Conductive Hydrogel-Elastomer Hybrid Circuits

To form conductive hydrogel circuit board pattern on Ecoflex substrate, thin PETE film (70 μm thickness) with predetermined circuit board pattern was prepared using laser-cutting machine (Epilog Mini/Helix). As a template for hydrogel pattern on elastomer, the film with circuit board pattern was assembled with thin Ecoflex substrate (1 mm thickness) treated with benzophenone solution as previously described. Thereafter, PAAm-alginate pre-gel solution was poured onto the assembly and covered with a glass slide, followed by ultraviolet irradiation for an hour. After ultraviolet irradiation, the glass cover and the PETE film were removed from the Ecoflex substrate leaving robustly bonded PAAm-alginate hydrogel pattern. The hydrogel pattern was made to be ionically conductive by submerging the hybrid in concentrated sodium chloride solution (3 M) for 6 h. To light up a LED on the conductive hydrogel circuit pattern, each ends of pattern were connected to a functional generator (5 V peak-to-peak voltage at 1 kHz).
The electrical property of the conductive hydrogel–elastomer hybrid under deformation was measured using the four-point method19 . The ionically conductive hydrogel pattern with 50 mm in length, 1 mm in width and 200 μm in thickness was bonded on thin Ecoflex substrate (1 mm thickness) following the abovementioned method. The two ends of the hydrogel pattern were connected in series with a function generator and a galvanometer, and the voltage between two ends were measured with a voltmeter connected in parallel (Supplementary Fig. 10a). The ratio of the measured voltage to the measured current gave the electric resistance of ionically conductive hydrogel pattern. The rate of stretch was kept constant at 100 mm min using a mechanical testing machine. Cyclic extension of the conductive hydrogel–elastomer hybrid was done by mechanical testing machine based on predetermined numbers of cycles.