| Common diseases of the human musculoskeletal system include degenerative joint degeneration,spinal degeneration,limb trauma,bone defects,osteoporosis and bone tumors.Bone implants are one of the modalities for the treatment of musculoskeletal system,whose main function is to replace all or part of the joint,bones,cartilage or musculoskeletal system.At present,implants used in clinical practice are mainly standardly manufactured by traditional machining and mold forming.However,the personalized skeleton features of human bones cause the mismatch between standardized implants and personalized bones,thereby affecting the treatment effect.Selective laser melting(SLM)technique provides tremendous advantages in the direct fabrication of fine and complex structural parts,making it particularly suitable for the individual formation of metallic bone implants.However,the bone implant applications face the problems of modulus mismatch between the metal material and bone tissue,low interfacial adhesion and poor osseointegration,meanwhile structure needs the internal pore gradients control and optimized topological properties.Therefore,this paper aims at the problems of material and structure preparation in the application of metal bone implants,and mainly focus on the structural optimization,surface coating modification of stucture,metal matrix composites,graded materials and gradient structure preparation.The research work is concluded as follows.In view of the stress shielding effect caused by the modulus mismatch between metal and bone tissue and the need of topological performance optimization of implant in human bone structure,a new topology optimization method based on unit cell is proposed.The effect of multi-type cell topologies on the mechanical properties of Co-Cr porous structures prepared by SLM was studied by finite element simulation and experimental methods.Through the uniaxial compression simulation,the stress distribution of multi-type cell topologies was obtained.It can be found that different cell topologies showed different degrees of stress concentration and area.The inclined load-bearing struts provided bending resistance while the horizontal beams acted as reinforcement,which can reduce the stress concentration degree.The prepared multi-type Co-Cr porous structures possessed a modulus and strength range of 7.18-16.57 GPa and 271.53-1279.52 MPa,respectively,satisfying the requirements of cortical bone and cancellous bone.Initial cell responses indicated that cells could adhere and stretch on the surface of Co-Cr porous structures fabricated by SLM,and the exchange of information between the cells occurred.Aiming at the problems of large surface area of metal porous implants which easily results in postoperative infection,a porous structure with an antibacterial drug-loading functional coating is proposed.Using the topologically optimized Co-Cr porous structure in the previous chapter,a functional coating of silk fibroin/gentamicin(SFGM)was constructed on the surface to promote faster and better bone formation and growth into the pores and prevent postoperative infection.By optimizing the coating preparation process parameters,a relatively complete,continuous and uniform conformal coating was obtained on the inner and outer surfaces of the porous structure with an average thickness of 2.30 ?m.Anodic electrochemical corrosion occurred on the surface of the porous structure during the construction of the coating,however,the degree of corrosion was small and the compression and fatigue properties of the structure were not reduced.The elastic modulus was 2.58 GPa,matching the modulus of cancellous bone in the femur.The initial cell surface reaction of the coated porous structure was better than that of the uncoated structure surface.The release of Co and Cr ions of the coated porous structure was less than that of the uncoated structure surface.The cell cytotoxicity of the coated porous structure was smaller than that of the uncoated structure surface.Moreover,the adhesion and growth of bacteria could be effectively inhibited by local slow release of antibiotics in a week.In view of the significant difference in the mechanical properties between the biocompatible coating and the metal material,making it easy to experience failure in the long term,a biocompatible Ti-Nb alloy prepared by SLM is proposed.It is expected to reduce the modulus and improve the biological activity of Ti matrix by the Nb addition.The effect of Nb content on the phase composition,microstructure,mechanical properties,in vitro apatite formation ability and corrosion resistance of SLM-processed Ti-Nb alloy was systematically studied.It was found that the increase of Nb content suppressed the martensite transformation and finally promoted the formation of ?(Ti,Nb)solid solution.When Nb content is 45 at.