Complexes of Ca(II) with polymers as precursors for preparation of amorphous calcium phosphate

Ca(II) complexes with poly(ethylene glycol) (PEG), poly(N-vinyl-2-pyrrolidone) (PVP) and poly(vinyl alcohol)(PVA) were formed in aqueous solutions. The Ca(II) complexes were used as Ca precursors to react with sodium phosphates to prepare amorphous calcium phosphate (ACP). The experimental results showed that ACP precipitates could be formed in the presence of PVA or PEG while only crystalline calcium phosphate precipitates (hydroxyapatite) was formed in the presence of PVP. When Ca ions were coordinated with PEG, the viscosity of the system was not obviously influenced, which was appropriate to separate the precipitates. Hence, PEG is considered to be an optimal polymeric complexing agent to prepare ACP. The Ca(11) complexes with the polymeric complexing agents can favour ACP formation, which is attributed to the coordinated polymeric complexing agents remaining in the structure of ACP, i.e. in aggregated Ca-9(PO4)(6) clusters. The existence of the coordinated complexing agents depresses the conversion process from ACP to crystalline hydroxyapatite, hence, ACP can survive in the mother solution.