Hierarchically Interpenetrated and Reentrant Microcellular Frameworks for Stretchable Lithium Metal Batteries.

With the rapid development of human-friendly wearable devices, energy storage components are required to have skin-like stretchability or free-form to fit closer and more comfortably to the human body. This study introduces a hierarchically interpenetrated reentrant microcellular structure combined with 2D cellular graphene/MXene/carbon nanotubes (CNTs) and 3D cellular melamine foam. This composite structure works as a stretchable framework of lithium metal composite electrodes to provide stretchability for lithium metal electrodes, which are promising as next-generation energy storage systems. The interpenetrated but independent cellular structures successfully obtain stable structural deformability from the nonconductive and deformable melamine foam, while at the same time, high electrical conductivity, lithiophilicity, and mechanical stability of the graphene/CNT/MXene network serve as a lithium deposition support during the electrodeposition of lithium. The reentrant structure is fabricated by radial compressing the hierarchical cellular structures to take advantage of the structural stretchability of the accordion-like reentrant frameworks. The lithium-deposited composite electrodes exhibit much lower overpotential during Li stripping and plating than lithium metal foil anodes and show stable electrochemical performances under 30% of mechanical strain. The reentrant microcellular electrodes offer great potential for advanced designs of lithium metal electrodes for stretchable batteries with high energy density.

Intrinsically Stretchable and Printable Lithium-Ion Battery for Free-Form Configuration.

For next-generation wearable and implantable devices, energy storage devices should be soft and mechanically deformable and easily printable on any substrate or active devices. Herein, we introduce a fully stretchable lithium-ion battery system for free-form configurations in which all components, including electrodes, current collectors, separators, and encapsulants, are intrinsically stretchable and printable. The stretchable electrode acquires intrinsic stretchability and improved interfacial adhesion with the active materials via a functionalized physically cross-linked organogel as a stretchable binder and separator. Intrinsically stretchable current collectors are fabricated in the form of nanocomposites consisting of a matrix with excellent barrier properties without swelling in organic electrolytes and nanostructure-controlled multimodal conductive fillers. Due to structural and materials freedoms, we successfully fabricate several types of stretchable lithium-ion battery that reliably operates under various stretch deformations with capacity and rate capability comparable with a nonstretchable battery over 2.5 mWh cm-2 at 0.5 C, even under high mass loading conditions over 10 mg cm-2, including stacked configuration, direct integration on both sides of a stretch fabric, and application of various electrode materials and electrolytes. Especially, our stretchable battery printed on a stretch fabric also exhibits high performance and stretch/long-term stabilities in the air even with wearing and pulling.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics.

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Stretchable Lithium-Ion Battery Based on Re-entrant Micro-honeycomb Electrodes and Cross-Linked Gel Electrolyte.

Stretchable energy storage devices are of great interest because of their potential applications in body-friendly, skin-like, wearable devices. However, stretchable batteries are very challenging to fabricate. The electrodes must have a degree of stretchability because the active materials occupy most of the volume, and the separator and packaging should also be stretchable. Here, an all-component stretchable lithium-ion battery was realized by leveraging the structural stretchability of re-entrant micro-honeycomb graphene-carbon nanotube (CNT)/active material composite electrodes and a physically cross-linked gel electrolyte, without using an inactive elastomeric substrate or matrix. Active materials interconnected via the entangled CNT and graphene sheets provided a mechanically stable porous network framework, and the inwardly protruding framework in the re-entrant honeycomb structure allowed for structural stretching during deformation. The composite network consisting solely of binder-free, highly conductive materials provided superior electron transfer, and vertically aligned microchannels enabled facile ion transport. Additionally, the physically cross-linked gel electrolyte increased the mechanical stability upon deformation of the electrodes and was effective as a stretchable separator. The resulting stretchable battery showed a high areal capacity of 5.05 mAh·cm-2, superior electrochemical performance up to 50% strain under repeated (up to 500) stretch-release cycles, and long-term stability of 95.7% after 100 cycles in air conditions.

Dynamically Stretchable Supercapacitor for Powering an Integrated Biosensor in an All-in-One Textile System.

Textile-based electronics have attracted much attention as they can perfectly combine the functionality of wearable devices with the soft and comfortable properties of flexible textile fibers. In this work, we report a dynamically stretchable high-performance supercapacitor for powering an integrated sensor in an all-in-one textile system to detect various biosignals. The supercapacitor fabricated with MWCNT/MoO3 nanocomposite electrodes and nonaqueous gel electrolyte, along the course direction of the fabric, exhibits stable and high electrochemical performance under dynamic and static deformation, including stretching in real time, regardless of the strain rate. The strain sensor created along the wale direction of the fabric shows a high sensitivity of 46.3 under an applied strain up to 60%, a fast response time of 50 ms, and high stability over 10 000 cycles of stretching/releasing. Finally, the supercapacitor and strain sensor are integrated into an all-in-one textile system via liquid-metal interconnections, and the sensor is powered by the stored energy in the supercapacitor. This system sewed into cloth successfully detects strain due to joint movement and the wrist pulse. This work demonstrates the high feasibility of utilizing the fabricated stretchable all-in-one textile system for real-time health monitoring in everyday wearable devices.

