Structural effects of phosphate groups on apatite formation in a copolymer modified with Ca²⁺ in a simulated body...

Organic-inorganic composites are novel bone substitutes that can ameliorate the mismatch of Young's moduli between natural bone and implanted ceramics. Phosphate groups contribute to the formation of apatite in a simulated body fluid (SBF) and the adhesion of osteoblast-like cells. Therefore, modification of a polymer with these functional groups is expected to enhance the ability of the organic-inorganic composite to bond with bone. Two phosphate groups have been used, phosphonic acid (-C-PO₃H₂) and phosphoric acid (-O-PO₃H₂). However, the effects of structural differences between these phosphate groups have not been clarified. In this study, the apatite formation of copolymers modified with Ca²⁺ and either -C-PO₃H₂ or -O-PO₃H₂ was examined. The mechanism of apatite formation is discussed based on analytical and computational approaches. The copolymers containing -O-PO₃H₂, but not those containing -C-PO₃H₂, formed apatite in the SBF, although both released similar amounts of Ca²⁺ into the SBF. Adsorption of HPO₄ ^2- from -O-PO₃H₂ in the SBF following Ca²⁺ adsorption was confirmed by zeta-potential measurement and X-ray photoelectron spectroscopy. The measurement of the complex formation constant revealed that the -O-PO₃ ^2-Ca²⁺ complex was thermodynamically unstable enough to convert into CaHPO₄, which was not the case with -C-PO₃ ^2-Ca²⁺. The formation of CaHPO₄-based clusters was found to be a key factor for apatite nucleation. In conclusion, this study revealed that modification with -O-PO₃H₂ was more effective for enhancing apatite formation compared with -C-PO₃H₂.