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With the rapidly growing interest and usage in wearable electronics, considerable attention has been paid to the environmental concern caused by electronic waste and the toxicity of constituent materials. In this study, we report the facile fabrication of a fully biodegradable and stretchable serpentine-shaped wire supercapacitor by using a water-soluble molybdenum (Mo) wire, polyvinyl-alcohol-based biodegradable gel polymer electrolyte, and biodegradable elastomer—poly(1,8-octanediol-co-citrate) (POC). A thin oxide layer grown on a Mo wire via simple anodization drastically improves the electrochemical capacitance by inducing pseudocapacitance. As a result, the fabricated supercapacitor exhibits areal capacitance of 4.15 mF cm−2 at 0.05 mA cm−2, energy density of 0.37 µWh cm−2, and power density of 0.8 mW cm−2. The design of the serpentine-shaped wire supercapacitor encapsulated with POC gives mechanical and electrochemical stability against deformations of repetitive stretching. The biodegradable property of the supercapacitor is confirmed by the measurements of the change in mass of its constituent materials with elapsed time in water. Furthermore, the transient electrochemical performance of the fabricated wire supercapacitor in water over a certain period of time is observed to depend on the encapsulation.

We report a fabrication of a high-performance wire-type supercapacitor through surface modification of carbon fiber with ionic liquid, nanomaterials, and gel electrolyte containing ionic liquid. Coating of Au nanoparticles onto carbon fiber increases both surface area and electrical conductivity. Dip-coating of mixture of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), carbon nanotubes, and electropolymerization of polypyrrole (Ppy) onto Au coated fiber for pseudocapacitance results in high capacitance. The use of propylene carbonate-poly(methyl methacrylate)-[EMIM][TFSI] gel electrolyte gives high operation voltage. Such wire-type supercapacitor exhibits a high voltage of 2.5 V, an areal capacitance of 38.49 mF cm−2, and a maximum energy and power density of 24.7 μWh cm−2 and 3.52 mW cm−2, respectively. In addition, the cyclic stability of the supercapacitor is dramatically enhanced by using 2-naphthalene sulfonic acid as a dopant in electropolymerization of Ppy. Encapsulation with a thermally shrinkable tube endows the supercapacitor with mechanical stability and waterproof features when it is bent, folded, twisted, even in water. This work demonstrates high potential of such wire-type supercapacitor as a flexible energy-storage device for various applications, especially those that require high voltage.

We report on the fabrication of an interactive display system consisting of a stretchable array of electrochromic devices (ECDs) and temperature and strain sensors for visual display of skin temperature and wrist movement. The electrochromic device is prepared by coating poly(3-hexylthiophene) and tungsten trioxide as electrochromic materials on a flexible and transparent electrode of indium-tin-oxide-coated polyethylene terephthalate, and the subsequent assembly of the prepared electrodes using a gel-type electrolyte of Li-mixed 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(methyl methacrylate). The fabricated ECD exhibits three different colors due to the redox reactions of the electrochromic materials at the corresponding potentials: magenta, violet, and blue at −1.5 V, 0.5 V, and 1.5 V, respectively. By using patterned liquid metal Galinstan interconnections, a stretchable array of the ECDs is fabricated based on a strain relieving mechanism with an island-bridge design, resulting in mechanical stability to repeated stretching of up to 30%. A high-performance strain sensor is fabricated by embedding fragmentized graphene foams inside poly(demithylsiloxane) thin film, and a temperature sensor is prepared by embedding multi-walled carbon nanotubes/polyaniline composite in poly(vinyl alcohol) film. With the integrated system, the strain due to wrist bending and the skin temperature measured by the sensors are displayed as changes in the color pattern of the ECD array aided by an Arduino circuit. This work demonstrates the high potential of our system for application to wearable healthcare devices providing real-time visual display of varied bio-signals.

