Stretchable Electrodes Research Articles

A nerve-adhesive stretchable electrode for stable neural signal recording and stimulation

A nerve-adhesive stretchable electrode for stable neural signal recording and stimulation

Stretchable Energy Storage with Eutectic Gallium Indium Alloy

AbstractThe integration of electronics with the human body or wearables necessitates the evolution of energy storage devices capable of seamless adaptation to the conformability of the skin and textiles. This work focuses on developing an intrinsically stretchable electrode prepared by sedimenting the liquid metal particles in a conductive stretchable matrix. The liquid metal‐based electrode can be stretched to ≈900% strain, and its conductivity increases by extending to 250% and retaining its initial conductivity at 500% strain. Benefitting from these properties, the assembled all‐solid‐state energy storage device provides high stretchability of up to 150% strain and a capacity of 0.42 mAh cm−3 at a high coulombic efficiency of 90%. The charge storage mechanism is investigated by probing the electrode/electrolyte interface, uncovering the intricate gallium‐bis(trifluoromethane)sulfonimide (Ga‐TFSI) complexation during electrochemical cycling through in situ Raman spectroscopy, ex situ X‐ray photoelectron spectroscopy (XPS) analyses, and density functional theory (DFT) calculations. This work offers a promising avenue for the advancement of stretchable batteries.

A Stretchable Conductive Material with High Fatigue Resistance for Strain Sensors.

Intrinsically stretchable conductive materials based on elastic substrates and conductive components play important roles in biomedical applications, such as exercise rehabilitation monitoring and disease prediction. A persistent challenge is to combine high fatigue resistance with excellent mechanical properties in stretchable conductive materials. Herein, we present a stretchable conductive material with both good fatigue resistance and high tensile properties (∼3170%) based on poly(acrylic acid)-phytic acid-trehalose-polypyrrole (denoted as PPTP). The as-prepared PPTP hydrogel electrode showed no obvious cracking or delamination after 400 loading and unloading cycles and maintained good electrical signal transmission function after 1000 cycles. We further collected stable signals for human motion and handwriting using the stretchable hydrogel electrode as a strain sensor, demonstrating the potential application of the PPTP stretchable hydrogel electrode in biomedicine.

Biodegradable, Self‐Adhesive, Stretchable, Transparent, and Versatile Electronic Skins Based on Intrinsically Hydrophilic Poly(Caproactone‐Urethane) Elastomer

In biomedical sciences, there is a demand for electronic skins with highly sensitive tactile sensors that have applications in patient monitoring, human–machine interfaces, and on‐body sensors. Sensor fabrication requires high‐performance conductive surfaces that are transparent, breathable, flexible, and easy to fabricate. It is also preferable if the electrodes are easily processable as wastes, for example, are degradable. In this work, the design and fabrication of hydrophilic silanol/amine‐terminated poly(caprolactone‐urethane) (SA‐PCLU) elastomer‐based breathable, stretchable, and biodegradable electrodes are reported. Ag nanowires dispersed in water are sprayed onto the intrinsically hydrophilic electrospun SA‐PCLU that became embedded into the scaffold and formed conformal hydrophilic polyurethane‐based conductive networks (HPCN). The electrodes are used to fabricate capacitive, curvature, and strain sensors, all having monomaterial composition. In addition to displaying particularly good transparencies at low sheet resistances, stretchability, hydrophilicity, and tight and conformal bonding with the target surface, the electrodes also allow the evaporation of perspiration, making them suitable for epidermal sensors for long‐time use. The application of the HPCN electrodes in flexible electronics and bionic skin applications is demonstrated through gesture monitoring experiments and swelling sensors.

Nanosilica-Reinforced Poly(dimethylsiloxane) Stretchable Transparent Electrodes and Multifunctional Applications.

