Study On The Chitosan/Hydroxyapatite Biomimetic Bone Materials

Biology evolvement during the past 350 million years designed and is designing the macrostructure and microstructure, shape and function of biology organs, which is the thesaurus of ideas and knowledge. Biomimetics referenced to learning from nature to explore the basic law of natural process and find solution to designed problem. Bone is composed of assembled collagen fibers and ordered hydroxyapatite nanoparticles, especially bone is living material. It is the hierarchal structure that contributes to the excellent mechanical properties of bone, such as strength and hardness of hydroxyapatite, and toughness of collagen. Chitosan and hydroxyapatite were widely used in biomedical application, especially bone reparation materials due to their biocompatibility and bioactivity. On the base of evaluation composing, structure and mechanical properties of bone, bone was chosen as the prototype to design chitosan/hydroxyapatite biomimetic composite via various methods, and the relationship between microstructure and performance of composite is disccussed.1. Chitosan/tri-calcium phosphate and chitosan/hydroxyapatite composite prepared by in situ precipitationIn order to endow chitosan with osteoinductive and ostoeconductive ability, chitosan/tri-calcium phosphate (CS/TCP) and chitosan/hydroxyapatite (CS/HA) composite were generated through in situ precipitation. The results of mechanical properties of composite indicate that bending strength and shear strength decrease as the TCP (or HA) content increase, at same time the modulus of composite is improved. The bending strength of chitosan/inorganic filler composite is significantly improved as high as 50%-60% when incorporation the crosslink and inorganic fillers simultaneously, which is defined as the synergetic enhancing effect.2. Chitosan/hydroxyapatite nanocomposite via in situ hybridizationIn order to solve the problem of hydroxyapatite nanoparticles aggregation in chitosan matrix, in situ hybridization technology was chosen to prepare CS/HA nanocomposite. The key of in situ hybridization is that a pre-precipitated CS hydrogel membrane was used to obstruct the CS/HA precursor from NaOH aqueous solution. The CS hydrogel membrane controlled the process of CS precipitation and transformation of HA from the HA precursor simultaneously and tardily when changing the pH of the system. XRD, TEM and SEM were performed to investigateCS/HA nanocomposite. Results indicated that the in-situ formed calcium phosphate in CS was exactly HA, and that nano-size HA granules were dispersed in CS matrix uniformly, and that the structure of CS/HA composite prepared by in-situ hybridization is layered structure. The bending strength of CS/HA and CS/HA modified by cross-linking were 86MPa and 102MPa, respectively, which is much higher than that of cancellous bone.3. Chitosan/magnetite nanocomposite via in situ hybridizationHA particles were distributed in chitosan matrix homogenously through in situ hybridization, however the HA granules of bone are oriented under the control of collagen fibers. Chitosan/magnetite nanocomposite was synthesized via in situ hybridization induced by magnetic field in ambient condition. XRD and TEM results indicated that magnetite particles with the size of approximately 10-20nm were dispersed in chitosan homogenously, and even assembled to form nanowire under the influence of the external magnetic field which mimetic the magnetite chain inside of magnetotatic bacteria. The result of magnetization indicated that magnetite nanoparticles were superparamagnetic.4. Chitosan/hydroxyapatite/magnetite nanocomposite via in situ hybridizationOn the base of experience from preparing the CS/HA and CS/magnetite nanocomposite, the superparamagnetic CS/HA/magnetite nanocomposite was prepared via in situ hybridization, and the inorganic fillers were oriented when applying the external magnetic field. The bend strength of composite is 105MPa, and is as high as 77% strength of fresh femoral bone of rabbit.5. Chitosan hydrogel with biomimetic structure prepared by in situ precipitation The chitosan was labeled by fluorescein isothiocyanate (FITC) as fluorescentprobe, which enable FITC labeled chitosan to be visible in the laser scanning confocal microscopy (LSCM). The fluorescence spectrum was chosen to confirm that the FITC were labeled on chitosan successfully. The scanning electron micrograph (SEM) and LSCM were carried out to characterize the microstructure of FITC labeled chitosan/chitosan hydrogel prepared by in situ precipitation. Chitosan hydrogel with hierarchal architecture, i. e. the chitosan hydrogel is not only composed a series of concentric ring similar to annul ring of wood, but also the spoke wire pattern of chitosan fibers along the gradient of OH'. It is the first time to find the Liesegang ring phenomenon in ordered chitosan hydrogel system when prepared by in situ precipitation. The chitosan hydrogel with biomimetic structure can be illustrated byLiesegang ring.6. Biomineralization of chitosan via alternate soakingThe hydroxyapatite dispersant in the natural bone varies with the function of the part, and the rich HA content surface of implant can enhance the bone formation due to easy formation bone-like apatite after implantation. Alternate soaking was investigated to precipitation hydroxyapatite on chitosan hydrogel with biomimetic structure. HA layer with gradient distribution was coated chitosan hydrogel tightly in a short period of time, and the thickness of HA coating can be controlled by cycles of alternate soaking.