Wearable systems that monitor muscle activity, store data and deliver feedback therapy are the next frontier in personalized medicine and healthcare. However, technical challenges, such as the fabrication of high-performance, energy-efficient sensors and memory modules that are in intimate mechanical contact with soft tissues, in conjunction with controlled delivery of therapeutic agents, limit the wide-scale adoption of such systems. Here, we describe materials, mechanics and designs for multifunctional, wearable-on-the-skin systems that address these challenges via monolithic integration of nanomembranes fabricated with a top-down approach, nanoparticles assembled by bottom-up methods, and stretchable electronics on a tissue-like polymeric substrate. Representative examples of such systems include physiological sensors, non-volatile memory and drug-release actuators. Quantitative analyses of the electronics, mechanics, heat-transfer and drug-diffusion characteristics validate the operation of individual components, thereby enabling system-level multifunctionalities.

Transparent electrodes have been widely used for various electronics and optoelectronics, including flexible ones. Many nanomaterial-based electrodes, in particular 1D and 2D nanomaterials, have been proposed as next-generation transparent and flexible electrodes. However, their transparency, conductivity, large-area uniformity, and sometimes cost are not yet sufficient to replace indium tin oxide (ITO). Furthermore, the conventional ITO is quite rigid and susceptible to mechanical fractures under deformations (e.g., bending, folding). In this study, the authors report new advances in the design, fabrication, and integration of wearable and transparent force touch (touch and pressure) sensors by exploiting the previous efforts in stretchable electronics as well as novel ideas in the transparent and flexible electrode. The optical and mechanical experiment, along with simulation results, exhibit the excellent transparency, conductivity, uniformity, and flexibility of the proposed epoxy-copper-ITO (ECI) multilayer electrode. By using this multi-layered ECI electrode, the authors present a wearable and transparent force touch sensor array, which is multiplexed by Si nanomembrane p-i-n junction-type (PIN) diodes and integrated on the skin-mounted quantum dot light-emitting diodes. This novel integrated system is successfully applied as a wearable human-machine interface (HMI) to control a drone wirelessly. These advances in novel material structures and system-level integration strategies create new opportunities in wearable smart displays.

Carbon nanotubes (CNTs) are a promising material for use as a flexible electrode in wearable energy devices due to their electrical conductivity, soft mechanical properties, electrochemical activity, and large surface area. However, their electrical resistance is higher than that of metals, and deformations such as stretching can lead to deterioration of electrical performances. To address these issues, here a novel stretchable electrode based on laterally combed CNT networks is presented. The increased percolation between combed CNTs provides a high electrical conductivity even under mechanical deformations. Additional nickel electroplating and serpentine electrode designs increase conductivity and deformability further. The resulting stretchable electrode exhibits an excellent sheet resistance, which is comparable to conventional metal film electrodes. The resistance change is minimal even when stretched by ≈100%. Such high conductivity and deformability in addition to intrinsic electrochemically active property of CNTs enable high performance stretchable energy harvesting (wireless charging coil and triboelectric generator) and storage (lithium ion battery and supercapacitor) devices. Monolithic integration of these devices forms a wearable energy supply system, successfully demonstrating its potential as a novel soft power supply module for wearable electronics.

Oxide nanomembrane hybrids with enhanced mechano- and thermo-sensitivity for semitransparent epidermal electronics are developed. The use of nanomaterials (single wall nanotubes and silver nanoparticles) embedded in the oxide nanomembranes significantly enhances mechanical and thermal sensitivities. These mechanical and thermal sensors are utilized in wheelchair control and hypothermia detection, which are useful for patients with strokes.

Abstract

Wearable mechanical and temperature sensors based on oxide nanomembrane hybrids are developed by D.-H. Kim and co-workers on page 992. Ultrathin and optically invisible materials make wearable devices fully compatible with natural human skin. Nanomaterials embedded in oxide nanomembranes dramatically enhance the mechanoand thermosensitivity, which enables successful demonstrations of the wheelchair control and hypothermia detection.

In article number https://doi.org/10.1002/adfm.201704353, Sangkyu Lee, Dae‐Hyeong Kim, and co‐workers report a stretchable electrode for wearable energy harvesting and storage. The cover shows an integrated energy supply system (left) and a serpentine‐patterned stretchable electrode based on a nanocomposite of vertically aligned carbon nanotubes (VACNTs) and poly(dimethyl siloxane) (PDMS). The process of combing CNTs by applying shear force using a slide glass is illustrated at the bottom.