Numerical Investigation About The Influences Of Scaffold Morphology On The Cell Biomechanical Environment In The In Vitro Culture

Bone defect is an important reason for physical disability, which is caused by trauma, tumor, congenital malformations and bone infections. In recent years, bone tissue engineering technology has become an important treatment for bone defects. Bone tissue engineered scaffolds can be obtained by in vitro culture. In vitro culture conditions, scaffold materials, and morphological characteristics are key factors to obtain bone-scaffold complexes. Seed cells adhered on the surface of scaffold survive in a complex mechanical environment. Bone tissue engineered scaffold can transfer mechanical stimuli to seed cells to promote their proliferation and differentiation by the support structure in the in vitro culture, and provide temporary mechanical support for the growth of bone tissue. However, the mechanical environment parameters cannot be accurately measured within the scaffold by the current laboratory technology, and this problem can be solved by finite element method. Therefore, in this study, the finite element models of scaffolds with different morphologies and materials were established based on micro-CT imaging in combination with finite element method, the mechanical stimuli in scaffolds were analyzed quantitatively, and then the differentiation results of BMSCs on the surfaces of the scaffolds were simulated. This study can be divided into the following three parts:The cell differentiation results of scaffold models based on the idealized unit cell models of trabecular bone were obtained in the first part. The three-dimensional pore morphology parameters of five scaffold models were measured and the differentiation results of cells were predicted in the in vitro culture conditions. The three-dimensional bone scaffold architectures with 65% porosity were designed using Rhino software. PDLLA linear elastic material was used, and the apparent compressive strains between 0% and 5% were applied to simulate an unconfined compression test. Strain distributions were analyzed on the wall surfaces of the solid models. The interstitial fluid flow at inlet velocities ranging between 0.01 and 1 mm/s was applied to interconnected pores, simulating a steady state flow in the scaffold, and the shear stress distributions on the surface of the scaffolds were calculated. Differentiation of BMSCs on the surfaces of the scaffolds with different morphologies was predicted according to the mechanoregulation theory. The results show that fluid shear stress and strain distribution depend on the distribution of pores within scaffold. The differentiation results of BMSCs on scaffold surface will be affected by inlet velocity, compressive strain, and the morphology of the scaffold. When the axial compressive strain was 0.5- 5%, and the inlet velocity was within the range of 0.01-1 mm/s, bone and cartilage differentiation area can reach more than 90% for all the scaffolds. This study shows that different levels of mechanical stimuli can be generated as a result of different scaffold morphologies under compressive loading and fluid flow to satisfy the mechanical requirements for different bone defect sites.In the second part, correlations between the microarchitectural parameters and the mechanical parameters, as well as the cell differentiation parameters were analyzed according to the animals? cancellous bone morphologies. Cancellous bones from male rats and bulls were scanned using a Micro-CT system. Afterward, 1 mm3 3D cube scaffold models with porosity of approximately 65% were created from the scanned cancellous bone structures. The solid phase of the scaffolds was then modeled as a linear elastic material of PDLLA. A uniaxial strain varying between 0.5% and 5% was applied, and a Newtonian flow with an inlet velocity ranging between 0.01 and 1 mm/s was simulated concurrently. The effects of these mechanical stimuli under different initial conditions on the differentiation of BMSCs adherent on each structure surface were determined according to the mechanoregulation theory. The scaffolds with different morphologies were shown to produce different distributions of strains and fluid shear stresses. Highly heterogeneous stress distributions were observed on the scaffolds with irregular morphology, and stress concentration appeared at small holes. Cell differentiation on the scaffold was more sensitive to the inlet velocity than the axial strain. More than 90% of the bone differentiation zone was found on the surfaces of all scaffolds when the inlet velocity applied to the scaffolds varied between 0.01 and 1 mm/s and the axial strain varied between 0.5% and 5%. For the scaffolds with rat cancellous bone structures, the stimulus area for bone differentiation on the 60%- 90% scaffold areas was larger than that of the scaffolds with bull cancellous bone structures. Cartilage differentiation on the scaffolds with bull cancellous bone structures that constitute more plate-like trabeculae was more pronounced than on those with rat cancellous bone structures. The stimulus area corresponding to the cartilage differentiation on the 60%- 90% scaffold areas was also larger in the scaffolds with bull cancellous bone structure than in those with rat cancellous bone structures. In this study, the scaffold structures were established on the basis of cancellous bone microarchitectures. Three principal components(PCs) that can reflect scaffold morphology were extracted. Correlations between the microarchitectural parameters and the mechanical parameters, as well as the cell differentiation parameters were analyzed, and the regressions among the PCs, mechanical parameters, and cell differentiation parameters were obtained. This study can help gain more insight into the mechanical environments that approximate the in vivo mechanical environments of BMSCs under the in vitro culture conditions. This study also provides a theoretical basis for scaffold design and bone defect repair in clinics.For biomaterial scaffolds prepared by different preparation processes, the mechanical environment with various loading parameters of perfusion culture conditions was compared. Ti O2 scaffold was reconstructed based on Micro-CT imaging, pore morphology parameters of scaffold were measured, and the apparent morphology was observed by SEM. The validity of the approach for modeling was verified by permeability, and the results were compared with other studies. In addition, the mechanical stimuli within scaffold were analyzed quantitatively, and comparative analyses on the other three commercial bone graft substitutes(Bio-Oss, Cerabone, Maxresorb) were performed. The results show that the mechanical property of Ti O2 is relatively inferior, but the wall shear stress(WSS) is significantly higher than the other scaffolds that are made of commercial bone substitute materials. In addition, the irregular morphology led to heterogeneous stress distributions, and fluid shear stresses were generated within biological response range of BMSCs.The mechanical properties of scaffolds, biomaterials, and histology were evaluated in this study, the results can provide important theoretical basis for designing in vitro culture conditions within bioreactor and repairing specific bone defect in clinics. Three different forms of scaffold models(idealized scaffold, animal cancellous bone scaffold, as well as synthetic biomaterial scaffold) can be established as the basis of in vitro perfusion culture experiments, and provide a more intuitive basis for preparation and clinical application of scaffold in the in vitro culture.