Uniform and multi-morphology graded TPMS structures: Design strategies, 3D printing and mechanical properties

Bone regeneration remains a challenge and designing scaffolds to replicate the natural bone structure (complex hierarchical network) while providing adequate mechanical properties is highly required. Triply periodic minimal surfaces (TPMS) have attracted considerable interest for their smooth surfaces (without sharp edges connections), lightweight, enhanced surface area, and tunable mechanical performance. This study adopted I-graph wrapped package (IWP), Neovius, primitive, and face-centered cubic rhombic dodecahedron (F-RD) TPMS for scaffold designs (3-6 mm cell size, 70 % porosity) and fabricated from polylactic acid using stereolithography 3D printing technique. Furthermore, multi-morphology graded (MMG) lattice structure designs were proposed, combining two TPMS unit cell types, which showed high mechanical properties. Gradient transitions were introduced using sinusoidal patterns along a single axis (sin(x)), along two axes (sin(x+y)), and a linear diagonal transition (x+y) to form complex structures and enhance performance. The compression tests were performed to examine the deformation behavior, mechanical properties, and energy absorption characteristics. Among uniform structures, IWP and F-RD lattice exhibited the highest surface area to volume (SA/V) ratio of 7.44-10 mm²/mm³, whereas IWP and Neovius showed higher yield strength (9-15 MPa) and strain energy (5.96-16.44 MJ/m³). Moreover, deformation in IWP and primitive shifted from bulging to curve bending, while Neovius and F-RD changed from bulging to shear zone formation. Furthermore, MMG lattice structures (created from IWP/Neovius) exhibited modern SA/V (6-9 mm²/mm³) and improved yield strength (12-22 MPa). The deformation began with bulging and shear zones, progressing to crushing with angular cracks, confirmed through electron microscopy imaging.