Response of hydrogels and microgels to nanoparticle crowding

Internal degrees of freedom of cross-linked polymer networks enable compressible hydrogels and microgels to swell or deswell by absorbing or expelling solvent in response to environmental changes. Macromolecules in solution can influence the swelling response by acting as crowding agents, with practical relevance for drug delivery and biosensing. We extend the Flory-Rehner theory of polymer network swelling to incorporate nanoparticle crowders as another implicit species that can perturb a gel through volume exclusion, including the entropic cost of crowder penetration from polymer field theory. Within the free-energy landscape of asymmetric ternary mixtures, we numerically solve equilibrium relations equating solvent and crowder chemical potentials inside and outside the gel. Independently, we perform Monte Carlo simulations, including trial moves that allow gels to swell and take up crowders. We investigate how crowder size, solution concentration, cross-link density, and solvent quality influence the swelling of polymeric gels, the partitioning of crowders between the inside and outside, and the structural properties of microgel suspensions governed by Hertzian elastic pair interactions. Our results show that nanoparticle crowding promotes deswelling, thereby weakening interparticle correlations, and that, with increasing concentration of crowders, polymeric gels reach a maximum loading capacity that is dependent on crowder size and polymer cross-link density.