(393d) Designing Bioreactors for Regenerating Large Tissues

Bioreactors are widely utilized in tissue engineering as a way to distribute nutrients within the biodegradable porous structures and to provide homogeneous physical forces required by the regenerating tissue. However, the fundamental concepts in developing these reactors are not well defined. For example, many tissues (for example skin, bladder, and cartilage) have a high aspect ratio (large surface area relative to the thickness of the matrix) and contain multiple cell types. Further, tissue regeneration is a dynamic process where the porous characteristics change due to proliferation of cells, de novo deposition of matrix components, and degradation of the porous architecture. These changes affect the transport characteristics of nutrients. In this regard, previously, our group reported on flow through bioreactors where fluid distribution/consumption was regulated by convective flow [1, 2]. Those studies were carried out at constant volumetric flow rate by compensating for the frictional loss by increased inlet pressure. However, when the tissue heals, reduction in pore size significantly increases pressure drop, resulting in very high inlet pressure and alternative designs are necessary. Further, the maximum flow rate allowable through the porous structures also depends on the strength of the porous structure.

The objective of this study was to evaluate reactor designs which produce lesser pressure drop and allowed higher flow rates that would produce shear stresses similar to physiological conditions. For this purpose, two new split channel flow models (referred as Design 8 and Design 9) were designed using a COMSOL 3.5a Multiphysics package and analyzed for homogeneous stress distribution nutrient distribution based on consumption characteristics of smooth muscles cells. Design 8 was an extension of previous flow-through reactor (2 mm thick and 10 cm diameter scaffold) with an increased thickness (4 mm overall) that allowed the medium to flow over the scaffold. Design 9 had a cavity which housed the scaffold (2 mm thick and 10 cm diameter) and the nutrients flow over it. Steady State conditions were assumed with no slip condition of the walls. Outlet pressure condition (boundary condition) was set to be atmospheric pressure. The fluid flow was defined by the Brinkman equation on the porous regions using the pore characteristics of Chitosan-Gelatin scaffold formed at -80°C (85 µm and 120 pores/mm2). Simulations were carried out with six decreasing pore sizes but using the number of pores per unit area that was determined experimentally (120 pores/mm2). Oxygen and glucose consumption profiles were obtained by solving the continuity equations with Michaelis Menton rate law for smooth muscle cells. The glucose and oxygen concentration profiles were assessed in different planes/levels of the scaffold.

It was found that the shear stress levels were significantly less in the new designs than the values observed for flow-through reactors. It was observed that the oxygen and glucose concentration profiles changed significantly for Design 8 as pore size was changed. However, no change was observed for Design 9. In Design 8, the mass transfer inside the scaffold occurred due to convection as well as diffusion. When the pore size is 85 µm, convective mass transfer is dominant but when the pore size is reduced to 10 µm diffusion mass transfer becomes dominant and therefore we see a drop in concentration of oxygen. In Design 9 the mass transfer inside the scaffold occurred only due to diffusion as the scaffold is placed at a lower level and nutrients flow over it. Since, constant diffusivity was used in all the simulations, the concentration did not change with reduced pore size. Thus, there is a need to assess the diffusivity of nutrients in changing pore architecture as the hindrance coefficient significantly depends on the pore architecture. In this regard, three porous structures of Chitosan-Gelatin with different pore architecture (controlled by total polymer concentrations) were generated by controlled rate freezing and lyophilization technique. Digital micrographs were obtained using light microscopy in hydrated conditions and analyzed for pore size using Sigma Scan Pro Image analysis software. Further, diffusion of glucose was assessed in these structures. Obtained diffusivities were plotted as a function of pore size. These values are incorporated into the simulation to assess the nutrient consumption patterns in the bioreactor.

References

[1] Lawrence BJ, Devarapalli M, Madihally SV. Flow Dynamics in Bioreactors Containing Tissue Engineering Scaffolds. Biotechnology/ Bioengineering. 2008.

[2] Devarapalli M, Lawrence BJ, Madihally SV. Modeling Nutrient Consumptions in Large Flow-Through Bioreactors for Tissue Engineering. Biotechnology/ Bioengineering. ePub March 19 2009 (hard copy in press).