Figure 3.

Bistable system response, critical feed acid concentration for successful light-triggering and resulting actuation strain. (a) The observed bistability between the high pH kinetic equilibrium and low pH thermodynamic equilibrium states. Error bars ± 1 standard deviation. Illumination near the end of the high pH branch of bistability triggers a jump to the low pH state where actuation occurs. (b) Successful triggering only occurs above a critical feed acid concentration (9.5 mM – 9.75 mM), as shown for illumination of systems with feed acid concentrations ranging from 7.75 mM to 10.25 mM. (c) The resulting actuation strain response (44.7%), calculated as the change in hydrogel length normalized to the unstrained, swollen hydrogel length (l ₁ −l ₂ )/l ₁ × 100, is comparable with that of biological muscle. Photo inset of artificial muscle actuation states, black sphere adhered to hydrogel and used for tracking (1) prior to illumination, the hydrogel of extended length l ₁ is completely transparent (2) at the end of illumination the hydrogel still has not contracted (3) at the end of actuation it has now actuated to length l ₂. Scale bar 5 mm. The clear swollen hydrogel only becomes visible through the accumulation of iodine after triggering. All results with 12 mm long, 1 mm diameter (unswollen dimensions) micro-porous hydrogel in place (actuation blocking stress of approximately 23 kPa).