cycles to 100 % strain. This work demonstrates a scalable approach towards the processing of high performance MXene-based fiber electrodes that could be used in the future for fiber-based energy storage devices and other textile-based wearables.
5:00 PM - EP04.03.03
Wearable Organic Memory Fiber for Low Voltage Operation and Conformable Data Storage
Minji Kang1,Tae-Wook Kim1
Korea Institute of Science and Technology1Show Abstract
Wearable electronics have attracted attention as an emerging technology to realize practical e-textile and smart garments. For future wearable automatic systems, robust data storage media are required to record and process the electrical signals generated under various unpredictable strain conditions. Here, we report an organic field-effect transistor (OFET)-type memory integrated on a thin and flexible metal wire. A new fiber coating technique by using a capillary tube and controlling the solution viscosity allows the formation of a thin and uniform organic ferroelectric film on the wire. The uniform morphology imparts excellent switching stability, long-term retention time and low-voltage operation to the flexible fiber organic memory devices. The fiber-shaped memory achieves reliable data storage even under tough environments when sewn in a stretchable textile fabric, providing the possibility of the practical, wearable fiber memory for recording electronic signals in smart textile applications.
5:00 PM - EP04.03.04
Stretchable Location Sensor Based on Transparent AgNWs Electrodes
Hang Guo1,Liming Miao1,Haobing Wang1,Ji Wan1,Xiaoliang Cheng1,Haixia Zhang1
Institute of Microelectronics, Peking University1Show Abstract
In the recent years, the smart electronic skin (E-skin) has attracted extensive attention due to the human skin provides remarkable sensor networks such as touch, temperature, vibration and pressure sensors, etc. There are many wearable electronic devices integrated with glasses and watches in the consumer electronics with the rapid development. However, all of these devices face an important issue about the stretchability which could sustain complex deformation and conformal contact with irregular surfaces. The stretchable materials are mainly required as a substrate such as organic polymer in order to make the devices stretchable. Apart from these, nanomaterials, such as carbon nanotubes, graphene and metal nanowires have been utilized to realize stretchable electronics. And the special structure designs have also played a very important role in improving stretchability. Wave, island-bridge structure, porous structure and grid structure are widely applied in these novel designs.
Herein, we developed a transparent conductive membrane which is uniform and stable even under elongation through spin-coating AgNWs solution on the stretchable substrate. The motion detection and location is indispensable for human-machine interfaces. However, as most devices work in digital method, there has been limited progress in the resolution enhancement that may lead to an increase in the number of electrodes. Therefore, the stretchable analog devices play an essential role in wearable devices, which owns higher resolution with a few electrodes. we demonstrate these new properties by using PDMS/AgNWs thin film.
A novel stretchable, transparent location sensor has been demonstrated in this work. Through the spin-coating process, the AgNWs are deposited on the surface of the PDMS substrate, which enhances the transparency and stretchability of the device. It can be attached to the human skin with conformal coverage and keep the function under the elongation of 60%. When the sensor is pressed, the positon of the contact point can be acquired by the relative resistances in the analog method, which can enhance the resolution without increasing in the number of electrodes. All of the excellent properties makes the sensor have much potentials in the wearable devices and smart robots.
5:00 PM - EP04.03.06
Soft Electronic and Optoelectronic Devices with Dynamic 3D Architectures Controlled by Heat-Responsive Polymers
Cheng Zhang1,Jian Lin1
The University of Missouri1Show Abstract
Three-dimensional (3D) architectures qualitatively extend the functionalities of electronic and optoelectronic devices. Complex functional 3D structures have been fabricated by 3D printing, stress-controlled folding, and mechanically-guided assembly. However, the potential of 3D functional architectures built by previous methods is restrained by their static or human-intervention required dynamic structures. Herein, we demonstrate a soft 3D scaffold integrated with electronic and optoelectronic devices which can be dynamically tuned by responsive polymer substrates. The 2D films are selectively bonded onto the responsive substrates which can respond to heat simulation and actuate the assembly of the 2D films to 3D architectures. The assembled structures are dynamically controlled by the heat. The dynamic assembly is highly controllable by both experimental study and numerical simulation. Soft electronic and optoelectronic devices, i.e., a strain sensor array and a photodetector array, are integrated to the 2D films and also assembled to 3D structures. The strain sensors monitor the motion of the assembly. The photodetectors can measure both intensity and direction of incident light attributing to its 3D architectures. Moreover, the photodetector array has the capability of tracking lights incoming from all directions because its spatial configuration can be dynamically tuned by heat.
5:00 PM - EP04.03.07
A Mxene-Based Wearable Biosensor System for High-Performance In Vitro Perspiration Analysis
King Abdullah University of Science and Technology1Show Abstract
Wearable electrochemical biosensors for sweat analysis (e.g., glucose, lactate) present a promising means for noninvasive biomarker monitoring. However, the sweat-based measurement of the developed devices still poses many challenges: easy degradation of enzymes and biomaterials upon regular testing; the corresponding poor shelf life of the all-in-one working electrodes patterned by traditional techniques (e.g., electrodeposition, screen printing); limited detection range and sensitivity of the enzyme-based biosensors caused from oxygen deficiency in sweat. Herein, we develop a wearable multifunctional biosensor incorporating MXene/Prussian blue (Ti3C2Tx/PB) composite for durable and sensitive detection of glucose and lactate in sweat. Due to the metallic conductivity and hydrophilic nature of the MXene, greatly improved electrochemical activity was observed on the Ti3C2Tx/PB composite in comparison to previously reported Graphene/PB and CNTs/PB composites. During in-vitro perspiration monitoring, the physiochemistry signals (glucose and lactate level) could be measured simultaneously, showing high sensitivity and acceptable repeatability. A unique modular architecture design enabled a simple exchange of the specific sensing electrode to tailor to the desired analytes. Furthermore, an implemented solid-liquid-air three-phase interface design led to superior sensor performance and stability, with typical electrochemical sensitivities of 35.3 μA mM-1 cm-2 for glucose and 11.4 μA mM-1 cm-2 for lactate using artificial sweat. Hence, this approach represents an essential step towards the realization of ultra-sensitive enzymatic wearable biosensors for early disease identification and personalized medical applications.
5:00 PM - EP04.03.08
Electrochemically Stable and Adherent PEDOT Coatings for High Quality EMG Recording
Nicolò Rossetti1,Ada Lee1,Prabhjot Kaur Luthra1,Côme Bodart1,Fabio Cicoira1
École Polytechnique de Montréal1Show Abstract
Conductive polymers coatings on metal electrodes are an efficient solution to improve neural signal recording and stimulation due to their mixed electronic-ionic conduction and biocompatibility . However, only a few studies have been reported on conductive polymers coatings on metallic wire electrodes for muscle signal recording . In this work, we developed mechanically and electrochemically stable invasive electrodes for muscle signal recording in small animals based on stainless steel multi-stranded wires coated with the conductive polymer PEDOT.
PEDOT doped with LiClO4 was galvanostatically electropolymerized on stainless steel wires using three different solvents: propylene carbonate (organic), acetonitrile (organic) and water (inorganic). The coatings adhesion to the metallic substrate was tested through ultrasonication and the electrochemical stability was evaluated through phosphate buffer solution soaking test and autoclave sterilization.
The solvent played a key role on the adhesion of the PEDOT coating, with organic solvents giving the best mechanical stability. Electrodes prepared with these solvents possessed excellent electrochemical stability and survived sterilization and prolonged soaking without major changes in electrochemical properties.
A solution for high quality invasive muscle signal recording in small animals based on conductive polymers has been demonstrated.
 R. Balint, N. J. Cassidy, and S. H. Cartmell, "Conductive polymers: Towards a smart biomaterial for tissue engineering," Acta Biomaterialia, vol. 10, no. 6, pp. 2341-2353, 2014.
 S. Kim, L. K. Jang, M. Jang, S. Lee, J. G. Hardy, and J. Y. Lee, "Electrically Conductive Polydopamine–Polypyrrole as High Performance Biomaterials for Cell Stimulation in Vitro and Electrical Signal Recording in Vivo," ACS Applied Materials & Interfaces, vol. 10, no. 39, pp. 33032-33042, 2018.
5:00 PM - EP04.03.09
3D-Printed Hydrogel with Superior Stability for Energy Harvesting and Physiological Monitoring
Min Wu1,Shengjie Gao1,Yixiu Wang1,Ruoxing Wang1,Wenzhuo Wu1
Purdue University1Show Abstract
Hydrogels have attracted much attention recently as a promising class of biomaterials. Traditional hydrogel bioelectronics are limited in several aspects, including the poor ability of the hydrogel to retain water under cold or hot environment, complicated device fabrication procedure, and complex external power requirement to drive the device. Here we propose an approach to directly fabricate hydrogel by 3D printing method, and further prepare wearable devices for self-powered sensing applications. With the addition of glycerol, the hydrogel would be able to resist a water loss and can tolerate a temperature with a wide range from -20 oC to 60 oC. The 3D printed hydrogel is able to deliver a stable triboelectric output, which can be explored as self-powered sensors for physiological monitoring. The facile 3D printing method, coupled with the water loss prevention properties of the hydrogel and its capability of converting mechanical vibration from human body into distinguishable electrical signals would enable the application of 3D printed hydrogel for electronic skin and human-machine interface.
