NOVELTY - Photo-responsive hydrogel comprising a copolymer comprising N-isopropylacrylamide, a polymerizable derivative of benzospiropyran, a crosslinking agent and an acid having a pKa of less than 6, is claimed. The hydrogel is operably responsive to exposure to water so as to undergo spontaneous protonation.    USE - The photo-responsive hydrogel is useful as a microvalve (claimed) in microfluidic platforms, sampling devices (environmental sensors), biomedical microfluidic devices, autonomous environmental sensor platforms, continuous-flow microfluidics, digital microfluidics, DNA chips (microarrays), molecular biology applications, evolutionary biology, cellular biophysics, microbial behaviour, acoustic droplet injection and fuel cells.    ADVANTAGE - The hydrogels possess excellent biocompatibility and have a degree of flexibility which is very similar to natural tissue, thus enabling them to be used in the field of microfluidic devices. The photo-responsive engineered hydrogels can be made to shrink, thus releasing a percentage of their water content upon light irradiation.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) preparing the hydrogel comprising (a) mixing N-isopropylacrylamide, the crosslinking agent, the polymerizable derivative of benzospiropyran and the acid, (b) dissolving the mixture in an organic solvent, (c) adding an initiator, and (d) polymerizing with white or UV light;    (2) a microvalve comprising a channel, where the photo-responsive hydrogel is placed in the channel to provide functionality to the channel, and the microvalve is operably responsive on exposure of the hydrogel to white light to effect opening of the valve and on exposure of the hydrogel to water, preferably in darkness, to effect closing of the valve; and    (3) operating a microvalve comprising the photo-responsive hydrogel, comprising (i) exposing the hydrogel to white light to effect opening of the valve and (ii) re-setting the valve by exposing the hydrogel to water, preferably in darkness, to effect closing of the valve.