(11g) Bioreactor Design Configurations for Regenerating Human Bladder

For tissue engineering to be successful, building scaffolds of the shape similar to the tissue to be placed is important. Further, designing of advanced culture system with appropriate cells is necessary to distribute nutrients within the biodegradable porous scaffold and to provide homogenous physical forces. However, the fluid dynamics with the scaffold of anatomically relevant structure are not well defined. In this study, we focused on designing of bioreactor for the bladder tissue regeneration. Effects of reactor geometry and fluid schemes on pressure drop, shear stress, velocity distribution, and nutrient consumption patterns during tissue regeneration were explored. In addition, when the tissue heals, reduction in pore size significantly increases pressure drop by proliferation of cells, resulting in very high inlet pressure and alternative designs are necessary. Geometries with a spherical shape of bioreactors were simulated using the computational fluid dynamic software (COMSOL multiphysics 3.5a). Previously, our group reported that blending gelatin with chitosan showed significant influence on scaffold properties and cellular behavior (Huang, Onyeri et al. 2005). The fluid flow was defined by the Brinkman equation on the porous regions using the pore characteristics of gelatin-chitosan scaffold. Based on previous scaffold characteristics, 85 µm average pore size and 120 pores/mm2 porosity were initially selected. Further, the thickness of the bladder scaffold was 3 mm, similar to the human bladder, and overall hold up volume within the spherical shape scaffold was 600 mL. Steady State conditions were used with no slip condition of the walls. Outlet pressure condition (boundary condition) was set to be atmospheric pressure. Simulations were carried out with increasing the flow rate from 0.5 to 5 mL/min and decreasing pore sizes from 85 to 20 µm as increase as the cell number. Oxygen and glucose consumption profiles were obtained by solving the continuity equations with Michaelis Menton rate law for smooth muscle cells. Two different inlet shapes i) straight entry at the center (design 1), ii) entry with an expansion (design 2) were simulated to evaluate shear stress distribution at the inlet within the porous structure. To mimic bladder configuration, two inlets (Design 3) instead of one was also simulated. Based on these results Design 2 provided the uniform shear stress at the inlet and nutrient distribution. Using Design 2, three different locations of the scaffold within the reactor were investigated: i) attached to the interior with a 3mm open flow area (Design 2a), ii) flow through with no open flow area (Design 2b) and iii) porous structure suspended in the middle with 1.5 mm open flow area on either side (Design 2c). Then simulations were performed by varying the flow rate. Design 2c showed fluid distribution on both inside and outside of the scaffold and the highest consumption of oxygen and glucose, appropriate maximum shear stress and pressure drop, and Peclet number < 1. Further, decrease in porosity by cell proliferation suggested that increase in flow rate is necessary to provide the minimum consumption of the oxygen.