5:00 PM - EP04.03.10
Extruded Liquid Metal Wires at Room Temperature via Electrochemical Oxidation
Minyung Song1,Michael Dickey1
North Carolina State University1Show Abstract
Compared to other liquids (e.g. water, ionic liquids, organic liquids), liquid metals have outstanding thermal and electrical conductivity, as well as distinct optical properties (i.e. reflectivity). As a result, they are excellent candidates for stretchable and soft metallic components such as antennas, electrodes, and interconnects. Thus, there is great interest in facile methods to create and pattern these liquids into useful components. The large tension and low viscosity of liquid metals presents a challenge: the metal tends tend to adopt a spherical shape as it exits a nozzle. There are a number of strategies to address this challenge that require the metal to directly contact other solid or visco-elastic surfaces. Here we report a way to create wires by directly extruding the metal from a nozzle. To control the shape of the liquid metal, we used electrochemical control of surface oxidation because the formation of the surface oxide significantly lowers the interfacial tension and allows the metal to adopt shapes that overcome Rayleigh-Plateau instabilities (i.e. it does not bead up).
To create these wires, we pump liquid metal into electrolyte (1M NaOH). As it emerges from the nozzle, the liquid metal forms large spherical drops to minimize the surface area. However, this droplet size diminishes with an increase of potential, which indicates a decrease in interfacial tension. At a critical potential, the interfacial tension drops significantly, and the liquid metal forms wires. At larger values of electric potential (>0.8V), the oxide layer gets thicker, causing the wire to deform and turn into unusual shapes. We successfully analyze each distinct regime of EGaIn as a function of flow rate and electric potential to identify conditions that form wires. The ability to deposit or remove the oxide electrochemically enables unprecedented control over interfacial activity (well beyond what is possible using electrocapillarity), which provides a means to reconfigure and manipulate the shape and position of liquid metal. This method requires minimal energy and provides rapid and reversible control of interfacial tension over an enormous range compared to conventional molecular surfactants. By adjusting the electrical potential and flow rate, the configuration of liquid metal can be manipulated on demand, which offers the possibility to create actuators or soft and stretchable analogues of rigid conductors.
5:00 PM - EP04.03.11
Electronic Skin with Autonomous Self-Healability
Jiheong Kang1,Zhenan Bao1
Stanford University1Show Abstract
Electronic skin devices capable of monitoring physiological signals and displaying feedback information through closedloop communication between the user and electronics are being considered for next-generation wearables and the ‘Internet
of Things’. Such devices need to be ultrathin to achieve seamless and conformal contact with the human body, to accommodate strains from repeated movement and to be comfortable to wear. Recently, self-healing chemistry has driven important advances in deformable and reconfigurable electronics, particularly with self-healable electrodes as the key enabler. Unlike polymer substrates with self-healable dynamic nature, the disrupted conducting network is unable to recover its stretchability after damage. Here, we report the observation of self-reconstruction of conducting nanostructures when in contact with a dynamically crosslinked polymer network. This, combined with the self-bonding property of self-healing polymer, allowed subsequent heterogeneous multi-component device integration of interconnects, sensors and light-emitting devices into a single
multi-functional system. This first autonomous self-healable and stretchable multi-component electronic skin paves the way for future robust electronics.
5:00 PM - EP04.03.12
Fast Self-Healing and Conductive Hydrogels as Soft Strain Sensor
Yujie Chen1,Hua Li1,Hezhou Liu1
Shanghai Jiao Tong University1Show Abstract
Remarkable process achieved for conductive hydrogels has been witnessed in recent years. However, hydrogels easily damaged during the course of use, which limits their applications as soft conductors. Here, two different methods are presented to obtain multi-functional hydrogels which have fast self-healing property, conductive capability and stain-sensitive performance. In the first method, the nanocomposite hydrogel (PVA-PDA-pRGo) is designed and synthesized from polyvinyl alcohol (PVA), polydopamine (PDA) and grapheme oxide (GO). Dynamic diol-borate eater bonds built from polyvinyl alcohol (PVA) and borax mainly allow nanocomposite hydrogels to display decent self-healing behaviors in mechanical (restore 92.89% of original tensile strength within 60s), electrical (restore 96.7 ± 2% of original resistance within 4.2s) and rheological recovery experiments without any external stimuli. Graphene oxide (GO) is partially reduced under the oxidative self-polymerization of dopamine (DA), providing conductivity for nanocomposite hydrogels (2.7 mS cm-1). In the second method, physically crosslinked oxidized sodium alginate (OSA)/poly(acrylic acid) hydrogel (OSA/PAA-Fe3+) are synthesized by one-pot free radical polymerization. Both hydroxyl and carboxyl groups on oxidized sodium alginate chains were involved in coordination with Fe3+, which endowed the hydrogel with double network structure. OSA/PAA-Fe3+ hydrogels have excellent mechanical properties, like tensile strength up to 0.1MPa and elongation up to 1500%. The self-healing experiment showed that the self-healing efficiency of OSA/PAA-Fe3+ hydrogels can reach about 90% within 4 hours at room temperature. Furthermore, all of the hydrogels prepared by these two ways demonstrate strain sensitivity in the designed LED bulb circuit. In view of no apparent anaphylaxis of nanocomposite hydrogels to human skins, the soft stain sensor prepared by designed hydrogel can be fabricated to detect human activities, such as bending and sitting. Our work offers an effective approach to synthesize a fast self-healing, conductive and strain-sensitive hydrogel applied as soft strain sensor for human movement monitoring.
5:00 PM - EP04.03.13
Planting Carbon Nanotubes onto Supramolecular Polymer Matrixes for Waterproof Non-Contact Self-Healing
Bo Li1,Ning Ma1
Harbin Engineering University1Show Abstract
Supramolecular polymers show unique and excellent properties due to the reversible and designable nature of the non-covalent interactions. Herein, ureido-pyrimidinone (UPy)-based supramolecular polymers were employed to fabricate the thermo-responsive composite materials with multi-walled carbon nanotubes (MWCNTs) by planting the MWCNTs onto the supramolecular polymer matrices via a simple surface spraying procedure. The MWCNTs coating on the surface of the supramolecular polymer matrices gave the composite film superhydrophobic and conductive properties, and it had a non-contact healable ability underwater under 808 nm near-infrared light (NIR) irradiation. Moreover, the UPy-based supramolecular polymers acted as thermo-responsive matrices to guarantee the self-healing properties at a relatively low temperature, such as body temperature (33 °C–34 °C). The supramolecular polymer/ MWCNTs composite materials exhibited excellent strain sensitivities and could be used to prepare human motion-monitoring devices. Meanwhile, non-contact IR or body temperature self-healing property will greatly extend the life of the device. It provides a promising possibility for the development of the next generation of health monitoring system applied in underwater environments. This line of research may find a promising practical application in healable wearable devices used in particular conditions. And we hope this scheme can provide a new avenue for the design and preparation of functional devices with supramolecular polymeric materials. (This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation.)
5:00 PM - EP04.03.14
Pressure-Sensitive Rectifier Array for High Resolution E-Skin Tactile Sensor
Insang You1,Minsik Kong1,Unyong Jeong1
Pohang University of Science and Technology1Show Abstract
E-skin tactile sensors mimicking human mechanoreceptors have been studied intensively and expected to be applied to various future electronics such as haptic device, robot tactile sensor, human-machine interface, prosthetic applications and implantable electronic devices. To imitate the human cutaneous tactile sensors, E-skin is characterized by extreme mechanical flexibility or deformability and excellent recognition of force distribution. The mechanical flexibility of the E-skin has been successfully achieved using ultrathin substrates or stretchable interconnection between the rigid sensor units. Those device structures minimize the stress accumulated on the device units so that E-skin can operate under repeated stretches. The breakthroughs open the possibility of fabricating printed active matrix tactile sensors.
In order for E-skin to sense the spatial distribution of mechanical stimulation, a large number of sensors must be integrated to provide enough spatial resolution. For this purpose, some advances have been made in the fabrication of transistor-operated tactile sensor arrays. Although the transistors could prevent electrical crosstalks between the pixels and give large on-off ratios, the complexity of the transistor-based device is high. Active matrix structure is advantageous for individual pixel operation especially for highly integrated devices such as display or memory. However, in terms of E-skin tactile sensors, a passive device with a simple two-terminal structure would be more desirable to achieve high flexibility or deformability simultaneously with enough spatial resolution. Using a rectifying diode can be another approach to remove the electrical crosstalks. Someya and coworkers fabricated a pressure sensor matrix in which each cell was composed of a resistor-selector combination. They developed a mechanically robust organic diode and integrated a pressure sensitive elastomer resistor on top of the diode layer, which was the first demonstration using the rectifying diode for a flexible tactile sensor. This type of sensor matrix effectively avoided the electrical crosstalks. In terms of E-skin devices, the rectifier-based approach deserves more studies in the directions of simple device structure, high spatial resolution, and high sensitivity in wide range of pressure.
In this study, we propose a simple pressure sensor matrix for E-skin, in which each pixel is separated geometrically from each other and operated by the diode current sensitively changing upon applied external pressure. Therefore, each pixel has the two-terminal one-selector structure which is the pressure sensor itself. We used position-registered conductive microparticles (MPs) to achieve the one-selector device structure. Research on the arrangement and application of assembled MPs has been conducted over the past three decades, but there has been no report showing the feasibility of MPs-based integrated electronic devices. This is because it is difficult to assemble the MPs in a large area with accurate position registration and the concept of using the MPs in the electronic devices has not been developed yet. We have fabricated a highly flexible MP-based pressure sensor matrix that can precisely sense the distribution of external pressure. Since each pixel of the sensor can independently sense the pressure, we use the sensor matrix a flexible electronic scale and as an artificial fingertip to read Braille.
