Actuator designs using environmentally responsive hydrogels

Environmentally responsive ( ER) hydrogels are hydrogels that can experience an abrupt volume change and hence hold or release a large amount of water under the change of certain environmental conditions, such as temperature, pH, or an electric field. Because of their unique capability to achieve a large yet reversible volume change, hydrogels have been widely used in microfluidics and biomedical applications, such as hydrogel sensors and actuators for microfluidic channels, novel drug delivery systems, and scaffolding materials for tissue engineering. In most applications, ER hydrogels are used to create an isotropic volume change or one-dimensional motion by constraining the hydrogel in the other two directions. In this study, using finite element based simulations, actuator designs are demonstrated that can generate a variety of shape changes by carefully patterning ER hydrogels on a non-ER hydrogel matrix. In these designs, as the environment changes, the ER hydrogels undergo volume change under the constraints imposed by the non-ER matrix, causing an eigenstrain (mismatch strain) and hence the deformation of the non- ER matrix. By properly controlling the locations of the ER hydrogel sections with respect to the non- ER hydrogel matrix, one can achieve a desired deformation of the composite structure. As examples, designs of a single material actuator, one linear spring composite actuator, and two coil composite actuators are demonstrated. The feasibility to produce these actuators and the possible applications are discussed at the end of the article.