Poly L-lactic Acid Composite Nano-hydroxyapatite Artificial Bone Tissue Repair Materials Preparation And Properties Research

【Objective】Poly-L-lactide (PLLA), one of biomaterials, are widely used as Bone defect repair and fracture fixation materials for its excellent biocompatibility, biodegradable absorbent and a certain mechanical strength. While the poor brittleness of pure PLA material limits the further application of PLLA. PLLA-based biomaterial lost its initial mechanical strength after implantation in the body for 4 to 8 weeks, thus it can not provide effective support for new bone tissue. Moreover, its acidic degradation products are likely to result in inflammatory response. Nano-hydroxyapatite powders (n-HA), as the inorganic filler in polymer-based biomaterials, can improve the stiffness, biological activity and degradation rate of the PLLA-based implant material, and reduce the inflammation response caused by the acid degradation products. By using self-made PLLA and n-HA for raw materials, prepared PLLA / n-HA composite biomaterials, and biomechanical properties of composite materials, such as biodegradability and biocompatibility aspects of research to look to find the applicable and the ideal artificial bone tissue repair material.【Methods】Based on n-HA enhancing PLLA polymer materials, PLLA/n-HA composites were fabricated via melt blending by using the raw material PLLA and nano-hydroxyapatite prepared in our laboratory. The mechanical properties of PLLA blends,morphology and PLLA/n-HA biocomposites, and the degradation behaviors in vitro were studied. 1. L-lactide (LLA) with high purity was prepared by combination of distillation technology and washing-recrystallization,(99.95%),and ring-opening polymerization using high molecular weight poly-L-lactic acid.2. By chemical coprecipitation hydroxyapatite, get a better dispersion of n-HA, elongated rod-like, particle size of about 30-50 nm, 30-100 nm in length between the crystal has good Performance. Modified n-HA surface and the silane coupling agent produces a weak chemical bond, the surface covered by a layer of coupling agent active molecules.3. The PLLA/n-HA composites were prepared by melt blending. From PLLA / n-HA strip samples basically opaque, milky white.4. Phosphate buffered saline (PBS) for degradation of the media, research PLLA and PLLA / n-HA in PBS in the bio-composite materials in vitro degradation.【Results】1. High-purity L-lactide prepared (LLA) and PLLA Preparation andperformance test results L-lactide (LLA) with high purity was prepared by combination of distillation technology and washing-recrystallization. PLLA with high molecular weight was synthesized via cationic ring-opening polymerization using prepared L-lactide as the monomer and stannous octoate (Sn(Oct)2) the initiator. The most appropriate factors, such as polymerization time, temperature and initiator concentration, for preparation of PLLA with high molecular weight were obtained in our experiments. The most appropriate factors are as follows: the reaction time is 24 h, the temperature is 140℃, and the ratio of the monomer to the initiator is 12000:1. Poly-L-lactide with viscosity-average molecular weight is about 65×104 and narrow molecular weight distribution is obtained under the above conditions. The variation of the mechanical strength of PLLA with molecular weight was investigated. The results show that, both bending strength and modulus of PLLA samples grow with the increase in molecular weight. And the bending strength and modulus can be reached 176 MPa and 2.78 GPa, respectively. The fracture surfaces of PLLA samples reveal that, with the increase in molecular weight, the toughness of the sample gradually increases, and the spherical crystallization of PLLA can be observed distinctly on the fracture surface.2. PLLA-nHA Composite Materials and performance resultsThe PLLA/n-HA composites were prepared by melt blending. The effects of nano-hydroxyapatite on the mechanical properties of composites were studied. The results illustrate that the bending strength and modulus, the tensile modulus, and the shear strength of composites are obviously higher than those of PLLA samples after incorporation of n-HA powders. When incorporating n-HA powders of 20 wt%, the bending strength and shear strength of PLLA/n-HA composites could be reached 175 MPa and 123 MPa, respectively. The bending strength and tensile modulus increase with the growth of n-HA content; and are up to 6.8 GPa and 3.5 GPa, respectively. The rod-like nano-hydroxyapatite particles filled in PLLA/n-HA composites play an important role in enhancement of PLLA/n-HA materials.3. The degradation behaviors in vitro of PLLA and PLLA/n-HA composite materials were studied.The water uptake, and mass loss, and molecular weight as well as distribution, and mechanical properties, and the fracture morphology, and pH values of PBS during degradation of PLLA and PLLA/n-HA composite were investigated. The results indicate that the water uptake and mass loss gradually become larger with prolonging degradation time; and at the same time, the water uptake of PLLA/n-HA is significantly higher than that of PLLA; on the contrary, the mass loss is significantly lower than that of PLLA. It is the fact that the degradation rate of PLLA/n-HA sample reduced due to incorporation of n-HA powder. Therefore, the pH values of PBS for PLLA/n-HA samples are higher than those for PLLA samples. The mechanical strength of samples gradually reduces as the degradation time prolongs. In the early term (before 10 weeks), the degradation rate of PLLA/n-HA samples is relatively low while keeping considerably high mechanical strength. 【Conclusion】1. L-lactide ring-opening polymerization of high molecular weight PLLA,the most appropriate factors are as follows: the reaction time is 24 h, the temperature is 140℃, and the ratio of the monomer to the initiator is 12000:1. Poly-L-lactide with viscosity-average molecular weight is about 65×104 and narrow molecular weight distribution is obtained under the above conditions. High molecular weight PLLA (viscosity average molecular weight of 65 million) of the bending strength and modulus were 176 MPa and 2.78 GPa.2. The results illustrate that the bending strength and modulus, the tensile modulus, and the shear strength of composites are obviously higher than those of PLLA samples after incorporation of n-HA powders. When incorporating n-HA powders of 20 wt%, the bending strength and shear strength of PLLA/n-HA composites could be reached 175 MPa and 123 MPa, respectively.3. PLLA and PLLA / n-HA composites in PBS bio-degradation process in the description of PLLA / n-HA material implanted early in the degradation of small, compared with pure PLLA material to keep more good initial mechanical strength for bone Tissue growth to provide a good supporting role.