Stretchable Silicone Research Articles

Stretchable and Elastic Triboelectric Nanogenerator with Liquid-Metal Grid-Patterned Single Electrode for Wearable Energy-Harvesting Devices.

Triboelectric nanogenerators (TENGs) have garnered significant attention as efficient energy-harvesting systems for sustainable energy sources in the field of self-powered wearable devices. Various conductive materials are used to build wearable devices, among which, gallium-based liquid metal (LM) is a preferred electrode owing to its fluidity and metallic conductivity even when strained. In this study, a stretchable, elastic, and wearable triboelectric nanogenerator is designed using a single electrode fabricated by embedding LM grid patterns into a stretchable silicone substrate through a two-step spray-coating process. Contrary to conventional double-electrode TENG that is challenging to integrate to human body, the LM grid-patterned single-electrode TENG (LMG-SETENG) has a simplified design and provides more flexibility. The LMG-SETENG can generate voltages of up to 100V via triboelectrification upon contact with the human body, even under various degrees of strain, owing to the fluidity of the LM electrode. The generated energy can be utilized as a sustainable energy source to power various small appliances. Moreover, the proposed LMG-SETENG can be utilized in soft robotics, electronic skin, and healthcare devices.

Elastic Tactile Sensor Glove for Dexterous Teaching by Demonstration.

We present a thin and elastic tactile sensor glove for teaching dexterous manipulation tasks to robots through human demonstration. The entire glove, including the sensor cells, base layer, and electrical connections, is made from soft and stretchable silicone rubber, adapting to deformations under bending and contact while preserving human dexterity. We develop a glove design with five fingers and a palm sensor, revise material formulations for reduced thickness, faster processing and lower cost, adapt manufacturing processes for reduced layer thickness, and design readout electronics for improved sensitivity and battery operation. We further address integration with a multi-camera system and motion reconstruction, wireless communication, and data processing to obtain multimodal reconstructions of human manipulation skills.

Graphite- and Graphene-Based Polymer Nanocomposites for Flexible Sensors and Actuators in Health Care and Soft Robotics Applications

Flexible sensors and actuators have broad applications in the fields of wearable electronics for health, sports, functional textiles, robotics and cobot applications. Graphene-or graphite-based polymer nanocomposites are promising materials for the development of soft sensors and actuators. This study investigates strain sensing properties of silicon rubber with various graphene filler concentrations (8wt%-12wt%). Current-voltage characteristics have been measured under various strains. We obtain that the sensor’s electrical resistance, for a given voltage, can be approximated by a linear fit of the logarithmic resistance as function of the extension ratio of the sensor. The obtained mechanically induced logarithmic resistor behavior of the polymer nanocomposite is highly promising for the development of electronic sensing and control. Furthermore, thin film graphite layers were investigated on highly stretchable silicone membranes.

Printable, adhesive, and self-healing dry epidermal electrodes based on PEDOT:PSS and polyurethane diol

Printable, self-healing, stretchable, and conductive materials have tremendous potential for the fabrication of advanced electronic devices. Poly(3,4-ethylenedioxithiopene) doped with polystyrene sulfonate (PEDOT:PSS) has been the focus of extensive research due to its tunable electrical and mechanical properties. Owing to its solution-processability and self-healing ability, PEDOT:PSS is an excellent candidate for developing printable inks. In this study, we developed printable, stretchable, dry, lightly adhesive, and self-healing materials for biomedical applications. Polyurethane diol (PUD), polyethylene glycol, and sorbitol were investigated as additives for PEDOT:PSS. In this study, we identified an optimal printable mixture obtained by adding PUD to PEDOT:PSS, which improved both the mechanical and electrical properties. PUD/PEDOT:PSS free-standing films with optimized composition showed a conductivity of approximately 30 S cm−1, stretchability of 30%, and Young’s modulus of 15 MPa. A low resistance change (<20%) was achieved when the strain was increased to 30%. Excellent electrical stability under cyclic mechanical strain, biocompatibility, and 100% electrical self-healing were also observed. The potential biomedical applications of this mixture were demonstrated by fabricating a printed epidermal electrode on a stretchable silicone substrate. The PUD/PEDOT:PSS electrodes displayed a skin-electrode impedance similar to commercially available ones, and successfully captured physiological signals. This study contributes to the development of improved customization and enhanced mechanical durability of soft electronic materials.

