Structural and Dynamic Features of Thermoresponsive Microgels around the Volume Phase Transition Temperature

Sustained drug delivery requires the use of multi-functional devices with enhanced properties, including responsivity to external stimuli (such as temperature, pH, ionic strength), ability to target specific receptors, enhanced bioadhesion to cells and biocompatibility. Microgels represent one such multifunctional suitable as drug delivery and switchable microdevices. The fabrication of a stable colloidal aqueous suspension of biocompatible microgel spheres is based, for instance, on a poly(vinyl alcohol)/poly(methacrylate-co-N-isopropylacrylamide) network [1]. These microgel spheres undergo an entropy driven volume phase transition around physiological temperature, this process being driven by the incorporation of NiPAAm residues in the network. In this study the microgel was loaded with the anti cancer drug, doxorubicin. Upon microgel de-swelling, a marked increase in the amount of doxorubicin released was noted. Sieving and size exclusion effects were studied by laser scanning confocal microscopy with microgel particles exposed to fluorescent probes with different molecular weights (Figure 1). In this contribution we focus on some fundamental issues regarding modifications of the network structure at a nanoscopic level and of the diffusive behavior of water associated with the polymer network around the volume phase transition temperature (VPTT) [2]. Observations carried out at room temperature and at 40 °C (i.e. below and above the VPTT), provided an evaluation of the variation of the average pore size (from 5 nm to 3 nm). The diffusive behaviour of water molecules closely associated to the polymer network around the VPTT was investigated quasi-elastic neutron scattering. Nanostructured changes around VPTT of the microgel particles was probed in direct and reciprocal space, i.e. small angle neutron scattering (SANS) (Figure 2) and scanning transmission X-ray microscopy (STXM), respectively. A transition of the microgel interface from brush-like to smooth surface was evidenced by a power law change from 2 to 4 (Porod’s law).