Marc Ramuz, MINES Saint-Étienne
Roozbeh Ghaffari, Northwestern University/Epicore Biosystems Inc/MC10 Inc
Pooi See Lee, Nanyang Technical University
Cunjiang Yu, University of Houston
EP04.04: Implantable Electronics and Sensors
Wednesday AM, April 24, 2019
PCC North, 200 Level, Room 222 A
8:00 AM - *EP04.04.01
Soft Implantable Devices for Electrophoretic Drug Delivery
University of Cambridge1Show Abstract
Neurological disorders strike millions of people each year, accounting for an annual economic cost of hundreds of millions of dollars that include not only direct expenses such as hospitalization and prolonged care, but also indirect ones caused by subsequent productivity losses. The most common treatment is oral or intravenous administration of pharmaceutically active compounds. These methods, however, require repeated administration and allow the drug to interact with tissues indiscriminately, causing side effects. Moreover, drug delivery in the brain is often precluded by a variety of physiological and metabolic obstacles such as the blood–brain barrier (BBB). The pursuit of novel administration strategies for efficient delivery of drugs to the central nervous system with greater precision and fewer side effects than conventional methods is an ongoing target of the biomedical community. We will discuss a novel class of devices that are implanted past the BBB and deliver the drug where and when is needed. These devices use a microfluidic channel to carry a drug solution into the brain where they use electrophoresis to deliver the drug without the solvent. They offer excellent spatiotemporal resolution, low-voltage operation, and high ON/OFF ratios. In vivo validation in a rodent model of epilepsy shows that these devices can stop, or even prevent seizures, paving the way for a new treatment option for neurological disorders.
8:30 AM - *EP04.04.02
Brain-Implanted Flexible and Stretchable Integrated Circuit System for Comprehensively Monitoring Brain Activities from Cerebral Cortex to Deep Brain Regions
Osaka University1Show Abstract
To totally understand brain-function networks, monitoring of brain signals by placing brain-signal sensors at various positions from the cerebral cortex to deep brain regions using advanced technology is important. However, the brains of most primates with advanced brain activities are small and are easily compressed by these sensors. Therefore, the development of a thin, flexible, and lightweight sensor system is indispensable.
In this study, we have developed a brain-implanted multichannel system for monitoring brain activities using (1) thin, flexible, and stretchable integrated circuits and sensors and (2) a highly conducting and flexible material with high stretchability like rubber and an organic integrated circuit technology using this material. Furthermore, we added optical functions on the basis of the technology of thin light-emitting diodes, successfully developing a minimally invasive brain-function monitoring system for optogenetics.
9:00 AM - EP04.04.03
Dense Conformal Electrode Array for Mormyrid Fish Electroreceptor Stimulation
Caroline Yu1,Christine McGinn1,Krista Perks2,Sarah Thompson1,Narumi Wong1,Johnny Li1,Nathaniel Sawtell2,Ioannis Kymissis1
Columbia University1,Columbia University Medical Center2Show Abstract
In neuroscience, weakly electric fish are used to study how motor signals influence sensory processing [1-3]. Specifically, understanding how information is encoded in the precise timing of electroreceptor spikes relative to sensory inputs will significantly contribute to behavioral studies. Previously, stimulating the electroreceptors of weakly electric mormyrid fish has been restricted to dipole current sources . In this work, we present a dense flexible electrode array that conforms to the fish’s skin and is seamlessly integrated with a robust user interface. The electrode array consists of 96 square electrodes with a side length of 250 μm and diagonal spacing of 1 mm. The pattern was chosen to match the hexagonal pattern of the fish’s electroreceptors. Gold electrodes were photolithographically patterned on parylene-covered spin-coated PDMS, as shown in multiple previous studies [5,6]. A second parylene layer was deposited and patterned to encapsulate the electrode traces and avoid crosstalk between electrodes. Chemical surface modification was completed to match the surface energy of the fish’s skin for several days. A multiplexer based circuit was used to individually select electrodes. A LabVIEW program allows a user to choose an electrode and synchronize the electrode’s information with neurophysiological data acquisition software. Local field potential was measured in the electrosensory lobe of the hindbrain while individually driving electrodes. Results demonstrate that this system is satisfactory for more complex neuroscience experiments.
Ref:  C. C. Bell, Science, 214, 450-453, 1981.  A. Kennedy et al., Nat. Neurosci., 17, 416-422, 2012.  T. Requarth et al., Neuron, 82, 896-907, 2014.  C. C. Bell, J. Neurophysiol, 63, 303-318, 1990.  M. Ochoa et al., Biomed Microdevices, 15, 437-443, 2013.  K. Meacham et al., Biomed Microdevices, 10, 259-269, 2008.
9:15 AM - *EP04.04.04
Flexible and Stretchable Organic Artificial Synapses for Sensory and Motor Nervous Systems of Bio-Inspired Electronics
Seoul National University1Show Abstract
Neuromorphic electronics is promising to process complex real-world problems such as visual information, speech recognition, and body movement control based on their compactness, fault tolerance, and high-energy efficiency. Artificial synapses are rapidly emerging for neuromorphic electronic devices that emulate biological nervous systems. Emulating learning and memory functions of the brain has been the main focus in neuromorphic electronics, but mimicking the complicated biological sensory and motor nervous systems that operate sequential functions related to proprioception, signal processing, and a motor response is also a challenging issue especially to realize bio-inspired soft robotics and electronics. Herein, we demonstrate flexible and stretchable organic artificial synapses for sensory and motor nervous systems of bio-inspired electronics. Sensory and motor electronic nerves are demonstrated by integrating organic synapses with sensory and motor organs. The sensory organs detect stimuli and fire artificial neural signals which will be transmitted to organic synapses as presynaptic action potentials. The flexible and stretchable organic synapses which are favorable for soft robots generate a post-synaptic response which will stimulate motor neurons and muscles. Thus, we realize i) a hybrid reflex arc system composed of artificial pressure-sensory nerves and biological motor nerves in a detached insect leg, and ii) artificial sensorimotor nervous system composed of an artificial light-sensory receptor and electronic neuromuscular system with an artificial muscle fiber. In addition, we demonstrate that these neuromorphic systems are promising to develop human/machine interface by distinguishing braille characters and conducting wireless optical communication. Our flexible and stretchable organic artificial synapses for sensory and motor nervous systems can be used for human-like soft robots and prosthetics which help people with neurological disabilities.
9:45 AM - EP04.04.05
Temporary Tattoo Electrode Records Brain Activity
Francesco Greco4,1,Laura Martinengo Ferrari1,Usein Ismailov2,Jean-Michel Badier3,Esma Ismailova2
Istituto Italiano di Tecnologia1,Ecole Nationale Supérieure des Mines de Saint Etienne2,Aix Marseille University3,Graz University of Technology4Show Abstract
The adoption of a thin-film substrate is a key aspect for the development of skin-contact imperceptible devices, in the so–called cutaneous or epidermal electronics. Together with reducing the thickness, lowering the mismatch in the mechanical properties of the device, with respect to the skin’s own, plays a major role. These properties reflect in devices inherent bending stiffness, determining their ability to flex to small curvature radius, following the skin relief profile in conformal adhesion. Temporary Tattoo Electrodes (TTEs) were developed to meet these requirements. TTEs are all made by polymers and their overall thickness is < 1-2 µm. They are fabricated by inkjet printing of the conductive polymer PEDOT:PSS on top of commercially available temporary tattoo paper. Recently, ultraconformable TTEs have been adopted in the recording of many electrophysiological signals and compared with standard Ag/AgCl electrodes. Wet Ag/AgCl electrodes are routinely used in electrophysiology (EP) thanks to their high signal quality. On the other hand, these electrodes exhibited many disadvantages which impose severe restrictions in all EP applications. The major issues are related to their limited time stability, due to gel drying, and to their cumbersome nature. With TTEs we showed the recording of electrocardiography (ECG) and electromyography (EMG) demonstrating their outstanding capabilities in interfacing with human body (1,2).
Now, we investigated the recording of electroencephalography (EEG) signals, the most challenging scenario due to the weakest signal and the lowest frequency range in EP (typical 10 - 100µV signal amplitude, frequency components 0.5Hz - 100Hz). EEG monitoring was performed in a clinical environment with the aim to open for new perspectives in EEG. TTEs have been thus tested in different clinical conditions, to cover the most of EEG diagnostics. We firstly validated TTE with the recording of alpha waves, the most known and studied brain rhythm. TTEs were also compared with Ag/AgCl electrodes, through Power Spectral Density (PSD) assessment, showing a good match over the whole signal spectrum. TTEs were further evaluated in auditory evoked potentials (AEP) measurements, in comparison with the standard Ag/AgCl electrodes, exhibiting comparable at all results. Furthermore TTEs’ compatibility with magnetoencephalography (MEG) equipment have been performed. EEG/MEG are frequently used in combination and at the best of our knowledge it is the first time that a dry electrode exhibits complete compatibility with MEG.
The successful evaluations of TTEs in the EEG field paves the way for future development of imperceptible alternatives to standard electrodes in clinical practice.
1. Zucca, A., et al. "Tattoo Conductive Polymer Nanosheets for Skin–Contact Applications." Advanced healthcare materials 4.7 (2015): 983-990.
2. Ferrari, L.M., et al. "Ultraconformable Temporary Tattoo Electrodes for Electrophysiology." Advanced Science 5.3 (2018).
