A theoretical model for thermal-sensitive microgel with pnipam core and elastic shell

Poly (N-isopropylacrylamide) (PNIPAM) microgels are widely used in drug delivery due to their fast response to temperature. In order to get a better biocompatibility, PNIPAM microgels are typically coated with a layer of biocompatible material, resulting in composite microgels with core-shell structure. In a composite microgel prepared recently, for example, a microsphere of PNIPAM gel is enclosed by a phospholipid membrane, and the composite microgel exhibits a substantial volume transition in response to temperature changes. Here we develop a theoretical model to describe the thermal-responsive behavior of this composite microgel. In particular, we treat the phospholipid membrane as an elastic layer behaving like rubber-like elastomers and adopt the form of the free-energy function for nematic gels (which refer to anther species of thermal-sensitive gels whose behavior has been intensively studied) as that for PNIPAM gels. We show that the thermal-responsive behavior of the composite microgel can be markedly influenced by the membrane. By investigating the state of stress on the interface, we further predict that when the coating membrane is stiff and thin, wrinkles are expected to occur on the outer surface of the composite microgel after the volume transition.