Soft and stretchable transparent electrodes still face many challenges, requiring a balance between stretchability, conductivity, transparency, and stability. In this paper, poly(dimethylsiloxane) (PDMS) elastomer is selected, and amphoteric nonionic surfactants (Triton X-100) are introduced to improve the interfacial bonding between nanosilica (SiO2) and the elastic matrix. At the same time, a fluorosilane-modified glass template is used to induce nanosilica enrichment on the membrane surface. Based on the dual effects of nanoenhancement and surface regulation, a PDMS transparent nanocomposite membrane with good stretchability, surface hydrophilicity, and easy in situ polymerization of polydopamine can be obtained. Silver nanowires (AgNWs) and poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT:PSS) conductive polymers can be assembled layer by layer on the surface of the nanocomposite membrane by simple spin coating and heat treatment. The transmittance of the soft and stretchable composite membrane can reach 63% in the visible light range, and the strength and elongation at break increase to 2.30 MPa and 150%, respectively. The sheet resistance of the stretchable transparent dry electrode is as low as about 6 Ω/sq. After being placed in the air for 3 months, the resistance increment is less than 5%, showing good environmental stability. The stretchable transparent electrode prepared based on the material-structure-preparation integrated method has brilliant application potential in wearable thermal management, electrochromism, strain sensing, and electromagnetic shielding.

Stretchable Ag/AgCl Nanowire Dry Electrodes for High-Quality Multimodal Bioelectronic Sensing.

Bioelectrical signal measurements play a crucial role in clinical diagnosis and continuous health monitoring. Conventional wet electrodes, however, present limitations as they are conductive gel for skin irritation and/or have inflexibility. Here, we developed a cost-effective and user-friendly stretchable dry electrode constructed with a flexible network of Ag/AgCl nanowires embedded in polydimethylsiloxane (PDMS). We compared the performance of the stretched Ag/AgCl nanowire electrode with commonly used commercial wet electrodes to measure electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG) signals. All the signal-to-noise ratios (SNRs) of the as-fabricated or stretched (50% tensile strain) Ag/AgCl nanowire electrodes are higher than that measured by commercial wet electrodes as well as other dry electrodes. The evaluation of ECG signal quality through waveform segmentation, the signal quality index (SQI), and heart rate variability (HRV) reveal that both the as-fabricated and stretched Ag/AgCl nanowire electrode produce high-quality signals similar to those obtained from commercial wet electrodes. The stretchable electrode exhibits high sensitivity and dependability in measuring EMG and EEG data, successfully capturing EMG signals associated with muscle activity and clearly recording α-waves in EEG signals during eye closure. Our stretchable dry electrode shows enhanced comfort, high sensitivity, and convenience for curved surface biosignal monitoring in clinical contexts.

High-Performance Intrinsically Stretchable Organic Photovoltaics Enabled by Robust Silver Nanowires/S-PH1000 Hybrid Transparent Electrodes.

Intrinsically stretchable organic photovoltaics (is-OPVs) hold significant promise for integration into self-powered wearable electronics. However, their potential is hindered by the lack of sufficient consistency between optoelectronic and mechanical properties. This is primarily due to the limited availability of stretchable transparent electrodes (STEs) that possess both high conductivity and stretchability. Here, a hybrid STE with exceptional conductivity, stretchability, and thermal stability is presented. Specifically, STEs are composed of the modified PH1000 (referred to as S-PH1000) and silver nanowires (AgNWs). The S-PH1000 endows the STE with good stretchability and smoothens the surface, while the AgNWs enhance the charge transport. The resulting hybrid STEs enable is-OPVs to a remarkable power conversion efficiency (PCE) of 16.32%, positioning them among the top-performing is-OPVs. With 10% elastomer, the devices retain 82% of the initial PCE after 500 cycles at 20% strain. Additionally, OPVs equipped with these STEs exhibit superior thermal stability compared to those using indium tin oxide electrodes, maintaining 75% of the initial PCE after annealing at 85°C for 390 h. The findings underscore the suitability of the designed hybrid electrodes for efficient and stable is-OPVs, offering a promising avenue for the future application of OPVs.

