Swelling under Constraints: Exploiting 3D-Printing to Optimize the Performance of Gel-Based Devices

Stimuli-responsive hydrogels that swell under constraints such as spatial geometric confinement are employed in many applications, including biomedical devices and actuators, to perform mechanical work. Due to the heterogeneous deformations that arise from these constraints, the computation of the swelling-induced stress poses numerical difficulties. This work proposes a simple experimental method based on 3D-printing technologies to characterize this stress. The swelling is investigated under two types of geometric confinements—transversely constraining and elastically constraining boxes. In the former, the box enforces uniaxial swelling of the gel. The results show that the longitudinal deformations decrease as the transverse stretches increase. The elastically constraining box comprises soft walls with various stiffnesses and sizes that deform in response to the swelling-induced stress exerted by the gel. By employing elastic plate theory, a method to determine this stress is developed. The results reveal that: 1) the maximum volumetric deformation is achieved by free swelling and 2) the stress gels exert depends on the wall stiffness and non-linearly decreases as the gel nears its freely swollen state. The insights from this work can be used to optimize the performance of swelling-based systems and characterize the stress generated due to other stimuli such as pH and temperature.