Preparation And Properties Of Hydroxyapatite/High Density Polyethylene Bone Substitute Composite

In order to enhance mechanical properties and improve the biocompatibility, the HA/HDPE composite with multi-scale structure was prepared by the process of structure design, surface modification, in situ biomineralization, and solid-state extrusion. And its microstructure, mechanical properties, biocompatibility and the mechanism of toughening and reinforcement were investigated systematically in this paper.1. Based on the microstructure characteristics of human being's bone, the HA/HDPE composite structure was designed from multi-scale structure: on the atom scale, the interfacial strength was high; the HA nanoparticles dispersed in the HDPE matrix homogeneously on the nano-scale; and on the micro-scale, the oriented and arrayed HA/HDPE composite fibers were formed. Its aims were to enhance the mechanical properties by the co-operate action on the multi-scales.2. The biomineralization mechanism of HA was studied systematically, results showed that the -COOH and the -NH₂ functional groups had high polarity and charged density, which led to increase supersatuation and lower the interfacial energy, they controlled the nucleation and crystal growth of HA by the "interfacial molecular recognization" of electrostatic accumulation, structural correspondence and stereochemical requirements. The inorganic ions and amine acids controlled the crystal growth and morphology of HA by absorbing on the surface of HA or exchanging with the lattice ions of HA. And the crystal growth of HA followed the surface diffusion controlled screw growth mechanism.3. The process of in situ biomineralization was applied to prepare HA/HDPE powders for the first time. FTIR analysis indicated that the HA nanoparticles grew on the HDPE with chemical bond and dispersed in polymer on the level of nanometer dimension, which improved the dispersion of HA in the HDPE matrix and efficiently enhanced the interfacial strength.4. The process of solid-state extrusion was applied to prepare the HA/HDPE composite, and its effect on microstructure and mechanical properties of HA/ HDPE composite was investigated by means of Fourier transformed infrared spectroscopy (FTIR), scanning electron microscope (SEM), Instron testing machine and Ceast Impact tester at room temperature. Results show that: (1) The HA/HDPE was a multi-scale microstructure composite. On the nano-scale, the HA particles dispersed in the matrix homogeneously; the HA/HDPE composite fibers was oriented and arrayed on the micro-scale. (2) HA/HDPE composite has the better mechanical properties. The bend strength, tensile strength and Yong's modulus reach 165MPa, 245MPa and 18.1GPa, respectively, which meet the mechanical requirements for cortical bone. Tensile fracture analysis and energy dispersive X-ray analysis (EDX) results indicated that the interfacial bonding strength was high, in the higher interfacial bonding strength condition, the produced interfacial stress from the contraction of matrix can strain-induces the crystallization of matrix to forming the extended-chain crystal structure in the area surrounding the filler, as a result, the mechanical properties of composite were improved greatly. On the other hand, the oriented and allied HA/HDPE fibers were formed, which was the main role to enhance the mechanical properties.5. The toughening and reinforcement mechanism of HA/HDPE composite was investigated. Results show that the co-operate actions of multi-scale was the main role to enhance the mechanical properties. On the nano-scale, homogeneous dispersion of HA and high interfacial strength between HA and HDPE leads to the high crystalline, litter crystal size and the formation of oriented extended-chain crystal structure. Its result in the HDPE deformation and HA deadhesion during the fracture process, which can pin the development of microcracks, expend large energies and prohibit the formation of large crack. On the micro-scale, the formation of oriented HA/HDPE fibers can reduce greatly the activity volume and crease the fracture energy, which leads to the strength high. On the other hand, the composite fiber can expend energies largely by fracture, pullout and crackbowing, which enhance the strength and toughening to a great extend.6. The simulating experiment in vitro was operated. Results show that the HA/HDPE composite can induce the carbonic-hydroxyapatite mineralize on the composite, and its influence on the concentration and acid/alkali value of simulate body fluid was very little, which indicate that the composite have good osteoinduction and osteoconduction. The biocompatibility of the HA/HDPE composite was evaluated by flow cytometry after L929 incubated with extraction of the HA/HDPE composite, implanted in animal, compared with the materials applied in clinch. The results showed that there are no significant differences between two groups (P>0.05), which demonstrated that the HA/HDPE composite had a good biocompatibility.