Structural Effects of Sulfur-Containing Functional Groups on Apatite Formation on Ca-Modified Copolymers in a Simulated Body Environment.

Chemical modification with specific functional groups has been the conventional method to develop bone-bonding bioactive organic-inorganic hybrids. These materials are attractive as bone substitutes because they are flexible and have a Young's modulus similar to natural bone. Immobilization of sulfonic acid groups (-SOH) onto the polymer chain is expected to produce such hybrids because these groups induce apatite formation in a simulated body fluid (SBF) and enhance the activity of osteoblast-like cells. Sulfinic acid groups (-SOH), which are derivatives of -SOH, can also induce apatite nucleation. However, the structural effects of such sulfur-containing functional groups on apatite formation have not been elucidated. In the present study, apatite formation on Ca-modified copolymers containing -SOH or -SOH was investigated in a simulated body environment. The copolymer containing Ca and -SOH promoted Ca release into the SBF and formed apatite faster (1 day) than the copolymer containing Ca and -SOH (14 days). In contrast, when they were not modified with Ca, the copolymer containing only -SOH deposited the apatite faster (7 days) than that containing only -SOH (>7 days) in the solution with Ca concentration 1.5 times that of SBF. The former adsorbed larger amounts of Ca than the latter. The measured stability constant of the complex indicated that the interaction of -SO···Ca was more stable than that of -SO···Ca. It was found that both the release and adsorption of Ca governed by the stability played an important role in induction of the apatite formation and that the apatite-forming ability of sulfur-containing functional groups drastically changed by the coexistence of Ca.