In order to improve tissue formation in scaffolds, various mechanical parameters have been explored such as fluid flow, hydrostatic pressure, and substrate deformation. It is thought that fluid flow is the dominant mechanical stimulus for physiological bone cell behavior. Porous seeded cell constructs cultured in perfusion bioreactors have demonstrated increased improvement in cell expansion and tissue growth. Two distinct flow mechanisms are thought to be responsible for the increased response generated by cellular scaffolds cultured in perfusion bioreactors. Firstly, shear forces have shown to improve osteoblastic differentiation, cell proliferation, matrix deposition, extracellular matrix calcification and mechanical properties of the cellular construct. The second mechanism to which the success of flow perfusion cultured scaffolds is attributed to is enhanced nutrient and waste transport.

The variety of perfusion system arrangements, the inherently random internal architecture of porous scaffolds and the differences in cell-biomaterial interactions due to the various surface chemistry and cell types render any analytical estimation of the local shear forces and transport properties implausible, while the current scaffold architecture design is mostly empirical and trial-and-error based. The situation is further complicated by the transient nature of the tissue growth process within the scaffold while it is being cultured. Cell adhesion strengths change with time as cell coverage grows from an initial monolayer coverage of the scaffold to cells adhering to layers of other cells and excreted extracellular matrix. Expansion of cells and tissue during the culturing period within the porous network of the scaffold creates a continuously changing geometry. Thus the biological environment inside the scaffold can be visualized as a continuously changing 3D matrix of tissue that dynamically responds to mechanical stresses. Therefore, as a primary goal of this work the local shear force distributions as well as the nutrient transport are explored throughout the culturing process for various scaffold geometries. Based on the obtained results, the 3D porous scaffold structure is analyzed for optimum fluid shear and nutrient transport for osteoblastic cells cultured in a perfusion bioreactor.

Micro-Computed Tomography with 10 µm resolution is used to obtain 3D structure of the scaffolds. Histological staining is used for validation of the µCT images. Flows of osteogenic media through the cell seeded cylindrical scaffolds are modeled via fluid dynamics simulations (Lattice Boltzmann Method – LBM). Experimentally obtained pressure drops across the scaffolds are used as an input for the fluid dynamics simulations. High performance computing in conjunction with a house hybrid MPI/Open MP parallelized scheme is employed in order to take advantage inherent LBM parallelizability. The code is validated against analytical solutions for flows pressure driven flows in channels, pipes, ducts and an infinite array of spheres. Macroscopic mass transfer is modeled using the Lagrangian Scalar Tracking method (LST) in conjunction with the LBM algorithm. In LST, the motion of scalar markers is used to synthesize the scalar profile. The trajectories of these markers are composed by a convection part (obtained using the velocity field from the LBM simulations) and a diffusion part (i.e., Brownian motion obtained from a mesoscopic Monte-Carlo approach). This method is resourceful in terms of computational efficiency, in that it can be used to simulate various thermal boundary conditions and Schmidt number fluids with a single flow field resulting from an LBM simulation. Scaffolds are characterized based on their geometric characteristics (such as tortuosity, surface area to volume ratio, and porosity, pore size, and pore connectivity/anisotropy) in order to parameterize the obtained results as a function of different scaffold structures. Special attention is devoted to porosity of the scaffold since it has been shown to play a detrimental role in the internal geometry of the scaffolds. The study as performed at different time points throughout a two week culturing point. The theoretical framework developed allows for improved tissue generation via: maximum solid-fluid contact area for uniform cell adhesion, efficient nutrient and waste transport inside the scaffolds for promotion of cell survival and favorable mechanical stimulation of cell proliferation.