Skin-Inspired Ultra-Tough Supramolecular Multifunctional Hydrogel Electronic Skin for Human-Machine Interaction

Abstract

Multifunctional supramolecular ultra-tough bionic e-skin with unique durability for human-machine interaction in complex scenarios still remains challenging. Herein, we develop a skin-inspired ultra-tough e-skin with tunable mechanical properties by a physical cross-linking salting-freezing-thawing method. The gelling agent (β-Glycerophosphate sodium: Gp) induces the aggregation and binding of PVA molecular chains and thereby toughens them (stress up to 5.79 MPa, toughness up to 13.96 MJ m^-3). Notably, due to molecular self-assembly, hydrogels can be fully recycled and reprocessed by direct heating (100 °C for a few seconds), and the tensile strength can still be maintained at about 100% after six recoveries. The hydrogel integrates transparency (> 60%), super toughness (up to 13.96 MJ m^-3, bearing 1500 times of its own tensile weight), good antibacterial properties (E. coli and S. aureus), UV protection (Filtration: 80%-90%), high electrical conductivity (4.72 S m^-1), anti-swelling and recyclability. The hydrogel can not only monitor daily physiological activities, but also be used for complex activities underwater and message encryption/decryption. We also used it to create a complete finger joint rehabilitation system with an interactive interface that dynamically presents the user's health status. Our multifunctional electronic skin will have a profound impact on the future of new rehabilitation medical, human-machine interaction, VR/AR and the metaverse fields.

Keywords: Flexible electronics; Human–machine interaction; Knuckle training; Supramolecular; Ultra-tough hydrogel.

Fig. 1

Structural properties of PGC supramolecular hydrogel. a Microscopic composition and skin-like properties of PGC hydrogel. b Production process of PGC hydrogel

Fig. 2

Characterization of PGC hydrogel. Different hydrogels of a FTIR spectra and b tensile curves. c Strain–stress curves of PGC hydrogel with different Gp concentrations. d Strain–stress curves of PGC hydrogel with different PVA concentrations (10%, 15%, 20%, 25%, 30%, 35%). e Immersion in different ionic solutions for the same time (CaCl₂, NaCl, sodium citrate). f Comparison plots of the hydrogel in this work with other tough/supramolecular hydrogels by toughness versus tensile strength. SEM images of g PVA and h PGC hydrogels at same magnifications. Tensile deformation including: i stretching, twisting stretching, knotting stretching, crossing stretching and j puncture resistance. k Lifting an object 1500 times heavier than itself without damage. Scale bars: 2 cm in (i–k)

Fig. 3

Conductivity and the UV shielding properties of PGC hydrogel. a Electrical conductivity of PGC hydrogels in different ionic solutions. b Cyclic stretching at different speeds. c Relative resistance changes under different cycling tensile strains at a fixed tensile speed of 2 mm s⁻¹. d PGC hydrogel's antifatigue conductivity of tensile loading–unloading cycles in 3,000 s. e Gauge factor under the strain range of 0–370%. f Comparison plots of the hydrogel in this article with other recently reported works by tensile strength versus conductivity. g UV absorption diagram of PGC hydrogel. h UV–vis transmittance spectra, i UV–vis absorbance spectra, and j UV–vis transmittance at 550 and 365 nm of the PGC hydrogel with different TA contents. (***p ≤ 0.001)

Fig. 4

Antibacterial activity of PVA, PGP, PGC and PGC₆ against a S. aureus, b E. coli. The diameter of inhibition circle of different hydrogels: c S. aureus, d E. coli (n = 5). The OD data of e S. aureus, f E. coli, the photographs in the figure are their corresponding bacterial suspension after 24 h incubation (n = 3). (*p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001, n.s.: no significant difference)

Fig. 5

Responsiveness of PGC sensors to various life activities: a coughing; b frowning; c ECG monitoring; d finger bending with different angles (30°, 60° and 90°); The flexion movements of the e wrist, f elbow and g knee joints. h PGC hydrogel as electronic skin for human–machine interaction. Scale bars: 2 cm in h

Fig. 6

PGC hydrogel for underwater sensing applications. Swelling behavior of PGC hydrogels in a water and b simulated seawater. c PGC hydrogels for potential applications such as underwater or encrypted communications. d Operational framework of the code-breaking system. The operation of “001101”, “010011”, “010001” and “011001” in e water and f simulated seawater, respectively

Fig. 7

PGC sensor for finger joint muscle training. a Diagram of the training-analysis process. b, c Force-stress-resistance three-dimensional curve and resistance real-time change curve for 3/3.5 mm type sensor. d Flow chart of the operation of MST analysis software. Tester’s resistance rate of e change-time, f traction force–time curve