A Constitutive Model of Water-Triggered Shape Memory Hydrogels and Its Finite Element Implementation

Shape memory hydrogel is a type of hydrogel whose shape can transform between a temporary shape and its initial shape when exposed to external stimuli, such as water, temperature, and pH. Over the last decade, shape memory hydrogels have gained increasing interest owing to their distinct properties; however, constitutive models to describe their shape memory mechanism are still lacking. In this paper, we propose a constitutive model for water-triggered shape memory hydrogels based on the transition between the sparse and dense phases. In the model, the shape memory process is identified using two internal variables: the frozen deformation gradient and dense phase volume fraction. To validate the model for describing shape memory effects, we implemented the model in the finite element method using a user-defined element (UEL) subroutine in ABAQUS. To verify the accuracy of the proposed UEL, we simulated the water-triggered shape memory effects in different recovery processes under different uniaxial loads. Furthermore, we investigated the water-triggered shape memory behavior of a self-bending bilayer structure and a four-arm gripper structure using both experiments and simulations. Good agreement was observed between the simulation and experimental results.