Studies On Preparation And Properties Of Thermotropic Liquid Crystal Polymer-based Biomimetic Bone

As the mechanical properties of inorganic/organic composites are too low to be used asload-bearing bone, hydroxyapatite (HA) powder and thermotropic liquid crystalline polymer(Vectra A950, VA) are selected as the filler and matrix of the biomimetic composite,respectively. VA has a self-oriented structure similar to the collagen fibrils of natural bone andsuperior mechanical properties over common organic materials. As a natural component ofbone, HA has excellent bioactivity and mechanical properties, which is widely used in thebiomimetic composite artificial bone.First, the twin-screw extrusion and injection molding parameters of pure VA were optimizedby orthogonal method. Second, the optimal parameters were used to prepare the HA/VAcomposites. Third, the microstructure and mechanical properties of HA/VA composites withdifferent HA contents were studied. The results reveal that the composites with HA/VA massratios below10/100show a core-shell structure. The HA particles are mainly dispersed in thecore part, while the skin layer is composed of highly oriented VA microfibrils. The modulusand strength of the composites are comparable or even higher than those of natural bone. Withthe increase of HA content, the core-shell structure disappeareddisappears, but the defectsseriously increases. At the HA/VA mass ratio of20/100, the composite mechanical properties,fracture strain in particular, significantly decreases, due to the poor interfaces between HAparticles and VA matrix. Therefore, HA powder surface was modified by polyacrylic acid(PAA) and then compounded with VA using the same method.The rheological behavior, microstructure and mechanical properties of HA-PAA/VAcomposites were studied. The FTIR and XPS spectra indicate that the HA particle surface issuccessfully coated with PAA, and its loading is around2.2%measured by TGA. The tensiletest results show that the PAA modified HA/VA composite with the mass ratio of20:100has astrength of138MPa (P<0.01), modulus of9.3GPa (P<0.01), and fracture strain of7.2%(P<0.01), which have been increased by200%,26%, and350%, respectively, compared with those of the unmodified composite. Therefore, PAA can effectively improve the interfaceadhesion between HA and VA, and significantly enhance the mechanical properties of HA/VAcomposite to the level of natural load bearing bone. In vitro mineralization experiments andMTT assay (3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide assay) were usedto evaluate the bioactivity and biocompatibility of the composite. The results show that HApowder in the composite can induce the formation of an apatite layer on the composite surface,and promote the proliferation of osteosarcoma cells (MG-63) as well. Hence, the HA/VAcomposite may be a potential bone defect repair material or a bone replacement material.As the mechanical properties of VA are significantly decrease by compounding with HApower, and the HA powder is mainly used to improve the bioactivity of VA, we proposed toprepare the HA coating on VA substrate by plasma spraying. The optimal plasma sprayingparameters are as follows: plasma gun using Ar as the primary gas and H2as the secondarygas with flow rates of100scfh and10scfh, respectively; spraying voltage (V)35V andspraying current (I)860A; spray distance (d)10cm and scanning speed (v)500mm/min; andHA powder feeding rate (r)20g/min. Microscopic analyses show that the HA coating with athickness of16.5μm is uniform and dense without delamination and pores at thesubstrate/coating interface. XRD patterns reveal that the HA powder is partially decomposedto tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), and calcium oxide due to thehigh plasma temperature. During in vitro mineralization, TCP reacts with TTCP to yield HA,and the HA deposition on the surface of VA increases with the mineralization time, whichindicates excellent bioactivity of HA plasma sprayed VA surface. In addition, in vitro cellcompatibility tests (MTT assay) show that HA coatings promote the proliferation of MG-63cells. Therefore, the plasma sprayed HA coating can improve the biocompatibility of VA. Thiswork provides a valuable exploration on improving the biocompatibility of polymer surfaceby plasma spraying bioactive coating.