Preparation Techniques Of Hydroxyapatite Ceramics With Different Forms And Porous Structures

Hydroxyapatite (HA), which has similar crystal structure and Ca/P ratio to the main inorganic component of normal bone and can form a chemical bond with live tissues, has internationally recognized as a bioactive ceramic for clinical applications. Due to the bittleness of HA ceramics, they are not suitable to be used in clinical load-bearing sites. So that the application of HA ceramics have been restricted. Therefore, it is significant to improve the mechanical properties of HA ceramics. Except as the tissue engineering scaffold material, HA with different morphology and pore structure is widely used in ionic exchange, catalytic carrier, biomedicine application, et al.In this paper, a series of HA ceramics with different morphology and pore structure, such as porous spheres, dense spheres, fiber-accumulated scaffolds, et al., were fabricated by using the sol-gel method. The self-adhering and self-filled process were used to enhance the mechanical strength of HA scaffolds. Stereomicroscope (SM), scanning electron microscopy (SEM), X-ray deffractomerer (XRD) and mechanical tester were used to study the properties of HA ceramics with different morphology and pore structure. Their biological properties were evaluated by the experiments in vivo and vitro. Main conclusions are drown as follows:(1) Using the HA slurry prepared via a precipitation process as the raw materials, porous and dense spheres with the size of150-1500um had been fabricated successfully by sol-gel method. Porous HA sphere with the porosity of20%hold a compressive strength of6.9±0.3MPa and dense HA spheres with the porosity of4.7±0.6%hold a compressive strength of8.9±0.4MPa were fabricated via a specific dried and sintered process.(2) The factors, such as HA loading, stirring rate, stirring time and stirring temperature, which influence the property and morphology of these spheres were investigated. The stirring rate is the major influencing factor for the size and sphericity and the HA loading is the major influencing factor for the micro-structure of spheres. The other technical parameters have slight effects on the the structure. The pore structure can be controlled combined with other pore-forming method, for example, particle leaching method. The mean size decreases obviously and the sphericity becomes better as the stirring rate increases when the other preparation process keep constant. The mean size increases and the sphericity becomes worse as the HA loading and stirring temperature increases. After2h, stirring time has a slight effect on the mean size, but sphericity becomes better as the stirring time increases.(3) Using the micro-sized HA powders as the raw materials, the gelated spheres and fibers were obtained by sol-gel method. Translucent HA spheres and fibers were obtained after rinsed, dried, and pressureless sintering and hot isostatic pressing. The HA sphere with the porosity of20%hold a grain size of2.2μm and a compressive strength of6.9±0.3MPa.(4) Spheres with different density was biomimetic mineralized and cultured with mesenchymal cells. The mineralization results show that the amount of HA formed on the porous surface is more than that of dense and translucent HA ceramics. Compared with translucent HA ceramics, the mount of crystallines formed on the surface of dense HA is more, but the size is smaller. The results of cell cuture show that the proliferation of cells on porous HA ceramics is better than dense and translucent HA ceramics, in contast, the lever of cell differentiation is lower. All of them have good cell compatibility and have no cytotoxicity. HA scaffolds with different micro-pore structure and same macro-pore structure were fabricated by accumulating spheres. These scaffolds were implanted in the abdominal cavity of dog for1and3months. The results show the ectopic bone formation of the scaffolds accumulated by porous HA spheres is much better that that of the scaffolds accumulated by dense HA spheres.(5) Nano-HA slurry was mixed with sodium alginate (SA) to obtain a mixed suspension and then directly injected into a fibrous structure in a CaCl2solution. Subsequent pressing in a mold and sintering allowed the formation of HA fiber-deposited scaffolds (DS) with good interconnectivity. The DS green was immersed into the mixed suspension and centrifuged to obtain the green of HA fiber-self-adhering scaffolds (BS). The diameter of fibers can be tuned by changing the needle gauge and the porosity of these scaffolds can be controlled by varying the level of compression. The compression strength is improved obviously via self-adhering process. When their porosity is50%, the compression strengths of DS and BS are2.9±0.4MPa and13.2±0.6MPa, respectively. Bio-mineralization and cell culture confirm that both of DS and BS have a good bioactivity. The scaffolds treated by self-adhering process have a more rough surface and have a clearer trend of cells grown into scaffolds.(6) HA fibers were added into HA/SA suspension with was spheres. The mixture was filled in a mold and immersed in a CaCl2solution. Paraffin spheres were removed by soaking in n-hexane. HA fiber-self-filled porous scaffolds were obtained after washed, dried and sintered. The interconnectivity increases with the paraffin wax spheres increase. Many micro-and submicron-sized pores exist on the wall of macro-pores. In the fiber-filled scaffolds, HA fibers distribute in the matrix uniformly and the interface between fibers and matrix is tightly bound without obvious separation. The compressive strength of the scaffolds decreases as the addition of paraffin wax spheres increase. The addition of fibers can improve the compressive strength obviously, and the difference between the fiber-filled scaffolds and the scaffolds without fibers increase as the addition of paraffin wax spheres increase.