| Bone tissue engineering provides an effective solution for the treatment of bone defects,and scaffold is the key in bone tissue engineering.In the design of scaffolds,the composite structure,whose morphology and function are closer to those of the natural bone,can be obtained by using different materials or unit cells.Reasonable composite structure design can not only make the mechanical and biological properties of the scaffold became better,but also improve its service life.The analysis of mechanical properties and permeability of scaffolds can provide reference for the design of composite scaffolds.In this study,a new design method of composite scaffolds was proposed,and two unit cells with similar boundary structures were used to make composite structure’s transition at the boundary more natural and have less stress concentration.In the two kinds of unit cells,the unit cell with higher yield strength is used in the support structure of the outside region to improve the mechanical properties of the whole scaffold,and the cell with better permeability is used in the inside structure to improve the transport capacity of the whole scaffold.In this study,the following research has been carried out: Above all,cube unit cells and D-cube unit cells were used to design composite structure with 60% porosity,the quantity ratios of cube cells was 21:4,13:12,9:16.And the size was 16×16×16mm~3.The composite scaffold model and uniform scaffold with the same size and porosity were developed in the 3D modeling software(NX 11.0).Then the porosity,specific surface area and barrier free flow area of each model were calculated in NX 11.0.The finite element method was used to conduct compressive simulation analysis to explore the differences of elastic moduli and yield strengths of different porous structures.In terms of flow properties,the velocity profile,pressure profile and wall shear stress distribution of the flow field in the scaffold were analyzed by computational fluid dynamics,and the permeability of the scaffold was calculated based on Darcy’s law.Stereolithographic 3D printing was selected to manufacture the scaffold.The rate of flow of the scaffold was measured by fluid experiment,and the permeability of the scaffold was calculated by Darcy’s law.The simulation results were compared and verified with the experiments.In addition,a composite structure scaffold was designed using TPMS structural cells as the unit cells.The TPMS composite scaffold was developed by Mathematica software.The influence of control equation parameters on porosity was analyzed,compression simulation analysis and hydrodynamic analysis were completed.It was shown that compared with the uniform structure bone scaffold,the bone scaffold with composite structure has a great improvement in the strength and wall shear stress distribution,and the specific surface area becomes larger,but the elastic modulus increases slightly while the permeability decreases.Compared with the bone scaffold with uniform structure,yield strength of the bone scaffold with the quantity ratio of cube cells as 21:4 increased by 60%,and the elastic modulus was only increased by 20%.However,compared with the uniform structure,the permeability of the composite scaffold decreased by about 25%.The elastic modulus of the composite TPMS scaffold is 17.6GPa,which are comparable to human bone.Compared with composite CAD scaffold with the same porosity,composite TPMS scaffold owns almost twice the strength and specific surface area,and the permeability is about 10%higher.The experimental results of permeability measurement were positively related to the simulation results.This study concluded that reasonable design of composite structure can increase the strength and specific surface area of the scaffold,which may show better wall stress distribution for the growth and differentiation of bone cells.The composite structure design method proposed in this study is not only suitable for the design of composite CAD scaffold,but also suitable for the design of composite TPMS scaffold. |
