Symposium SM3 : Soft Materials for Compliant and Bioinspired Electronics

Electrophysiological recordings of neuronal activity are necessary for medical purposes. Measuring neurological rhythm on the scalp (electroencephalography, EEG) allows in particular to localize and study epileptic sources. It is also used as a key tool for Brain-Computer Interfaces to control prosthesis. However, actual technologies used in cutaneous recordings of brain activity present critical limitations such as instable contact with the skin causing high impedance at the sensor interface, evaporation of the gel and low signal to noise ratio (SNR). Active devices like transistors have shown the advantage of providing increased SNR due to local amplification compared to passive electrodes. Organic ElectroChemical Transistors (OECTs) have already demonstrated their enhanced capabilities to transduce low amplitude biological signals and their great compatibility with biological interfaces. Past works showed the success of the OECT to record electrophysiological signals. By tuning the geometry of the transistor’s channel we achieved to low-pass the signal and eliminate high-frequency noise. In this work, we integrated the OECT on a fully flexible circuit to use it as a miniaturized and conformable cutaneous sensor for EEG recordings. In this configuration the brain activity is directly gating the OECT and so modulation of the channel current matches with neuronal activity. The contact with the skin is assisted with a biocompatible ionic liquid gel which is also used to enhance transistor’s doping/de-doping in long-term recordings. To compare the performance of our active sensors the signals from medical electrodes and OECTs were recorded in similar configurations. We recorded a broad-range of brain waves using different clinical protocols and demonstrated the OECTs capabilities in in-situ signal amplification and filtering. This study shows the great potential of the OECTs in clinical EEG monitoring and promises to advance actual state-of-the-art in comprehension of the brain function.

The organic electrochemical transistor (OECT), capable of amplifying small electrical signals in an aqueous environment, is an ideal device to utilize in organic bioelectronic applications involving for example neural interfacing and diagnostics. Currently, most OECTs are fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene)-based suspensions such as PEDOT:PSS and are therefore operated in depletion mode giving rise to devices that are permanently on with non-optimal operational voltage.

With the aim to develop and utilize efficient accumulation mode OECT devices, we discuss here our recent results regarding the design, synthesis and performance of novel intrinsic semiconducting polymers. Covering key aspects such as ion and charge transport in the bulk semiconductor and operational voltage and stability of the materials and devices, we have elucidated important structure-property relationships. We illustrate the improvements this approach has afforded in the development of high performance accumulation mode OECT materials.

Various flexible and stretchable materials have been reported for use in electronic devices or biomedical applications. Technology monitoring neural signals for a long time has developed with flexible materials. But flexible dry electrode for electrical stimulation reaches the limit because high impedance of conductive polymer or weak mechanical property of thin metal layer. Here, we propose a novel method for fabricating flexible and stretchable electronic devices using a porous elastomeric substrate.
Pressurized steam was applied to an uncured polydimethylsiloxane (PDMS) layer for production of porous structure. An electroplated metal anchor had a key role in bonding porous structure of elastomers, and metal could be stably patterned and electroplated for practical uses. The proposed method solved most of problems using stretchable porous PDMS and metal electroplating on metallic porous surfaces. The method to construct porous PDMS by steam is unique and advantageous for the mass production of porous structure through a simple and cost-effective process. The proposed technology was applied to develop dry electrode and multi-channel microelectrodes that could be used as a long-term wearable bio-signal monitor and electrical stimulator.
The electrical conductivity of the metal patterns on the porous PDMS was robust under extensions (~30%) and after a durability test over 1,000 cycles. Although the electrical conductivity was deteriorated owing to fatigue by the repeated stretching motions, electrical disconnections were not observed. These results demonstrated that the porous electrodes maintained robust electrical conductance under stretching cycles. The electrode was also resistant to bending. Although the electrode was seriously bent along a sharp edge, electrical breakdown was not observed. The higher change in resistance based on the angle and thickness is clearly demonstrated. The metal layer is thin and deposited on the surface only. Therefore, for a larger bending angle, the thin metal layer expands much more. At the edge of the bend, the increase in bending angle causes a much larger extension of the thin metal layer. As the thickness increases, the metal layer extends much more. After all bending tests, the normalized resistance of the electrode regained its initial resistance, demonstrating the usability in foldable electronic devices.
In summary, biocompatible metal-patterned porous PDMS with high flexibility and stretchability were successfully fabricated as electrical stimulator using simple and cost-effective techniques. Biomedical applications require stable operation under a variety of conditions, and the materials must not induce skin or tissue incompatibility. Our method satisfied most of these requirements. The robustness of the metal electroplating on porous substrate is another crucial feature of our method, and it will be widely useful for neural electrodes. Such electrodes are useful in brain-to-machine interfaces.