Hybrid microstructure-based stretchable biosensors for multi-physiological signal sensing

Wearable biosensors provide continuous, real-time physiological monitoring of biochemical markers in biofluids such as sweat, tears, saliva, and interstitial fluid. However, achieving high stretchability and stable biochemical signal monitoring remains challenging. Here, we propose a hybrid microstructure (HMS) strategy to fabricate highly stretchable multifunctional biosensors capable of detecting sweat electrolyte concentrations, pH levels, and surface electromyography (EMG) signals. By integrating a HMS, stable conductivity under large strains is ensured. Stretching tests up to 5000 cycles demonstrated the electrodes’ stretchable stability and reliability. The high-performance electrodes were used for EMG monitoring on human skin. Additionally, active materials were coated onto the stretchable electrodes to create multifunctional sweat sensors capable of monitoring pH as well as calcium, sodium, and potassium ions (Ca2+, Na+, K+). The electrodes reliably maintained their functionality under 60% strain, providing new insights into the fabrication of stable, highly stretchable biosensors.

Fully Implantable Wireless Cardiac Pacing and Sensing System Integrated with Hydrogel Electrodes.

Cardiac pacemakers play a crucial role in arrhythmia treatment. Existing devices typically rely on rigid electrode components, leading to potential issues such as heart damage and detachment during prolonged cardiac motion due to the mechanical mismatch with cardiac tissue. Additionally, traditional pacemakers, with their batteries and percutaneous leads, introduce infection risks and limit freedom of movement. A wireless, battery-free multifunctional bioelectronic device for cardiac pacing is developed. This device integrates highly conductive (160Sm-1), flexible (Young's modulus of 80kPa is similar to that of mammalian heart tissue), and stretchable (270%) soft hydrogel electrodes, providing high signal-to-noise ratio (≈28dB) electrocardiogram (ECG)recordings and effective pacing of the beating heart. The versatile device detects physiological and biochemical signals in the cardiac environment and allows for adjustable pacing in vivo studies. Remarkably, it maintained recording and pacing capabilities 31 days post-implantation in rats. Additionally, the wireless bioelectronic device can be fully implanted in rabbits for pacing. By addressing a major shortcoming of conventional pacemakers, this device paves the way for implantable flexible bioelectronics, which offers promising opportunities for advanced cardiac therapies.

A super-stretchable conductive film with strain-insensitive conductivity for stretchable EMI shielding materials and wearable capacitive strain sensors

Strain-insensitive conductive films as stretchable electromagnetic interference (EMI) shielding materials and stretchable electrodes are highly desired in wearable electronics. However, fabricating super strain-insensitive conductive films under a tensile strain higher than 400 % is still a great challenge. Herein, a super-stretchable conductive film based on the crumple-structured Ti3C2Tx nanosheets-single walled carbon nanotubes/stretchable substrate double-layers is designed for the stretchable EMI shielding materials and electrodes. The resulting film exhibits a strain-insensitive electrical conductivity as high as 3.01 × 103 S/m even at a strain up to 500 %, which endows the film with a high and stable electromagnetic interference shielding efficiency (EMI SE) value of ∼45 dB. More interestingly, the EMI SE value of the film remains nearly constant even after 2000 cycles of 500 % tensile strain, indicating the excellent long-term service stability as a stretchable EMI shielding material. Moreover, a capacitive strain sensor with extra-wide sensing range, ultra-high stability, and excellent durability is successfully achieved by employing the as-prepared films as stretchable electrodes. This work proposes a convenient strategy of strain-insensitive conductive film aiming to design stretchable EMI shielding materials and electrodes for wearable electronics.

Stretchable Sweat Lactate Sensor with Dual-Signal Read-Outs.

