Fast pH- and ionic strength-responsive hydrogels in microchannels

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