Packaged Elastomeric Optical Fiber Sensors for Healthcare Monitoring and Human‐Machine Interaction

Elastomeric polymer optical fiber (EPOF) enabled flexible optical sensors have been attracting intensive research interest due to their excellent stretchability, sensing performance, and anti‐electromagnetic interference. However, a soft EPOF without effective protection tends to be damaged, which may undermine its long‐term stability, and thus, limit its practical applications. In this work, a strategy for packaging a section of EPOF is proposed, a light‐emitting diode (LED), and a photodiode (PD) into a stretchable and flexible silicone patch for both healthcare and human‐machine interaction applications. The silicone package not only protects the EPOF from environmental disturbance, providing a compact sensing approach for conformal attachment on flexible or curved surfaces but also offers an effective manner to adjust the sensitivity and working range of the EPOF sensors. Typically, an EPOF sensor is sensitive to pressure, strain, or bending. As a proof‐of‐concept demonstration, a laryngoscope equipped with a packaged EPOF sensor that can bear a pressure of 60 MPa is developed. It can effectively avoid teeth damage by real‐time monitoring the contact force during the operation. Furthermore, a data glove with five packaged EPOF sensors for hand gesture recognition and remote control of a micro‐vehicle is fabricated.

Multi-scale-filler reinforcement strategy enabled stretchable silicone elastomer with synergistically enhanced thermal conductivity and mechanical strength

As soft electronic devices and robotics advance towards high power density and miniaturization, integrating superior thermal and mechanical properties has become a big challenge for functional elastomers. Herein, a multi-scale-filler reinforcement strategy was reported for constructing stretchable, mechanically strengthened and thermally conductive silicone elastomers. The multi-scaled Al2O3/vinyl and methyl co-modified fumed SiO2 (VM-SiO2) co-filled liquid silicone rubber composite exhibited a high thermal conductivity of 1.25 W m−1 K−1 at ∼55 vol% filler loading, 555.7% higher than that of the matrix. The finite element simulation demonstrated that the “bridging” effect of small-scaled fillers between gaps of large-scaled spheres in the matrix lowered the interfacial thermal resistance. Meanwhile, stemming from the strong interfacial interactions between the VM-SiO2 and matrix, up to 12.4 times higher elongation at break (7.17%) and 317.7% increase of tensile strength (3.00 MPa) were reached. We further demonstrated the potential application of the multi-scaled Al2O3/VM-SiO2 co-filled LSR composites for thermal management materials.

A late-model electrode structure for intrinsically flexible stretchable supercapacitor

In this work, a late-model intrinsically flexible stretchable supercapacitor (IFSS) was proposed, using liquid metal with excellent electron transfer capability as flexible collector, combined with stretchable silicone and well-designed kirigami MXene electrode based on laser cutting process. Systematic mechanical and electrochemical characterization suggested that the steady electrochemical output can still be maintained when the assembled device undergoes different degrees of stretching range from 0 to 100% even under bending or twisting. These results demonstrate that an effective integration of printed liquid metal and silicone with excellent flexibility and stretchability provides a potential avenue for the development of intrinsically stretchable devices.

Highly stretchable and sensitive silicone composites with positive piezoconductivity using nickel powder and ionic liquid.

