Design,Manufacturing,and Mechanical-biological Performance Evaluation Of Porous Bone Scaffold Based On Triply Periodic Minimal Surface

Bone defect is a common disease in orthopedics,which is always caused by trauma,infection,and tumors.Although human bone possesses the capability of self-repair,a bone scaffold is needed for treatment when the defect size is too large to heal.Human bone shows hierarchical,heterogeneous and anisotropic structural features.Thus,bone scaffold is always designed as porous structure to acquire bionic properties of human bone.Although porous bone scaffolds have been used in clinics,challenges still exist.For instance,fractures,stress shielding and insufficient fatigue resistance are seen in bone scaffold after implanting into human body due to the mechanical mismatch between porous scaffold and human bone.Besides,the significant structural mismatch between porous bone scaffold and human bone also defers bone growth inner the scaffold and affects osseointegration.Given this,porous bone scaffolds were designed based on triply periodic minimal surface(TPMS)in this study.By controlling the topological structure of TPMS,the mechanical properties,mass-transport performances and structural bionics of TPMS-based scaffolds were quantitatively manipulated to meet the various implanted requirements of different bone positions.Meanwhile,for solving the low manufacturing accuracy of the metal TPMS-based scaffolds fabricated by additive manufacturing(AM),proper post-treatments were adopted,then the fatigue resistances of the metal TPMS-based bone scaffolds were improved by reducing the manufacturing defects.This thesis can be mainly divided into the following four parts.(1)In order to build porous bone scaffold satisfying strength requirement of the load-bearing bone positions and possessing better biological properties,two new types of TPMS-based scaffolds called AD and AG were designed on the basis of changing the topological structures of diamond(ID)and gyroid(IG)through manipulating the trigonometric functions in their expressions.The gradient density ID(FD)and the gradient density IG(FG)were designed for comparison.The porosities of ID,IG,FD,FG,AD,and AG were set to 60,70,and 80%.AD with 80% porosity(AD80)and FD with 80% porosity(FD80)were manufactured through electron beam melting(EBM)using Ti6Al4 V powder.Microcomputer tomography(Micro-CT)scanning,electron microscopy(SEM)test,uniaxial compression and permeability test were conducted to the manufactured scaffolds.Computational fluid dynamics(CFD)simulations were performed to all the designed scaffolds.In combination with the numerical and experimental results,the structural features,manufacturing accuracies,mechanical properties,permeability,and flow behavior of each scaffold were systematically analyzed.Results show that with the same porosity,the manufacturing accuracies and the permeability of AD and AG are higher than ID,IG,FD,and FG.The mechanical properties of AD are higher than other scaffolds.Although the wall surface area of AD is smaller than ID and FD,it is still larger than IG and FG,which can provide sufficient space for cell adhesion.AD is an appropriate candidate for solving the contradiction between superior mechanical strength and high permeability and exhibits excellent biological property.In addition,the study shows that when the porosity is constant,the mechanical properties and the permeability of a porous scaffold mainly depend on its topology structure,while there is little effect of adding a gradient change to the uniform structure.The topology structure of a TPMS-based scaffold is controlled by its implicit expression.Therefore,the form of the trigonometric function in the implicit expression can be appropriately manipulated to establish a proper structure that meets the mechanical and biological requirements.(2)To improve the endurance limits of TPMS-based bone scaffolds,the posttreatment process combing hot isostatic pressing(HIP)and electropolishing(ELP)was proposed and conducted in TPMS-based bone scaffolds with various structures,which means the fatigue properties of the TPMS-based bone scaffolds were tried to be enhanced from three aspects: changing the topological structures of TPMSs,improving surface qualities and eliminating internal defects.The typical stretching-dominated scaffold AD and the typical bending-dominated scaffold IG were selected and their porosities were set to 80%.These scaffolds were fabricated by EBM using Ti6Al4 V powders.HIP and ELP were conducted in the manufactured scaffolds.Then,compression–compression fatigue tests were performed and fatigue parameters such as fatigue life,accumulated strain rate,fatigue strength and fatigue ratio were obtained.The techniques of SEM,Micro-CT,optical microscopic(OM),X-ray diffraction(XRD)and electron backscattered