In the healthcare industry, long-term monitoring of bioelectrical signals is one of the key issues with continuous and inconspicuous recording. The bioelectrical signals such as neural activity, heart rate, and muscle contraction as well as body motion are major detected biopotentials from human body. There are critical challenges to record the bioelectrical signals without noise and dermatitis due to the difficulties of conformal contact and cytotoxicity of skin contact materials. Commonly used wet and dry electrodes should be periodically replaced for continuous monitoring, because of contact dermatitis onto the attached sites and reduced signal quality by hydrogel evaporation.
In this study, we developed the soft conductive materials for skin-like patch-type electrodes and the innovative electrode systems for electrocardiogram (ECG) and electroencephalogram (EEG) recording. The outside of patch is composed of adhesive polydimethylsiloxane (PDMS), functioning self-adhesion on skin. We fabricated the inside of electrode with soft conductive materials of carbon nanotubes (CNT) as an electrical conductive nanocomposite in flexible and elastic elastomer matrix, PDMS, by a simple and cost-effective method with isopropyl alcohol and sonication. This CNT/PDMS materials provided high flexibility, elasticity and conductivity and also good biocompatibility.
The ECG patch-type electrode maintained conformal contact on skin and was stretched along with muscle extension and contraction, sustaining stable signal quality. And this ECG patch could be used several times by attaching and detaching. The recording site of the ECG patch was precordial because of specific ECG measurement of Lead I, II, and III. The feasibility test of the ECG patch showed good coherence of the signals between the conventional wet electrode (Ag/AgCl) and our ECG patch with over 0.95 value of Lead I, II, and III. The CNT/PDMS EEG sensor could be easily equipped on the eye glasses and provided for continuous recording system. In the recording site on the eye glasses, T3 and T4 of 10-20 system, we observed alpha rhythm wave in the range of 8 to 14 Hz and N100 auditory evoked potential with the distinct peak at 100 ms.
In this paper, we present epidermal bioelectrical signal sensors which can provide conformal contact on skin and long-term monitoring by using biocompatible conductive elastomer materials. We expect that our flexible, elastic and conductive CNT/PDMS materials can be applied for long-term, inconspicuous, and noiseless signal recording of the various biomedical devices.

The tactile sensor for the artificial skin should emulate features of touch feeling in the biological skin. It basically requires flexibility and stretchability to be worn and folded, and a high sensitivity in the pressure range of 100~100,000 Pa for the human perception. The tactile sensation is a biological recognition of shape, hardness, roughness, which are manifested through experiences obtained by the complex responses of the cutaneous mechanoreceptors reacting pressure and vibration generated by a touching surface. The purpose of the tactile sensor is to materialize the functions of mechanoreceptors with the sensor device. The force sensor has been generally regarded as an element for recognition of touching, and thus method for achieving the tactile sensor. However this approach is only available for the limited function of tactile recognition characterized with the static responses defining amounts and distributions of pressure. Texture recognition is another critical factor for the tactile sensation in which a dynamic consideration is involved.
To recognize the surface texture of touched materials, an accelerometer has been introduced as a sensor element to obtain identification of surface texture with detection of the vibrations on interacting surface. Unfortunately it is limited because of rigid nature. Moreover spatial resolution for texture recognition is far from the requirement of capability for the human skin which is determined by distribution density of the mechanoreceptors. It could be the ideal situation if the force sensor represents detecting capabilities of the surface texture as the dynamic responses beyond detecting of force strength. Indeed, force sensors were applied for an artificial finger of the robotic system to provide recognition of touch feeling including surface texture information obtained from sliding motion on the surface of object. However, these sensors suffer difficulties to represent human perception capability for the recognition of surface texture. Moreover, an integration issue is still critically remained for the surface texture recognition.
Here, we demonstrate bioinspired tactile sensor using single layer graphene (SLG) in the single sensor architecture to recognize the surface texture due to roughness of interacting surfaces. Incorporation of two-dimensional nature of SLG to the force sensor reveals that the resistance changes due to the local deformation induced at the local area of the SLG is distinguishable in the resistance of the entire sensor. With introduction of microstructures inspired by the human finger print, spatial resolution is easily achievable, and the surface texture is successfully defined through analysis of the fast Fourier transform. This work provides a simple method utilizing single sensor for surface texture recognition in the level of human sensation without the matrix architecture which requires high density integration technology with force and vibration sensor elements.