Stretchable electronic devices with self-healing functions that can improve durability are highly recommended as next-generation personal instruments for economic and sustainable society. Here, we report a fabrication of self-healing strain-responsive electrochromic display based on a multiple crosslinked network hydrogel (MCNH) consisting of both hydrophilic and hydrophobic domains. After optimizing the mechanical and self-healing properties of the hydrogel with variation of the chemical crosslinker, N,N'-methylenebisacrylamide, and the ionic crosslinker CaCl2, an extreme mechanical stretchability of up to 2000% strain and shape recovery, and a self-healing efficiency of 83.5% after 8 h at room temperature are obtained. The MCNH-based strain sensor exhibits a fast and linear resistance response with a coefficient of determination of 0.997 over a wide strain range of 100%. The strain sensitivity of the hydrogel remains stable even after 10 repeated self-healing cycles at a single location. As a display application, a novel two-dimensional electrochromic device is fabricated using a hydrogel without depositing an electrochromic material (ECM) on the electrode. ECM-containing gel electrolyte exhibits electrochromic properties through the migration of ions to the electrodes. Coloration/discoloration occurs at a potential bias of 1.7 V with a transmittance change of 76.1% at 547 nm through the chemical oxidation/reduction of ethyl viologen ions in the hydrogel matrix. An integrated system comprising a self-healing strain sensor and an ECD attached to the skin is demonstrated to visually express the applied strain due to finger bending, aided by an external circuit. Such a strain-responsive ECD system preserves a stable performance with the self-healed sensor after a complete bisection. These results suggest the potential application of our newly synthesized hydrogel to various skin-attachable self-healing, and stretchable devices with high durability.

The aim of this paper is to report on the fabrication of a stretchable array of high-performance solid-state micro-supercapacitors (MSCs), which can be charged with integrated, commercial Si-based solar cells (SCs). This would facilitate the powering of an integrated strain sensor. The planar MSCs comprised electrodes of potentiostatically deposited polypyrrole, on spray-coated multi-walled carbon nanotube film, and a gel-type electrolyte of LiCl/polyvinyl alcohol with a redox additive of 1-methyl-3-propylimidazolium iodide. The fabricated MSC achieved an areal capacitance of 5.17 mF cm−2. After 5000 charge/discharge cycles, the MSC retained 80% of their initial capacitance. A strain sensor was fabricated utilizing a composite film of fragmentized graphene foam and polydimethylsiloxane. Such fabricated twelve parallel connected MSCs, a strain sensor, and SCs were integrated on a single deformable polymer substrate with embedded stiff platforms of negative epoxy series resist (SU-8) via long serpentine interconnections of polyimide encapsulated Ti/Pt metal film for mechanical stability under stretching. After 1000 repetitive biaxial stretching/releasing cycles by 30%, no noticeable change was observed in the charge/discharge behavior of the MSC array. Furthermore, both the photo-charge/discharge characteristics and electrochemical performance remained stable. When the whole integrated system was attached to the wrist, the integrated strain sensor could detect both externally applied strain and the arterial pulse using the energy stored in the MSCs from the SCs.

Display devices that can visually present various bio-signals are a core future technology required for skin attachable wearable electronics. Quantum dot light emitting diodes (QD-LEDs), which can be used as high performance display devices, exhibit many advantages such as a narrow bandwidth, high color purity, high environmental stability, easy and fast processing, and a high brightness at low voltage. In this study, we report on the fabrication of a stretchable QD-LED array for the visual display of body movement and skin temperature signals from skin-attached sensors. After forming an array of a rigid NOA63 islands, an array of CdSe/ZnS QD-LEDs is fabricated on top of that island array and electrically connected using liquid metal Galinstan interconnections. Finally, the array of QD-LEDs on the NOA63 islands is transferred onto a stretchable elastomer substrate of Ecoflex/PDMS. Of particular importance, for stable performance of the QD-LEDs under deformation, in replacement of the conventionally used ITO, a flexible and highly transparent electrode of Au grid/Ethylene glycol doped PEDOT:PSS is used. Such a design architecture can minimize the strain applied to the QD-LEDs by concentrating the strain onto the soft elastomer film substrate and the Galinstan interconnections. As a result, the fabricated QD-LED array exhibits stable operation under both 50% uniaxial and 30% biaxial strains. After attachment of the stretchable QD-LED array onto skin, the extent of knee bending, and changes in skin temperature are displayed as changes in the pattern of the QD-LED array. This work demonstrates the potential application of our stretchable array of QD-LEDs for easy and daily monitoring of health conditions through a visual display.

