Mechanical Properties of 2D Re-Entrant Gradient Structures Produced by Additive Manufacturing

In this study, experimental and numerical methods have been used in order to obtain the elastic modulus of the gradient structure with auxetic unit cells (negative Poisson’s ratio). Stress shielding is the most significant issue at the bone-implants interface. The difference in elastic modulus between bone and implant provides stress shielding. The development of mechanical properties in the porous structure can provide opportunities to resolve this mechanical properties mismatch between bone and implant. In this study, firstly, the uniform auxetic unit cells with negative Poisson’s ratio were introduced, and their mechanical properties were investigated by numerical and experimental methods in the x and y direction, and the results were verified. Then, numerical methods were used to achieve the uniform distribution of the elastic modulus in the gradient structure. Therefore, in the gradient structure, the elastic modulus in the outermost layer (assumed the contact surface with the bone) is considered lower to reduce the stress shielding at the bone-implant contact surface. In the next layers approaching the center of gradient structure, the elastic modulus is increased gradually in order to increase the mechanical properties of the whole porous structure. It is noted that the difference in elastic modulus between two contact layers is approximately 15% to reduce stress shielding. The results showed that stress shielding could be reduced by using the gradient structure. In addition, using negative Poisson’s ratio unit cells can establish good surface contact between the bone and implant.