Study On Nano-carbonated Hydroxyapatite And Its Organic Biocomposites

Hydroxyapatite/organic polymer biomedical composite is a hot topic of hard tissue biomaterials research. Hydroxyapatite (HA), the main mineral composition of bone, can form strong bone-bonding with natural bone and has been used extensively for biomedical applications and bone regeneration based on its high biocompatible, bioactive and osteoconductive properties, which has been the research focus of bone repair and substitute materials in resent 20-30 years. However, due to the brittleness, high modulus and fatigue failure in vivo, it is restricted in clinic for un-load bearing bone repair. In order to improve the mechanical property of HA material for hard tissue replacement, study on HA and polymer composite is highlighted. The combination of (HA) bioactivity and (polymer) toughness may result in a new load-bearing bioactive material with good mechanical property, excellent bioactivity and biocompatibility. Carbonate-substituted nano-hydroxyapatite (CHA) is synthesized with calcium nitrate tetrahydrate, diammonium hydrogen phosphate and diammonium hydrogen carbonate. The results show that all of the CHA are nanograde crystals and different' synthetic conditions have obviously effect on the morphology, crystallinity and the size of these nanoapatite crystals. The size and crystallinity of the apatite crystals increase with reaction temperature and hydrothermal treatment. In hydroxyapatite structure, the carbonate group can substitute the hydroxyl and the phosphate ions, called A-type CHA and B-type CHA respectively. The presence of carbonate in the apatite lattice causes a decrease in crystallinity and crystal size. Crystallinity and crystal size of HA are higher than those of CHA at same conditions. CHA85 prepared at 85 "C is B-type CHA in which [CO3 OH] tetrahedral-coordinations substitute [PO4] tetrahedral-coordinations. Both CHA60 prepared at 60 °C and CHA140 treated hydrothermally at 140°C are AB mixed-type CHA in which [CO3 OH] tetrahedral-coordinations substitute [PO4] tetrahedral-coordinations and [CO3] triangle-coordinations substitute [OH] in column sites. B-type is the main carbonate substitution in CHA60 and A-type is dominant in CHA 140. n-CHA/HDPE and n-CHA/HDPE-g-MAH nano-composites are prepared directly using n-CHA crystals slurry and co-solution method under normal atmosphere pressure. The results show that n-CHA crystals are nano-grade and disperse uniformly in the composites. The morphology, crystal structure and crystallinity as well as crystal size of n-CHA are similar to bone apatite, ensuring excellent biocompatibility and bioactivity of the composites. In n-CHA/HDPE composite, only mechanical mixing exists between the filler and the matrix. On the contrary, strong molecular interactions and chemical bonding are present between the n-CHA crystals and the HDPE-g-MAH polymer. However, the compressive strength of the two composites molded by means of hot pressing are lower than that of cortical bone. They are suitable for low load-bearing applications. n-CHA/PA66/HDPE and n-CHA/PA66/HDPE-g-MAH nano-composites are prepared by co-solution method. The results show-that n-CHA crystals keep original morphological structure and distribute uniformly in polymer alloy matrix with reinforcement effect, this is similar to the natural bone in which n-HA uniformly distribute in the collagens. In n-CHA/PA66/HDPE composite, two kinds of bonding modes are present between n-CHA and PA66: hydrogen bonding between —NH—in PA66 and —OH in CHA; electrostatic attraction between Ca2+ in CHA and" —COO" group in PA66. There is no chemical bonding between HDPE and PA66 or n-CHA. In n-CHA/PA66/HDPE-g-MAH composite, besides interface chemical bonding between n-CHA and PA66, a graft copolymer HDPE-g-MAH-co-PA66 iscreated both because of reaction of the end amine groups in PA66 with carbonyl groups in MAH of HDPE-g-MAH and because of recombination of the macroradicals generated under action of shearing stresses. The presence of copolymer HDPE-g-MAH-co-PA66 as compatibilizer significantly influences the morphology of the polymer alloy, inducing a finer dispersion and promoting interfacial adhesion. Many kinds of bonding modes are present among the three phases of n-CHA, PA66 and HDPE-g-MAH and stable chemical interface bondings form in n-CHA/PA66/HDPE-g-MAH composite. An optimum mechanical property is achieved with W pA66: WHDPE= 7:3 or W PA66: WHDPE-g-MAH = 7:3 in the composites. n-CHA/PA66/HDPE and n-CHA/PA66/HDPE-g-MAH composites have excellent mechanical properties and biocompatibility with low water sorption rate, are promising biomaterials for load-bearing bone replacement. The bioactivity of n-CHA/PA66/HDPE-g-MAH composite is investigated in simulated body fluid (SBF). The results show that a bone-like apatite layer is formed on the surface of the composite immersed in SBF. The apatite is poorly crystallized nonstoichiometric bone-like apatite and contains carbonate ions, which play an important role in bone regeneration. For PA66 and HDPE-g-MAH, there are no Ca-P deposition on their surface after immersion in SBF. It can be concluded that the introduction of n-CHA endows the composite with a high bioactivity.