The Biocompatibility Of Poly-L-lactide/poly-L-lactide Grafted Nano-hydroxyapatite Composite (PLLA/PLLA-gHA)

Internal fixation of fractures and osteotomies is classically performed using metallic devices. Although these are claimed to be inert, complications may occur as a result of their permanent presence in the body, such as migration, growth disturbances, stress shielding, infection and secondary removal operation. For all these problems, absorbable devices have been developed. Among the various absorbable polymers, the polyesters polylactide (PLA), polyglycolide (PGA), and their copolymers (P[L/DL]LA and PLGA) are the most important because of their biocompatibility, adjustable mechanical properties, adjustable degradation rates, and resorbability through biologic processes. Several resorbable plate and screw fixation systems are now available for use in the maxillofacial skeleton. These systems allow initial stable fixation of bone segments during the bone-healing phase and then gradually are reabsorbed through physiologic processes. The advantage is that no permanent implants remain that might affect facial growth, create imaging artifacts, or be susceptible to infection and extrusion.But the bioabsorbable devices are not yet an ideal internal fixation system, mainly for the two reasons. The first one, although the polymers have been successful used in non-loading bone, they are not strong enough for cortical bone repairing. So it needs to improve their strength and modulus. The second one, there is still some kind of inflammatory reaction during the degradation course of the polymers. In order to improve the bioabsorbable material's properties, new generation of multifunctional bioactive absorbable osteofixation devices is under research and development. Among all these compounds, the composites of hydroxyapatite (HA) particles and biodegradable polymers have attracted much attention. Many studies showed that HA/PLA composite can combine the osteoconductivity and biocompatibility exhibited by HA ceramic and PLLA, respectively. After mixing with PLLA, n-HAP particle can equally reinforce the materials in three dimensions.But in an ordinary PLA/HA blending system, only physical adsorption is achieved between HAP particles and PLA matrix. After transplanted to human body the interfacial layer between the filler and the polymer matrix was destroyed firstly, thus HA particles may easily disengage from the organic matrix, resulting in a sharp decrease of mechanical properties in a short time, so its mechanical properties are low and its load-bearing applications are limited. Consequently, improvement of the interfacial adhesion between the HAP particles and the PLLA matrix has become the key technique in preparing HAP/PLLA composites.In the present study, a novel method to modify the surface of n-HAP particles was introduced. PLLA was directly grafted onto the hydroxyl group of the surface of n-HAP particles by ring-opening polymerization of L-lactide in the presence of stannous octanoate (Sn (Oct) 2) as catalyst. The PLLA-gHAparticles could be more uniformly dispersed into chloroform than the non-grafted one. When they are blended with PLLA via a solution process they can be uniformly dispersed in the PLLA matrix and show improved adhesion with PLLA matrix. Therefore, the PLLA / PLLA-gHA composites exhibit better mechanical properties than simple HA/PLLA blends.Our study was focus on observing the biocompatibility of the new composite of PLLA / PLLA-gHA. The two aspects of the composite's biocompatibility, safety and validity, were tested by a series of experiments in vitro and in vivo. All the results were also compared and analyzed with the PLLA.The experiment was divided into four parts:1. An introduction of the preparation of PLLA / PLLA-gHA and their properties. PLLA was directly grafted onto the hydroxyl group of the surface of n-HA particles by ring-opening polymerization of L-lactide in the presence of stannous octanoate (Sn (Oct) 2) as catalyst. And the PLLA-gHA was further blended with PLLA matrix to form PLLA / PLLA-gHA. The dispersion and mechanical properties of the composite were tested.2. Biological evaluation of PLLA / PLLA-gHA composite. According to the demands of International Standard Ogzanation (ISO 10993) and Technical Evaluation Standards of Biomedical Materials and Medical Instruments promulgated by Chinese Boardof Health (GB/T 16886). Experiments of cell toxicity in vitro, acute toxicity, hemolysis, and implantation into muscles were investigated to evaluate the biocompatibility of the composite and compared with PLLA.3. A study on the biocompatibility of PLLA / PLLA-gHA coculture with rat osteoblasts. Rat osteoblasts were seeded on the PLLA / PLLA-gHA and PLLA composite, the adherent cell number and morphological changes of osteoblasts were observed by cell arithmometer and scanning electon microscopy. The osteoblasts were also cocultured with the leaching liquor of PLLA / PLLA-gHA and PLLA. The proliferation of the cells was determined by inverted microscope and MTT method. The cells were also stained for alkaline phosphatase (ALP) activity and matrix mineralization.4. An experimental study of PLLA / PLLA-gHA and PLLA plate to guided the regeneration of the rabbits'cranial bone defects. The rabbit's skull defect model was prepared, and the defect was covered with PLLA / PLLA-gHA and PLLA plate. At 4, 8 and 12 weeks after operation, samples were harvested and examined by gross observation, three dimension CT scanning, and tissue pathological examination to evaluate the bone conduction properties of PLLA / PLLA-gHA.And the results were also compared with PLLA.By all the experiments mentioned above, we try to afford a biological and clinical foundation for the new synthetic bone internal fixation material.Main Results and Conclusions are as follows:1. In the study, we synthesized a new bone repairing material PLLA / PLLA-gHA, grafting with PLLA successfully modified the surface of HA nano-particles. The PLLA-gHA particles could be more uniformly dispersed into chloroform than the non-grafted one. When they are blended with PLLA they can be uniformly dispersed in the PLLA matrix and show improved adhesion with PLLA matrix. The PLLA /PLLA-gHA composites exhibit better mechanical properties than simple PLLA / HA blend.2. The results of experiments on cell toxicity test in vitro, acute toxicity, hemolysis, implantation into muscles in rabbits met the demands of (ISO,10993) and GB/T, which show the biological safety for clinical application.3. The adherent number of osteoblasts on the surface of PLLA /PLLA-gHA was higher than that of PLLA, and the osteoblasts are more easily to adherent and spread on PLLA /PLLA-gHA. The proliferation rate, alkaline phosphatase activity and mineralization of osteoblasts in the leaching liquor of PLLA /PLLA-gHA was also higher than that of PLLA. The results show the PLLA /PLLA-gHA material has excellent biocompatibility for rat osteoblast.4. The composite of PLLA /PLLA-gHA can effective guided the bone regeneration of the rabbit's cranial bone defects.5. The new composite of PLLA /PLLA-gHA showed satisfactory biocompatibility. It was not only safety for clinical use, but also can promote the adherent, proliferation and mineralization of rat osteoblast. At the same time, The PLLA /PLLA-gHA showed good osteoconducion property.