Structural Effects of Sulfur-Containing Functional Groups on Apatite Formation on Ca²⁺-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 (-SO₃H) 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 (-SO₂H), which are derivatives of -SO₃H, 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 Ca²⁺-modified copolymers containing -SO₂H or -SO₃H was investigated in a simulated body environment. The copolymer containing Ca²⁺ and -SO₃H promoted Ca²⁺ release into the SBF and formed apatite faster (1 day) than the copolymer containing Ca²⁺ and -SO₂H (14 days). In contrast, when they were not modified with Ca²⁺, the copolymer containing only -SO₂H deposited the apatite faster (7 days) than that containing only -SO₃H (>7 days) in the solution with Ca²⁺ concentration 1.5 times that of SBF. The former adsorbed larger amounts of Ca²⁺ than the latter. The measured stability constant of the complex indicated that the interaction of -SO₂^-···Ca²⁺ was more stable than that of -SO₃^-···Ca²⁺. It was found that both the release and adsorption of Ca²⁺ 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 Ca²⁺.