Conductive rubber composites are mixtures of stretchable rubber and conductive materials. They can achieve conductivity and high elasticity and are used in soft robots and wearable devices. In general, these composites exhibit high electrical resistance owing to their bonds between the fillers breaking during elongation. However, there are several types of composite materials that decrease resistance by increasing contact between the conductive materials during elongation through optimization of the shape and size of the filler. These composite materials can rapidly decrease the resistance and are expected to be applicable to switch in electric circuits and sensors. However, to use such composite materials in circuits, the electrical resistance at the time of resistance reduction must be sufficiently low to not affect the electric circuit. To achieve this, a considerable amount of filler must be mixed; however, this reduces the elasticity of the composite. Simultaneously achieving elasticity of the composite and a sufficient decrease in the resistance is challenging. This study developed a conductive rubber composite gel by mixing silicone rubber, ionic liquid, and metal filler. Consequently, the composite achieved an elongation rate of over six times and a decrease in the resistance of less than 1/105. In addition, this composite material was used as a switch circuit wherein an electric circuit is turned on and off according to elongation through a connection to a DC power source.

TRPC1 channels underlie stretch-modulated sarcoplasmic reticulum calcium leak in cardiomyocytes.

Transient receptor potential canonical 1 (TRPC1) channels are Ca2+-permeable ion channels expressed in cardiomyocytes. An involvement of TRPC1 channels in cardiac diseases is widely established. However, the physiological role of TRPC1 channels and the mechanisms through which they contribute to disease development are still under investigation. Our prior work suggested that TRPC1 forms Ca2+ leak channels located in the sarcoplasmic reticulum (SR) membrane. Prior studies suggested that TRPC1 channels in the cell membrane are mechanosensitive, but this was not yet investigated in cardiomyocytes or for SR localized TRPC1 channels. We applied adenoviral transfection to overexpress or suppress TRPC1 expression in neonatal rat ventricular myocytes (NRVMs). Transfections were evaluated with RT-qPCR, western blot, and fluorescent imaging. Single-molecule localization microscopy revealed high colocalization of exogenously expressed TRPC1 and the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA2). To test our hypothesis that TRPC1 channels contribute to mechanosensitive Ca2+ SR leak, we directly measured SR Ca2+ concentration ([Ca2+]SR) using adenoviral transfection with a novel ratiometric genetically encoded SR-targeting Ca2+ sensor. We performed fluorescence imaging to quantitatively assess [Ca2+]SR and leak through TRPC1 channels of NRVMs cultured on stretchable silicone membranes. [Ca2+]SR was increased in cells with suppressed TRPC1 expression vs. control and Transient receptor potential canonical 1-overexpressing cells. We also detected a significant reduction in [Ca2+]SR in cells with Transient receptor potential canonical 1 overexpression when 10% uniaxial stretch was applied. These findings indicate that TRPC1 channels underlie the mechanosensitive modulation of [Ca2+]SR. Our findings are critical for understanding the physiological role of TRPC1 channels and support the development of pharmacological therapies for cardiac diseases.

Extremely Soft, Stretchable, and Self-Adhesive Silicone Conductive Elastomer Composites Enabled by a Molecular Lubricating Effect

Softness, adhesion, stretchability, and fast recovery from large deformations are essential properties for conductive elastomers that play an important role in the development of high-performance soft electronics. However, it remains an ongoing challenge to obtain conductive elastomers that combine these properties. We have fabricated a super soft (Young's modulus 2.3-12 kPa), highly stretchable (up to 1500% strain), and underwater adhesive silicone conductive elastomer composite (SF-C-PDMS) by incorporating dimethyl silicone oil as a lubricating agent in a cross-linked molecular network. The resultant SF-C-PDMS not only exhibits superior softness but also can readily recover after a strain of 1000%. The initial resistance only decreases by 8% after 100000 cycles of tensile fatigue test (100% strain, 0.5 Hz, 15 mm/s). This multifunctional silicone conductive elastomer composite is obtained in a one-step preparation at room temperature using commercially available materials. Moreover, we illustrate the capabilities of this composite in motion sensing.

Invariable resistance of conductive nanocomposite over 30% strain.

