Using osmotic deswelling of microgel particles to control the mechanical properties of pH-responsive hydrogel composites

The ability to tune the mechanical properties of hydrogel composites through the rational design of their constituents has excellent potential for providing advanced hydrogels for regenerative medicine applications. In this study we investigate the morphology and mechanical properties of a new family of pH-responsive hydrogel composites. They were prepared by adding crosslinking monomers (X) to concentrated dispersions of pH-responsive poly(EA/MAA/BDD) (ethylacrylate, methacrylic acid and butanediol diacrylate) MG particles. These MG/X dispersions formed physical gels. The dispersions were then polymerised to give pH-responsive hydrogel composites containing MGs, which are termed here as MG/H-X. The MG particles swelled strongly when the pH exceeded 6.0. The added crosslinking monomers were either ethyleneglycol dimethacrylate (EGD) or poly(ethylene glycol) dimethacrylate (PEGD). The latter had molecular weights in the range of 330 to 750 g/mol. If X had a molecular weight of 330 g/mol, or more, osmotic deswelling of the MG particles occurred. This strongly affected both the gelation behaviour of the MG/X dispersions and the mechanical properties of the chemically crosslinked MG/H-X hydrogel composites. For these new hydrogel composites X acted in two ways; it directed the morphology through osmotic deswelling of the MG particles and also provided a source of crosslinking. The storage modulus of the hydrogel composites was tuneable over a range of five orders of magnitude. The MG/H-X hydrogel composites showed pH-dependent swelling and mechanical properties. The hydrogel composites could also be prepared using poly(EA/MAA/EGD) and poly(EA/MAA/PEGD550) microgels. The simple method introduced here should enable construction of a wide range of pH-responsive hydrogel composites containing microgels with morphologies that can be tuned at the length scale of the MG particles, i.e., about 100 to 500 nm.