Ecoflex

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.

All tests were performed in ambient air at room temperature. The hydrogels and hydrogel–elastomer interfaces maintained consistent properties over the time of the tests (that is, approximately a few minutes), during which the effect of dehydration is not significant. The interfacial toughness of various hydrogel–elastomer hybrids was measured using the standard 90°-peeling test (ASTM D 2861) with mechanical testing machine (2 kN or 20 N load cells; Zwick/Roell Z2.5) and 90°-peeling fixture (Test Resources, G50). All elastomer substrates were prepared with 2.5 cm in width, 7.5 cm in length and 1 mm in thickness. polydimethylsiloxane and Ecoflex were adhered on borosilicate glass plate using oxygen plasma treatment (Harrick Plasma PDC-001). Latex and polyurethane were adhered on glass plate by with epoxy adhesives. VHB was simply adhered onto glass plate as it was provided in two-sided tape form. Hydrogels were bonded onto elastomer surfaces following the abovementioned procedure with the size of 100 × 15 × 3 mm (length × width × thickness). As a stiff backing for the hydrogel, PETE film was bonded onto the hydrogel with cyanoacrylate adhesive. The resultant samples were tested with the standard 90°-peeling test with a constant peeling speed of 50 mm min⁻¹. The measured peeling force reached a plateau (with slight oscillations), as the peeling process entered steady state. The interfacial toughness Γ was determined by dividing the plateau force F by the width of the hydrogel sheet W.
To investigate the effect of elastomer surface treatment on interfacial toughness and failure modes of hydrogel bonded on elastomers, the same 90°-peeling test was performed using PAAm-alginate tough hydrogel and polydimethylsiloxane substrate with the same sample size and testing conditions. The surface treatment time for polydimethylsiloxane substrate was fixed to 2 min, while the concentration of benzophenone in the surface treatment solutions was varied from 2 wt.% to 10 wt.%. As PAAm-alginate tough hydrogel cannot be successfully cured on top of polydimethylsiloxane with the surface treatment solution containing <5 wt.% of benzophenone due to the effect of oxygen inhibition, 2 wt.% of glucose and 0.02 wt.% of glucose oxidase were added as an oxygen scavenger into the prescribed PAAm-alginate pre-gel solution.
For uniaxial-tensile tests of hydrogel–elastomer hybrids, PAAm-alginate tough hydrogel and PAAm common hydrogel with size of 50 × 20 × 3 mm (length × width × thickness) were bonded onto Ecoflex substrate following the abovementioned procedure. For physically attached samples, the same size of PAAm-alginate tough hydrogel was simply put onto the Ecoflex substrate without any other treatment. The stretching of hybrids was carried out using the mechanical testing machine (2 kN; Zwick/Roell Z2.5) with grip-to-grip separation speed of 100 mm min⁻¹.

The MWCNT and dispersant (polyvinylpyrrolidone, PVP) were precisely quantified to provide a mass ratio of 10:1 [38 (link)]. The MWCNT and PVP were dispersed into the hexane solution by an ultrasonic cleaner (30 min) (GW0303, GW Ultrasonic Instruments Co., Ltd., Shenzhen, China). Ecoflex part A was then incorporated into the solution and agitated at 1000 rpm for 30 min at high speed. Ecoflex part B was then added and the mixture was stirred for an additional 30 min. The resulting liquid mixture was poured into a mold. After the evaporation of hexane, the uncured MWCNT and Ecoflex mixture was then vacuum-dried in an oven for 30 min and heated at 60 °C for 1 h to achieve complete curing.

The beaker, glass rod, and Petri dish were cleaned with hexane and then dried in a heated oven at 60 °C for 30 min. Subsequently, they were removed from the oven and cooled. Ecoflex parts A and B were mixed at a ratio of 1:1 and stirred for 15 min. The resulting mixture was then poured into a Petri dish and put into a vacuum oven for 15 min to eliminate air bubbles. The Ecoflex fluid was then cured at room temperature for 3 h.