An Anti‐Freezing, Ambient‐Stable and Highly Stretchable Ionic Skin with Strong Surface Adhesion for Wearable Sensing and Soft Robotics

Abstract

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.

Engineering Self-Adhesive Polyzwitterionic Hydrogel Electrolytes for Flexible Zinc-Ion Hybrid Capacitors with Superior Low-Temperature Adaptability

“…We speculated that the strongest adhesion occurred on carbon cloth (31.5 kPa), which was related to the both rough porous and hydrophilic surface of the carbon cloth that facilitated the penetration of PSA and TCs and produced more physical interlocks thereby, , whereas for the metal substrates (Zn and Al), the adhesion/cohesion behavior of TC-Zn/PSA hydrogels was driven by the coordination and hydrogen bond between the carboxyl groups and catechol groups with the metal surface . Alternatively, the OH groups and NH 2 groups available on keratin and ceramides in the outermost layers of skin could form hydrogen bonds and dipole–dipole interaction with OH-groups from TCs and COOH-groups and zwitterionic groups from PSA, respectively, accounting for the strong adhesion to the skin. , Indeed, the extraordinary adhesiveness allowed TC-7.5/PSA hydrogels (adhesion area of 2.5 × 2.5 cm 2 ) to easily lift a 500 g (8.0 kPa) weight without the interfacial detachment. It could also firmly adhere on diverse substrates in daily life (Figure S12).…”

“…In contrast, the mass of the original PSA and TC/PSA hydrogel rapidly decreased to 52% of the original mass, leaving a severely shrunken xerogel without any elasticity (Figure S17). This large mass reduction was ascribed to the fact that the hydrogel matrix without ZnCl 2 had a higher internal vapor pressure of water than that of the environment …”

“…Considering that this work was mainly focused on the design, characterization, and proof-of-concept demonstration of the PZHE, further research is urgently needed to increase the integration level and broaden the scope of application of the PZHE. For instance, the PZHE is engineered for all-in-one energy storage devices to enhance the interfacial integration between the gel electrolyte and electrode. ,,− More importantly, similar to other types of energy storage devices based on the gel electrolyte, the repertoire of PZHEs could be expanded to other energy storage devices, such as Zn-ion batteries, ,− Zn-MnO 2 batteries, ,,,,, Zn-air batteries, ,,− and supercapacitors. ,,,, In addition, the PZHE is potentially a superior conductive material to fabricate stretchable sensors − or triboelectric nanogenerators, − which can achieve seamless integration with untethered wearables and soft robots. , …”

Transparent, stretchable and anti-freezing hybrid double-network organohydrogels

“…Moreover, hydrogels have a high transparency [20,21]. Accordingly, ionic conductive hydrogels could resolve the concerns of the significant increase in resistance upon deformation, but their tolerance window of temperature is too narrow to support large-scale application and long-term stability in reality [22][23][24][25]. This is mainly due to freezing and evaporation of water in dry or subzero environments.…”

Accurate Prediction of Knee Angles during Open-Chain Rehabilitation Exercises Using a Wearable Array of Nanocomposite Stretch Sensors

“…High-deflection strain gauges are desirable for biomechanical measurement applications because of their large strain capacity and sensitivity to changes in strain [ 16 , 17 ]. There are many types of high-deflection strain gauges used in biomechanics including liquid metal [ 18 , 19 , 20 ], gel-based [ 21 , 22 ], polymer optical fiber [ 23 , 24 , 25 ], and piezoresistive sensors. The most common filler materials used in high-deflection strain gauges are derivatives of carbon [ 26 , 27 , 28 , 29 , 30 ], due to the favorable mechanical properties, high electrical conductivity, and high thermal conductivity of carbon-based polymers.…”