Monolithic intercalated PNIPAm/starch hydrogels with very fast and extensive one-way volume and swelling responses to temperature and pH: prospective actuators and drug release systems.

Remarkable monolithic (non-porous) hydrogels based on poly(NIPAm-co-sodium methacrylate) intercalated by starch were prepared, and were found to display very fast and extensive one-way solvent (water) release, induced by both pH and temperature. With centimeter-sized 3D specimens, the achieved response times were as short as 4 min (for 70% water release), in combination with very large volume responses (shrinking ratios up to 15). The response time can be tuned from minutes, over tens of minutes, up to hours. The pH-induced deswelling is always slower than the temperature-induced one, but at the highest starch content, ca. 5.5 min are needed for 70% completion of the pH-triggered process. Simultaneous temperature- and pH-stimuli expectedly also lead to very fast water release. The unique intercalated structure and the temperature-dependent hydrogen bridging between the intercalated phases, as well as between these phases and water, were found to play the key role in the ability of the gels to rapidly release water and shrink, which was deeper elucidated in this work. The hydrogels are of interest as soft actuators, but also for chemical release systems or for drug release applications. The latter was successfully tested with theophylline as the drug.