A Flexible Loudspeaker Using the Movement of Liquid Metal Induced by Electrochemically Controlled Interfacial Tension.

A flexible liquid metal loudspeaker (LML) is demonstrated consisting of a gallium-based eutectic liquid metal (Galinstan) and basic aqueous electrolyte (NaOH(aq) ). The LML is driven by liquid metal motion induced by the electrochemically controlled interfacial tension of the Galinstan in NaOH(aq) electrolyte under an applied alternating current (AC) voltage. The fabricated LML produces sound waves in the human audible frequency band with a sound pressure level of ≈40-50 dB at 1 cm from the device and exhibits mechanical stability under bending deformation with a bending radius of 3 mm. Various sounds can be generated with the LML from a single tone to piano notes and human voices. To understand the underlying mechanism of sound generation by the LML, motion analyses, sound measurements, and electrical characterization are conducted at various frequencies. For the first time, this work suggests a new type of liquid metal-based electrochemically driven sound generator in the field of flexible acoustic devices that can be applied to future wearable electronics.

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.

Skin-Like, Dynamically Stretchable, Planar Supercapacitors with Buckled Carbon Nanotube/Mn-Mo Mixed Oxide Electrodes and Air-Stable Organic Electrolyte.

For practical applications of high-performance supercapacitors as wearable energy storage devices attached to skin or clothes, the supercapacitors are recommended to have stable mechanical and electrochemical performances during dynamic deformations, including stretching, due to real-time movements of the human body. In this work, we demonstrate a skin-like, dynamically stretchable, planar supercapacitor (SPS). The SPS consists of buckled manganese/molybdenum (Mn/Mo) mixed oxide@multiwalled carbon nanotube (MWCNT) electrodes; organic gel polymer electrolyte of adiponitrile, succinonitrile, lithium bis(trifluoromethanesulfonyl)imide, and poly(methyl methacrylate); and a porous, elastomeric substrate. The addition of an Mn/Mo mixed oxide to the MWCNT film produces an 8-fold increase in the areal capacitance. The use of an organic solvent-based electrolyte enhances the operation cell voltage to 2 V and air stability to one month under ambient air conditions. The fabricated planar supercapacitors are biaxially stretchable up to 50% strain and maintain ∼90% of their initial capacitance after 1000 repetitive stretching/releasing cycles. Furthermore, the SPS exhibits stable electrochemical performance under dynamic stretching in real time regardless of the strain rate and performs reliably during repetitive bending/spreading motions of an index finger while attached to skin.

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.

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.