Artificial joints such as hip joint and knee joint have been popularly used for replacementofdamaged joints. To achieve a successful outcome offixation of the joints, surroundingbone tissues should have sufficient bone density. However bone density of elderlypatients is generally lower than that ofhealthy people due to diseases such as osteoporosis.This also makes it difficult to tightly fix the artificial joints to the bone. Thereforerecovery of the bone density is the important factor that governs success on theimplantation ofthe artificial joints. As a treatment to increase bone density ofthe patients,injectable bioactive filler with bone-bonding ability, Le. osteconductivity, can be anattractive candidate, since the osteoporosis can be recovered with minimal invasion. Sucha filler material should have not only osteoconductive property but also bioresorbabilityafter implantation in the bone tissue. It is known that osteoconduction on bioactive _materials is achieved by formation of biologically active hydroxyapatite (so-calledbone-like apatite) layer on their surfaces through chemical reaction with surroundingbody fluid. Heterogeneous nucleation of the hydroxyapatite is triggered by a catalyticeffect of specific functional groups on the material surfaces. In addition, release of Ca2+ions into the surrounding body fluids allows to enhance the apatite nucleation byincreasing degree of supersaturation with respect to the apatite. In this study, weattempted to develop injectable bioactive filler started from polyglutamic acid (pGA)through chemical cross-linkage, since PGA is abundant in carboxyl group that iseffective for inducing the apatite nucleation in body environments. The cross-linkage leadPGA hydrogel, whereas PGA without cross-linkage easily dissolved in aqueous media.The potential of the bone-like apatite formation was examined on the PGA gels due toexposure to a simulated body fluid (SBF) proposed by Kokubo and his colleagues. Aftersoaking in SBF for 7 days, trace amounts of particles were formed on the PGA gel, butwere not identified to bone-like apatite with X-ray diffraction. Formation of bone-likeapatite layer could be detected with X-ray diffraction on the PGAgels treated with CaChaqueous solution of 0.01 moldm-3 or more, followed by soaking in SBF for 7 days. Therate ofapatite formation increases with increasing concentration ofthe CaCh in the priortreatment. These results indicate that chemically cross-linked PGA gel has a potential toinduce the heterogeneous nucleation ofhydroxyapatite in body environment, and that thepotential would be remarkably enhanced by prior treatment ofthe gel with CaCh aqueoussolution.
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