No abstract

While microgels and nanogels are most commonly used for the delivery of hydrophilic therapeutics, the water-swollen structure, size, deformability, colloidal stability, functionality, and physicochemical tunability of microgels can also offer benefits for addressing many of the barriers of conventional vehicles for the delivery of hydrophobic therapeutics. In this review, we describe approaches for designing microgels with the potential to load and subsequently deliver hydrophobic drugs by creating compartmentalized microgels (e.g., core−shell structures), introducing hydrophobic domains in microgels, leveraging host−guest interactions, and/or applying "smart" environmentally responsive materials with switchable hydrophobicity. In particular, the challenge of promoting hydrophobic drug loading without compromising the inherent advantages of microgels as delivery vehicles and ensuring practically relevant release kinetics from such structures is highlighted, with an eye toward the practical translation of such vehicles to the clinic.

While photopolymerization has been broadly used to fabricate hydrogels, the kinetics of the structural evolution of such hydrogels during photopolymerization and how the kinetics relate to ultimate hydrogel properties are not well-understood. Herein, small-amplitude oscillatory shear rheology (bulk scale) and time-resolved very small-angle neutron scattering (vSANS, microscale) are used in tandem to investigate the kinetics of the photopolymerization of methacrylated starch building blocks with different concentrations, charges (cationic (+), anionic (−), or neutral (0)), and morphologies (soluble branched starch, starch nanoparticles, or combinations thereof). Starch nanoparticles (SNPs) enabled the fabrication of much denser hydrogels than soluble starch but took longer to gel due to the reduced conformational mobility of the polymerizable methacrylate groups on the SNPs. The addition of charge (cationic or anionic) increases the bulk gelation time while significantly reducing the observed changes in the fluid scale and correlation length, suggesting less covalent crosslinking and inherent SNP deformation during photogelation; indeed, the fluid exponent analysis suggests that charged SNPs deswell upon crosslinking, consistent with the behavior of microgels in colloidal crystals, while uncharged SNPs swell due to competition between inter-and intraparticle crosslinking. The combination of shear rheology and vSANS measurements can thus inform the design of new photopolymerizable hydrogels with targeted comprehensive properties.