EP04.05: Soft and Stretchable Systems and Applications I
Wednesday AM, April 24, 2019
PCC North, 200 Level, Room 222 A
10:30 AM - EP04.05.01
Hydrogels Sense and Heal Better with MXene
Yizhou Zhang1,Kanghyuck Lee1,Husam Alshareef1
King Abdullah University of Science and Technology1Show Abstract
Conductive hydrogel-based strain sensors hold great promise for wearable electronics, point-of-use medical sensors, and soft robotics. However, their sensitivities are generally low and they suffer from signal hysteresis and fluctuation due to the viscoelastic property, which can compromise their sensing performance. In our recent work, we prepared MXene (Ti3C2Tx)-based Poly (vinyl alcohol) (PVA) hydrogel (M-hydrogel) via a simple method . The obtained M-hydrogel sensor exhibits high strain sensitivity with remarkable stretchablity, instantaneous self-healing ability, excellent conformability, and adhesiveness to various surfaces including the human skin.
More importantly, the M-hydrogel shows much higher sensitivity under compressive strains than tensile strains. This asymmetrical strain sensitivity coupled with viscous deformation (self-recoverable residual deformation) is for the first time proposed to add new dimensions to the sensing capability of hydrogels: both direction and speed of motions on the hydrogel surface can be detected conveniently. Based on this effect, the M-hydrogel shows superior sensing performance in advanced sensing applications such as recognition of signature and vocal signals. Thus the traditionally disadvantageous viscoelastic property of hydrogels is turned into an advantage for sensing, which reveals new prospects for hydrogel sensors.
 Y.-Z. Zhang, K. H. Lee, D. H. Anjum, R. Sougrat, Q. Jiang, H. Kim, H. N. Alshareef, "MXenes Stretch Hydrogel Sensor Performance to New Limits”, Science Advances, 4(6), eaat0098 (2018).
10:45 AM - EP04.05.02
Bioimpedance Spectroscopy with Conformal Polymer Electrodes and Its Application in Long-Term Health Monitoring
Jae Joon Kim1,Linden Allison1,Trisha Andrew1
University of Massachusetts Amherst1Show Abstract
Conformal polymer electrodes can be vapor printed directly onto the surfaces of living organisms without damaging their health and self-sustenance. These polymer electrodes can then be used as contact pads for bioimpedance spectroscopy, which reveals detailed information about the health of the organism. Notably, polymer electrodes can be created directly on living plants, meaning that bioimpedance measurements can be performed on-demand, throughout the growth cycle of a plant. Vapor-printed polymer electrodes, unlike their adhesive thin-film counterparts, do not delaminate from microtextured living surfaces as the organism matures and do not observably attenuate the natural growth pattern and self-sustenance of the plants investigated herein. Deep tissue damage caused by dehydration and UVA exposure can be reliably detected throughout the life cycle of a plant. Long-term plant health monitoring will find strategic use in food farming, crop management, and biohazard signaling.
11:00 AM - EP04.05.03
3D Designed Ion Selective Sensors
Chao Bao1,Woo Soo Kim1
Simon Fraser University1Show Abstract
Requirement of real-time health monitoring boosts the development of wearable electronics. Especially cutting-edge technologies of integrated circuit board have been successfully utilized to obtain flexible devices. Here we report development of 3D ion selective field sensors integrated by 3D printing as ion selective field effect transistors (ISFETs). Separately fabricated FETs and ion selective membranes are both verified before hybridization. Then, the combined ISFETs show high sensitivity to ammonium (NH4+), potassium (K+), and calcium (Ca2+) ions with linear responses depending on ion concentrations. Moreover, high selectivity of these ISFETs has also been proved during the interference tests, not only by the separate interference ions testing, but also by artificial saliva with multiple interference ion concentrations. Additionally, unique wireless protocols for the 3D printed ISFET has also been implemented. The AC output signal converted from DC input during this signal transmission process is identified as the ion concentration. This new wireless sensing protocols open the potential of the 3D printed ISFETs in the field of wireless health monitoring sensors.
11:15 AM - *EP04.05.04
Stretchable Conductive Nanocomposite for Implantable and Wearable Bioelectronics
Institute for Basic Science1,Seoul National University2Show Abstract
Intrinsically stretchable conductors form a vital component of advanced soft bioelectronics. And novel nanocomposites based on conductive nanomaterials have been used in diverse applications of implantable and wearable bioelectronics. Among many nanomaterials for the composites, silver nanowires (Ag NWs) have been popular. However, achieving highly conductive and soft composites simultaneously is challenging. Furthermore, because bioelectronics is necessarily exposed to biofluids, preventing Ag NW oxidation and Ag ion leaching is a significant challenge. Here we have achieved a highly conductive, biocompatible, and soft nanocomposite by using silver-gold (Ag-Au) core-sheath NWs and polystyrene-butadiene-styrene (SBS) elastomer. We synthesized ultralong Ag NWs encapsulated with a smooth and uniform Au sheath and then mixed them with the polymer. Phase separation of Ag-Au NWs and SBS occurs, which forms microstructures in the composite, reduces Young’s modulus, and increase softness and conductivity of the composite. We used the nanocomposite to fabricate a customized multi-channel soft cardiac mesh for the diseased swine heart. The mesh could be also fabricated as a wearable device which monitored electrophysiological signals and applied electrical and thermal stimulations on the human skin.
EP04.06: Liquid-Material Embedded Soft Structures II
Wednesday PM, April 24, 2019
PCC North, 200 Level, Room 222 A
1:30 PM - *EP04.06.01
From Particles to Parts—Multi-Phase Metallic Particle Additives for Sensing and Tunable Materials
Yale University1Show Abstract
Particles made from liquid or low-melting-point alloys can be leveraged to create a new class of functional responsive composite materials. In this talk, I will present two types of metallic particles: eutectic gallium-indium particles and Field’s metal particles. Eutectic gallium-indium particles have been used to achieve printable, soft, and stretchable electronics. More recently, we have demonstrated the role of Field’s metal particles in enhancing the range of stiffness properties in a thermally responsive epoxy, and in creating variable elasticity and switchable anisotropy when embedded in silicone elastomer. These responsive material properties derived from metallic particle additives enable new capabilities for wearables, soft electronics, and soft robotics.
2:00 PM - EP04.06.02
Electrical Control of Shape in Liquid Crystalline Elastomer Nanocomposites
Tyler Guin1,2,Timothy White3,Amit Naskar2
Air Force Research Laboratory1,Oak Ridge National Lab2,University of Colorado Boulder3Show Abstract
Liquid crystal elastomers (LCEs) are soft, anisotropic materials that exhibit large, reversible shape changes in response to external stimuli. The director (average alignment) of an LCE can be localized into pixels via photoalignment. By rationally designing the director profile, out-of-plane shape transformations are possible when the material is activated. In this talk, single-wall carbon nanotube – LCE nanocomposites are presented, and their distinctive electromechanical actuation explored. When a DC electric field is applied through the material thickness, the composite with contract along the alignment direction. By localizing the orientation of the LCE and SWNT via photoalignment, complex 3-D shapes can be electrically triggered. Additionally, a brief explanation of the novel electrostrictive mechanism will be presented.
2:15 PM - EP04.06.03
Mechanical Tunability of Core-Shell Liquid Metal Nanoparticles for Self-Healing Electronics
Nicholas Morris1,2,Zachary Farrell1,2,Carl Thrasher1,Christopher Tabor1
Air Force Research Laboratory1,UES, Inc.2Show Abstract
A vision for many potential applications of flexible and stretchable electronics has led to new and diverse research approaches in the previous decade, such as intrinsically stretchable organic electronics and strain relief geometries in otherwise non-compliant traditional materials. A third approach is to engineer materials to respond and adapt to strain by repairing themselves through self-healing mechanisms. In this work, we utilize the stimulus-responsive characteristics of a colloidal room temperature eutectic gallium-indium alloy (EGaIn) to achieve electronic self-healing triggered by strain and mechanical damage. Unlike organic conjugated systems, EGaIn possesses the high conductivity of a metal and while its fluidic properties allow it to flow and adapt to induced strain. We utilize EGaIn’s intrinsic ability to grow a native passivating viscoelastic oxide skin to produce liquid metal core-shell nanoparticles, serving as small reservoirs of the fluidic metal core, which upon rupture provide the potential to reconnect damaged electronic components and restore electrical connectivity. We report on our ability to modify the mechanical properties of the core-shell nanoparticles by correlating changes in stiffness, shell modulus, and ultimately the rupture force required to induce healing with various chemical treatments of the oxide shell (Farrell et al., Langmuir 2018). Flat-punch nanoindentation techniques with in situ electrical characterization were developed to experimentally characterize single particle stiffnesses and the critical buckling loads associated with particle rupture which would lead to electrical restoration. The modulus of the oxide shell was estimated using Reissner’s elastic thin plate theory, with estimated moduli between 0.37-1.38 GPa depending on functionalization. A fifty-fold increase in critical buckling load was measured across the various particle systems, providing a large range of engineering control and allowing application specific mechanical properties for implementation in self-healing coatings, stretchable conductors, and push-to-connect low temperature solders. In light of the standalone nature of the particle system, we anticipate their addition to existing electronics in a variety of ways, such as chemical binding through ligand anchoring, spray coating, or drop casting; imparting healing functionality and mechanical robustness to traditional electronic materials.
EP04.07: Robotics, Prosthetics and Eskin I
Wednesday PM, April 24, 2019
PCC North, 200 Level, Room 222 A
3:30 PM - *EP04.07.01
Emerging Self-Healing Material and System Platforms for Electronic Skins in Wearables and Robotics
National University of Singapore1,Institute of Materials Research and Engineering2,Biomedical Institute for Global Health Research and Technology3Show Abstract
The multi-disciplinary efforts in the last few decades have enabled mechanically softer and skin-like electronic devices1–5. These exciting progress opens multiple doors of opportunity to utilize innovative materials and devices intimately with epidermal surfaces, wearable technologies and robotics. In this talk, I will discuss some of the new material platforms we are developing for more robust human-machine interactions, such as self-healing stretchable optoelectronics devices. At the same time, I will also present our efforts in developing new system architectures to scale the density and number of such devices for rapid environment perception. Such system architectures can be broadly applied in electronic skins, and has broad potential for continued applications in wearables, brain-machine interfaces, prosthetics and robotics.