Highly Durable and Flexible Transparent Electrode for Flexible Optoelectronic Applications.

A highly-durable, highly-flexible transparent electrode (FTE) is developed by applying a composite made of a thin metal grid and a doped conducting polymer onto a colorless polyimide-coated NOA63 substrate. The proposed FTE exhibits a transparency of 90.7% at 550 nm including the substrate and a sheet resistance of 30.3 Ω/sq and can withstand both moderately high-temperature annealing (∼180 °C) and acidic solution (70 °C, pH 0.3) processes without performance degradation. The fabricated FTE yielded good mechanical stability under 10 000 cycles of bending deformations at a bending radius less than 1 mm without degradation of electrical conductivity. The high durability of the proposed FTE allows for the fabrication of flexible energy harvesting devices requiring harsh conditions, such as highly flexible perovskite solar cells (FPSCs) with a steady-state power conversion efficiency (PCE) of 12.7%. Notably, 93% of the original PCE is maintained after 2000 bending cycles at an extremely small bending radius of 1.5 mm. The FPSCs installed on curved surfaces of commercial devices drive them under various environments. The applicability of the proposed FTE is further confirmed via the fabrication of a flexible perovskite light-emitting diode. The proposed FTE demonstrates great potential for applications in the field of flexible optoelectronic devices.

Wire-Shaped Supercapacitors with Organic Electrolytes Fabricated via Layer-by-Layer Assembly.

A wire-shaped supercapacitor (WSS) has structural advantages of high flexibility and ease of incorporation into conventional textile substrates. In this work, we report a thin reproducible WSS fabricated via layer-by-layer (LbL) assembly of multiwalled carbon nanotubes (MWCNTs), combined with an organic electrolyte of propylene carbonate (PC)-acetonitrile (ACN)-lithium perchlorate (LiClO4)-poly(methyl methacrylate) (PMMA) that extends the voltage window to 1.6 V. The MWCNTs were uniformly deposited on a curved surface of a thin Au wire using an LbL assembly technique, resulting in linearly increased areal capacitance of the fabricated WSS. Vanadium oxide was coated on the LbL-assembled MWCNT electrode to induce pseudocapacitance, hence enhancing the overall capacitance of the fabricated WSS. Both the cyclic stability of the WSS and the viscosity of the electrolyte could be optimized by controlling the mixing ratio of PC to ACN. As a result, the fabricated WSS exhibits an areal capacitance of 5.23 mF cm-2 at 0.2 mA cm-2, an energy density of 1.86 µ W h cm-2, and a power density of 8.5 mW cm-2, in addition to a high cyclic stability with a 94% capacitance retention after 10 000 galvanostatic charge-discharge cycles. This work demonstrates a great potential of the fabricated scalable WSS in the application to high-performance textile electronics as an integrated energy storage device.

Highly Conductive, Stretchable, and Transparent PEDOT:PSS Electrodes Fabricated with Triblock Copolymer Additives and Acid Treatment.

Here, we report on a highly conductive, stretchable, and transparent electrode of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fabricated via modification with triblock copolymer, poly(ethylene glycol)- block-poly(propylene glycol)- block-poly(ethylene glycol) (PEO20-PPO70-PEO20, Pluronic P123), and post-treatment with sulfuric acid. The fabricated electrode exhibits high transparency (89%), high electrical conductivity (∼1700 S/cm), and minimal change in resistance (∼4%) under repetitive stretch-release cycles at 40% tensile strain after stabilization. P123 acts as a secondary dopant and plasticizer, resulting in enhanced electrical conductivity and stretchability of PEDOT:PSS. Furthermore, after sulfuric acid post-treatment, P123 helps the electrode to maintain its stretchability. A successful demonstration of the stretchable interconnection was shown by stretching the P123-modified PEDOT:PSS electrodes, which were connected with light-emitting diodes (LEDs) in series. Finally, a stretchable and transparent touch sensor consisting of our fabricated electrodes and an LED array and stretchable semitransparent supercapacitor were presented, suggesting a great potential of our electrodes in the application to various deformable devices.

Skin-Attachable, Stretchable Electrochemical Sweat Sensor for Glucose and pH Detection.

