Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Abstract

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.

Keywords: Biodevices; Bioelectronics; Biomaterials; Robotics.

Graphical abstract

Figure 1

Skin-like hydrogel devices (A) Schematic of the human skin that resists physical deformation due to the elastin fiber and collagen in the dermis layer, maintains the body temperature due to the fat cells in the hypodermis layer, holds water due to the hygroscopic substance (i.e., pyrrolidone carboxylic acid), and transports ionic signal directionally within sensory neurons. (B) Main features of hydrogels (e.g., toughness, ionic conductivity, anti-dehydration, anti-freezing, adhesive, and self-powering) desired for practical use as wearable sensors, soft robotics and energy harvesting devices.

Figure 2

Two typical DN hydrogel structures and their synthesis strategies (A) By combining two different hydrogel networks, tough DN hydrogels can be created. Reprinted and adapted with permission from Ref. (Gong, 2014), 2014 American Association for the Advancement of Science. (B). Classical two-step polymerization method to prepare chemically–chemically cross-linked DN hydrogels. Reprinted and adapted with permission from Ref. (Chen et al., 2015), Royal Society of Chemistry. (C) The composition of a typical physically–chemically cross-linked alginate-PAAm hydrogel. (D) Preparation of the physically–chemically cross-linked alginate-PAAm DN hydrogel.

Figure 3

Conducting-polymer-based hydrogels (A) Process for fabricating conductive PEDOT:PSS-based IPN hydrogels. (B) PEDOT:PSS/acrylic acid mixture casted into different silicone soap molds. Reprinted and adapted with permission from Ref. (Feig et al., 2018). (C) Dry-annealing and swelling processes of pure PEDOT:PSS with DMSO as the additive. (D) Robust laminate of pure PEDOT:PSS hydrogel pattern. Reprinted and adapted with permission from Ref. (Lu et al., 2019). (E) Schematic of injectable RT-PEDOT:PSS hydrogels. Reprinted and adapted with permission from Ref. (Zhang et al., 2020b). (F) Schematics of morphing electronics enable neuromodulation in growing tissue. Reprinted and adapted with permission from Ref. (Liu et al., 2020b).

Figure 4

Miscellaneous novel conductive gels (A) Photographs of a zwitterionic PIL-based soft gripper holding a cup of ice water. Reprinted and adapted with permission from Ref. (Liu et al., 2020c), Royal Society of Chemistry. (B) Photographs of a click-ionogel stretched above liquid nitrogen (at about −50°C). Reprinted and adapted with permission from Ref. (Ren et al., 2019), 2019 American Association for the Advancement of Science.

Figure 5

Hydrogel adhesives (A) The tough bonding between hydrogel and solid surfaces. (B) Photograph of the peeling process of a tough hydrogel with its long-chain network chemically anchored on a glass substrate. Reprinted and adapted with permission from Ref. (Yuk et al., 2016b), Nature publish group. (C) The tough bonding between hydrogel and soft materials. Reprinted and adapted with permission from Ref. (Yuk et al., 2016a), Nature Publish Group. (D) Design of hydrogel tough adhesives. (E) In-vivo test on a beating porcine heart with blood exposure. (D and E) Reprinted and adapted with permission from Ref. (Li et al., 2017a), American Association for the Advancement of Science. (F). Tissue adhesive takes the form of a dry DST. (G). The dry cross-linking mechanism for the DST integrates the drying of interfacial water by hydration and swelling of the dry DST, temporary cross-linking, and covalent cross-linking. (H) The DST can take on various shapes owing to its high flexibility in fabrication. (F–H) Reprinted and adapted with permission from Ref. (Yuk et al., 2019b), Nature Publish Group.

Figure 6

Hydrogel-based biomechanical sensors (A) Capacitance-based ionic hydrogel pressure sensors. Reprinted and adapted with permission from Ref (Sun et al., 2014). (B) Graded intrafillable architecture-based ionic hydrogel pressure sensor. Reprinted and adapted with permission from Ref. (Bai et al., 2020). (C) Triboelectric nanogenerator-based ionic hydrogel pressure sensor. Reprinted and adapted with permission from Ref. (Pu et al., 2017). (D) Resistance-based ionic hydrogel strain sensor. Reprinted and adapted with permission from Ref. (Cheng et al., 2019).

Figure 7

Hydrogel-based temperature sensor (A) Schematic illustration of synthesis process of a hybrid hydrogel composed of PNIPAM and conductive polymers. Reprinted and adapted with permission from Ref. (Shi et al., 2015). (B). Schematic illustration of the plasmonic microgels in the PAAm hydrogel under swollen and shrunk states and schemes of a sensor array patch attached to human skin at different positions (neck and hand). Reprinted and adapted with permission from Ref. (Choe et al., 2018). (C) Three principles of stretchable elastomeric thermocouple. Reprinted and adapted with permission from Ref. (Wang, 2019).

Figure 8

Hydrogel-based biochemical sensor (A) Schematics of organohydrogel-based humid sensors based on the mechanism of hygroscopic EG/glycerol molecules capable of adsorbing water molecules (via forming hydrogen bonds), which improves both the water-holding and humidity sensing capabilities of the organohydrogel. Reprinted and adapted with permission from Ref. (Wu et al., 2019a). (B) Poly(acrylic acid) ionic hydrogel for pH sensing Reprinted and adapted with permission from Ref. (Yin et al., 2016).

Figure 9

Hydrogel based multimodel sensation (A) Strain and humidity sensing by an ionic diode-based artificial skin. Reprinted and adapted with permission from Ref. (Ying et al., 2020a). (B) Strain and temperature sensing of a three-layer biomimetic skin. Reprinted and adapted with permission from Ref. (Lei and Wu, 2018).

Figure 10

Ionic hydrogels for soft robotics (A) Highly stretchable electroluminescent skin for optical signaling and tactile sensing. Reprinted and adapted with permission from Ref. (Larson et al., 2016). (B) Soft somatosensitive actuator innervated with multiple soft sensors (curvature sensor, the inflation sensor and the contact sensor). Reprinted and adapted with permission from Ref. (Truby et al., 2018). (C) Ionic hydrogel-based stretchable electrodes for dielectric elastomer actuators. Reprinted and adapted with permission from Ref. (Keplinger et al., 2013).