1. Heikenfeld, J. et al. Wearable sensors: Modalities, challenges, and prospects. Lab Chip 18, 217–248 (2018).
2. Wang, S. et al. Skin electronics from scalable fabrication of an intrinsically stretchable transistor array. Nature 555, 83–88 (2018).
3. Tee, B. C. K. et al. A skin-inspired organic digital mechanoreceptor. Science (80-. ). 350, 313–316 (2015).
4. Gao, W. et al. Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis. Nature 529, 509–514 (2016).
5. Tan, Y. J., Wu, J., Li, H. & Tee, B. C. K. Self-Healing Electronic Materials for a Smart and Sustainable Future. ACS Appl. Mater. Interfaces 10, 15331–15345 (2018).
4:00 PM - EP04.07.02
Stretchable, Transparent and Breathable Epidermal Electrode for Health-Related Applications
Guang Zhu1,You Jun Fan1,Lu Liu1,Xin Li1
Beijing Institute of Nanoenergy and Nanosystems, CAS1Show Abstract
Recently emerged electronic skins with applications in on-body sensing and human−machine interfaces call for the development of high-performance skin-like electrodes. In this presentation, a family of recently developed stretchable, transparent, and breathable epidermal electrode will be introduced, which is composed of a nanofibers scaffold and a silver nanowires network.
Through techniques such as vacuum filtration or electro-spinning/electro-spraying, the silver nanowires are embedded into a scaffold made of polymer nanofibers. Optical transmittance of 84.9% at 550 nm wavelength is achieved at a significantly low sheet resistance of 8.2 Ωsq−1. The resistance of the epidermal electrode only slightly increases by less than 0.1% after being bent for 3000 cycles at the maximum curvature of 300 m−1 and by less than 1.5% after being dipped in saline solution for 2500 cycles. Attributed to silver nanowires with hierarchical dimensions, great robustness against tensile strain is achieved. The reinforcement from the nanofiber-based scaffold as a backbone maintains the connections among the Ag nanowires by undertaking most of the loaded stress. The epidermal can still maintain its conductivity when experience a strain of over 300%.
The epidermal electrode can form tight and conformal bonding with genuine skin through Van Dal Walls force. Because of the conformal contact as well as the high conductivity, the epidermal electrode shows a contact impedance 50% lower than commercial gel-based electrode. As a result, the epidermal electrode exhibits apparently higher noise resistance again body motions when measuring physiological signals such as ECG, making it promising to be used in ambulatory on-body monitoring for healthcare applications. Besides, the epidermal electrode allows the evaporation of perspiration, making it suitable for long-time use.
4:15 PM - EP04.07.03
Fully Wirelessly-Operated Soft Actuators with Environmental-Sensing Capability
Byungkook Oh1,Young-Geun Park1,Sangyoon Ji1,Woon Hyung Cheong1,Jang-Ung Park1
Yonsei University1Show Abstract
The most important function in soft robotics is to travel through harsh spaces and receive the real-time sensing signals wirelessly based on today’s smart technologies. However, to date, the reported soft robotics has the limitation of low degrees of freedom of movement because tethered systems should be connected to soft actuators to initiate the actuation. Although few research groups have demonstrated an untethered soft actuator, the reported untethered soft actuators only mimicked the movement of people or animals without any environmental-sensing capability. The key requirements for designing human-like soft robots with somatosensory systems are to operate the soft actuators without supporting equipment and to have wireless environmental-sensing capability. Fabricating fully wirelessly-operated soft actuators with environmental-sensing capability has advantages of improving degrees of freedom of movement and adapting them to various applications like biomedical fields. For the Internet of Things (IoT)-based smart wireless environment, providing wireless environmental-sensing systems has highlighted the need for using the electronic sensors and integrating them to a soft actuator body. However, the conventional electronic sensors are rigid, the integration of the electronic sensors to the soft actuator body has challenges for poor adhesion between electronic circuits and the soft actuator body and low reliability and stability of electronic sensors due to their rigid properties. Thus, improving the reliability of electronic sensors is essential for the advances in soft robotic fields. Herein, we report an unconventional approach for demonstrating fully wireless soft robotics with wireless tactile-sensing systems. The electrothermally-actuated phase transition soft actuator is composed of the entirely soft and flexible body frame. A liquid-metal particles are embedded in the soft actuator matrix so that the rate of initiating the actuation of the soft actuators increases. Soft actuators we designed are integrated to a pressure sensor and temperature sensor with improved stability and reliability due to conformal actuating properties, causing the actuation strain to distribute throughout the entire soft body actuated. However, it still has a problem of generating of electron-hole pairs (EHPs) in pressure sensors as the temperature of flexible heaters increases. The generation of EHPs results in the increase in the output current. Thermally excited charge carriers can affect the sensing signals of pressure sensors. Evading the generation of EHPs is an important feature for the sensory system to receive pressure sensing information exclusively. To that end, we designed the configuration of pressure sensor and temperature sensor above the heater film. By integrating them in parallel above the soft body frame, only tactile sensing signals can be received with calibration steps using sensing signals of the temperature sensor. It provides the pressure sensors are independent of the increasing temperature of heaters. Our soft actuators can be actuated wirelessly with remote temperature control systems interconnected to heaters. And, they can also receive real-time sensing feedback wirelessly by connecting the wireless pressure sensory module to electronic sensors. Our demonstration of wireless actuation of soft actuators with wireless environmental-sensing capability suggested that the soft actuator has an important feature for providing the capability of the use in the harsh spaces.
4:30 PM - EP04.07.04
Liquid Crystal Elastomers for Soft and Stretchable Bioelectronics
Jimin Maeng1,Hyun Kim1,Mahjabeen Javed1,Taylor Ware1
The University of Texas at Dallas1Show Abstract
Achieving implantable devices that reside in human body with chronic stability has long been a goal of the bioelectronics community. A major challenge in achieving long-term stability lies in the difficulty in establishing reliable interconnection of the implantable devices to the outside world. For example, implantable neural devices in the periphery undergo significant strain due to repeated muscle flexion and extension. Furthermore, device encapsulation can be significantly weakened under such strain, causing infiltration of physiological fluids into devices that leads to chronic failure. Therefore, efforts toward developing implantable electronics based on high barrier and strain tolerant materials are needed.
Here, we present a liquid crystal elastomer (LCE)-based, soft, and stretchable electronics architecture for their potential as a platform for chronic implantable bioelectronics. The unique shape-morphing capability of LCE is leveraged to fabricate electronic devices in strain tolerant three-dimensional (3D) shapes. Specifically, we employ LCEs as substrates for electronic devices that are flat during processing but then morph to 3D shapes for use. A variety of thin-film electronic components, including conducting traces and metal-insulator-metal (MIM) capacitors, is fabricated on surface-aligned LCE substrates by using photolithographic microfabrication processes. These devices are shown to morph their shapes from 2D to 3D as programmed, upon the release of the LCE films from the carrier wafer. The 3D helical cable conductors and capacitors created by this method demonstrate strain tolerance up to 100% with less than a 5% change in resistance or capacitance. We will discuss the mechanical and electrical properties of these cables with various encapsulation coatings including parylene-C, Al2O3/parylene-C bilayer, and LCE. Our preliminary results suggest that the LCE-based soft electronics technology proposed herein may open up a new avenue for developing chronic implantable bioelectronics and neural interfaces.
4:45 PM - EP04.07.05
3D Printing Flexible Silicones, TPUs and Nylon Materials for Actuating Devices and Motors
Case Western Reserve University1Show Abstract
The application of silicones and thermoplastic polyurethanes (TPUs) in the number of electronic and flexible device applications has multiplied through the years with its excellent composition control and the ability to prepare various forms primarily through the injection molded and thermo-formed processing shapes. This means that their thermo-mechanical properties can be tuned towards more mechanically compatible properties to the human body including elastomeric properties. However, in a number of prosthesis devices including artificial body parts, sensors, implants, etc. 3D printing is emerging to be an alternative for additive manufacturing with its rapid-prototyping advantage and the ability to incorporated new polymer and nanocomposite in processing methods such as fused deposition modeling (FDM), stereolithographic apparatus (SLA) and viscous solution printing (VSP). It should be a viable method for the limited and specific production of flexible electronic devices that are fit even for personalized healthcare This talk will highlight our work on 3D Printed Silicones, 3D printed TPU, etc. and 3D Printed nylons, to form basic material compositions flexible <span style="background-color:rgb(246, 213, 217)">electronic nanocomposite materials containing graphene, carbon nanotubes, carbon black,e etc. </span> Structure-property relationships correlated with the formation of improved modulus and flexural strength with increasing amounts of electronic nanofiller components.
EP04.08: Poster Session II: Soft and Stretchable Electronics—From Fundamentals to Applications
Pooi See Lee
Wednesday PM, April 24, 2019
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - EP04.08.01
NIR Absorbing Ionic Dyes for Transparent Photo-Actuators
Minsu Han1,Lim Hanwhuy1,Jongun Hwang1,Eunkyoung Kim1
Yonsei University1Show Abstract
Photothermal materials absorbing near infrared light have received strong attention for optical filters, sensors, energy harvesters, and photothermal theragnosis agents. In particular, NIR absorbing ionic dyes containing multi aromatic units have high transparency and optical selectivity, so they are challenging materials as a transparent photothermal material. However, the researches on these materials are rather limited due to the multi-step synthetic process as well as difficulty in the characterization of molecules. Herein, we report synthesis and application potential of NIR absorbing ionic dyes as a photo-actuating electrode. NIR absorbing ionic dyes were synthesized through modification of aromatic amines over >80% yields, and characterized by spectroscopic methods. The ionic dyes showed strong absorption of NIR light at ~ 1000 nm region, while they were transparent in visible region (%T > 90) both in solution and film state (Fig. 1(a)). Thin film of the ionic dyes was applied to a photothermal actuator, which showed large temperature rise upon exposure to a NIR light (Fig. 1(b)). The temperature rise, transparency, and photo-actuation of the transparent actuator of the ionic dyes will be presented.
