Remotely triggered release from composite hydrogel sponges

A thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel crosslinked with methylene bisacrylamide (MBA), with the ability to undergo a volume change of approximately 50%, was used to fabricate a hydrogel sponge with oil microdrops encapsulated within its structure by the polymerization of an O/W emulsion. By adding 1.5 wt.% of iron oxide nanoparticles to the hydrogel structure it is possible to trigger the hydrogel volume change remotely by exposing the composite to an alternating magnetic field in the radiofrequency range, which heats up the nanoparticles. The increase of the hydrogel temperature leads to its shrinking and subsequently the encapsulated oil droplets are squeezed out. It was shown that the oil is released gradually during cyclic volume change (repeated heating and cooling) and the amount of oil content released during the first 8-15 shrinking cycles was in the range 1-3% v/v per cycle. The total amount of the released oil phase was in the range 18-24% v/v and it was found to be affected by the size of the encapsulated microdroplets, with the larger ones being squeezed out preferentially.