Design and Performance Evaluation of TPMS-Based Dual-Layer Gradient Porous Structures for Bone Scaffolds

This study investigates and compares properties of various P-type Triply Periodic Minimal Surface (TPMS) porous structures for bone scaffold design. At first, six cases of homogeneous single/dual-layer structures, axial single/dual-layer gradient structures and radial single/dual-layer gradient structures with the same average porosity are developed. Dual-layer gradient structures are selected for further design due to more similar pore and stress distributions to human bones, reduced maximum stress, higher yield strength and greater variations in yield strength and elastic modulus (E). The mechanical and permeability properties of ten cases of axial and radial dual-layer gradient structures with the same overall porosity but different inner and outer layer porosities are then further designed and studied. The results show that yield strength is within 112.75–139.97 MPa, E ranges from 11.15 to 13.01 GPa, the permeability (K) falls within 1.51–10.01 × 10⁻⁹ m² and the average wall shear stress (WSS_avg) varies between 6.18 and 9.11 mPa. The yield strength, E and K of radial dual-layer gradient structures are higher and WSS_avg is lower than those of axial dual-layer gradient structures. Moreover, with increase in inner average porosity ($\overline{P}$) and decrease in outer $\overline{P}$, the yield strength, E and K gradually decrease while WSS_avg gradually increases for both types of structures. In particular, the radial dual-layer structure with the lowest porosity of 27.5% in the inner layer and highest porosity of 42.5% in the outer layer has superior mechanical and permeability properties. The findings offer direct guidance for the structural design of bone implants, enabling performance customization for different applications.