Mechanics of multi-stimuli temperature-responsive hydrogels

• Mechanics of multi-stimuli temperature-sensitive pNIPAm hydrogels • Light diffusion, photo-thermal heating, solvent equilibrium and polymer mechanics are considered in determining the coupled governing equations • Light-driven deformation of hydrogels is considered • FE numerical modeling of the multi physics problem is developed • Parametric numerical tests and experimental validations are performed The temperature-sensitivity of a certain class of elastomeric gels (cross-linked elastomers keen to uptake a fluid into their network, shown for example by poly-N-isopropylacrylamide (pNIPAm) hydrogels), is an interesting property allowing a temperature-controlled swelling. Being the elastomer-fluid affinity sensible to the temperature variation, this implies the possibility to control the amount of fluid uptaken by the material by properly changing the temperature. This capability is particularly interesting in biomedical applications or in the development of sensors whose responsiveness is often required to depend on the environmental temperature. Further, the incorporation of photo-thermal particles into the gel enables the use of light for controlling the material response. In this way, smart untethered multi-stimuli sensors or actuators can be obtained. In the present study, we consider the mechanical behavior of temperature-sensitive hydrogels and, relying on a theoretical multiphysics-based model accounting for light diffusion, heat generation and transfer, fluid absorption, and mechanics, the response of morphing elements is studied. Light-induced morphing due to photo-thermal effect is also considered and mathematically modeled. Validation and parametric simulations of the emerging deformations confirm the soundness of the approach and demonstrate the wide range of morphing functionalities obtainable by harnessing the temperature-dependent sensitivity of pNIPAm.