Thermoresponsive hydrogels based on a phosphorylated star-shaped copolymer: mimicking the extracellular matrix for in situ bone repair

A type of four-arm star-shaped copolymer (star-PAA(PEA)-PNIPAM), consisting of the thermoresponsive block (poly(N-isopropylacrylamide), PNIPAM) and the phosphorylated functional block O-phosphoethanolamine (PEA) grafted poly(acrylic acid) (PAA(PEA)), is synthesized by atom transfer radical polymerization (ATRP) and subsequent modification. Owing to the unique superiority of the star-shaped structure, star-PAA(PEA)-PNIPAM can transform from the sol to gel state in response to the physiological temperature (37 °C) at a relatively low polymer concentration (>0.5 wt%). In addition, because of the enriched phosphorylated functional groups, the hydrogel formed by star-PAA(PEA)-PNIPAM can mimic the acidic extracellular matrix protein to adsorb calcium ions and mineralize in situ, both in in vitro and in in vivo experiments. Meanwhile, it is favorable for cell adhesion and proliferation due to its appropriate three-dimensional interspace. Thus, the biocompatible star-PAA(PEA)-PNIPAM hydrogel has great potential for bone repair applications.