The dependence of the electrical resistance on materials’ geometry determines the performance of conductive nanocomposites. Here, we report the invariable resistance of a conductive nanocomposite over 30% strain. This is enabled by the in situ–generated hierarchically structured silver nanosatellite particles, realizing a short interparticle distance (4.37 nm) in a stretchable silicone rubber matrix. Furthermore, the barrier height is tuned to be negligible by matching the electron affinity of silicone rubber to the work function of silver. The stretching results in the electron flow without additional scattering in the silicone rubber matrix. The transport is changed to quantum tunneling if the barrier height is gradually increased by using different matrix polymers with smaller electron affinities, such as ethyl vinyl acetates and thermoplastic polyurethane. The tunneling current decreases with increasing strain, which is accurately described by the Simmons approximation theory. The tunable transport in nanocomposites provides an advancement in the design of stretchable conductors.

Fabrication, characterization and modelling of the fabric electrode-based highly stretchable capacitive strain sensor

Highly stretchable and capacitance-based strain sensors were fabricated utilizing highly stretchable silicone elastomer as dielectric film sandwiched between the conducting and stretchable cloth-based top and bottom electrodes. Electromechanical characterizations for the proposed sensor after multiple and reversible loading and unloading cycles up to 150% applied strain were conducted followed by the modelling of the sensing performance. It has been shown that the sensor exhibits reproducible electromechanical performance under multiple reversible stretching and relaxation cycles. There was a very small deviation (< 10%) in the observed values of the capacitance upon the applied strain of 150% up to the 1000 loading and unloading cycles. Tensile measurement for the sensor under various applied strains, not only exhibits almost constant force indicating negligible Mullins effect and negligible time-dependent changes at a fixed strain up to 100% suggesting that viscoelastic losses can also be neglected. From uniaxial tensile experiments, constitutive model of the dielectric elastomer sensor was validated. For the fitting of the experimental data obtained from the uniaxial tensile measurement, a variety of strain energy formulations have been compared for predicting the constitutive behaviour of sensor using the Mooney– Rivlin, Neo-Hookean, Yeoh and Ogden models, where 2nd term of the Ogden model was found to explain the experimental tensile data most accurately. To explain some observed losses at 150% of applied strain, the Prony series-based time dependent model was used for predicting stress at any value of time and it was found that experimental data was well fit with 4th order of Prony series.

Soft silicone‐based neural interface to modulate bladder function: chronic studies in awake behaving cats

Direct bladder wall stimulation has been attempted for decades to restore bladder function in people with spinal cord injury and other voiding dysfunctions. However, these efforts were limited by co‐activation of the urethra, legs and other pelvic organs at stimulus intensities that evoked bladder contractions. Neural interfaces for the detrusor muscle itself face several challenges due to its structure and the volume changes it undergoes in normal function. We designed a stretchable silicone net that can be placed around the bladder body to anchor a soft electrode array that interfaces directly with the base of bladder to generate bladder contractions.We created implantable versions of the electrode nets and tested them in chronic experiments. We implanted 5 healthy cats (4 females, 1 male) and tested them with and without anesthesia for 2‐3 months. A pressure catheter was implanted into the bladder to control volume and measure pressure.Bladder wall stimulation through various electrode configurations (monopolar, bipolar), temporal patterns (single electrode, sequential stimulation of multiple electrodes) and stimulus intensities were able to generate complete bladder emptying. In behaving cats, bladder wall stimulation at many different stimulus intensities elicited efficient voiding at physiological bladder pressures (med: 41, IQR: 27‐60 cmH2O). However, there was little relationship (r&lt;0.28) between stimulation amplitude and bladder pressure itself. Rather, stimulation amplitude had an effect on the time between stimulation onset and bladder contraction onset. At low stimulation amplitudes, there was a longer time between stimulation onset and voiding (med: 35s, IQR: 13.5‐68.2s) than occurred at higher amplitudes, which evoked more rapid voiding (med: 23s, IQR: 10‐45s). This contrasts with anesthetized tests, where stimulation at 3 ± 2 mA evoked increases in bladder pressure up to 35 cmH2O. These results suggest that stimulation of the bladder wall recruits sensory pathways leading to an urge to void, but that can be suppressed by the animals. For most of the duration of the implants, stimulation evoked functional voiding with consistent electrical stimulation thresholds. However, after several months there was a failure of an implanted bond that limited the overall duration of the implants.Chronic experiments demonstrate that these electrode nets can be used as a neural interface to generate or initiate comfortable, complete bladder emptying in awake behaving cats. Future efforts include understanding the effects of evoked sensations on voiding and improving the integrity of the bonding between the flexible conductors and the lead wire.

