Effect of phosphate functional groups on the calcification capacity of acrylic hydrogels

The incorporation of negatively charged groups into the structure of synthetic polymers is frequently advocated as a method for enhancing their calcification capacity required in orthopedic and dental applications. However, the results reported by various research groups are rather contentious, since inhibitory effects have also been observed in some studies. In the present study, phosphate groups were introduced in poly(2-hydroxyethyl methacrylate) (PHEMA) by copolymerization with 10% mol of either mono(2-acryloyloxyethyl) phosphate (MAEP) or mono(2-methacryloyloxyethyl) phosphate (MMEP). Incubation of these hydrogels for determined durations (1-9 weeks) in a simulated body fluid (SBF) solution induced deposition of calcium phosphate (CaP) deposits of whitlockite type. After 9 weeks, the amount of calcium deposited on the phosphate-containing polymers was four times lower than that found on PHEMA, as determined by X-ray photoelectron spectroscopy (XPS). Samples of copolymer HEMA-MAEP were implanted subcutaneously in rats and evaluated after 9 weeks. No CaP deposits could be detected on the copolymer by XPS or energy dispersive X-ray spectroscopy, while PHEMA samples were massively calcified. It was concluded that the presence of phosphate groups decreased the calcification capacity of the hydrogels, and that in the conditions of this study, the phosphate groups had an inhibitory effect on the deposition of CaP phases on HEMA-based hydrogels.