Solid-fluid mixture microstructure design of composite materials with application to tissue engineering scaffold design

The ability to design the material microstructure brings the use of composite materials into the next generation. In this paper, we report pioneering research to implement the computational material microstructure design into the internal architecture design for a tissue engineering scaffold. A tissue engineering design postulate is that scaffolds should match specified healthy tissue stiffness, while concurrently providing sufficient porosity for cell migration and tissue regeneration. Employing the inverse homogenization method and the adaptive topology optimization method, a complex 3D microstructure can be designed to perform with the anisotropic elastic stiffness and porosities analogous to a native bone specimen. Besides the elastic stiffness from its solid part, fluid in the porous region also plays an important role in tissue engineering. The flow of fluid through the pores brings nutrients to cells in the tissue matrix and also removes their waste. Fluid permeability of cylinderical trabecular bone grafts was found to predict clinical success. Deriving from Darcy's Law, we developed software to calculate the homogenized fluid permeability of 3D cancellous voxel models, which were directly reconstructed from micro-CT images. Furthermore, an Evolutionary Structural Optimization (ESO) algorithm was utilized to maximize fluid permeability in the microstructure. The fluid optimization scheme was then collaborated with solid phase optimization through Multidisciplinary Design Optimization (MDO) to create an integrated solid-fluid mixture microstructure design. In addition, to ensure the fabrication feasibility, we also implemented a post-optimization process to enhance design results by improving the dynamic stiffness to eliminate weak connections and checkerboard pattern. The design scaffolds were then built by an indirect solid freeform fabrication (SFF) technique using various bio-compatible materials and ready for further investment. This computational design scheme based on topology optimization was developed in order to provide a general solution for tissue engineering scaffold internal architecture design and fabrication. Hence, the subsequently engineered scaffold will provide a biomimetic mechanical environment, while maximizing fluid permeability and maintaining sufficient porosity for tissue ingrowths.