Porous PNIPAm hydrogels: Overcoming diffusion-governed hydrogel actuation

A custom centrifugal melt-spinning technique was used to prepare randomly arranged 3D fibre networks from commercially available shellac flakes. These fibre networks were implanted into thermally actuating poly(N-isopropylacrylamide)-alginate hydrogels and then removed chemically to yield an interconnected porous gel structure. Pore diameter was capable of being controlled through the fibre-spinning temperature, where it was shown that shellac fibres spun at a lower temperature yielded larger diameter pores in the resultant gels. These gels demonstrated a fast actuation, with a 77 % volume loss of a cylindrical sample in just 30 s when immersed in a 60 °C water bath. It was shown that the volume change mechanism overcame the well-known dimensional constraint, which has previously governed hydrogel swelling/de-swelling, through minimisation of the water diffusion distance from the gel to the porous network. With such rapidly actuating materials, potential applications for these hydrogels lie in thermally responsive valves and artificial muscles for soft robotics or microfluidics.