Design And Mechanical Properties Of Negative Poisson’s Ratio Composite Structure For Bone Implants

Negative Poisson’s ratio metamaterials are derived from their unique pore structure,and their cross sections expand(or contract)inversely under the action of uniaxial tensile(or compressive)forces.Due to its abnormal deformation mechanism,it has a lower elastic modulus and higher compressive strength,which has a huge application prospect in the field of orthopedic implants.In order to solve the problem of "stress shielding" effect in bone implants,this paper conducts research on the design and mechanical properties of negative Poisson’s ratio composite structures,and explores their applicability in artificial bone implants.The specific research content is as follows:(1)Design and mechanical properties of novel negative Poisson’s ratio structures with complex chiral and concave structures.A three-dimensional composite negative Poisson’s ratio cellular structure based on chiral and concave structures was designed.Samples of porous implants with negative Poisson’s ratio were prepared using laser selective melting forming technology.The effects of cellular structure and porosity on mechanical properties such as strength and elastic modulus were studied through compression tests,and the matching degree between the composite structure and the mechanical properties of human bone was evaluated.The results show that the mechanical properties of the composite structure are directly related to the cellular structure and porosity.At a porosity of 65% to 85%,its elastic modulus is 1.12 GPa to 3.77 GPa,and its yield strength is 45.25 MPa to 195.81 MPa,which is between the concave structure and the chiral structure.It is expected to achieve a spanning connection of mechanical properties from cancellous bone to dense bone.(2)Study on the influence of structural parameters of composite negative Poisson’s ratio on mechanical properties.Based on the response surface method,a three factor and three level experimental scheme was designed using the size ring to diameter ratio,concave angle,and rod diameter of the composite negative Poisson’s ratio cell as parameters.Based on mechanical compression tests and response surface method,the influence of structural parameters on elastic modulus,yield strength,and Poisson’s ratio was analyzed,and a mathematical model was established.Taking the mechanical properties of human bone trabeculae as the response value,the optimal structural parameters were optimized.The results show that the elastic modulus and yield strength are positively correlated with the rod diameter and cell size ring to diameter ratio,while negatively correlated with the concave angle;Poisson’s ratio has a positive correlation with rod diameter and concave angle,and a negative correlation with rod diameter and cell size ring to diameter ratio.Taking a trabecular bone with an elastic modulus of 3.65 GPa and a yield strength of 119 MPa as the target,the structural parameters of the cell were optimized to obtain a cell size to ring diameter ratio of 0.9,a concave angle of 60.1°,and a rod diameter of 0.546 mm.Experiments have verified that the error of its elastic modulus is 1.9%,which can meet the customized design requirements of cellular structures.(3)Design and mechanical properties of composite negative Poisson’s ratio gradient structures.Based on the composite negative Poisson’s ratio cell structure,three gradient specimens with different porosity were designed.The relationship between mechanical properties such as yield strength and elastic modulus and the gradient structure was revealed through compression tests.The results show that under the same porosity,the elastic modulus and yield strength of the axial and radial gradient structures are similar,significantly higher than those of the unidirectional gradient structure,with a difference of no more than 13% from the uniform structure.Axial and radial gradient structures can still meet the mechanical performance requirements of bone implants while improving material delivery capabilities,and direction is not sensitive to the impact of mechanical properties.