As part of increased efforts to develop wearable healthcare devices for monitoring and managing physiological and metabolic information, stretchable electrochemical sweat sensors have been investigated. In this study, we report on the fabrication of a stretchable and skin-attachable electrochemical sensor for detecting glucose and pH in sweat. A patterned stretchable electrode was fabricated via layer-by-layer deposition of carbon nanotubes (CNTs) on top of patterned Au nanosheets (AuNS) prepared by filtration onto stretchable substrate. For the detection of glucose and pH, CoWO4/CNT and polyaniline/CNT nanocomposites were coated onto the CNT-AuNS electrodes, respectively. A reference electrode was prepared via chlorination of silver nanowires. Encapsulation of the stretchable sensor with sticky silbione led to a skin-attachable sweat sensor. Our sensor showed high performance with sensitivities of 10.89 µA mM-1 cm-2 and 71.44 mV pH-1 for glucose and pH, respectively, with mechanical stability up to 30% stretching and air stability for 10 days. The sensor also showed good adhesion even to wet skin, allowing the detection of glucose and pH in sweat from running while being attached onto the skin. This work suggests the application of our stretchable and skin-attachable electrochemical sensor to health management as a high-performance healthcare wearable device.

Fabrication of High-Sensitivity Skin-Attachable Temperature Sensors with Bioinspired Microstructured Adhesive.

In this study, we demonstrate the fabrication of a highly sensitive flexible temperature sensor with a bioinspired octopus-mimicking adhesive. A resistor-type temperature sensor consisting of a composite of poly(N-isopropylacrylamide) (pNIPAM)-temperature sensitive hydrogel, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and carbon nanotubes exhibits a very high thermal sensitivity of 2.6%·°C-1 between 25 and 40 °C so that the change in skin temperature of 0.5 °C can be accurately detected. At the same time, the polydimethylsiloxane adhesive layer of octopus-mimicking rim structure coated with pNIPAM is fabricated through the formation of a single mold by utilizing undercut phenomenon in photolithography. The fabricated sensor shows stable and reproducible detection of skin temperature under repeated attachment/detachment cycles onto skin without any skin irritation for a long time. This work suggests a high potential application of our skin-attachable temperature sensor to wearable devices for medical and health-care monitoring.

A skin-integrated transparent and stretchable strain sensor with interactive color-changing electrochromic displays.

In this study, we report on the development of a stretchable, transparent, and skin-attachable strain sensor integrated with a flexible electrochromic device as a human skin-inspired interactive color-changing system. The strain sensor consists of a spin-coated conductive nanocomposite film of poly(vinyl alcohol)/multi-walled carbon nanotube/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) on a polydimethylsiloxane substrate. The sensor exhibits excellent performance of high sensitivity, high durability, fast response, and high transparency. An electrochromic device (ECD) made of electrochemically synthesized polyaniline nanofibers and V2O5 on an indium-tin-oxide-coated polyethylene terephthalate film experiences a change in color from yellow to dark blue on application of voltage. The strain sensor and ECD are integrated on skin via an Arduino circuit for an interactive color change with the variation of the applied strain, which enables a real-time visual display of body motion. This integrated system demonstrates high potential for use in interactive wearable devices, military applications, and smart robots.

Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin.

A stretchable polyaniline nanofiber temperature sensor array with an active matrix consisting of single-walled carbon nanotube thin-film transistors is demonstrated. The integrated temperature sensor array gives mechanical stability under biaxial stretching of 30%, and the resultant spatial temperature mapping does not show any mechanical or electrical degradation.

Stretchable Array of Highly Sensitive Pressure Sensors Consisting of Polyaniline Nanofibers and Au-Coated Polydimethylsiloxane Micropillars.

We report on the facile fabrication of a stretchable array of highly sensitive pressure sensors. The proposed pressure sensor consists of the top layer of Au-deposited polydimethylsiloxane (PDMS) micropillars and the bottom layer of conductive polyaniline nanofibers on a polyethylene terephthalate substrate. The sensors are operated by the changes in contact resistance between Au-coated micropillars and polyaniline according to the varying pressure. The fabricated pressure sensor exhibits a sensitivity of 2.0 kPa(-1) in the pressure range below 0.22 kPa, a low detection limit of 15 Pa, a fast response time of 50 ms, and high stability over 10000 cycles of pressure loading/unloading with a low operating voltage of 1.0 V. The sensor is also capable of noninvasively detecting human-pulse waveforms from carotid and radial artery. A 5 × 5 array of the pressure sensors on the deformable substrate, which consists of PDMS islands for sensors and the mixed thin film of PDMS and Ecoflex with embedded liquid metal interconnections, shows stable sensing of pressure under biaxial stretching by 15%. The strain distribution obtained by the finite element method confirms that the maximum strain applied to the pressure sensor in the strain-suppressed region is less than 0.04% under a 15% biaxial strain of the unit module. This work demonstrates the potential application of our proposed stretchable pressure sensor array for wearable and artificial electronic skin devices.

