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.
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.
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