5:00 PM - EP04.08.02
Thermally Self-Healing Electrochromic Film and Devices Derived from Reversible Diels–Alder Polymer
Yi Wang1,Chunyang Jia1
Chengdu University1Show Abstract
The cracking of electrochromic materials due to aging or reiterative bending is a major problem which noticeably degrades the performances of electrochromic devices. In this research, a novel self-healing electrochromic polymer PPFMA was synthesized, which integrated the electrochromic triphenylamine and self-healing Diels-Alder groups. The PPFMA film showed all superior electrochromic properties, such as stable color changes (yellow to green to blue) with a maximum optical contrast of 42.7 % at 1050 nm and high coloration efficiency of 108.3 cm2 C-1. Meanwhile, the polymer PPFMA also showed excellent self-healing performance, cracks cut on the film surfaces (bleached or colored states) could be self-healed at 110 °C within 4 min, and the self-healing rate was about 80 %. The results indicate that the polymer PPFMA has the mutual independent bifunction of electrochromic and self-healing, it is a promising material for prolonging the service life of electrochromic device.
5:00 PM - EP04.08.03
Highly Robust, Transparent and Breathable Epidermal Electrode
Chinese Academy of Sciences1Show Abstract
Recently emerged electronic skins with applications in on-body sensing and human-machine interface call for the development of high-performance skin-like electrodes. In this work, we report a highly robust, transparent, and breathable epidermal electrode composed of a scaffold-reinforced conductive nano-network (SRCN). Solution-dispersed silver nanowires, through facile vacuum filtration, are embedded into a scaffold made of polyamide nanofibers. Optical transmittance of 84.9% at 550 nm wavelength is achieved at a significantly low sheet resistance of 8.2 Ω sq-1. The resistance of the SRCN only slightly increases by less than 0.1% after being bent for 3,000 cycles at the maximum curvature of 300 m-1 and by less than 1.5% after being dipped in saline solution for 2,500 cycles. The excellent robustness is attributed to the reinforcement from the nanofibers-based scaffold as a backbone that maintains the connections among the silver nanowires by undertaking most of the loaded stress. The SRCN not only forms tight and conformal bonding with target surface but also allows the evaporation of perspiration, making it suitable as an epidermal electrode for long-time use. Furthermore, fine and clean-cut circuit patterns with line width on the micrometer scale can be readily prepared, paving the basis for fabricating sophisticated functional electronic skins.
5:00 PM - EP04.08.04
Patterned Transfer of Silver Nanowire Electrode by Using UV Curable Pressure Sensitive Adhesives
KeumHwan Park1,Ye-seul Song1,Hee-Jin Lee1,Woongsik Jang2,Dong Hwan Wang2,Youngmin Kim1
KETI1,Chung-Ang University2Show Abstract
Recently, studies on the fabrication of stretchable electrodes using AgNW have attracted much attention. Among the many methods, stamping transfer process is most considered as an advanced technology which can overcome the limitations of existing methods such as lithography, and wet coating. In this study, UV curable pressure sensitive adhesives (PSAs) consisting of acrylic adhesives and UV curable oligomers are prepared. The novel UV curable oligomers are synthesized through two steps. The aminolysis of the cyclic carbonate with amine-capped polypropylene glycols (PPGs) afforded multifunctional alcohols in the first step. Next, the reaction between the alcohols and methacrylic isocyanates produced multi-arm PPG methacrylates.
These compounds were mixed with the acrylic adhesive to generate two kinds of UV curable PSAs. The peel strength of the two PSAs on th PC substrate was 1,113 gf/in, and 991 gf/in, respectively. Interestingly, the peel strength of the PSAs was reduced by two orders of magnitude after UV irradiation. The initial tack forces of the PSAs also were dramatically reduced. The reduced adhesion of the PSAs was attributed to the high storage moduli induced by UV treatment. The UV curing of PSAs was confirmed bymonitoring the disappearance of an IR absorption band corresponding to H-C=C stretching by FTIR spectroscopy.
Finally, AgNW and UV curable PSA were mixed and coated to make a film.
The sample was irradiated with UV through a mask to produce a pattern in which only the exposed portion to lost its adhesion. The adhesive portion was transferred to the opposite substrate through the stamping transfer process,
and the sheet resistance was measured as nearly same to that before the transfer. Stretching and bending tests were performed on the fabricated samples, which demonstrated that they are well suited for the production of stretching electrodes.
5:00 PM - EP04.08.05
A Hybrid PVDF/PDMS Electronic Skin for Accurate Touch Localization
Keith Behrman1,Caroline Yu1,Pedro Piacenza1,Johnny Li1,Matei Ciocarlie1,Ioannis Kymissis1
Columbia University1Show Abstract
Flexible tactile sensors that can precisely and rapidly detect touch and provide location are of great importance for e-skin development . Piezoelectric sensors are a great candidate because of their high voltage response, speed and sensitivity . Current implementations use dense taxel arrays to accurately detect touch with high precision [3,4]. However, these solutions suffer from complex designs, slow response rates, expensive detection equipment or large wire arrays required to address individual sensors. The work presented here improves on current work by achieving high spatial resolution with a simple fabrication scheme and a minimal number of sensing elements. The sensing circuitry for the device utilizes a single charge-amplifier and a multiplexer controlled by a microcontroller to achieve high-speed measurements at a low cost.
Yu et al., have shown that multiple sensing modalities can be combined within a single PVDF sensor and utilizing compressible compliant layers, such as foam, can improve device sensitivity . Here we iterate on this design by stacking two layers of PVDF with patterned electrodes to create a dense sensing area encapsulated and sandwiched by PDMS layers. The deformable nature of the PDMS offers increased sensitivity by acting as a compliant layer. PDMS can be used in a release mold to create a flexible skin that can be attached to existing grasping fingers, or the PDMS can be directly bonded to custom finger architectures to improve adhesion to the substrate while being fully integrated into the finger.
PVDF pre-coated on both sides with sputtered Cr/Au (40/250nm) was lithographically patterned and wet-etched utilizing backside alignment techniques to form electrode arrays on a 15x15mm active area. The electrode arrays were electrically connected with heat-seal connectors before being stacked and encapsulated in PDMS. This sensor consists of two PVDF arrays with 16 electrode pairs to create a grid pattern. The electrodes were individually addressed with a multiplexer as seen in Geng et al., to simplify the circuit by using only one charge amplifier while maintaining a high sampling frequency .
The sensor was characterized with a robotic indenter by collecting individual sensor readings across a dense grid of indentation locations to train a kernel ridge regressor to predict locations against a test set of randomized locations. The prediction accuracy improves at deeper indentation depths and has a minimized prediction error of 0.78 mm with a standard deviation of 0.61 mm. Compared to a 15x15 array of individual taxels, this design achieves comparable spatial resolution with 14x fewer sensing elements. The sensor can also be implemented as a tri-modal sensor to capacitively sense object proximity, measure pressure by implementing strain gauges along with localization data from the grid. This rich information set can be used to augment robotic grasping applications.
 R. Dahiya, IEEE Trans. Robot, 2010, pp. 1-20.  R. R. Reston and E. S. Kolesar, IEEE Conf, 1990.  B. Choi et al., Diffus. Defect Data Pt.B Solid State Phenom, (2007), pp. 229-234.  L. Seminara et al., IEEE Sens. J., (2013), pp. 4022-4029.  C. Yu et al., MEMS 2018, 886-888, 2018  Z. Geng et al., IEEE Instrum. Meas. Technol. Conf., (2011), pp. 74-78.
5:00 PM - EP04.08.06
Single-Crack-Activated Ultrasensitive Flexible Impedance Strain Sensor
Tsinghua University1Show Abstract
A development of single-crack-activated impedance strain sensors with unprecedented sensitivity is demonstrated first. The gauge factor of the device is beyond 108 in 10-4 strain range in comparison with the reported highest gauge factor 1.5 × 105 within 60% strain range, and the displacement sensitivity is 1.6 MΩ nm−1. The extremely high sensitivity is attributed to the transition region which has never been studied before. Multiple crack-based sensors, however, cannot work in the transition region due to complicated interaction among cracks, which essentially limits their sensitivity. Additionally, studying a precisely controllable single crack rather than multiple cracks is favorable for excluding other factors such as crack spacing, difference among cracks, and interaction among cracks, simplifying the model and facilitating better understanding of the underlying mechanism of the device. The device can satisfy requirements of mechanical flexibility, durability, and repeatability. In addition, the device developed is capable of measuring displacement in nanometers range or force in tens of nanonewton range, and has the potential to be applied in various fields, such as specific biomolecular recognition.
