Dual responsive physical gels prepared from binary dispersions of pH- and temperature-responsive microgel particles.

Concentrated dispersions of stimulus responsive microgels (MGs) can form physical gels when the MGs are swollen and there is insufficient space for the particles to move past one another. Such gels are usually constructed from one particle type. Here, we blend a pH-responsive poly(ethyl acrylate-co-methacrylic acid-co-1, 4-divinylbenzene) (denoted as PEA-MAA) MG dispersion with a temperature-responsive poly(N-isopropylacrylamide-co-methylene-bis-N, N'-acrylamide) (denoted as PNIPAM) MG dispersion. The purpose of our experiments is to investigate the morphologies and rheological properties of these new dual responsive physical gels. The experiments involved gelation phase diagram construction and rheology studies. Our findings show that whilst the PEA-MAA MG swelling dominates gel-forming behavior, there is synergy within the mechanical properties. The mixed PEA-MAA/PNIPAM MG gels show rapid shear-thinning and self-healing. The gels exhibit pH-dependent and temperature-dependent mechanical properties, with the storage modulus reaching a maximum at pH 8.0 and decreasing with increasing temperature. Surprisingly, the PEA-MAA MGs cause the temperature at which the PNIPAM MGs collapse to increase. These mixed binary MG gels provide four methods for tuning the mechanical properties: proportion of each MG, total MG concentration, temperature and pH. We also investigate the ability to release a model drug and study the effect of temperature on the release rate, providing proof-of-concept data for a future dual responsive injectable MG gel based controlled drug release system. We conclude that there are interactions, possibly involving entanglements, between the two MG systems that are responsible for the hypothesized synergistic behaviors present within these binary MG gels.