| Hydroxyapatite (HA) can be widely used for repair and replacement of damaged or traumatized bone tissues due to its good biocompatibility and bioactivity. With specific porous structure and good surface properties, spherical HA has been potentially applied on the field of separation and purification for bioactivity macromolecules, such as proteins, nucleic acids and enzymes. The lower crystallinity or amorphous HA microspheres (HAM) has broad application prospect not only because of its good flowability and high specific surface area, but also its biodegradable and excellent bioactivity. One of method reducing the crystallinity of HAM is through changing the properties of HA slurries and rapid solidification, which can retain much more characteristic of original HA slurry in the HAM. The low crystallinity HAM can be used for drug delivery, cell culture carrier, bone filter, bone repair and other fields because of its good properties. So far, hollow and porous HAM mainly has been fabricated primarily by the template method and then calcinated it. However, almost no degradable HAM with high crystallinity could be prepared using this mehod. The relevant reports about preparation and characterization of lower crystallinity of HAM are very rare.In this study, HAM with low crystallinity and high porosity were fabricated by the flame-drying method with self-made device using methane flame as the drying medium. The microstructure, phase component and other performances of HAM were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), laser diffraction particle size analyzer (LDPSA), surface area analyzer (SSA), and fourier transform infrared spectrum analyzer (FTIR). Besides, the parameters of the flame-drying method were systematically optimized in the process of HAM fabrication. Three kinds of HAM were fabricated by that flame-drying method using HA slurries with ammonia (AHAs), without ammonia (NAHAs) and under ice-water condition (IWHAs), respectively. The in vitro and vivo experiments of HAM were discussed for studying the biological behavior differences with different crystallinity and microstructure. Furthermore, taking the HAM by the NAHAs as the supplement, porous PLLA/HAM composite scaffolds were fabricated by the thermally induced phase separation technology. Then the effect of content of HAM in the composite scaffolds on their mechanical and biological properties was systematically researched in this paper.The results indicated that HAM has been retained more performances of HA crystal in the slurry under higher drying medium using self-made flame-drying device. According to these results, it could reduce the crystallinity of HAM and improve its structural properties through changing HA slurries parameters. In this paper, HAM by the AHAs has the regular spherical structure, smooth surface morphology, smaller particle size range and highest crystallinity which were compared with other two kinds of HAM. HAM by the NAHAs showed some hollow and burst open structural and has lower crystallinity. HAM by the IWHAs has the porous structure, larger particle size range and lowest crystallinity (29.8%). In addition, the specific surface area (167.16 m2/g) and the total pore volume (0.5054 ml/g) of HAM by the IWHAs were much higher than that of HAM by AHAs (52.523 m2/g and 0.2331 ml/g, respectively).Because the characteristic of HA slurry can significantly impact the properties of HAM by the flame-drying method, it is necessary to research the crystal process during procedure of synthesis HA. The results showed that HA crystallization and its crystal grain microstructure in the slurry were affected by the reaction time and temperature as well as sedimentation time. Among them, the reaction temperature is the main parameters of crystalline HA. High temperature could provide more free energy, so that HA crystallinity increased with the temperature increasing.The TEM microstructure of HAM by the flame-drying method showed that the properties of HA crystal in the original slurry would affect the HAM microstructure. The component particles of HAM by the NAHAs had the high crystallinity, and distributed as rod or strip like. For the HAM by AHAs, there were some non-transparent crystal existents in the microstructure morphologies. The TEM results indicated that some bubble-like defects were existed in the nano-particles of HAM by the IWHAs. In addition, there were lots of nano-pores included in the HAM by the IWHAs, which resulted in its soft microstructure.Three kinds of HAM by the flame-drying method showed different impacts at different calcining temperature. A little change took place on the surface morphologies, crystallinity and BET of HAM after sintering under 600℃. The grain of HAM became coarsening and fusion with the calcining temperature increasing above 800℃. Moreover, impure phases, such asα-TCP andβ-TCP, appeared when HA decomposed at 1000℃. And the BET and porosity of HAM were significantly decreased because small particles united together when calcined at higher temperature. However, for HAM by the IWHAs, obviously changes appeared, including the BET and total pore volume markedly decreased, even though at lower calcining temperature.In vitro experiment indicated that HAM fabricated by the flame-drying method has the good bioactivity and the potential application of adsorption and control release for bovine serum albumin (BSA). Three kinds of HAM showed different impacts in the physiological environment. Among them, the HAM by the IWHAs had the best bioactivity because of its lowest crystallinity and high total pore volume. In vivo data showed that HAM by the flame-drying method had non-toxic effect and could be well integrate with bone. Histology results demonstrated that the HAM had well osteoconduction and biodegradation after implanting into New Zealand rabbit femur for 4 months. The degradability of HAM by the IWHAs was much more serious compared with the other two kinds of HAM. In addition, SEM morphologies showed that some connective tissue grew into the HAM implants after implanting into rabbit's femur for 4 months.Polymer combined with HAM could form composite biomaterials which would modify the acidic environment due to degradability of HA and improve biological and mechanical properties. In this study, porous PLLA/HAM composite scaffolds were fabricated by the thermally induced phase separation technology. The results demonstrated that the HAM was uniformly incorporated into the PLLA/HAM composite porous scaffolds. As the HAM ratio was increased the porous composite scaffolds changed from ladder-like into isotropic structure. In addition, the mechanical property of PLLA/HAM composite scaffolds improved with increasing HAM ratio in the scaffolds. The compressive modulus reached to maximum (9.1MPa) when the HAM ratio was 30% in the composite scaffolds. While the compressive of plain PLLA scaffolds was only 4.4MPa. Bone-like apatites would be formed onto the surface of composite scaffolds after incubated into simulated body fluid (SBF) for a period of time. And the amount of deposition on the surface of scaffolds was increased with the HAM ratio increasing. Meanwhile, BSA adsorption of composite scaffolds was also improved as adding into HAM.In vitro experiment indicated that PLLA/HAM composite scaffolds improved the attachment, migration and differentiation of MC3T3 osteoblastic cells after culture for 4 weeks. It demonstrated that the PLLA/HAM composite scaffolds were superior to plain PLLA scaffold for bone tissue engineering. The histologies morphologies also showed that the cells could grow into and well integrate with PLLA/HAM composite scaffolds.
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