Electroencephalogram (EEG) is one of the most important method to diagnose diverse neurological diseases on the scalp. With the emerging advances of information and communication technology (ICT) and biomedical engineering, numerous interests about EEG-based applications are steadily rising in the area of brain computer interface (BCI) and ubiquitous (U)-healthcare. Continuous and insensible recording of EEG is highly required for applications. However, there are several problems in conventional EEG electrodes for unconscious and long-term continuous monitoring, including intrusiveness, poor biocompatibility and reducing signal quality due to conductive gel. Therefore, several research groups recently reported on dry electrodes by using diverse novel nanomaterials, such as carbon-nanotubes (CNTs), nanowires (NWs) and graphene. Nanocomposites which are combined of nanomaterials with polymers are actively studied and reported for flexible and stretchable electrodes. However, low conductivity, skin toxicity and unconscious recording for EEG are still challengeable.
In this paper, we developed highly conductive and flexible epidermal electrodes composed of CNTs, silver nanowires (AgNWs), and polydimethylsiloxane (PDMS) for continuous and insensible EEG recording to improve aforementioned problems. Previously, we developed biocompatible and conductive CNT/PDMS electrode for biomedical applications. However, EEG measurement requires extremely high conductivity to reduce contact impedance because the signal level is very small. Therefore, we added AgNWs to CNT/PDMS for improving conductivity and tested its electrical and mechanical performance by measuring the contact impedance and young’s modulus. The electrical properties of the CNT/AgNW/PDMS electrode are highly improved compare to our previous works, and the young’s modulus showed that the electrode is suitable for soft epidermis. Furthermore, the cytotoxicity test was conducted with HaCaT human keratinocyte cells for a week and showed little toxicity. The feasibility of the newly developed electrode was evaluated through two standard EEG paradigms including alpha rhythm detection, steady state visually evoked potential (SSVEP). In addition, the soft and flexible surface is perfect fitted to soft epidermis that it provides high signal to noise ratio and comfort insensible recording for patients.
All feasibility was successfully recorded with fabricated electrodes and good biocompatibility was also proved. These results indicated that the highly conductive and soft CNT/AgNW/PDMS-base electrodes can be used for insensible and continuous EEG recording and also it could be widely applied for clinical, U-healthcare and BCI fields.

A set of environmental problems caused by oil spills is now looming larger than ever before. A selective absorbent that absorbs (or rejects) one or more specific components from a multicomponent mixture of liquids is an exclusive cleanup method in small oil spills or in area that cannot be reached by skimmers, more often for removing final trace of oil. To be more effective in combating oil spill, a selective absorbent should have the Janus-like structure with which the absorbent is entitled to selective absorption ability. In detail, the Janus-like absorbent has one half of its surface (or volume) composed of hydrophilic (and oleophobic) groups and the other half composed of hydrophobic (and oleophilic) groups. We develop a selective absorbent through two physicochemical modifications on a polydimethylsiloxane (PDMS) sponge, thus achieving the Janus-like structure thereon. Our selective absorbent made of Janus-like PDMS sponge has incomparable, exceptional features as follows: high selectivity to oil and water using the Janus-like structure, selective absorption function on volume instead of on surface, high reusability enabling excellent recyclability, and low production cost. The fabrication of the selective absorbent begins with a PDMS sponge that is prepared by centrifuging a mixture of salt particles (having a diameter of 50 to 500 μm) and PDMS (having a mixing ratio of 10:1), followed by mixture crosslinking and salt dissolving with warm water. Next, exfoliated and fragmented graphite (EFG) particles are attached on the PDMS sponge to get superhydrophobicity/superoleophilicity thereon and a biocompatible surfactant, Silwet L-77, is added to the mixture of salt particles and PDMS, before the mixture crosslinking, to achieve superhydrophilicity/superoleophobicity thereon. The performance of the selective absorbent in capturing and separating oil from water depends on diverse factors such as surface geometry, surface physicochemistry, hydrostatic stability, etc. Here, we report an experimental effort in controlling the physicochemistry (i.e., hydrophobicity or hydrophilicity) of the Janus-like PDMS sponge through adjusting the size and amount of EFG particles and the concentration of Silwet L-77. The oil-absorbing behavior of the selective absorbent is also extensively discussed.