Innovations in wearable sweat sensors hold great promise to provide deeper insights into molecular level health information non-invasively. Lactate, a key metabolite present in sweat, holds immense significance in assessing physiological conditions and performance in sports physiology and health sensing. This paper presents the development and characterization of stretchable electrodes with ultrahigh active surface area of 648 % for lactate sensing. The as-printed stretchable electrodes were functionalized with an electron transfer layer comprising Toluidine Blue O and multi-walled carbon nanotubes (MWCNTs), and an enzymatic layer consisting of lactate dehydrogenase with β-Nicotinamide adenine dinucleotide as the cofactor for lactate selectivity. This sensor achieves a dual-signal read-out in which both electrochemical and fluorescence signals were obtained during lactate detection, demonstrating promising sensor performance in terms of sensitivity and reliability. We demonstrate the robustness of the dual-signal sensor under simulated conditions of physical deformation and shifted excitation. Under these compromised conditions, the performance of the stretchable electrodes remained largely unaffected, showcasing their potential for robust and adaptable sensing platforms in wearable health monitoring applications.

Multi-Regional Pelvic Floor Muscle Function Diagnosis System Based on Inflatable Stretchable Electrode Array.

Effective prevention and treatment of pelvic floor dysfunction (PFD) necessitates the identification of lesions within the complex pelvic floor muscle (PFM) groups associated with various symptoms. Here, we developed a multi-region pelvic floor muscle functional diagnosis system (MPDS) based on an inflatable stretchable electrode array, which aids in accurately locating areas related to PFD. Clinical diagnostic experiments were conducted on 56 patients with postpartum stress urinary incontinence (PSUI) and 73 postpartum asymptomatic controls. MPDS collects pelvic floor electromyography from all participants. By assessing EMG parameters such as activation time differences (ATD) and using Jensen-Shannon (JS) divergence to verify, with the aim of locating target muscle groups with functional abnormalities. Clinical test results showed that by observing the AT sequence of the PSUI group and the control group, muscle groups with functional abnormalities in the Pubococcygeus muscle (PC) and Puborectalis muscle (PR) regions could be preliminarily diagnosed. In the assessment of regional muscle contribution values based on JS divergence, it was verified that the contribution values of rapid contraction in the PC and PR regions of the PSUI group were relatively lower compared to those of the control group, which correlated with urinary control dysfunction. These experiments demonstrate that the MPDS helps in accurately locating target muscle groups with functional abnormalities, showcasing its potential in precise assessment of complex muscle groups such as PFM, which may improve diagnostic precision and reliability.

Stretchable, transparent and multifunctional PVC-gel heater: a novel approach to skin-mountable, wearable thermal devices

Electric heaters based on functional materials and innovative designs have been developed for various applications. In this paper, we propose a soft dielectric heater (SDH) using polyvinyl chloride-gel (PVC-gel) as the dielectric heater and hydrogel as stretchable electrodes. Under an AC voltage, the leakage current in the PVC-gel leads to continuous injection and discharge of charges, causing the polarized plasticizers and flexible PVC chains to vibrate and collide, thereby generating heat through dielectric heating. Furthermore, the SDH generates a uniform temperature distribution even under strains up to 400%. Besides, high transmittance over 86% across the visible range renders it suitable for wearable or skin-mountable heaters from an esthetic viewpoint. Its capacitor-like structure achieves a scalable design, enabling extension from a singular cell to a row/column addressable and pixelated array of heaters. The 5 × 5 SDH array can deliver varied thermal information and sensations while maintaining performance even when stretched.

Triboelectric Nanogenerator Made with Stretchable, Antibacterial Hydrogel Electrodes for Biomechanical Sensing.

Triboelectric nanogenerators (TENGs) have attracted widespread attention as a promising candidate for energy harvesting due to their flexibility and high power density. To meet diverse application scenarios, a highly stretchable (349%), conductive (1.87 S m-1), and antibacterial electrode composed of carbon quantum dots/LiCl/agar-polyacrylamide (CQDs/LiCl/agar-PAAm) dual-network (DN) hydrogel is developed for wearable TENGs. Notably, the concentration of agar alters the pore spacing and pore size of the DN hydrogel, thereby impacting the network cross-linking density and the migration of conductive ions (Li+ and Cl-). This variation further affects the mechanical strength and conductivity of the hydrogel electrode, thus modulating the mechanical stability and electrical output performance of the TENGs. With the optimal agar content, the tensile strength and conductivity of the hydrogel electrode increase by 211 and 719%, respectively. This enhancement ensures the stable output of TENGs during continuous operation (6000 cycles), with open-circuit voltage, short-circuit current, and transferred charge increasing by 200, 530, and 155%, respectively. Additionally, doping with CQDs enables the hydrogel electrode to effectively inhibit the Gram-negative bacterium Escherichia coli. Finally, the TENGs are utilized as a self-power smart ring for efficient and concise information transmission via Morse code. Consequently, this study introduces a creative approach for designing and implementing multifunctional, flexible wearable devices.

