Temperature-Dependent Nanoparticle Dynamics in Poly(N-isopropylacrylamide) Gels

Nanoparticle (NP) probes were used to characterize the local structure of N-isopropylacrylamide (NIPAAM), a thermoresponsive hydrogel, using single particle tracking (SPT). Swelling ratio, and thus gel network confinement, was varied by tuning polymer and cross-linker concentrations. Based on the swelling ratio, the volume phase transition (VPTT) was determined to be near 32 degrees C. In general, NPs were found to be localized by two barriers. A primary localization region of approximately 100 nm was attributed to attractive interactions between the NIPAAM strands and the poly(ethylene glycol) (PEG) brush grafted to the NP. As the polymer and cross-linker concentrations were reduced, or temperature approached the VPT, NPs escape the primary localization region and explore a larger secondary localization region (150-300 nm), ascribed to confinement by the gel network. As temperature was raised above the VPT, however, the increase in confinement due to the collapse of the NIPAAM strands dominated, causing NPs to become localized to a single region despite the higher temperature. This study of NP dynamics provides insight into controlling the release and loading of drugs in responsive hydrogel systems.