Kinetics of swelling of cylindrical functionally graded temperature-responsive hydrogels

Cylindrical hydrogels have a wide variety of applications in microfluidics; for example, they serve as micro-valves, micro-mixers, and micro-lenses. The main advantages of them can be mentioned as their autonomous functionality due to their responses to environmental stimuli and simple geometry. Furthermore, functionally graded hydrogels have recently found applications in hydrogel actuators. Therefore, in this work, the kinetics of swelling, shrinking, and force generation of cylindrical functionally graded temperature-responsive hydrogels are investigated. Kinetics of cylindrical structure is investigated analytically by developing a mathematical model based on available constitutive models of large deformation of hydrogels. The cross-link density of the hydrogel polymeric network varies along the radius of the cylinder linearly. In order to analyze the structure's behavior, the temperature is changed, and the response time of the structures with different distribution of cross-link density is investigated. In addition, to investigate the realistic actuators' performance, the cylindrical hydrogel is located inside a hollow cylinder, and the pressure of the hydrogel, which puts on the cylinder, is investigated. The results show that manipulation of cross-link density influences the response time of the cylindrical hydrogel; hence, it is a useful tool to manage the overall performance of cylindrical actuators.