Oxygen Transport in Bioreactors for Engineered Vascular Tissues - Computational Modeling in Tissue Engineering - page 278

engineered tissues lack this level of vasculature. In vitro mass transport limitations
can impact engineered tissue constructs due to the formation of nutrient or
metabolite gradients. These gradients can impact size or uniformity of engineered
tissues due to changes in metabolic activity or cell viability. Transport limitations
can also impact subsequent integration of these constructs in vivo.
Adequate supply of oxygen is critical due to its role in synthesis of ATP for cell
energy production and is required for extracellular matrix (ECM) production. High
cellular demand for oxygen coupled to low solubility and slow diffusion yield
gradients in this essential nutrient and can also lead to hypoxic (low oxygen) or
anoxic (zero oxygen) regions. Oxygen gradients have been measured in cartilage
[ 2 - 4 ] and cardiac tissue [ 5 , 6 ], and regions of low dissolved oxygen (DO) were
associated with dead or dying cells. Stem cells, which are typically cultured in low
oxygen environments, were shown to die when exposed to anoxia for 5 days [ 7 ].
Oxygen limitations have also been evaluated for their impact on cell viability and
proliferation. Demol et al. recently conducted a theoretical analysis of oxygen
gradients in fibrin gels and coupled DO to a model of cell growth, predicting the
highly populated cell layer on the outer periphery observed in many fibrin-based
engineered tissues [ 8 ].
One method aimed at improving uniformity of oxygen delivery to cells is based
on bioreactor culture. Bioreactors serve to improve mass transport by flowing or
mixing culture medium around cell monolayers. Mass transport is also improved
during bioreactor culture of engineered tissues by either eliminating external
gradients to maximize diffusional driving force or providing convective mass
transfer through tissues. Bioreactors are also frequently used to provide mechan-
ical stimulation, and while there is some degree of convective transport to the cells
within the tissue construct to augment diffusional transport [ 9 - 14 ], improved
transport is often neither intentional nor controlled. Axial perfusion bioreactors for
engineered vascular grafts [ 15 - 17 ] cause transmural flow through the construct
wall as culture medium is pulsed through the lumen, but this forced convective
transport associated with transmural flow is again incidental and not controlled.
Some recent designs, however, have focused on improved nutrient delivery via
controlled convective transport through the tissue. For example, Khong et al.
applied interstitial flow to the culture of liver tissue [ 18 ]. Their work demonstrated
enhanced enzyme activity, urea synthesis and albumin secretion in liver slices up
to 1 mm thick using a needle-perfusion chamber. Forced convection has also been
investigated in cardiac tissue engineering [ 19 ] to examine cardiomyocytes
entrapped in Matrigel. The interstitial flow of medium improved the density of
viable cells throughout the thickness, whereas static culture resulted in constructs
with only a 100- to 200-lm thick surface layer containing viable cells around an
acellular core. In a subsequent study using collagen constructs [ 20 ], oxygen
concentration and cell viability decreased linearly from the construct surface and a
physiological density of live cells was present only within the first 128 lm.
However, interstitial flow of medium significantly increased oxygen concentration
within the construct. Interstitial flow has also been applied to the culture of liver
tissue. Chung et al. developed a detailed model to predict how changes in