Highly Stretchable Silicone Elastomer Applied in Soft Actuators.

In this work, a highly stretchable silicone elastomer is incorporated into dielectric elastomer actuators (DEAs) in order to decrease operation voltages by applying high prestretches. Results show that the fabricated DEAs (5 mm diameter circle active region) can be actuated to a lateral strain of 30% at 4.3kV for a 122µm thick prestretched film, and to a lateral strain of 2.5% at only 250V for a 6.9µm thick prestretched film. Due to the significant viscous component of the silicone elastomer, the DEAs respond more slowly (2-14 s to reach 90% of full strain) and show greater strain changes over time compared to conventional silicone-based DEAs. While this inherent viscosity is not universally favorable, it can be advantageous in applications where actuator damping is desirable. The studied DEAs' mean lifetimes under DC actuation range significantly-from 0.9 h to more than 123.0 h-depending mainly on initial electrical fields (17.8-36.3V µm-1 ). For instance, DEAs with a 150µm initial thickness and a prestretch ratio of 3 show 1.4-2.6% lateral strains for the mean lifetime (123.0 h) at only 300V. Given the strains achieved at low voltage, such DEAs show promise for applications that do not require fast response speeds.

One reaction to make highly stretchable or extremely soft silicone elastomers from easily available materials

Highly stretchable, soft silicone elastomers are of great interest for the fabrication of stretchable, soft devices. However, there is a lack of available chemistries capable of efficiently preparing silicone elastomers with superior stretchability and softness. Here we show an easy curing reaction to prepare silicone elastomers, in which a platinum-catalyzed reaction of telechelic/multi-hydrosilane (Si–H) functional polydimethylsiloxane (PDMS) in the presence of oxygen and water leads to slow crosslinking. This curing chemistry allows versatile tailoring of elastomer properties, which exceed their intrinsic limitations. Specifically, both highly stretchable silicone elastomers (maximum strain of 2800%) and extremely soft silicone elastomers (lowest shear modulus of 1.2 kPa) are prepared by creating highly entangled elastomers and bottle-brush elastomers from commercial precursor polymers, respectively.

Preparation of carbon black/silicone rubber composites with large-area-homogeneous-low electrical-resistance used as electroplating matrix.

Conductive carbon black (CCB) is an important filler in stretchable conductive silicone rubber (CSR) composites. However, due to the active oxygen-containing groups on CCB, introducing it into silicone rubber (SR) may cause SR to not completely cure. Surface modification of CCB may ease the problem but at the cost of reducing the electrical conductivity of pristine CCB. In this work, the curing and crosslinking performance of CCB/SR is detected in detail, the hydroxyl groups (-OH) carried by CCB can react with the silicon-hydrogen group (Si-H) with the existence of Pt catalyst, causing insufficiency of the hydrosilylation reaction thus hindering the solidifying process of silicon rubber. To take advantage of this reaction, more hydrogen silicone oil (PHMS) possessing silicon-hydrogen bonds is introduced into the system to improve the curing degree as well as fix the CCB in the crosslinked network. Due to the lock-in effect of CCB, the resistance of CSR samples is stable after several hundred bending cycles, and the composite's tensile strength is three times that of the pure SR samples. Besides, the size of the composites can expand to dozens of centimeters or even a few meters with uniform electric conductivity. This composite has resistance as low as 10.20 Ω and is suitable to make electroplating mold, and a rapid plating rate of 2.4 mm/24 h can be achieved. Meanwhile, the overall properties make this CSR composite have potential applications in mold manufacture, flexible electronics, and other related fields.

Direct ink writing of tough, stretchable silicone composites.

