Developing an analytical solution for photo-sensitive hydrogel bilayers

In this article, a new conceptual design of light-sensitive switches made of a soft bilayer is introduced. The bilayer structure consists of a photothermal-sensitive hydrogel strip attached to another neutral incompressible elastomeric layer. The bilayer is assumed to be initially flat under a uniform light irradiation. Decreasing the light intensity causes the bilayer to bend due to the inhomogeneous swelling of hydrogel layer and results in the switch actuation. To enlighten the actuation mechanism and investigate the influence of various parameters on the switch's photomechanical response, an analytical method is developed to study the bilayer deformation due to the light-intensity variation under plane-strain condition. Additionally, the finite element analysis of the bilayer is conducted, implementing the relevant constitutive model into a finite element code. The deformation and stress inside the layers are studied both analytically and numerically for several cases. The numerical results verify the accuracy of the presented analytical method in this work. The influence of various material and geometrical parameters including each layer's modulus, the thickness ratio of the layers, and the aspect ratio of the bilayer is investigated. The analytical method is found to be useful for proper design of the light-sensitive hydrogel-based switches.