A ternary heterogeneous hydrogel with strength elements for resilient, self-healing, and recyclable epidermal electronics

Abstract

Epidermal sensing devices, which mimic functionalities and mechanical properties of natural skin, offer great potential for real-time health monitoring via continuous checking of vital signs. However, most existing skin-mounted electronics use a flexible film with high elastic modulus, which hinders physical activity and causes interfacial delamination and skin irritation. The compliance of hydrogel-based devices can firmly conform to complex, curved surfaces without introducing excessive interfacial stresses. However, most hydrogels still suffer from the weakness of stable and reproducible sensing. In this work, we report a skin-friendly epidermal electronic made of a resilient, self-healing, and recyclable polyvinyl alcohol (PVA) hydrogel. The hydrogel is reinforced through a ternary heterogeneous network for good mechanical robustness while maintaining high stretchability and exceptional conformability. Simultaneously, the abundant dynamic hydrogen bonds give the hydrogel rapid self-healing ability. The assembled hydrogel epidermal electronic is able to stably monitor multiple physiological signals as well as sense the strain level of the skin motion and joint bending. The unique, versatile, environmental and biological friendly epidermal electronics will have broad applications in health care, human-machine interface, augmented reality, and so on.

Conductive and Robust Cellulose Hydrogel Generated by Liquid Metal for Biomedical Applications

“…The development of conductive and robust hydrogels for use in bioelectronics, including tactile and strain sensors, electronic skin, and physiologic signal monitoring, has been a major focus of research. To satisfy the requirements of biocompatibility, biostability, high electrochemical conductivity, and mechanical stability, numerous studies have been conducted. ,,− In this study, we confirmed the biocompatibility of LMCs through live/dead cell staining (Figure a). Immunofluorescence images verified that 3T3 cells cultured for 1 and 3 days with PCH and LMC5 displayed sparse dead cells (red) and little difference compared to the control.…”

Wearable and implantable bioelectronics as eco‐friendly and patient‐friendly integrated nanoarchitectonics for next‐generation smart healthcare technology

“…Therefore, to apply bioelectronics to the skin, there are only a few requirements (e.g., sweat‐resistance, air‐permeable, elastic, flexible, and stretchable), and most failures do not cause serious problems for the user 202 . Compared to bioelectronics applied to internal organs, those applied to skin have simple functions (e.g., electrophysiological monitoring, drug delivery, epidermal chemistry monitoring, and wound healing) 203 and potential to decrease pollution by using bioelectronics systems (e.g., charged by actions without battery, 202 natural hydrogels, 203 recyclable epidermal electronics, 204 and starch‐based materials 205 ). However, although the operations (e.g., breathable conductive modeling, dermis insertion, epidermal combined electronics, and multichannel measuring) 206 and electronics (e.g., electrophysiological sensor, conductive microneedles, and photothermal electronic patch) 207 of bioelectronics applied to the skin are low‐risk, we can obtain many functions from skin‐on bioelectronics from various approaches 207 .…”

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

“…Cui et al constructed a double network nanocomposite hydrogels sensor with high strength (1.2 MPa) by incorporating tannic acid-coated cellulose nanocrystals (TA@CNC) as nanofillers into a chitosan-based conductive hydrogel, highlighting the NaCl solution immersion method (the mechanical properties of the hydrogel easily tuned by varying the immersion time). Additionally, Wang et al prepared biomass conductive hydrogels with three-dimensional interconnected porous network microstructures by a dual-network, dual-cross-linking process (Figure A). They employed PVA hydrogel with adjustable modulus as a model material system, restructured the polymer matrix of embedded hydroxypropyl cellulose (HPC) and conductive carbon nanotubes (CNTs), which were applied to enhance their mechanical and electrical properties, respectively; further cross-linked the existing PVA molecular chains in the aqueous system of HPC/CNTs with the chemical cross-linking agent, sodium tetraborate, to form an elastic PVA hydrogel; and increased the crystallinity of the PVA hydrogels by introducing a freeze–thaw physical cross-linking procedure, which protected the integrity of the network, improved the fatigue fracture resistance threshold, and consolidated the self-healing properties.…”

“…(A) (i) Mechanistic schematic of the ternary strength elements of PVA-S hydrogels, (ii) conducive strength of the PVA-S hydrogel: stretching, lifting test with a load of 1.5 kg, and in-plane extension, and (iii) tensile stress–strain curves of representative PVA-S hydrogels. Reproduced with permission from ref . Copyright 2022 Nature Publishing Group.…”

“…While hydrogels are recognized as excellent flexible material, their mechanical strength could be better (not as biological tissues rigid yet flexible), which may limit their implementation context and service lifetime. Concerning this, scholars introduced polymer units with diverse functions, combined weak and strong bonds, and constructed multinetwork structures .…”