Transparent, Ultra-Stretching, Tough, Adhesive Carboxyethyl Chitin/Polyacrylamide Hydrogel Toward High-Performance Soft Electronics

Highlights Hydrogel demonstrates superior merits of strain (1586%), self-adhesion (113 kPa for pigskin), high conductivity and transparency (92%). The wearable sensors with a gauge factor up to 18.54, wide pressure sensing range (0–600 kPa) enable the detecting, quantifying, and monitoring of human activities. The hydrogels were developed as electronic skin, high output stretchable CTA-TENGs and explored using as wearable keyboards for human-machine interaction. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-022-00980-9.


S1.1 Mechanical Property Test
Tensile and compressive measurements were tested on the hydrogels using a universal tensilecompressive tester (INSTRON instrument,Model 5576,USA). For tension, hydrogel membranes with a length of 50 mm, a width of 13 mm and a thickness of about 1.5 mm were measured at the speed of 50 mm· min -1 . For compression, columnar hydrogels with a height of about 10 mm and a diameter of 15.5 mm were tested at the speed of 2 mm min -1 . Young's modulus was calculated from the initial linear region of the stress-strain curves. The fracture energy (toughness) was calculated from full region of the stress-strain curves.

S1.2 Adhesion Performance Testing of Hydrogel Samples
The adhesion strength was determined by the lap-shear test using a universal test machine (INSTRON instrument,Model 5576,USA). The glass, plastic, wood, metal substrates and pig skin without contaminants were cut into rectangle with a length of 40 mm and a width of 15 mm. Hydrogel samples (10 × 10 × 1.5 mm 3 ) were sandwiched between two substrates with an area of 10 × 10 mm 2 . After preloaded by 1kg weight for 10 min, the specimens were tested by the standard lap-shear test at a velocity of 10 mm min −1 under ambient conditions. The adhesion strength was calculated by dividing the maximum force by the adhesion area. Additionally, the adhesion-strip cyclic tests were also conducted to evaluate the effect of a cycle load on the adhesion strength of the hydrogels.

S1.3 Conductivity Assessment
Ionic conductivity of the hydrogels was measured by the electrochemical impedance spectroscopy (EIS) using an electrochemical workstation 165 (CHI760E, CH Instruments Ins) operated in the frequency range of 100 to 100 kHz and the amplitude of 5 mV. The hydrogels were sandwiched between two carbon cloths for the measurement. The ionic conductivity (σ, S m −1 ) of the hydrogels was calculated according to the following equation: where L (m), S (m 2 ), and R (Ω) was the length between two carbon cloths, the contact area of the hydrogel with carbon cloths, and resistance obtained by the intercept at the real part in Nyquist plots, respectively.

S1.4 Electrical Measurement
The electrical signals of the hydrogels were recorded by a capacitance meter (CAPACITANCE TESTER, UC2652, UCE Technologies). The change in the relative resistance/capacitance of the hydrogel sensors was examined using the above-mentioned capacitance meter at a constant voltage of 1 V, on the basis of different strains, and human motions. Relative changes in resistance and capacitance were calculated as the following equations: where R0, C0 and R, C are the original resistance, capacitance at the strain of 0% at room temperature and the real-time resistance, capacitance at a certain strain, respectively.

S1.5 Characterization
1 H NMR spectra were recorded on a Bruker Avance-III 400 MHz spectrometer at room temperature. Field emission scanning electron microscopy (FESEM, Zeiss, SIGMA, Germany) was used to characterize morphologies of lyophilized hydrogels. Fourier transform infrared spectroscopy (FT-IR) of lyophilized hydrogels were tested by a Nicolet 170-SX (Thermo Nicolet Ltd., USA) in the wavenumber range from 4000 to 400 cm −1 . X-ray photoelectron spectra (XPS, ESCALAB250Xi, Thermo Fisher Scientific, America) analyses were recorded using a Kratos XSAM800 X-ray photoelectron spectrometer. Optical transmittance of the hydrogel films with a thickness of 1.5 mm was observed with a UV-vis spectrometer (UV-6, Shanghai Meipuda Instrument Co., Ltd., China) at a wavelength from 900 to 200 nm. Raman spectroscopy and spatial Raman mapping were performed using a Raman imaging microscope (Thermo Scientific DXR xi, USA). The wavelength of the excitation laser was 532 nm. The collected spectra were preprocessed using cosmic ray removal, noise filtering, and normalization techniques. The multivariate curve resolution (MCR) method developed by OMNICxi software was applied for calculating the proportion of interaction domains.

S2 Supplementary Videos
Video S1 (.mp4 format). Demonstration of the strong self-adhesive properties of the CTA hydrogel by vigorously swinging the hand.
Video S2 (.mp4 format). The proof-of-concept demonstrations of the CTA hydrogel as a human-machine interactive system.

Video S3
(.mp4 format). Demonstration of the CTA hydrogel in the field of tactile sensing as a tactile switch.

S3 Supplementary Figures
Scheme S1 Synthesis route of CECT           Tables   Table S1 Effect