Mechanics and physics of the light-driven response of hydrogels

Hydrogels are versatile and environmentally sensitive materials having appealing and tunable physical-chemical-mechanical properties, similar to those encountered in natural living tissues, which enable their use in a wide range of applications, especially in the biomedical field. In the present study, we investigate the temperature-driven responsiveness of one class of elastomeric gels, namely poly-N-isopropylacrylamide (pNIPAm), that allows for temperature-controlled swelling. In particular, we consider the mechanical behavior of temperature-sensitive hydrogels in which the temperature variation is the result of light-thermal conversion enabled by nanoparticles embedded in the material. Relying on a theoretical multi-physics-based model describing light diffusion, heat generation and transfer, fluid absorption, and mechanics, the morphing response of hydrogel elements is investigated. The complex interplay between fluid uptake (swelling) and mechanical deformation taking place at different temperatures is investigated. In particular, the light-driven swelling response of hydrogel under a time-dependent light stimulus is studied. It is shown that the self-cooling of the material influences the material responsiveness. Accordingly, we demonstrate the interplay between the generated and the dispersed heat, which is a key aspect for an efficient design of tunable devices and materials.