Fast pH- and ionic strength-responsive hydrogels in microchannels

A simple method for the preparation of pH- and ionic strength-sensitive hyd rogels that exhibit fast response times is described. Patterned hydrogel ob jects were prepared inside microchannels by photopolymerization of monomer mixtures containing water and surfactant. The swelling process, initiated b y an abrupt change in pH, exhibited second-order kinetics. Compared to conv entional hydrogels synthesized from a neat monomer mixture alone, the new h ydrogels swell and shrink faster by approximately one order of magnitude. F ully swollen hydrogels prepared from mixtures containing water and surfacta nt were observed to be sensitive to ionic strength changes. The effects of cross-linking density and acrylic acid composition on the response rate wer e investigated and were found to have little effect on response times. The fast response was thus attributed to the porous structure generated from th e surfactant phase template during the polymerization.