In this work, we report 3D printable soft composites that are simultaneously stretchable and tough. The matrix of the composite consists of polydimethylsiloxane containing octuple hydrogen bonding sites, resulting in a material significantly tougher than conventional polydimethylsiloxane. Short glass fibers are also added to the material. Prior to solvent evaporation, the material possesses a viscoelastic yield stress making it suitable for printing via direct ink writing. We mechanically characterize the printed composite, including fracture tests. We observe robust crack deflection and delay of catastrophic failure, leading to measured toughness values up to 2 00 000 J m-2 for specimens with 5 vol% glass fibers. The printed composites exhibit an unprecedented combination of stiffness, stretchability, and toughness.

Biocompatible, Electroconductive, and Highly Stretchable Hybrid Silicone Composites Based on Few-Layer Graphene and CNTs.

In this paper, we report a cost-effective and scalable approach to produce highly homogeneous graphene and CNT-based silicone composites with potential applications in diverse fields of research, including biosensors and wearable electronics. This approach includes the fabrication of hybrid fillers based on few-layer graphene and CNTs by water solution blending and manufacturing of graphene/CNT/PDMS composites through calendering in a three-roll mill. The influence of processing parameters, the graphene/CNT ratio, and hybrid filler loading was thoroughly investigated, and the optimal parameters for producing hybrid composites with superior electrical and mechanical properties were found. It was also confirmed that the graphene/CNT hybrid system exhibits a synergistic effect of non-covalent interactions between graphene sheets and CNT sidewalls. This synergistic effect prevents the aggregation of graphene sheets, facilitates the dispersion of graphene and CNTs in the silicone matrix, and contributes to the superior properties of hybrid composites compared to composites with either of these fillers alone.

Peel of elastomers of various thicknesses and widths

We peel a highly stretchable silicone (Ecoflex), thickness H and width B, sandwiched between two inextensible films. When the peel front advances steadily in the elastomer, the peel force reaches a plateau Fss, and the ratio 2Fss/B is commonly reported as the toughness of the elastomer. Our data show that this “peel toughness” is not always a material constant, but can depend on the thickness H and width B. We interpret the data in terms of two fundamental ideas in fracture mechanics. First, as the ratio B/H increases, the deformation in the elastomer changes from the plane stress to the plane strain conditions, so that the stress state ahead the peel front changes from biaxial to triaxial tension. We show that the elastomer under triaxial tension reaches higher stress and damages more than the elastomer under biaxial tension. Second, when the inelastic zone around a peel front is not much smaller than H, peel is under large-scale inelasticity conditions, and the material ahead the peel front deforms in nearly a homogeneous state. We show our data measured using various thicknesses collapse into a single curve on the plane with axes Γ∕H and B/H.

Manufacturing and characterization of novel silicone/natural fabric/graphene‐based functional composites for human body motion sensing

AbstractThis study focuses on the development of strain‐sensors utilizing biodegradable natural fabrics (cotton, flax, and wool) and graphene nanoplatelets (GN) through novel coating techniques. A systematic study on improving the electrical conductivity of natural fiber fabrics to be used as effecting sensing stimuli was conducted by varying the proportions of graphene. To further enhance the conductivity, GN was hybridized with conducting polymers, such as polypyrrole (PPy), polyaniline (PANI), and with carbon black (CB). The electrical conductivity, morphological, and mechanical characterizations were carried composites for detailed comparative analysis. The conductive composites were incorporated into flexible and stretchable silicone elastomer (Ecoflex) to be used as strain sensors. Electromechanical characterizations were carried out on the composite sensors and their performance on different human body motions, including finger, wrist, elbow, and knee movements were assessed. The results showed that the addition of hybrids, such as PPy increased the electrical conductivity in fabrics (for instance, cotton with a conductivity of 1.7 S.cm−1). However, the addition of PANI tends to have a negative effect on the conductivity due to poor ability to deposit on the fiber surface. It was observed that the thermal stability was improved with the addition of conductive particles which gives rise to excellent sensitivity and durability of the composite sensors.