Microgels: From responsive polymer colloids to biomaterials

Microgels are network polymer colloid particles that can swell in a good solvent or as a result of electrostatic repulsion between charged groups produced by pH-triggered neutralisation. They have attracted considerable interest as both model colloids and for their potential applications. This discussion reviews the properties of microgel particles and the current understanding of their structure. The review concentrates on the period after an earlier microgel review by Saunders and Vincent [Adv. Coll. Interf. Sci., 1999, 80, 1]. A key challenge for microgel research has involved elucidation of the internal particle structure. Most microgels prepared by emulsion or precipitation polymerisation have a core-shell structure. The segment density is usually highest in the core. Here, we discuss relationships between microgel structure and dispersion stability. The reasons for the exceptional stability of microgel dispersions are considered. There are a number of favourable structural features that make microgels candidates for biomaterial applications and these are discussed. The main potential biomaterial applications that have been investigated for microgels to date are drug delivery and regenerative medicine. Poly(NIPAM) (N-isopropylacrylamide) microgels have been extensively studied in the context of drug delivery. Regenerative medicine research for microgels is an emerging area. Recent work involving the use of gelled microgel dispersions to support biomechanically meaningful loads is considered. We conclude with a discussion of promising directions for microgel research as biomaterials.