Highly stable Ag@Au nanowires micromesh as transparent conductive network for stretchable electrochromic modulator

The stretchable electrochromic modulator is one of the most promising energy-saving optoelectronic devices that can be integrated into smart portable and wearable electronics. A lot of focus has been on developing transparent conductive electrodes of stretchable electrochromic modulators, but they generally suffer from poor chemical stability, low photoelectric balance, and unsatisfactory deformability. To address this challenge, we first develop Ag@Au nanowires (NWs) micromesh-based stretchable transparent conductive electrodes (STCEs) using a self-assembled approach, which displays superior photoelectric performance (8.5 Ω sq-1 with a transmittance of 81.5 %), good chemical stability in harsh electrolyte environment (acidic and alkaline) as well as robust mechanical stability (bending, folding, stretching and twisting). As a demonstration, the grown WO3 on Ag@Au NWs-based STCEs can deliver good electrochromic stability even after bending over 1000 cycles and stretching to 50 %. Moreover, highly soft Zn//WO3 and polyaniline (PANI)//WO3 electrochromic modulators were assembled using flexible Ag@Au micromesh STCEs, which still retain uniform electrochromism in neutral and acidic electrolytes respectively. Definitionally, the strategy of in-situ grown Au shell on the surface of AgNWs micromesh provides an innovative solution to fabricate high-performance STCEs for the related portable smart wearable electronic devices.

Utilization of in-situ self-supporting materials for the preparation of flexible and stretchable electrodes for the detection of nitric oxide released from cells

Real-time monitoring of nitric oxide from endogenously cells released is crucial for revealing their physiological activities or pathological processes. However, designing electrochemical sensors for real-time monitoring at the cellular level is challenging due to the simultaneous requirements of sensitivity, transient recording capability, and biocompatibility. Combining the universally self-supported grown highly active materials with stretchable conductive flexible substrates holds promise for achieving sensitive detection of cells and real-time monitoring of mechanically induced biomolecular release. However, existing manufacturing strategies for highly sensitive stretchable flexible electrochemical sensors still face significant challenges due to low material activity, high processing requirements, and the complexity of further designing membrane electrodes. Here, we designed a method of in-situ self-supported growth to grow Co-doped Ni(OH)2 structures on rigid Ti foil. Leveraging the synergistic effect of Co and Ni, sensitive detection of NO was achieved, enabling real-time monitoring of trace amounts of NO released by Raw 264.7 cells. Based on this material, using catalytic ink method, it was transferred to low-resistance flexible ITO film (LIF), enabling real-time monitoring of mechanically induced NO release from Human Umbilical Vein Endothelial Cells (HUVECs) cells.

Microconfined Assembly of High-Resolution and Mechanically Robust EGaIn Liquid Metal Stretchable Electrodes for Wearable Electronic Systems.

Stretchable electrodes based on liquid metals (LM) are widely used in human-machine interfacing, wearable bioelectronics, and other emerging technologies. However, realizing the high-precision patterning and mechanical stability remains challenging due to the poor wettability of LM. Herein, a method is reported to fabricate LM-based multilayer solid-liquid electrodes (m-SLE) utilizing electrohydrodynamic (EHD) printed confinement template. In these electrodes, LM self-assembled onto these high-resolution templates, assisted by selective wetting on the electrodeposited Cu layer. This study shows that a m-SLE composed of PDMS/Ag/Cu/EGaIn exhibits line width of ≈20µm, stretchability of ≈100%, mechanical stability ≈10000 times (stretch/relaxation cycles), and recyclability. The multi-layer structure of m-SLE enables the adjustability of strain sensing, in which the strain-sensitive Ag part can be used for non-distributed detection in human health monitoring and the strain-insensitive EGaIn part can be used as interconnects. In addition, this study demonstrates that near field communication (NFC) devices and multilayer displays integrated by m-SLEs exhibit stable wireless signal transmission capability and stretchability, suggesting its applicability in creating highly-integrated, large-scale commercial, and recyclable wearable electronics.

