Surface Morphology Changes and Micromechanics in PNiPAAm Based Micro- and Nanogels Particles

Gels thermal responsivity is usually associated to a Volume Phase Transition, VPT. However, different and equally relevant for biomedical applications, responsive behaviors to temperature can be found in micro- and nanogel particles. In this contribution we report on the morphological changes around physiological temperature of hybrid nanogel particles made by crosslinking via click chemistry an azido-derivative of hyaluronic acid, HA, and a derivative of poly(N-isopropyl acrylamide), PNiPAm, having propargyl functions at both ends, with interesting fallouts concerning targeting and drug delivery. In in vitro tests, doxorubicin loaded nanoparticles with a PNiPAAm/HA ratio of 33 % (w/w) increased its cytotoxicity on HT-29 tumor cells over the NIH 3T3 healthy fibroblasts, by a factor of two. In this HA/PNiPAAm nanogel, the hydrophobic – hydrophylic interplay of the two crosslinked polymer components gives rise to a reshuffling of the surface composition passing a quite narrow threshold around 33 °C, taken as the VPT temperature, VPTT, of the system. In this process, the room temperature surface structure is a collection of PNiPAAm and hyaluronate patches. Approaching the VPTT, the PNiPAAm hydrophobicity promotes the interparticle aggregation. Above the VPTT, the NiPAAm residues self-segregate in the particle core, leaving the charged HA moiety on the surface. This moieties redistribution, and not the size reduction, within the microgel particle, assessed by an atomic force microscopy, AFM, study of the surface, dynamic light scattering, DLS, zeta potential analysis and small angle neutron scattering, SANS, is at the base of the recognition of tumor cells via the interaction HA/receptors of overexpressed CD44 transmembrane proteins and of the specific drug delivery mechanism. In PNiPAAm hydrogels, drug release has been often coupled with the temperature response of the matrix. However, besides the temperature, responsiveness to other state variables, such as pressure, or to the combination of both temperature and pressure can be considered. Two microgels having PNiPAAm as the only polymer component but different architectures, obtained by precipitation polymerization, were investigated: a morphology with (i) an un-crosslinked core and a crosslinked shell of PNIPAM chains and a softer one, (ii), having crosslinked PNIPAM chains as particle core with a shell of tethered un-crosslinked PNIPAM chains to the core. Confocal microscopy was a useful tool to validate the different morphologies. Differential scanning calorimetry, DSC, and DLS were used to follow the microgels as a function of temperature, pressure and both, pressure being osmotically controlled with a high molecular weight non-interacting polymer. Under osmotic stress, soft microgels can be compressed above the VPTT, reaching extremely low hydration degrees. The microgel volume behavior as a function of pressure and temperature allowed to estimate the isothermal compressibility and thermal expansion coefficients for the different morphologies. The VPT enthalpy is scarcely affected by the network architecture, whereas the work on osmotically stressed PNiPAAm is strongly influenced by the morphology. As biological counterpart, a very low cytotoxicity was highlighted by MTT tests of all morphologies on HaCAT cell line, while it is noteworthy that the softer morphology (ii) of microgels enhances the phagocytosis by RAW 264 macrophages.