Structural Effects of Sulfur-Containing Functional Groups on Apatite Formation on Ca(2+)-Modified Copolymers in a Simulated Body Environment

[Image: see text] 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 (−SO(3)H) 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 (−SO(2)H), which are derivatives of −SO(3)H, 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(2+)-modified copolymers containing −SO(2)H or −SO(3)H was investigated in a simulated body environment. The copolymer containing Ca(2+) and −SO(3)H promoted Ca(2+) release into the SBF and formed apatite faster (1 day) than the copolymer containing Ca(2+) and −SO(2)H (14 days). In contrast, when they were not modified with Ca(2+), the copolymer containing only −SO(2)H deposited the apatite faster (7 days) than that containing only −SO(3)H (>7 days) in the solution with Ca(2+) concentration 1.5 times that of SBF. The former adsorbed larger amounts of Ca(2+) than the latter. The measured stability constant of the complex indicated that the interaction of −SO(2)(–)···Ca(2+) was more stable than that of −SO(3)(–)···Ca(2+). It was found that both the release and adsorption of Ca(2+) 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(2+).