A conductive polymer matrix composite (c-PMC) is a promising material for fabricating smart sensors that can detect either large deformation or crack (or crack propagation). Our c-PMC is polydimethylsiloxane (PDMS) blended with exfoliated and fragmented graphite (EFG) particles where PDMS and EFG particles are used as a host elastomer and conductive fillers, respectively. The c-PMC has incomparable, exceptional features of controllability in electrical conductance and mechanical compliance, high compatibility to soft and hard surfaces, adjustability in viscosity before crosslinking, etc. Since the smart sensors are made of c-PMC that incorporates high electrical conductivity of graphite (1250 S/cm) into high mechanical deformability of PDMS (Young’s modulus of 750 kPa), the sensors are fabricated through airbrushing the solution with stencils and have the following noticeable benefits: adjustable measurement range and sensitivity of up to 50% (for large strain sensor) or less than 1% (for structural crack detection sensor), high conformation to flat and free-curved surfaces, and controllable sensing area per sensor. Here, we develop two types of smart sensors using c-PMC where one is a large strain sensor and the other is a structural crack detection sensor. The preparation of the c-PMC solution begins with the microwave-assisted graphite exfoliation, which expands the graphite by irradiating microwave at 700 W and 2.45 GHz. Then, the exfoliated graphite is ultrasonicated to get EFG particles. After chemical reduction by hydroiodic acid, the EFG particles are mixed with PDMS diluted with chloroform, thus making c-PMC solution with different EFG particle wt% of 0 to 40. The c-PMC solution is airbrushed with stencils at a pressure of 200 kPa and a working distance of 100 mm, followed by thermal drying on a hotplate, to fabricate two types of smart sensors. At first, we investigate the effect of EFG particle (or EFG/PDMS) wt% on the electrical conductance and mechanical compliance of c-PMC after crosslinking, thus experimentally showing the controllability in electrical conductance and mechanical compliance of c-PMC by adjusting EFG particle wt%. Next, two types of smart sensors are intensively explored to quantitatively characterize their performances including sensitivity, linearity, hysteresis, and temperature/humidity characteristics, etc. Extrapolation of this paper to other c-PMCs might help us to develop c-PMC-based smart sensors detecting physical properties, heretofore not possible, and also to fabricate the high-performance smart sensors in large quantities for a low price.

Stretchable interconnects can be made by embedding microcracked conductive gold films into soft elastomeric substrate such as polydimethylsiloxane (PDMS). Upon stretching, such interconnects demonstrate significant changes in resistance that can be utilized for specific applications, such as sensing pressure, normal contact force, shear force or bending of the sensor. Here we report on the first component of this study, which investigated the effect of bending on resistance with a focus on two main factors: the direction of the bending (i.e. tension or compression) and the strain, which is dependent on the distance of the film from the neutral plane and the bending radius. Resistive sensors were produced by embedding two parallel or perpendicular thin microcracked gold films in PDMS at the same distance from, but on opposite sides of, the neutral plane. The sensors were flexed over varying bending radii while constant voltage was supplied and the gold film’s resistance variation was monitored in real time. The same procedure was repeated after flipping the sensors, i.e. the gold conductor that was compressed earlier would now be stretched and vice versa. Experimental results demonstrated that the bending direction determines the direction of the change in resistance: the resistance of the interconnects increases when the gold film is on the stretched side of the neutral plane, decreases when the film is on the compressed side, and remains invariant when the film is on the neutral plane. When stretched, the magnitude of this change generally increases consistently with the bending radius, but when compressed the magnitude of the decrease in resistance is not as substantial and is less dependent of the bending radius.
The second component of this study will characterize the resistance change of the stretchable interconnects due to applied normal and shear forces while the sensor is wrapped around a curved surface. Embedding multiple interconnects in one sensor will allow the sensor to simultaneously detect said forces along with bending. Such sensors – soft, elastomeric, capable of concurrent detection of pressure, normal and shear stresses – can be adapted for numerous biomedical applications such as enhanced prosthetic tactile perception, plantar pressure sensor, etc. Additional understanding of the variation of the resistive behaviors of the interconnects will provide guidance on the fabrication of said sensors to meet the requirements for aforementioned applications.

Currently 4^th generation stretchable microelectrode arrays (sMEA) are used to record field potentials from neural tissue. This is accomplished by having an array of 28 microelectrodes which can be stretched reversibly due to their microcracked gold morphology. sMEA’s with a high density of recording cites require the microelectrodes adopt angled geometry. Effect of angles on mechanical and electrical properties was studied on single unencapsulated angled microcracked gold conductors. Structures were 2 cm in length and 100/75 µm in nominal width, with either sharp or round angle present at midpoint of 45^o, 90^o, 135^o and 180^o. Microcracked gold lines were produced by depositing a metal stack of 3 nm chromium and 25–100 nm of gold on a poly(dimethylsiloxane) substrate. We characterized the shadowing effects on the width and the defects of the conductors for different shadow mask orientations in a thermal evaporator. Additionally, the rupture strain, which is defined as the strain at electrical failure, was evaluated by applying cyclic strain of increasing magnitude. Mechanical and electrical model was developed that relates angle and the type of angle in the thin gold line to the rupture strain. The model will help facilitate 5^th generation of sMEAs by providing guidance for the design of higher density of recording cites.