%,the Ti-Nb alloy showed a relatively high strength of 1030 MPa and a hardness of 356 HV0.1,97.32% and 52.32% higher than that of the cast alloy,respectively.It is attributed to the combined effect of grain refinement and solid solution strengthening.Compared to Ti,the addition of Nb significantly increased the apatite formation ability and corrosion resistance of Ti-Nb alloys.The former is due to the fact that ? phase may have the ability to induce the formation of apatite,while the latter is caused by an increase in the amount of Nb2O5 formed in the passivation film.Aiming at the problems of low binding capacity between metal and bone tissue and poor bone integration ability,it is proposed the Ti-HA composites in situ prepared by SLM to improve the osteointegration ability through the addition of HA.The effect of HA addition on the phase composition,microstructure evolution,surface properties and mechanical properties of the composites was systematically investigated.The metallurgical mechanism of the composites during the SLM was illustrated.It is found that Ti5P3,Ca3(PO4)2,Ca Ti O3 and Tix O phases were generated through the reaction of Ti and HA during SLM.With the increase of HA content,a novel microstructure evolution of the composites can be presented as follows: relatively long lath-shaped grain(Ti)? short acicular-shaped grain(Ti-2%HA)? quasi-continuous circle-shaped grain(Ti-5%HA).The Ti-n HA composites exhibited a remarkable improvement of microhardness from 336.2 to 600.8 HV and the nanohardness from 5.6 to 8.3 GPa compared to those of pure Ti,due to the integrated effects of grain refinement and solution strengthening.The elastic modulus of Ti-HA composite indicates a bit improvement of 3.7% higher than that of Ti due to the microstructure refinement.While the ultimate tensile strength of Ti-HA composite declines substantially to 289 MPa owing to fragile phases produced during SLM.Aimed at the commonly stratification in the manufacture of multi-type materials and the internal stress caused by the difference of physical properties between heterogeneous materials,the Ti-HA gradient material with quasi-continuous ratio prepared by SLM is further proposed.The mechanisms of pore formation and crack formation in gradient materials,the interface and formation mechanism of gradient materials,and the variation of fracture toughness and hardness in gradient materials were systematically studied.The results showed that with the increase of HA from 0 wt.% to 5 wt.%,the porosity of gradient material increased from 0.01% to 3.18% with heterogenous pore size and distribution,and the crack density and crack count increased 1700% and 1250%.The mechanism of pore generation is the combination of pore nucleation and elemental evaporation,while the brittle phase formed by the reaction of Ti and HA causes the solidification cracks.The interface boundaries between each gradient layer were identified.The microstructure grain morphology evolution follows long lath-shaped grains ? long acicular-shaped grains ? short acicular-shaped grains ? continuous circle-shaped grains ? quasi-continuous circle-shaped grains.The Ti/HA gradient materials exhibited a wide range of nano-hardness from 5.11 to 8.36 GPa,Vickers hardness from 3.42 to 5.67 GPa and Young's modulus from 134.24 to 156.26 GPa.The compressive strength and yield strength were 871.92 MPa and 799.38 MPa,respectively,showing a strong resistance to deformation.The fracture toughness ranged from 3.41 MPa m1/2 to 0.88 MPa m1/2 for Ti/HA layers from 2 wt.% to 5 wt.% HA ratios,and the increase of micro-pore appears to be the most significantly factor responsible for the reduction of toughness.Aiming at the problems of delamination and stress concentration caused by discontinuous bearing struts at the interface between gradient layers in the design and manufacture of gradient porous structures,a preparation method of gradient porous structure with continuous space surface of strut is proposed.Gradient porous structures based on a novel Schwartz diamond unit cell were fabricated by SLM,and could provide a wide range of volume fraction from 7.97% to 19.99% without significant manufacturing defects.The experimental porosity is only 3.61% lower than the design model,indicating that the structure is in good agreement with the original design geometry.The strut size of gradient porous structures significantly increased from 483 ?m to 905 ?m,along with the large variation in volume fraction.The stress distribution of the gradient porous structure showed obvious stratification in different gradient layers,and the struts in the gradient layer with high porosity presented the larger strain.The gradient porous structures had the elastic modulus and yield strength range of 0.28-0.59 GPa and 3.79-17.75 MPa respectively,which can be adjusted by adjusting the volume fraction.The mathematical relation model between gradient porosity and compressive mechanical properties of gradient porous structure was established.The formulas of relative modulus,relative intensity and relative density of gradient porous structure were obtained to predict and design the mechanical properties of SLM-processed Schwartz diamond gradient porous structure. |
PDF Full Text Request