Microstructured thermo-responsive double network granular hydrogels.

Many hydrogels respond to external stimuli such as changes in temperature, pH, or salt concentrations by changing their degree of swelling, and hence mechanical properties, rendering them attractive actuators. Unfortunately, response rates of many of these hydrogels are limited because they rely on the diffusion of water, which is relatively slow within the gel. Here, we introduce thermo-responsive granular hydrogels which combine accelerated response rates with load-bearing properties. To accelerate the response to temperature changes, we formulate poly(-isopropylacrylamide) (PNIPAM) microgels with connected pores by leveraging phase separations. To impart the porous hydrogel load-bearing properties, we formulate them as thermo-responsive double network granular hydrogels (TDNGHs). We demonstrate that the granular structure combined with the open micropores located within the microfragments increase the response-rate of these gels 3-fold compared to that of bulk counterparts. Moreover, the granular material exhibits 18-fold enhanced work of fracture compared to the bulk. The granular structure adds an additional benefit: it renders them 3D printable. We co-process thermo-responsive hydrogels with a non-responsive counterpart to fabricate a bilayer, which lifts up to 85% of its weight if heated and 3D print a butterfly as a bilayer structure that bends its wings when exposed to elevated temperatures.