Mechanical and degradation properties of triply periodic minimal surface (TPMS) hydroxyapatite & akermanite scaffolds with functional gradient structure

High porosity and high mechanical strength are both important for bone scaffolds, but they are always contradictory, which can be tackled by reasonable structural design. In this paper, we proposed a series of functional gradient lattice structures with the local porosity increasing continuously from the inside out. Based on the triply periodic minimal surface (TPMS), four types of gradient scaffolds were designed with the same porosity (65%) and fabricated by digital light processing (DLP) technology. Finite element (FE) analysis and compressive tests were conducted to investigate the mechanical property, and degradation test was conducted to investigate the biological property. Results showed that gradient structures can effectively enhance the overall compressive strength at the same porosity level. Scaffolds with exponential structure have the highest compressive strength and elastic modulus, reaching 4.45 +/- 0.52 MPa and 0.580 +/- 0.034 GPa respectively, almost twice that of uniform structure. Gradient structure has a trivial effect on degradation performance. The degradation rates of all scaffolds can reach more than 5% after immersing for one day. Surface morphology of distinct parts of a scaffold are almost the same due to the highly connected internal structure. These results confirmed that gradient structure can improve mechanical property while maintaining the degradation rate, which have great advantages in bone scaffold engineering.