On earth, ocean covers approximately 71% of the surface areas, with only 5% being explored by human activities. Exploring the abundant resources in the sea requires improvement on underwater vehicles, especially on their equipped sensors such as acoustic wave detectors and fluid velocity meters, with a much-improved reception to weak sound from farther distances and the capability of finding objects without revealing self-identities. We present hydrogels with a deformable network of Metal Nanoparticles (MNP) as Stimuli-Sensitive Supercapacitors (S³). Multiple merits make this hydrogel-based S³ unique and interesting, including perfect acoustic impedance match to saline water, ultra-high sensitivity to surface perturbations (1.0 nF/kPa; at least 2 orders of magnitude higher than state-of-the art), and unmatched potential in signal amplification once the S³ is coupled with a transistor.

Soft and flexible materials have had huge growth or development in the current era of smart & portable electronics. Softness of the material, easiness in processing and manufacturing, and enriched functionalities from microelectronic units all have promoted their vast growth and popularity throughout all ages of customers. Instead of focusing on the material issues along the human-computer interfaces, this current talk utilizes soft materials of hydrogels for its easiness in receiving a deformable network of metal nanoparticles. Once this superstructure is engineered at the skin depth of hydrogel, this bulk nanostructured material holds a high hope in underwater passive acoustic wave detections.

Integrating liquid metals with other low modulus materials, such as elastomers and gels, imparts high electrical conductivity into a composite without altering its mechanical properties. Combined, these materials form ‘softer than skin’ systems that are well-suited for forming conformal interfaces and ultra-compliant soft electronics. This work characterizes the behavior of a eutectic alloy of gallium and indium (75% Ga, 25% In, by weight, ‘EGaIn’), which remains a liquid at room temperature (M.P., 15.5 ^oC) and has low toxicity. These fluid metals are injectable into microfluidic systems, fibers, and capillary networks. Once injected, the metal remains in its place because of the adhesive nature of its thin native oxide. Reversibly moving EGaIn through microchannels has implications for reconfigurable electronic circuits, frequency tunable antennas, and other opto-fluidic technologies.

This work explores microfabrication methods to reversibly move small volumes of EGaIn through microchannels by preventing adhesion of the oxide. Pre-wetting the channels with an aqueous solution prior to injecting the metal establishes a ‘slip-layer’ of water between the metal and channel wall. Thereafter, an applied electric field (~10-20 V/m) establishes a gradient of surface tension along the liquid metal-electrolyte interface to actuate the liquid metal by continuous electrowetting. Optical microscopy, and electrochemical impedance spectroscopy are utilized to characterize the electrohydrodynanic behavior of EGaIn under various conditions (e.g., composition and pH of electrolyte, applied voltage, and presence of oxide). However, evaporation of water can cause the slip-layer to disappear and impede CEW. Therefore, this work also explores strategies to create coatings that prevent oxide adhesion inside microchannels.

To fabricate stretchable solar cells using crystalline inorganic materials, recent studies have adopted serpentine interconnects which can isolate the solar cells from the applied strain. However, to increase stretchability of the devices the areal density of serpentine interconnects should be increased. Moreover, complicated process for the fabrication of serpentine interconnects will lead to high fabrication cost. In this presentation, we fabricated stretchable GaAs single-junction solar cells using strain isolation pads embedded in elastomeric substrate. Strain isolation pads are embedded in PDMS(polydimethylsiloxane) stretchable substrate like stepping-stones by chemical bonding formed between PDMS and strain isolation pads. Owing to the difference of Young’s modulus between SU-8 pads and PDMS substrate, deformation only occurs in PDMS substrate while the elastomeric substrate embedded SU-8 layer is stretching. SU-8 pads are not fractured or deformed by the external force, whereas PDMS substrate is easily deformable during stretching. Because SU-8 pads are completely isolated from external strain, GaAs solar cells that were directly transfer printed onto SU-8 pads can be protected from the external stress. Though photovoltaic characteristics of transfer-printed GaAs solar cells are investigated up to ~40% strain, the properties of solar cells are not degraded. These results clearly showed that GaAs solar cells on the strain isolation pads are not damaged during the stretching of the PDMS substrate. Our strain isolated stretchable substrate not only can isolate the solar cells from the external stress, but also has an almost flat surface topography. Therefore, the elastomeric substrate embedded strain isolation pads will have wide applicability to many different electronic devices