5:00 PM - EP04.08.07
Highly Stretchable Strain Sensors Comprising Double Network Hydrogels and Conducting Polymers Prepared by Microfluidic System
Jinhwan Yoon1,Dowan Kim1,Jin-woo Oh1,Soo Hyung Kim1
Pusan National University1Show Abstract
Strain sensors with high sensitivity and stretchability are highly required for the application of wearable or implantable sensor to detect the human motion. In this study, we have prepared a highly stretchable double network (DN) of soft polyacrylamide and brittle calcium-alginate microfibers containing poly(3,4-ethylenedioxythiphene)-poly(styrenesulfonate) (PEDOT:PSS) by using microfluidic devices. The resistance changes in response to the stretching of the microfiber due to the connection/disconnection of the PEDOT:PSS domain can be observed up to 400% of elongation with the resolution of 0.1%. Furthermore, these changes are fully reversible and repeatable over 10,000 cycles at 300% elongation. On the basis of these mechanical and electronic properties, the DN microfibers are envisioned to use stretchable strain sensor, demonstrating the detection of the human motion including bending of the finger and walking and running in real time. We also demonstrated that the developed sensor can be stably used in outdoor by measuring the growth of the bamboo planted in the garden.
5:00 PM - EP04.08.08
Fabrication, Characterization and Dielectric Spectroscopy of BaTiO3 Styrene Butadiene Styrene Stretchable Thin-Film Nanocomposites for Flexible Electronics
Suporna Paul1,Benard Kavey1,Gabriel Caruntu1
Central Michigan University1Show Abstract
Polymer ceramic nanocomposites exhibit performance characteristics superior to those of the parent materials as they harness the mechanical properties of the polymer and the dielectric properties of the ceramic. However, the rational design of flexible high-k dielectric nanocomposites with high filler loading is still challenging as the increase of the ceramic content deteriorates the mechanical properties of the nanocomposites. In this work, we investigated the fabrication of flexible electronics by dispersing BaTiO3 (BTO) colloidal nanocrystals with various sizes (10-20 nm) into the styrene butadiene styrene (SBS) matrix. The resulting SBS-BTO nanocomposite films contain up to 50% (weight) BaTiO3 nanocubes fillers and possess high energy density values along with excellent mechanical properties. It has been observed that the nanocomposite films can easily be peeled off from any type of substrate which is an indication of good flexibility and strength of these free standing films. We observed a linear increase in the dielectric constant with respect to fillers content, ranging from 6.1 to 24.1 for 0% to 40% respectively. The alternate current (AC) conductivity of the polymer remained below 10-8, which shows the polymer is insulative and can be used in flexible capacitor applications. Also, with scanning electron microscope, we observed that, the BTO cuboidal nanocrystals are uniformly distributed in the polymer matrix that can explain the reason behind of good flexible property of the film. The experimental results show that these nanocomposite thin films exhibit superior properties which make them attractive for implementation in high-performance capacitive storage devices, wearable technology and nanoelectronics.
5:00 PM - EP04.08.09
Transformable Crystalline Silicon Photovoltaics
Inchan Hwang1,Kwanyong Seo1
Transformable photovoltaics (TPVs) have been attracting much attention because of the limitation of the PV’s installation and a continuous power source for the portable and wearable electronic devices. At present, TPVs have been developed based on the perovskite and organic photovoltaics. However, these PVs have problems of the low efficiency and instability. In contrast, crystalline silicon (c-Si) PVs exhibit high efficiency and stability, but they are not able to be transformable because they tend to be fragile when an external force is applied. Therefore, a new strategy for the TPVs technology based on the crystalline silicon is required to realize the high efficiency and stable c-Si TPVs. In this research, we developed the c-Si TPVs through the interdigitated back contact (IBC) structured c-Si module and introduction of a stretchable electrode. Because all electrodes (positive and negative contact) of the IBC PVs are positioned on the rear side, we can easily fabricate the TPVs module using the stretchable electrode. The stretchable electrode is consisting of the combination of the carbon materials and elastic polymer. The stretchable carbon filler/ polymer composite is found to effectively retain its electrical conductivity, even when under the high strain of ≈200%. Due to the outstanding electrical conductivity and highly stretchable property of the carbon filler/ polymer composites, the fabricated IBC PVs can be transformable without the efficiency degradation. Thus, we expect that the proposed c-Si TPVs are possible to overcome the PV’s installation limitation and integrate with flexible or wearable electronic devices for the continuous power source.
5:00 PM - EP04.08.10
Effective Processing Strategies to Integrate Ag NWs with Polymer Semiconductors for High Performance Stretchable Field Effect Transistors
Runqiao Song1,Shanshan Yao1,Zheng Cui1,Jingyan Dong1,Yong Zhu1,Brendan O'Connor1
North Carolina State University1Show Abstract
Intrinsically stretchable transistors are a critical building block for an array of stretchable electronic applications. They also represent a model system where conductors, semiconductors, and insulators must all be stretchable and effectively integrated. Here, we present an intrinsically stretchable organic field effect transistor that incorporates Ag nanowires (NWs) as the source, drain, and gate electrode with a stretchable PDMS gate dielectric and stretchable DPP-4T polymer semiconductor. The Ag NWs provide a significant improvement in electrical conductivity compared to other candidate stretchable electrode materials such as carbon nanotubes and PEDOT:PSS. However, there are several challenges associated with roughness and patterning that can limit their application. Here, we demonstrate an effective processing strategy of embedding the Ag NWs in a PDMS elastomer allowing for high stretchability and high conductivity. This high conductivity enables the electrical leads and electrodes to be composed of the same material. The NWs are patterned using direct electrohydrodynamic jet printing, and through drop casting followed by laser ablation. While the NWs are embedded in the elastomer, we show that the surface of the NW network is able to effectively contact the semiconductor for low contact resistance transistor performance. Through this processing strategy we are able to demonstrate a stretchable transistor with a saturated field effect mobility that remains largely unchanged when under an applied strain of 40%, and remains functional under cyclic loading. As part of this work, the morphology of each active component is studied in detail along with detailed device characterization.
5:00 PM - EP04.08.12
Bioinspired Multi-Responsive Soft Actuators Controlled by Laser Induced Graphene
Heng Deng1,Cheng Zhang1,Jheng-Wun Su1,Yunchao Xie1,Chi Zhang1,Jian Lin1
University of Missouri1Show Abstract
By exploiting aligned cellulose fibrils as geometrically constraining structures, plants can achieve a complex programmable shape change in response to environmental stimuli. Inspired by this natural prototype, a series of manmade materials with aligned structures have been developed and employed in self-morphing materials. However, in these cases, the constraining materials are fabricated and aligned in separate processes. In botanic systems, a more efficient way is adopted, in which the aligned microstructures are simultaneously synthesized and aligned in one bottom-up process. Herein, we report a bioinspired bottom-up approach to fabricate laser induced graphene (LIG) structures which resemble the aligned microstructures of the cellulose fibrils in plants. Such LIG structures serve as geometrically constraining materials to precisely control the shape-changing behaviors of soft actuators made from polymer and LIG layers. Meanwhile, the LIG structures also serve as functional materials to absorb photo and electrical energy to stimulate motions of the soft actuators. Taking advantage of the geometrically constraining effect from the aligned LIG structures, a series of programmable actuations stimulated by electricity, light, organic vapor, and moisture were demonstrated. Furthermore, the soft actuators also act as soft grippers and walking robots upon different stimuli, indicating their potential applications in soft robotics, electronics, microelectromechanical systems, and others.
5:00 PM - EP04.08.13
Robust and Stretchable Polymer Semiconducting Networks—From Film Microstructure to Macroscopic Device Performance
Guoyan Zhang1,Elsa Reichmanis1
Georgia Institute of Technology1Show Abstract
Although stretchable polymer-based devices with promising electrical performance have been produced through the polymer blend strategy, the interplay between the blend film microstructure and macroscopic device performance under deformation has yet to be unambiguously articulated. Here, we will discuss the formation of robust semiconducting networks in blended films from a thermodynamic perspective. Thermodynamic behavior along with the linear absorption and photoluminescence measurements predict the competition between polymer phase separation and semiconductor crystallization processes during film formation. Semiconducting films comprised of different pi-conjugated semiconductors were prepared and shown to have similar mechanical and electronic properties to films comprised of a model P3HT and PDMS blend. These results suggest that a film’s microstructure and therefore robustness can be refined by controlling the phase separation and crystallization behavior during film solidification. Fine-tuning a film’s electrical, mechanical, and optical properties during fabrication will allow for an advanced next-generation of optoelectronic devices.
5:00 PM - EP04.08.14
Stretchable/Flexible Transparent Conductors for Emerging Optoelectronic Devices and Epidermal Transducers
Huazhong Univ of Science and Technology1Show Abstract
Compared with continuous transparent conductive films, such as metal oxides that cannot effectively release the strain during bending or stretching, percolated network composed by conductive nanowires like silver nanowires (AgNWs) can be a ductile form, which also possesses excellent electro-optical performance and solution-processed manufacturing. However, this unique metal percolated structure also has issues, such as the binding strength with polymers and poor patterning performance. Therefore, the design of network and development of compatible techniques become important for practical applications in stretchable/flexible optoelectronic devices.
In our study, we firstly use finite element simulation to explore the electrical, optical and mechanical properties of the network, which guides us to optimize the fabrication of stretchable/transparent conductors. Then, AgNW transparent conductors with unique optoelectronic properties are experimentally realized and applied in different fields. For example, by virtue of their transparent and stretchable feature, we demonstrate a dual-mode electronic skin with the capability of tactile sensing and visualized injury warning . The soft properties also enable them to be stick mildly onto the perovskite solar cells conformally as good top electrode . We also develop cryo-transfer method by triggering the glass transition of elastomers to fabricate high-performance AgNWs/elastomer samples, which has the thickness of 8.4 μm and transmittance of 90.8% at sheet resistance of 13.2 Ω/sq, and could tolerate strain of 70%, overcoming the critical issue of incomplete transfer of AgNWs, the obtained ultrathin invisible conductors is used as epidermal electrophysiology transducers which shows better performance exceeding commercialized gel electrodes 
 ACS Appl. Mater. Interfaces 2017, 9, 37493.
