Bioreactors for Tissue Regeneration

Broad objectives is to develop the governing conditions for designing bioreactors during the process of regeneration.   Successful development of principles will significantly advance the regeneration of high quality tissues “off the shelf” by variety of contributions including a methodology to monitor the regenerative process non-destructively.

Background.  In a traditional cell culture, cells are populated in a batch culture on flat plates with a defined amount of nutrients which distribute within the porous structure by diffusion, dictated by Fick’s first law. As cells consume nutrients, relying on diffusion alone with depleting concentration gradient leads to starvation and non- uniform distribution. A way to improve the nutrient distribution is by constant mixing and/or constant replenishment via fluid flow.  In addition to improving the nutrient distribution, fluid flow also applies shear force on the cells, which is important for certain tissues.  Many parts of the body are exposed to stresses either due to the weight they carry (such as bone), the function they perform (such as bladder and cartilage) or due to the flow of fluid (lung, blood vessels).  In these cases, there is evidence that application of certain magnitude of stress helps alters how cells behave. One such effect is shown below on endothelial cells, which line the inner lumen of blood vessels.

Different types of bioreactors have been designed to regenerate tissues with the while applying mechanical stimuli.  However, the design principles in developing these bioreactors are not well developed. For example, selecting a specific shape of bioreactor, the inlets and outlet locations, and the media flow rates have remained random.  There is limited understanding of the nutrient distribution within these bioreactors during the entire process of tissue regeneration.  The kinetic models are often over-simplified because mass and structural complexities have been ignored.  Tissue regeneration is a dynamic process where the porous characteristics change due to cell growth, assembly of newly secreted matrix components, and degradation of the porous architecture. These changes affect the transport characteristics which ultimately determine the quality of the regenerated tissue. Non-uniform flow patterns within the reactor could lead to i) poor distribution of nutrients and ii) non-uniform shear stress distribution. These factors affect cellular colonization, proliferation, and function. Thus to develop improved quality tissues, one has to understand the influence of these factors.

Our approach. We design bioreactors for regenerating tissues using the tools and governing equations, proven to work in other engineering disciplines.  We use a set of integrated tools: i) computational fluid dynamic (CFD) software such as COMSOL and CFX to understand the effect of flow configurations on fluid distribution through the porous structure, ii) different scaffold preparations preparations, and iii) cell culture experiments in bioreactors with specifications identical to simulation. Some factors evaluated thus far include i) reactor shapes (rectangular, circular, spherical), ii) flow rate, iii) inlet-outlet location, iv) inlet-outlet size which regulate velocities, v) nutrient consumption (oxygen, glucose, estrogen) kinetics, vi) cell types (smooth muscle cells, chondrocytes, hepatocytes, fibroblasts, cord blood stem cells), vii) scaffold mechanical properties, and viii) scaffold permeability properties using Kozeny-Carman definition.  We also combine residence time distribution analysis with parametric models to calculate the outlet oxygen concentration.  We assess the effect of changing porous architecture due to cell growth and deposited matrix elements on fluid distribution, shear stresses and pressure drop. We measure diffusivities in porous structures, and account for changes in dimensions of the scaffolds using mechanical properties. We believe these efforts will help develop strategies to run the bioreactor during the entire regenerative cycle. Our current efforts are focused on integrating mechanical effects on nutrient distribution.