Stretchable Loudspeaker using Liquid Metal Microchannel.

Considering the various applications of wearable and bio-implantable devices, it is desirable to realize stretchable acoustic devices for body-attached applications such as sensing biological signals, hearing aids, and notification of information via sound. In this study, we demonstrate the facile fabrication of a Stretchable Acoustic Device (SAD) using liquid metal coil of Galinstan where the SAD is operated by the electromagnetic interaction between the liquid metal coil and a Neodymium (Nd) magnet. To fabricate a liquid metal coil, Galinstan was injected into a micro-patterned elastomer channel. This fabricated SAD was operated simultaneously as a loudspeaker and a microphone. Measurements of the frequency response confirmed that the SAD was mechanically stable under both 50% uniaxial and 30% biaxial strains. Furthermore, 2000 repetitive applications of a 50% uniaxial strain did not induce any noticeable degradation of the sound pressure. Both voice and the beeping sound of an alarm clock were successfully recorded and played back through our SAD while it was attached to the wrist under repeated deformation. These results demonstrate the high potential of the fabricated SAD using Galinstan voice coil in various research fields including stretchable, wearable, and bio-implantable acoustic devices.

Biaxially stretchable, integrated array of high performance microsupercapacitors.

We report on the fabrication of a biaxially stretchable array of high performance microsupercapacitors (MSCs) on a deformable substrate. The deformable substrate is designed to suppress local strain applied to active devices by locally implanting pieces of stiff polyethylene terephthalate (PET) films within the soft elastomer of Ecoflex. A strain suppressed region is formed on the top surface of the deformable substrate, below which PET films are implanted. Active devices placed within this region can be isolated from the strain. Analysis of strain distribution by finite element method confirms that the maximum strain applied to MSC in the strain suppressed region is smaller than 0.02%, while that on the Ecoflex film is larger than 250% under both uniaxial strain of 70% and biaxial strain of 50%. The all-solid-state planar MSCs, fabricated with layer-by-layer deposited multiwalled carbon nanotube electrodes and patterned ionogel electrolyte of poly(ethylene glycol) diacrylate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide having high-potential windows, are dry-transferred onto the deformable substrate and electrically connected in series and parallel via embedded liquid metal interconnection and Ag nanowire contacts. Liquid metal interconnection, formed by injecting liquid metal into the microchannel embedded within the substrate, can endure severe strains and requires no additional encapsulation process. This formed MSC array exhibits high energy and power density of 25 mWh/cm(3) and 32 W/cm(3), and stable electrochemical performance up to 100% uniaxial and 50% biaxial stretching. The high output voltage of the MSC array is used to light micro-light-emitting diode (µ-LED) arrays, even under strain conditions. This work demonstrates the potential application of our stretchable MSC arrays to wearable and bioimplantable electronics with a self-powered system.

High-density, stretchable, all-solid-state microsupercapacitor arrays.

We report on the successful fabrication of stretchable microsupercapacitor (MSC) arrays on a deformable polymer substrate that exhibits high electrochemical performance even under mechanical deformation such as bending, twisting, and uniaxial strain of up to 40%. We designed the deformable substrate to minimize the strain on MSCs by adopting a heterogeneous structure consisting of stiff PDMS islands (on which MSCs are attached) and a soft thin film (mixture of Ecoflex and PDMS) between neighboring PDMS islands. Finite element method analysis of strain distribution showed that an almost negligible strain of 0.47% existed on the PDMS islands but a concentrated strain of 107% was present on the soft thin film area under a uniaxial strain of 40%. The use of an embedded interconnection of the liquid metal Galinstan helped simplify the fabrication and provided mechanical stability under deformation. Furthermore, double-sided integration of MSCs increased the capacitance to twice that of MSCs on a conventional planar deformable substrate. In this study, planar-type MSCs with layer-by-layer assembled hybrid thin film electrodes of MWNT/Mn3O4 and PVA-H3PO4 electrolyte were fabricated; when they are integrated into a circuit, these MSCs increase the output voltage beyond the potential of the electrolyte used. Therefore, various LEDs that require high voltages can be operated under a high uniaxial strain of 40% without any decrease in their brightness. The results obtained in this study demonstrate the high potential of our stretchable MSC arrays for their application as embedded stretchable energy storage devices in bioimplantable and future wearable nanoelectronics.

Design and fabrication of novel stretchable device arrays on a deformable polymer substrate with embedded liquid-metal interconnections.

Stretchable devices are fabricated on a newly designed deformable substrate. Active devices attached on the stiff islands are electrically connected by an embedded EGaIn interconnection, which ensures protection from external damage. In this structure, the local strain in the active device area is estimated to be less than 1% under applied strain of 30% by analysis of the strain distribution using the finite element method.