Most hydrogel actuators and sensors are made via acrylate polymerizations. Because these chain reactions are inhibited by oxygen, it is difficult to print thin films or dots with good control. Epoxy curing chemistry is much less sensitive to experimental conditions. We have previously shown that hydrogels formed from reaction between water-soluble amines and epoxides can be readily printed. If the gel is filled with conducting carbon at a level close to the percolation threshold, the resistance changes as water is taken up or removed from the gel. In particular, a pH decrease results in ionization of amine groups and drives swelling of the gel. By incorporating an enzyme, such as glucose oxidase, that releases hydrogen ions when its substrate is present, a resistance change can be used to measure glucose concentrations. These gels also respond to stress with a change in resistance. By making the gel the anode or cathode of an electrolytic cell, they can also be formed as actuators that expand or contract as the pH changes locally. Epoxy chemistry has been little explored for gels. It is very versatile and could be used to make a wide range of gel composites with one or more phases, varying water contents, varying functional groups and a range of electrical conductivity.
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