A supramolecular biomimetic skin combining a wide spectrum of mechanical properties and multiple sensory capabilities

Abstract

Biomimetic skin-like materials, capable of adapting shapes to variable environments and sensing external stimuli, are of great significance in a wide range of applications, including artificial intelligence, soft robotics, and smart wearable devices. However, such highly sophisticated intelligence has been mainly found in natural creatures while rarely realized in artificial materials. Herein, we fabricate a type of biomimetic iontronics to imitate natural skins using supramolecular polyelectrolyte hydrogels. The dynamic viscoelastic networks provide the biomimetic skin with a wide spectrum of mechanical properties, including flexible reconfiguration ability, robust elasticity, extremely large stretchability, autonomous self-healability, and recyclability. Meanwhile, polyelectrolytes' ionic conductivity allows multiple sensory capabilities toward temperature, strain, and stress. This work provides not only insights into dynamic interactions and sensing mechanism of supramolecular iontronics, but may also promote the development of biomimetic skins with sophisticated intelligence similar to natural skins.

Fig. 1

Confirmation of multiple dynamic interactions in the supramolecular networks. a The chemical structure of the polyelectrolyte (PAA-co-DMAPS) synthesized in this work. b ATR-FTIR spectra (in the regions of 1800–1000 cm⁻¹) of the freeze-dried PAA-co-DMAPS copolymer and the supramolecular polyelectrolyte hydrogels equilibrated in D₂O with different NaCl concentrations and their corresponding second derivative curves. c Hydrodynamic diameters of the freeze-dried PAA-co-DMAPS copolymer in aqueous solutions (0.1 wt%) with different NaCl concentrations (0, 0.1, 0.5, and 1 M). The inset photo shows precipitates are observed in 1 M NaCl. d A photo of the PAA-co-DMAPS copolymer equilibrated in aqueous solutions (0.1 wt%) with different NaCl concentrations. e Swelling volume ratios of the polyelectrolyte hydrogels equilibrated in NaCl aqueous solutions with different concentrations. (error bars: standard deviations). f A corresponding photo of the hydrogels in e

Fig. 2

Viscoelasticity, reconfiguration ability, compliance, and autonomous self-healability of the dynamic supramolecular networks. a Frequency-dependent storage (G’) and loss (G”) moduli of the supramolecular polyelectrolyte hydrogel (the monomer mass ratio of AA: DMAPS is 4:1, equilibrated in 4 M NaCl). b A photo of the as-prepared transparent hydrogel; it is reshaped to adapt to the irregular surfaces of a prosthetic hand and shows compliance with the prosthetic finger’s locomotion (scale bar: 2 cm). c The direct observation of the reconfiguration and autonomous self-healing of the dynamic networks by time-resolved fluorescence microscopic images (scale bar: 20 μm). The white arrows indicate the chains in dynamic networks interdiffuse across the crack. d Schematic illustration of the dynamic process

Fig. 3

Strain and temperature sensations of the biomimetic skin. a Schematic design and two simplified equivalent electrical circuits of the biomimetic skin. b The capacitive response of the biomimetic skin upon mutual effect of strain and temperature. c The capacitance–strain curve of the biomimetic skin. Data are derived from average values of capacitance at different temperatures in b, and error bars (standard deviations) are very small suggesting the capacitive response is insensitive to temperature variation. d The resistive response of the biomimetic skin upon mutual effect of strain and temperature. e The reversible capacitance–strain curves and the theoretical prediction (red dash line). f The reversible resistance–strain curves and the theoretical prediction (red dash line). g The reversible resistance–temperature curves. h Capacitance–strain cycling curves in the strain range of 0–100%. i Resistance–strain cycling curves in the strain range of 0–100%. j Resistance–temperature cycling curves in the temperature range of 10–50 °C

Fig. 4

Applications in strain and temperature sensing and self-healability of the biomimetic skin. a A photo of the biomimetic skin attached to a prosthetic finger with the assistance of VHB tapes. b Capacitive signals monitor the finger’s movements. The inset photo is derived from Supplementary Movie 2. c Resistive signals when a person’s hand contacts the prosthetic finger. The inset photo is derived from Supplementary Movie 3. d An infrared image of the prosthetic hand after the removal of the person’s hand. e A photo of a biomimetic skin whose hydrogel layer is cut into half. f A photo of the biomimetic skin after self-healing. g The resistance and capacitance of the biomimetic skin before fracture and after self-healing. h The capacitance changes of the biomimetic skin before fracture and after self-healing when it is applied to detect a prosthetic finger’s bending–straightening movement (the capacitance increases when the finger bends and decreases when it straightens). (Scale bar: 2 cm)