Hyperelastic model for polyacrylamide-gelatin double network shape-memory hydrogels

A shape-memory double network hydrogel consists of two polymer networks: a chemically crosslinked primary network that is responsible for the permanent shape and a physically crosslinked secondary network that is used to fix the temporary shapes. The formation/melting transition of the secondary network serves as an effective mechanism for the double network hydrogel's shape-memory effect. When the crosslinks in the secondary network are dissociated by applying an external stimulus, only the primary network is left to support the load. When the secondary network is re-formed by removing the stimulus, both the primary and secondary networks support the load. In the past, models have been developed for the constitutive behaviors of double network hydrogels, but the model of shape-memory double network hydrogels is still lacking. This work aims to build a constitutive model for the polyacrylamide-gelatin double network shape-memory hydrogel developed in our previous work. The model is first calibrated by experimental data of the double network shape-memory hydrogel under uniaxial loading and then employed to predict the shape-fixing performance of the hydrogel. The model is also implemented into a three-dimension finite element code and utilized to simulate the shape-memory behavior of the double network hydrogel with inhomogeneous deformations related to applications. Graphic abstract A shape-memory double network hydrogel consists of a chemically crosslinked primary network and a physically crosslinked secondary network. The formation/melting transition of the secondary network serves as an effective mechanism for the shape-memory effect of the double network hydrogel. This work built a constitutive model for the polyacrylamide-and-gelatin double network shape-memory hydrogel. The model was first calibrated by experimental data and then employed to predict the shape-fixing performance of the hydrogel. The model was also implemented into a three-dimension finite element code and utilized to simulate the shape-memory behavior of double network hydrogel in complex geometries.