Reversible bidirectional bending of hydrogel-based bilayer actuators.

Temperature and pH responsive semi-interpenetrating network (semi-IPN) hydrogel-based bilayer actuators were fabricated by generating a poly(N-isopropylacrylamide) (pNIPAm)-based hydrogel in the presence of positively charged polyelectrolyte poly(diallyldimethylammonium chloride) (pDADMAC) on a layer of gold-coated polydimethylsiloxane (PDMS). The bilayers showed unique bidirectional bending behavior in response to solution temperature and pH, which is a result of the modulation of the hydrogel solvation state in response to solution temperature and pH changes. The behavior described here is vastly different than what is observed from bilayers composed of just pNIPAm. The direction and degree of bending of the device could be easily adjusted by tuning the composition of the hydrogel layer. We also showed that the bilayers could be used as stimulus-induced grippers and for controlled/triggered small molecule delivery, which can make the bilayers useful for various biomedical applications.