Functional design and fabrication of heterogeneous tissue engineering scaffolds

This research posits a fundamental change in design philosophy for tissue engineering scaffolds from a single material approach, in which a single material must be optimized for all criteria, to a philosophy more typical of mechanical engineers, in which independent functional criteria are performed by multiple components. The dominant approach to tissue engineering is a scaffold-guided strategy in which cells are harvested from a patient, seeded upon a biocompatible scaffold, cultured into a functional tissue within the scaffold, and reimplanted. Presented here is a strategy to construct hybrid scaffolds of multiple components, each a specialist material fulfilling a required scaffold function. These functional components include freeform fabricated structural poly-ϵ-caprolactone (PCL) meshes, fibrin filling the mesh to increase cell attachment, and a novel nutrient transfer component, a diffusion network, alginate threads or conduits to allow diffusion more rapidly or to greater depth within a scaffold. To characterize the structural component, PCL scaffolds of varying architectures were constructed, measured using micro-computed tomography (micro-CT), estimated for nutrient transfer characteristics, and cultured with fibroblasts to examine cell seeding, cell attachment, and porosity-dependent proliferation. To characterize the cell attachment component, fibrin, PCL structural scaffolds were filled with fibroblast-bearing fibrin to determine cell distribution and to demonstrate volumetric cell growth. Fibrin was also cultured with hepatocytes alone to demonstrate material dependent effects on cell behavior. Most novel was the alginate diffusion network component, developed during this research and not yet demonstrated in the literature. To test the rationale for diffusion networks, multiple models for oxygen diffusion through scaffolds were developed to calculate oxygen concentrations and cell distribution: an analytical model for cases with uniform cell distribution, a matrix model for homogenous scaffolds with non-uniform cell populations, and a time-iterative model for scaffolds possessing non-uniform cell populations and a diffusion network. These models predicted enhanced cell growth at lower levels within scaffolds possessing a diffusion network. Preliminary cell work was then performed using fibroblasts and hepatocytes on PCL/fibrin/alginate scaffolds with two diffusion network geometries. Combined architectural analysis, cell culture testing, and numerical simulation support both the use of multiple component scaffolds and the novel diffusion network component.