3D printing of liquid metal circuits on fabric for stretchable heater application

Abstract Liquid metals (LMs), due to their excellent stretchability, low resistance, and high thermal conductivity, has become an ideal material for preparing stretchable circuits. 3D printing technology can produce patterns with complex structures with high precision and is suitable for low-cost large-scale production of circuits. However, the high surface tension of liquid metals limits the practical application of 3D printing of LMs. In this work, we proposed a method for preparing a mixed ink based on thermoplastic polyurethane, Cyrene solvent, and LMs, and producing stretchable LMs-based electrodes through 3D printing on a fabric substrate. The stretchable LMs-based electrode prepared by this method shows a wide range of strain response and excellent tensile recovery performance, providing theoretical guidance for the application of LMs electrodes. In addition, the LMs-based heaters showed good Joule thermal response at low voltages, as well as good tensile stability, providing a new strategy for wearable heaters.

A Transparent, Patternable, and Stretchable Conducting Polymer Solid Electrode for Dielectric Elastomer Actuators

AbstractDielectric elastomer actuators (DEAs) offer versatile applications including haptics, soft robotics and smart lenses. However, due to the lack of conductive electrodes with low modulus and high stretchability, their use is limited by nonsolid electrodes, hindering integration with other systems. In this study, transparent and patternable solid electrodes, achieving actuation performance comparable to a commonly used non‐solid counterpart (e.g., carbon grease), with a stretchable and patternable conducting polymer composed of PEDOT:PSS and PEG‐PPG‐PEG diacrylate (P123DA) are reported. Varying the ratio of P123DA to PEDOT:PSS enables optimization of electrical and mechanical properties to achieve compliant solid electrodes in static and dynamic actuation. The ratio of P123DA to PEDOT:PSS is found to impact PEDOT:PSS nanofiber formation and the associated electrical and mechanical properties. Moreover, the resulting P123DA/PEDOT:PSS‐based solid electrode shows excellent optical transmittance exceeding 95%. This work highlights the potential of tuning different solid electrode properties to realize transparent, patternable, and stretchable solid electrodes, enhancing their applicability in diverse fields.

Tensile-endurable nanolayered polypyrrole templated by liquid crystal for high-performance stretchable supercapacitors

Ordered architectures favorable for energy storage are hard to build and preserve on stretchable electrodes owing to the elongation-induced variation in morphology. Herein, we demonstrate the construction of nanolayered polypyrrole (PPy) on stretchable polymeric electrodes via lyotropic liquid crystal (LLC) template for the first time. In situ graft polymerization of PPy onto swollen polyacrylamide-co-poly(2-hydroxy ethyl acrylate) in an aqueous Lα LLC gives stretchable electrodes with nanolayered PPy rooted in expandable wrinkles. The nanolayered PPy electrode could be stretched up to 330 % strain and keep intact after 2,000 stretch-release cycles owing to reversible unfolding of wrinkles. By virtue of the well-arranged PPy nanolayers, the stretchable electrode exhibits a high electrical conductivity of 154 S cm−1 and gives a capacitance of 1143 mF cm−2 after assembled into a supercapacitor with H2SO4 polyacrylamide hydrogel. In addition, the capacitance retention remains 94.6 % after 2,000 stretch-release cycles accompanied by galvanostatic charge–discharge. The LLC templating graft polymerization of conjugated polymers on elastic substrates paves a way to develop nanostructured stretchable electrodes for high-performance wearable supercapacitors.