In this report, high sensitivity pressure and temperature sensors are demonstrated based on a single common active material of poly(3-hexylthiophene-2,5-diyl) (P3HT)-coated elastic carbon foam (ECF) for bio-signal monitoring. A conductive microporous elastic foam is obtained via direct carbonization of a commercially available melamine foam at 700 °C and a subsequent dip coating with P3HT. The design incorporating interdigitated electrodes on the bottom of the P3HT-coated ECF is used to fabricate a high-performance pressure sensor with a high pressure sensitivity of 102.4 kPa−1 (

(Ni,Co,Mn)O 4 (NMC) oxides were prepared by conventional sintering (CS) and spark plasma sintering (SPS) using micro and nanopowders. Small hoping polaron theory was used in order to investigate effect of processing routes on electrical properties of NMC oxides as negative temperature coefficient (NTC) thermistors. Also, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques were utilized to analyze compositional and structural effects on the electrical properties of NMC compounds. Hopping conduction in NMC prepared by SPS and CS using nanopowder occurs via variable range hopping (VRH) mechanism, however conduction in NMC prepared by CS using micropowder follows nearest neighboring hopping (NNH) mode. Hopping distance and activation energy for the VRH mode were calculated using corresponding physical model.

We report on the fabrication of a high performance flexible micro-supercapacitor (MSC) for powering a vertically integrated skin-attachable strain sensor on a gecko-inspired micro-structured adhesive. Combined utilization of the mixed manganese/vanadium (Mn/V) oxide grown on MWCNT electrode and the sulfone-based electrolyte, PC/SL/LiClO4/PMMA, enhances both the capacitance and operation voltage up to 2 V of MSC. Thus, the fabricated MSC exhibits excellent electrochemical performance with an areal capacitance of 11.8 mF cm−2 and an areal energy density of 6.58 µWh cm−2 at an areal power density of 200 µW cm−2. The MSC shows mechanical stability over 1000 repetitive bends at a bending radius of 3.7 mm. The strain sensor made of fragmentized graphene foam embedded in PDMS film provides a high gauge factor of 12.6 up to 50% strain to detect strains due to various bio-signals. Our gecko-inspired adhesive made of PDMS micropillars inked with a mixture of PDMS and Silbione to have spatula tips exhibits not only high adhesion property but also high durability over repeated cycles of attachment-detachment and negligible skin irritation. After vertical integration of the MSC, strain sensor, and the adhesive film, bio-signals such as an arterial pulse, swallowing, and frowning of the brow are successfully detected using energy stored in the MSC.

We report on the fabrication of a rationally designed self-healing and flexible all-in-one system. Using a vacuum filtration method, an integrated system consisting of a planar supercapacitor and a multifunctional sensor is fabricated with gold nanosheet (AuNS) interconnections. A fast and reversible self-healing hydrogel based on the diol-ester bonding between poly(vinyl alcohol) (PVA) and borax is synthesized, and its mechanical stability is enhanced with the additional hydrogen bonding between PVA and agarose. Using an electrolyte of PVA/borax/agarose/NaNO3 and an electrode consisting of multiwalled carbon nanotubes (MWCNTs) coated on an AuNS current collector, a high-performance self-healing flexible planar supercapacitor is fabricated. The multifunctional sensor is made of MWCNTs with a zinc oxide nanowire composite. Both the supercapacitor and sensor recover their performance from bisectional damage via physical contact with a water supply. The self-healing supercapacitor shows mechanical stability for bending deformation even after repetitive self-healing cycles. Furthermore, the integrated system exhibits self-healing performance through stable sensing of NO2 gas and UV light using the stored energy in the supercapacitor after recovery from repetitive bisection of the sensor. This work demonstrates the high potential of our rationally designed flexible self-healing all-in-one system in high-performance wearable devices with high durability and longevity.

We report on the fabrication of a fractal-designed transparent and stretchable (TS) microsupercapacitor (MSC) as a skin-attachable energy storage device for driving an integrated strain sensor (SS). The MSC is designed to have a fractal structure with long and narrow patterns to ensure high transparency and stretchability. MnO2/Carbon nanotubes (CNTs) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(methyl methacrylate) ([BMIM][TFSI]/PMMA) are used as electrodes and a stretchable electrolyte for the MSC, respectively. The fabricated MSC exhibits high transparency of 79% and an areal capacitance of 12.6 mF cm−2 at a current density of 5 mA cm−2. The TS MSC shows excellent mechanical stability in that it can maintain stable electrochemical performance even after 2000 repeated stretching cycles up to 30%. The TS SS is facilely prepared via spray coating of Ag nanowires (NWs) and subsequent curing of PDMS, where the transparency and the electrical resistance are controlled by adjusting the deposited volume of Ag NWs. Wrist bending and a wrist pulse are detected by the SS driven with the stored energy of the MSC. This work demonstrates the possible application of our TS MSC to future skin-attachable electronics as an integrated energy storage device.