diffraction(EBSD)were adopted to characterize the surface qualities,internal defects and microstructures.The effects of topological structure and surface quality on the stress value and distribution of TPMS-based scaffolds under compressive loading were investigated by finite element analysis(FEA).Results show that topological structures,surface qualities and internal defects all affect the fatigue properties of TPMS-based scaffolds.Under cyclic compression loading,the fatigue ratios of IG and AD without undergoing post-treatment are 0.11 and 0.59,respectively;while they rise to 0.26 and 0.69 after HIP and ELP.Compared with surface qualities and internal defects,topological structures influence the fatigue properties of TPMSbased scaffolds more prominently.The fatigue properties of TPMS-based scaffolds can be enhanced by the post-treatment process combing HIP and ELP,and greater improvement will be obtained for bending-dominated structures than stretchingdominated structures.Thus,manipulating the topological structures and increasing the buckling components of TPMS-based scaffolds is an effective way to obtain porous scaffolds with high compression strength and excellent compression-compression fatigue resistances.Besides,HIP and ELP can also be adopted to further improve their fatigue lives.(3)To obtain porous scaffolds with bionic anisotropic structures similar to human bone,then to satisfy the requirement of various pore sizes during the different periods of cell growth,the orthotropic TPMS-based bone scaffolds were designed in this study.Orthotropic Schoen’s I-WP(OP)and orthotropic Gyroid(OG)were designed based on isotropic I-WP(IP)and isotropic Gyroid(IG).The trigonometric function of Sin(z)was added to the TPMS-unit expressions of IG and I-graphwrapped package(IP),then the orthotropic IG(OG)unit and orthotropic IP(OP)unit were obtained.The whole scaffolds of IG,IP,OG and OP were modeled by arraying their units.The four scaffolds with 70% porosity were fabricated by EBM using Ti6Al4 V powder,and then were scanned by Micro-CT to evaluate their manufacturing accuracies.The quasi-static behaviors of the fabricated scaffolds were acquired by uniaxial compression test along the three orthogonal directions.The morphologies of the fractured struts were scanned using SEM to analyze the differences in failure mechanism among the four scaffolds.Combined with homogenization approach and FEA,the elastic parameters of the designed scaffolds were calculated and the anisotropic degrees were further compared.CFD simulations were performed to the designed scaffolds and the mass-transport parameters such as wall shear stress(WSS)and permeability along the three orthotropic directions were obtained.Results show that OP and OG possess irregular pores and perform well in manufacturing accuracy compared with IP and IG.Similar to human bone,OP and OG exhibit orthotropic mechanical properties and mass-transport performances.OP and OG can also provide proper mechanical stimuli for cell growth;meanwhile,their yield stresses and permeability are still in the value range of human bone.Furthermore,the elastic modulus ratio of OG along the three orthogonal directions is close to the value of cancellous bone.OG also shows excellent loadbearing capacity and permeability,which is suitable to implant into load-bearing bone positions.The orthotropic porous scaffold design was achieved by manipulating the topological structure of TPMS.The designed scaffolds are advantageous for their bionic mechanical and mass-transport properties of human bone and can be the proper candidates in bone graft surgery.(4)In order to establish a bone scaffold with good osseointegration,two new kinds of gradient TPMS scaffolds,i.e.,two-way linear gradient G scaffolds(L-G)and D,G fusion scaffold(N-G)were designed based on the gyroid(G)-type and diamond(D)-type TPMS in this study.The structural mechanical parameters of the two kinds of scaffolds were obtained through the compression simulation.The mass-transport parameters were also obtained through the CFD simulation.The tissue differentiation areas of the two kinds of scaffolds were calculated based on the tissue differentiation theory.The results show that L-G scaffold has a better mechanical property than the NG scaffold.However,N-G scaffold is better than the L-G scaffold in biological properties such as permeability and cartilage differentiation areas.In this study,the mechanical and mass-transport properties of TPMS-based bone scaffolds were quantitatively manipulated by controlling their topological structures,which lay the theoretical foundation of personalized and bionic porous bone scaffold design.The post-treatments of HIP and ELP were adopted to further improve the fatigue properties of metal TPMS-based scaffolds,which provide technical support for advancing their clinical application.