Design And Biomechanical Study Of Gradient Irregular Porous Prosthesis Based On Additive Manufacturing Technology

Background:Although bone tissue has the regenerative capacity to heal itself,bone defects exceeding a critical size（usually>2 cm）are not self-repairing and typically require external intervention for treatment.Metal implants are commonly used clinically to fill the bone defect.However,the high elastic modulus of these implants produces a stress shielding effect that ultimately leads to a reduction of bone around the implant.Porous scaffolds with micropores have controlled biomechanical properties that significantly reduce the elastic modulus.And its micropores provide sufficient space for cell growth.Most porous scaffold designs are based on a regular structure.The typical characteristics of regular porous scaffolds are the repetition of specific units in three mutually perpendicular directions in space,which have the advantages of simple design,predictable mechanical properties,and easy porosity control.However,regular structures cannot be applied in all cases.The internal pore size and the diameter of the struts of the bone trabecular structures are not consistent and exhibit irregularities.Bone structures also have specific anisotropy within the organism,distributed in gradients according to the force characteristics,allowing the body to perform various actions in all directions;therefore,the design of gradient irregularities in bone scaffolds is key bionics.Therefore,this study aimed to design an irregular bionic scaffold for bone tissue engineering applications,comparing it with the leading histomorphometric indices of bone（porosity,cortical bone/trabecular unit thickness,and surface to volume ratio）.The scaffold’s mechanical properties and fluid distribution were also explored using finite element analysis,hydrodynamic analysis,and in-vitro experiments.Methods:1.According to the Voronoi-tessellation distribution,the parametric design of regular and irregular scaffolds was developed under Rhino software and the grasshopper plug-in.2.Statistical analysis was performed to establish the connections between the parameters controlling the irregular structure（number of random points and scaling factor）and the necessary properties of the support（porosity,strut diameter,and specific surface area）.3.Computational hydrodynamic analysis was performed after establishing fluid domains for the same porosity of the body-centered cubic（BCC）structure,the octet structure,and the irregular structure.The three structures’flow velocity,pressure,and permeability are analyzed in three directions（x-,y-,and z-）.4.The finite element models of homogeneous and gradient structures of the three structures are established,the loads and boundary conditions of simulated uniaxial compression are imposed,and the force concentrations and mechanical strengths of the four structures are analyzed.5.Three structures are manufactured by electron beam melting.In vitro cellular experiments are on the three structures,including cellular value-added,live-dead staining,and osteogenic differentiation assays.Results:The microstructure of the irregular scaffold can be adjusted by parameters to make it similar to the microstructure of the cortical/cancellous bone unit,and the scaling factor mainly determines its porosity.In contrast to the regular scaffold,the permeability of the irregular scaffold was highest in all three directions from 40-80%porosity,where the 70%homogeneous irregular scaffold had a permeability of 7.48×10^-8 m~2 and the irregular scaffold also had the highest permeability among the gradient scaffolds at 1.05×10^-7 m~2.The flow velocity of the irregular scaffold was distributed erratically inside the micropores,and the concentration points of flow velocity in the three directions were not in the same area.The results of the finite element analysis indicated that the yield strength was 135.9 MPa for the irregular scaffold,120.7 MPa and 153.5 MPa for the BCC and octet,respectively,and the modulus of elasticity was11.2 GPa for the irregular scaffold,10.2 GPa for the BCC,and 17.7 GPa for the octet.The yield strength of gradient irregular scaffold is 173.5 MPa,while the gradient octet scaffold is 24.8 MPa,and gradient BCC scaffold is 62.3 MPa.The in vitro results showed that the irregular structure had the highest cell proliferation rate among the three structures,with a rate of 295.3%on day 5,and the alizarin red staining showed that the irregular structure and the octet structure had better osteogenic induction ability than the BCC structure.Conclusion:1.the irregular scaffold can be modified to the bone microstructure by adjusting the parameters to achieve morphological mimicry.2.the mechanical strength of irregular scaffold,especially gradient irregular scaffold,is superior to that of the regular scaffold.3.Irregular scaffold is beneficial for cell adhesion and osteogenic differentiation.