Preparation And Properties Of Porous Titanium Alloy Tissue Engineering Scaffolds By Selective Laser Melting

Bone defect caused by trauma,disease and aging population is a common orthopedic disease.Traditional autologous and allograft bones cannot meet the clinical applications.In recent years,bone tissue engineering scaffolds with three dimensional perforation structure have become an effective method for the treatment of bone defects.Titanium alloy has become a promising scaffold material for bone tissue engineering due to its low density,high strength and good biocompatibility.Porous titanium alloy bone tissue engineering scaffold prepared by additive manufacturing can be used for bone repair in load-bearing parts.However,it should be noted that scaffolds are prone to bacterial infection during implantation,and it is extremely difficult to eradicate the infection with traditional methods once bacterial infection occurs before the bone tissue is re-implanted.Therefore,it is urgent to develop porous bone scaffolds with the combination of good mechanical properties and antibacterial performance.In this paper,selective laser melting(SLM)technique was used to fabricate porous titanium alloy(Ti6Al4V)scaffolds with different porosity.The SLM processing parameters and the microstructure of Ti6Al4 V was investigated.Chemical polishing and heat treatment were carried out on Ti6Al4 V scaffolds,and the compressive mechanical properties of scaffolds with various porosity were evaluated.Subsequently,gentamicin was loaded into the Ti6Al4 V scaffold using vacuum impregnation,and the drug release behavior in phosphate buffer was also investigated.The main results are as follows:(1)In the process of SLM forming Ti6Al4 V,it is found that the higher input energy density associated with lower scanning speed usually lead to the evaporation of molten metal,subsequently resulting in formation of keyholes in the sample.On the other hand,insufficient lower input energy density together with higher scanning speed was observed to be hardly capable of melting the powder completely.As a result,the SLM processing parameters were optimized as follows: a laser power of 200 W,a scanning speed of 1200 mm/s,a scan spacing of 80 μm and a layer thickness of 30 μm.Microstructure observations revealed that the a chessboard pattern appeared on the XOY section of the sample,which can be ascribed to the fact that the applied hatch angle of 90° gives rise to the perpendicular scanning paths between neighboring layers.The columnar β grains along building direction were noticeable due to epitaxial growth,and different martensite ranging several micrometers to nanometers precipitated inside these parent β grains.The microstructure of Ti6Al4 V was controlled by heat treatment,and the small α+β phase was formed at 750℃.At 980℃,part of the primary α phase changed into equiaxed crystals,forming a typical bimodal two-state microstructure.(2)Porous Ti6Al4 V scaffolds with different porosity were designed and fabricated.The compression tests showed that the elastic modulus and yield strength of porous titanium alloy scaffolds decreased and the fracture strain value increased when the porosity increased from 33% to 59%.Among them,the elastic modulus,yield strength and fracture strain of 59% porous scaffolds were measured to be 4.6 GPa,236 MPa and30.7% respectively.Further increase in porosity to ～83%,the fracture strain value of the scaffold decreased sharply to ～7.5%.The fracture morphology analysis shows that the samples with suitable porosity(～33% ～59%)displayed are ductile and brittle mixed fracture mode,and higher porosity(～83%)led to typical brittle fracture of the scaffold.(3)The scaffolds was impregnated with gentamicin loaded biodegradable chitosan loaded in virtue of vacuum impregnation,and in vitro drug release behavior in phosphate buffer was studied.The results showed that the drug(gentamicin)was released in such a fast way which was controlled by the diffusion mechanism in the initial stage up to ～ 184 h.After that,the drug released with a characteristic linear trend over a longer duration period(184-496 h),which might be dependent upon the biodegradation rate of chitosan,Meanwhile,an increase in porosity of the scaffolds was conducive to drug release in porous scaffolds,and ～96% of the drug was released in the scaffold with a porosity of 83% up to 496 h.