Natural living systems such as wood frogs develop tissues composed of active hydrogels with cryoprotectants to survive in cold environments. Recently, hydrogels have been intensively studied to develop stretchable electronics for wearables and soft robots. However, regular hydrogels are inevitably frozen at the subzero temperature and easily dehydrated, and have weak surface adhesion. Herein, a novel hydrogel-based ionic skin (iSkin) capable of strain sensing is demonstrated with high toughness, high stretchability, excellent ambient stability, superior anti-freezing capability, and strong surface adhesion. The iSkin consists of a piece of ionically and covalently cross-linked tough hydrogel with a thin bioadhesive layer. With the addition of biocompatible cryoprotectant and electrolyte, the iSkin shows good conductivity in wide ranges of relative humidity (15-90%) and temperature (−95-25 °C). In addition, the iSkin can adhere firmly to diverse material surfaces under different conditions, including cloth fabric, skin, and elastomers, in both dry and wet conditions, at subzero temperature, and/or with dynamic movement. The iSkin is demonstrated for applications including strain sensing on both human body and winter coat, human-machine interaction, motion/deformation sensing on a soft gripper and a soft robot at extremely cold conditions. This work provides a new paradigm for developing high-performance artificial skins for wearable sensing and soft robotics.

A diode-like artificial ionic skin for strain and humidity sensing with controlled ion mobility, high toughness, stretchability, ambient stability and transparency.

Summary Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Periosteum, a highly vascularized bilayer connective tissue membrane plays an indispensable role in the repair and regeneration of bone defects. It is involved in blood supply and delivery of progenitor cells and bioactive molecules in the defect area. However, sources of natural periosteum are limited, therefore, there is a need to develop tissue‐engineered periosteum (TEP) mimicking the composition, structure, and function of natural periosteum. This review explores TEP construction strategies from the following perspectives: i) different materials for constructing TEP scaffolds; ii) mechanical properties and surface topography in TEP; iii) cell‐based strategies for TEP construction; and iv) TEP combined with growth factors. In addition, current challenges and future perspectives for development of TEP are discussed.

The surface mucosa that lines many of our organs houses myriad biometric signals and, therefore, has great potential as a sensor–tissue interface for high-fidelity and long-term biosensing. However, progress is still nascent for mucosa-interfacing electronics owing to challenges with establishing robust sensor–tissue interfaces; device localization, retention and removal; and power and data transfer. This is in sharp contrast to the rapidly advancing field of skin-interfacing electronics, which are replacing traditional hospital visits with minimally invasive, real-time, continuous and untethered biosensing. This Review aims to bridge the gap between skin-interfacing electronics and mucosa-interfacing electronics systems through a comparison of the properties and functions of the skin and internal mucosal surfaces. The major physiological signals accessible through mucosa-lined organs are surveyed and design considerations for the next generation of mucosa-interfacing electronics are outlined based on state-of-the-art developments in bio-integrated electronics. With this Review, we aim to inspire hardware solutions that can serve as a foundation for developing personalized biosensing from the mucosa, a relatively uncharted field with great scientific and clinical potential.

Paper has been one of the most popular materials of choice for biomedical applications including for bioanalysis and cell biology studies. Regular cellulose paper-based devices, however, have several key limitations...

Paper‐based surface‐enhanced Raman scattering (SERS) substrates have gained growing interest as an eco‐friendly and low‐cost tool for chemical and biosensing. However, paper‐based SERS substrates often suffer relatively low signal spatial homogeneity because of their nonuniform hot‐spot distribution. In this paper, a nanofibrillated cellulose paper (nanopaper) based SERS multiwell plate is developed for trace chemical detection with high sensitivity and high signal homogeneity. The SERS plate is fabricated from ultrasmooth (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl‐oxidized NFC paper (TO‐nanopaper) through wax‐printing‐based multiwell patterning followed by silver nanoparticle (AgNP) growth based on a successive ionic layer adsorption and reaction (SILAR) process. Taking advantage of the abundance of carboxyl groups on the TO‐nanopaper, uniformly distributed and densely arranged AgNPs are successfully synthesized through the SILAR process on the NFC multiwell surface under ambient conditions. The SERS performance of the device is evaluated for testing two Raman marker chemicals, rhodamine B and 2‐naphthalenethiol, and picomolar detection limit and high Raman enhancement factor (up to 1.46 × 109) are achieved. The Raman signal mapping results show superior signal spatial homogeneity of the device with low variations (≤11%). The nanopaper‐based SERS device represents a promising SERS platform for chemical and biomolecule detections with high sensitivity and high repeatability.

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