Recent Advances in Bioinspired Hydrogels: Materials, Devices, and Biosignal Computing

Abstract

The remarkable ability of biological systems to sense and adapt to complex environmental conditions has inspired new materials and novel designs for next-generation wearable devices. Hydrogels are being intensively investigated for their versatile functions in wearable devices due to their superior softness, biocompatibility, and rapid stimulus response. This review focuses on recent strategies for developing bioinspired hydrogel wearable devices that can accommodate mechanical strain and integrate seamlessly with biological systems. We will provide an overview of different types of bioinspired hydrogels tailored for wearable devices. Next, we will discuss the recent progress of bioinspired hydrogel wearable devices such as electronic skin and smart contact lenses. Also, we will comprehensively summarize biosignal readout methods for hydrogel wearable devices as well as advances in powering and wireless data transmission technologies. Finally, current challenges facing these wearable devices are discussed, and future directions are proposed.

Keywords: bioinspired hydrogels; biosensors; biosignal computing; diagnostics; flexible electronics; wearable devices.

Figure 1.

Bioinspired hydrogel materials and their applications in recently developed wearable devices. They can be divided into the ionic conductive hydrogel, conductive polymer hydrogel, and conductive micro/nanocomposite hydrogel. Current hydrogel wearable devices are mainly “electronic skin”, reproduced with permissions from refs and . Copyright 2020 Royal Society of Chemistry and copyright 2020 Wiley, respectively; and “microneedle”, reproduced with permissions from refs and . Copyright 2019 Wiley, respectively, and “contact lens”, reproduced with permission from refs and . Copyright 2021 Elsevier and copyright 2020 American Society for Pharmacology and Experimental Therapuetics, respectively.

Figure 2.

Bioinspired hydrogel materials for wearable devices. (a) Schematic illustration of the skin-inspired bilayer conductive hydrogel. The hydrogel is made of a tough conductive layer containing hexadecyl methacrylate (HMA)-acrylamide (AMM) and a nonconductive adhesive layer comprising acrylate thymine (Ata)-acrylate adenine (AAa). The images display the adhesive property, conductivity, and toughness of the hydrogel and its application in real-time monitoring of neck movements. Reprinted from ref with permission from 2019 Elsevier. (b) Schematic illustration demonstrating the fabrication process of PAH and MAPAH fibers, and the picture of PAH fiber exhibits bead formation on the spider-like string. Adapted from ref with permission. Copyright 2018 from Nature Research. (c) Schematic illustration demonstrating MXene-PVP/PVA double-network hydrogel sensor fabrication process and its application for simulating Morse code and handwriting verification. Adapted from ref with permission. Copyright 2020, from AAAS. (d) Schematic illustration showing the fabrication process of Ag/TA incorporated CNC hydrogel with dynamic borate ester bonds and its structure in SEM image. The hydrogel was used as electronic skin to dial a number on a smartphone. Reprinted from ref with permission. Copyright 2019 Royal Society of Chemistry.

Figure 3.

Bioinspired hydrogel wearable devices. (a) Skin-inspired physical cross-linking ionic hydrogel with core–shell hybrid latex particles (HLPs) displayed excellent mechanical adaptability and could distinguish between various body movements and vocal patterns. Reproduced with permission from ref . Copyright 2019 American Chemical Society. (b) Octopus-inspired PDMS microstructure with integrated multilayered pNIPAM/PEDOT:PSS/CNT composite for skin compliant temperature sensing. Reproduced with permission from ref . Copyright 2018 American Chemical Society. (c) 3D bioprinting of skin-inspired Ca-PAA-SA-CNT ionic hydrogel for on-body strain sensing. Reproduced with permission from ref . Copyright 2021 American Chemical Society. (d) Schematic diagram of biological skin and self-powered biomimetic artificial skin for highly sensitive static and dynamic pressure sensing. Reproduced with permission from ref . Copyright 2020 American Chemical Society. (e) Schematic illustration of multifunctional MN array with a mussel-inspired PDA base, polymyxa derived poylmxin loaded in the PEDGA/SA tips, and octopus-inspired suction cups surrounding each needle for improved skin adhesion. Reproduced with permission from ref . Copyright 2020, AAAS. (f) Schematic illustration for fabricating drug-laden biomimetic hydrogels via imprinting by polymerizing the HEMA polymer in the presence of functional monomers inspired by the human carbonic anhydrase active site. Reproduced with permission from ref . Copyright 2011 American Chemical Society. (g) Schematic of contractive hydrogel mechanism inspired by a frog jumping for storing and releasing chemical energy by incorporating permanent and reversible bonding throughout the hydrogel network. Reproduced with permission from ref . Copyright 2020 AAAS. (h) Thermophilic bacteria-inspired PAAc hydrogel with temperature-dependent hydrophobic and ionic interactions for instant thermal hardening. Reproduced with permission from ref . Copyright 2020 Wiley.

Figure 4.

Represented hydrogel sensor with different signal readouts. (a) Hydrogel with programmed morphologies and motions which is realized by temporal control of polymerization and cross-linking reactions. Reproduced with permission from ref . Copyright 2018 Nature Publishing Group. (b) Photonic hydrogel skin can respond to multiple stimuli, including tension, pressure, and temperature, into an optical readout. Reproduced with permission from ref . Copyright 2021 American Chemical Society. (c) Hydrogel sensors with auditory abilities. The sound signal can be converted into a readable electrical signal by the hydrogel ear. Reproduced with permission from ref . Copyright 2013 American Chemical Society. (d) Hydrogel sensors with controllable and insulin release ability. Reproduced with permission from ref . Copyright 2017 American Chemical Society.

Figure 5.

Various wearable battery solutions. (a) Encapsulated coin cell batteries are suitable solutions to power wearable devices for the cases which require the entire stretchability is not very high. Reproduced with permission from ref . Copyright 2020 the Proceedings of the National Academy of Sciences. (b) A mussel-inspired fiber-like battery to display high flexibility. Reproduced with permission from ref . Copyright 2020 Wiley. (c) Typical bridge-island structured stretchable battery. Reproduced with permission from ref . Copyright 2016 American Chemical Society. (d) Stretchable battery providing power to a red LED under no strain, stretched 70%, folded, and returned to its original position. Reproduced with permission from ref . Copyright 2019 Nature Publishing Group. (e) Honeycomb-inspired wearable battery based on microstructure strain engineering technique. Reproduced with permission from ref . Copyright 2020 American Chemical Society.