 Adv. Funct. Mater. 2018, 28, 1705409.
 ACS Nano submitted.
5:00 PM - EP04.08.16
Fully Printed Carbon Nanotube Network Thin-Film Transistor Based Gas Sensors on Flexible Substrates
Satish Kumar1,Diego Vaca1,Jialuo Chen1,Woonhong Yeo1
Georgia Institute of Technology1Show Abstract
Aerosol jet printing (AJP) has been proven to be capable of printing microelectronic devices and relevant circuits with relatively high precision, repeatability, and scalability using a wide range of materials because it can handle inks viscosities in the range of 1-1000 Cp. In this work, fully-printed thin film transistors (TFTs) have been fabricated on flexible substrates. Metallic, semiconducting, and dielectric materials such as silver, single walled carbon nanotubes (SWCNTs), and xdi-dcs, have been printed successively with high quality using AJP. In this fabrication process, CNT network is printed in the channel region of TFTs as a semiconducting CNT solution. Highly uniform CNT network film is achieved by performing a multiple layer-by-layer deposition method. The printing of thin layer of xdi-dcs as gate-dielectric is realized by diluting with n-butanol solvent and plasma treatment for better surface wetting during printing. Fully printed CNT-TFTs showed a very stable performance with on/off current ratio as high as ~105, mobility around 5 cm2V-1s-1, negligible hysteresis, and good uniformity. More importantly, this device can be operated using bias voltages as small as 5V, which is much smaller than the previously reported values because of the improvement in both CNT network and dielectric layer (thinner). CNT-TFTs were used as gas sensors to detect NO2 and NH3 and their performance was compared with two-electrode CNT gas sensors. Our printing method has potential to further reduce the bias voltages for operation. This improvement is key for the application of printed microelectronics and CNT-TFTs based circuits on flexible substrates because of the relatively lower voltage operation and power dissipation.
5:00 PM - EP04.08.17
Water Permeable Sticky Patch with Serpentine Patterns for Detection of Electrophysiological Signals
Hyeokju Chae1,Sunkook Kim1,Srinivas Gandla1,Muhammad Naqi1
Sungkyunkwan University1Show Abstract
Skin patch based stretchable sensors for Physiological monitoring have drawn great attention due to their significant finding in many human-machine interaction applications. As of now, stretchable serpentine structures supported by biocompatible elastomeric substrates have been widely examined for conformable and strain relief stretchable electrophysiological sensors. However, the substrates are not breathable, which is crucial for next-generation wearable electronics. Additionally, the fabrication of these sensors mostly relies on conventional lithographic techniques followed by etching processes that are complex, expensive and entail multiple steps, which greatly impede the realization of low-cost and scalable manufacture electronics. The development of flexible structures, stretchable materials and novel processing techniques, can enable a viable solution for these electronics.
In this, we present a novel low-cost and rapid “cut and transfer” method to fabricate electrophysiological sensor using a benchtop programmable mechanical cutter plotter. Furthermore, the sweat dependent sensor performances have been avoided by introducing a water permeable patch possessing filamentary serpentine patterns that are stretchable, sticky, and conformable. Moreover, the sensors maintain the sensing performances irrespective to the deformations that are imposed. Overall, the fabrication method offers the chance of developing a new format of stretchable and flexible electronics.
5:00 PM - EP04.08.18
Influence of Bulk/Interface Anomalies Upon Resistive Switching in Dual Ion Beam Sputtered ZnO Based Memristive Devices
Amitesh Kumar1,Mangal Das1,Brajendra Sengar1,Vivek Garg1,. Aaryashree1,Sanjay Kumar1,Abhinav Kranti1,Shaibal Mukherjee1
Indian Institute of Technology Indore1Show Abstract
In this work, we report the effect of interface anomalies such as disorder-induced interface states, Schottky barrier formation/dissolution for a resistive switch or memristor. Distribution of bulk defects with applied bias governs switching and also attributes to formation/dissolution of interfacial oxide. The present work considerably contributes to further understand the conduction mechanisms of a wide range of resistive switches.
Al/ZnO/Al (AZA) device depicts forming-free bipolar resistive switching with memristive behavior . Bipolar against Unipolar resistive switching  is attributed to formation/dissolution of Schottky barrier. To further understand the effect of various other interface anomalies, we performed CV characterization of device in HRS and LRS states for different frequencies. As we can observe from Fig. 1(a), we may observe higher capacitance at a lower frequency which denote the presence of trap charges in bulk as at lower frequencies trap charges also make significant contribution to total capacitance. Further, we can also observe from CV characteristics of device for device in HRS and LRS at 1 MHz in Fig. 1(b) that for device in LRS and near SET voltage, the thickness of the oxide barrier reduces to minimum which leads to set device into LRS and hence total capacitance increases to maximum. As we vary the voltage and it comes near RESET voltage, the thickness of the oxide barrier will increase to its maximum and device will set into HRS and hence total capacitance will decrease to minimum. Fig. 2 (a) shows the I-V characteristics of AZA device. The device AZA shows excellent repeatibility for 250 set/reset cycles (Fig. 2(b)).
We believe our work could play a crucial role in further understanding of conduction mechanism of resistive switching devices in the future.
Acknowledgement: Authors are thankful to Sophisticated Instrument Centre (SIC) of IIT Indore for DIBS facility. Mangal Das and Gaurav Siddarth would like to thank Ministry of Electronics and Information Technology (MeitY), Government of India, for providing fellowship under Visvesvaraya PhD Scheme for Electronics and Information Technology (IT). Amitesh Kumar would like to thank Council of Scientific and Industrial Research (CSIR) for providing fellowship. Prof. Shaibal Mukherjee is thankful to MeitY for the Young Faculty Research Fellowship (YFRF) under the Visvesvaraya PhD Scheme for Electronics and IT.
5:00 PM - EP04.08.19
Impact of Metal/Semiconductor Junctions in the Resistive Switching of Yttria Based Memristive System
Amitesh Kumar1,Mangal Das1,Sanjay Kumar1,Biswajit Mandal1,Pawan Kumar1,Shaibal Mukherjee1
Indian Institute of Technology1Show Abstract
In this report, we study factors that dominate resistive switching (RS) in yttria (Y2O3) based memristive devices. Our experiment consists of two significant steps. First, we have studied the effect of crystallinity to find out the conducive state for RS in yttria film.
Subsequently, in the second step, the effect of metal/semiconductor interface on the RS behavior of yttria has been investigated via changing bottom electrodes (BE) in Al/Y2O3/BE-type device structure. We have fabricated five different types of devices; these are N1, N2, N3, P3, A3 by varying deposition temperature and BE material. Deposition temperature for N1, N2 and rest of the devices (N3, P3, A3) are 100, 200, and 300 oC respectively. Bottom electrodes, which are used in devices, are n-Si (N1, N2, N3), p-Si (P3) and Al (A3).
Dual ion beam system (DIBS) system is deployed to deposit yttria (~100 nm) on low-resistive Si substrate (BE) for devices N1 (100 °C), N2 (200 °C), N3 (300 °C), and P3 (300 °C). For device A3 (300 °C), a 250 nm of the SiO2 layer is deposited upon p-Si. Further, a 150 nm layer of Al (BE) is sputtered on SiO2. A 100 nm thick yttria layer (switching layer) is deposited on all substrates, using commercially-available sintered yttria target.
In the First phase of our study, To understand the effect of crystalline properties on the RS of yttria film in all these devices (N1, N2, and N3) X-ray diffraction (XRD) is performed. N1 and N2 films show high intensity (highly crystalline) peak near 2θ = 28.8° (m111) corresponding to monoclinic phase along with another low-intensity peak around 59.4° (c 631) corresponding to cubic phase. To study the current-voltage (I-V) characteristics, triangular voltage waveforms with peak voltages of -5 to 5 V is applied to devices N1, N2, and N3. The absence of resistive switching in N1 and N2 can be attributed to higher crystallinity (as compared to N3) of yttria thin films which are counterproductive for the realization of reliable resistive switching devices. Contrary to highly ordered and crystalline structure (N1, N2), amorphous films (N3) do not have long-range order and contain defects which may be a more favorable condition for RS.
In the second phase of our study, three devices N3, P3, and A3 are used to further understand the effect of the interface on the behavior of yttria-based RRAM. Devices P3 and A3 are fabricated under the same condition as N3 but with different BE. N3, P3, and A3 have n-Si, p-Si and Al as their bottom electrode, respectively. It is important to note that N3 has one Schottky interface between Al and Y2O3, while A3 has two Schottky interfaces between Al and Y2O3. P3 shows diode-like characteristics after forming, which turns out to be useless for RS. Diode-like characteristics of P3 may be attributed to p-n heterojunction between p-Si and n-yttria. Device A3, peak voltages of -2 to 2 V shows bipolar switching which may be originated due to the occurrence of Schottky behavior at both interfaces. Comparison between N3 (Unipolar) and A3 (Bipolar) shows that the transformation from unipolar to bipolar RS can be realized via moving from single Schottky interface (N3) to both-sided Schottky interfaces (A3) structure. It can be observed that by merely changing BE from n-Si (N3) to p-Si (P3) or Al (A3), the shape and slope of I-V characteristics change significantly.
Endurance measurements do not show much degradation even after ~29000 switching cycles for the A3 memory cell. The resistances for HRS and LRS states are measured using a read voltage of 0.1 V, to confirm the non-volatility of resistance states. Our device (A3) shows excellent retention as there is no significant switching degradation even after a duration of 1×105 s.