Mechanical design of 3D printed bone tissue scaffolds with tunable anisotropy

Additive manufacturing is enabling the design of intricate biomedical structures with tuned mechanics for bone tissue engineering. Tuning structures to mimic the effective anisotropic mechanical properties of bone, however, remains challenging due to difficulties in recreating bone’s hierarchical geometry and porous structure. Here, we introduce beam-based lattices with tunable unit cell aspect ratios and hierarchical pores to tailor the biomechanics of tissue engineering scaffolds for interbody spine fusion cages. BC-Tetra unit cells with beams along edges and diagonally from each corner to the center of a tetragonal unit volume were selected due to their mechanical efficiency and favorable geometry for tissue growth. Unit cells were designed with 500 and 800 µm diameter beams, porosities of 50 % and 70 %, and adjustable aspect ratios by tuning unit cell height. Scaffolds were printed using digital light processing with a biocompatible methacrylic polymer. Uniaxial mechanical compression experiments demonstrated that larger unit cell aspect ratios resulted in higher effective mechanical properties in the loading direction. Finite element analysis matched experimental trends and highlighted stress distributions for each tested lattice. Dimensional characterization demonstrated beams were printed larger than expected towards the center of the scaffold, that in turn decreased scaffold porosity while increasing stiffness. Large hierarchical voids were introduced to improve the consistency of printed beams throughout scaffolds and facilitate biological functioning. Mechanical testing demonstrated scaffolds of 40 % to 80 % porosity had stiffness from 3.9 to 8.4 kN/mm, suitable for vertebral bone fusion. These results enable improved design and fabrication of tissue scaffolds by providing new strategies for controlling anisotropy and hierarchy that could widely enhance regenerative medicine treatments.