Apatite-forming ability of polyglutamic acid hydrogels in a body-simulating environment

Abstract

Artificial joints can replace damaged joints provided the surrounding bone is sufficiently dense. However, elderly patients generally have reduced osteoporosis-associated bone density. Therefore, restitution of bone density is essential to ensure implantation. Injectable and resorbable bioactive fillers with bone-bonding ability (osteoconductivity) are promising, as osteoporosis can be reversed with minimal invasion. Osteoconduction occurs through the surface formation of biologically active hydroxyapatite via reactions with body fluids. Heterogeneous nucleation of the hydroxyapatite is catalysed by specific surface functional groups. In addition, release of Ca²⁺ ions into the surrounding fluids enhances apatite nucleation by increasing its degree of supersaturation. We tested injectable bioactive filler made from cross-linked polyglutamic acid (PGA). This has many carboxyl groups that facilitate apatite nucleation. An insoluble hydrogel can be formed by cross-linkage. We exposed PGA gels to a simulated body fluid for 7 days. Trace amounts of calcium phosphate were formed, but were not identified as bone-like apatite by X-ray diffraction. However, formation of a bone-like apatite layer was detected using pre-treatment with CaCl₂ solutions (>0.01 mol dm⁻³) dose dependently. Thus, this chemically cross-linked PGA gel could induce the heterogeneous nucleation of hydroxyapatite in a body environment, and this was enhanced by pre-treatment with CaCl₂.