Computational evaluation of light propagation in cylindrical bioreactors for optogenetic mammalian cell cultures

Light-inducible regulation of cellular pathways and gene circuits in mammalian cells is a new frontier in mammalian genetic engineering. Optogenetic mammalian cell cultures, which are light-sensitive engineered cells, utilize light to regulate gene expression and protein activity. As a low-cost, tunable, and reversible input, light is highly adept at spatiotemporal and orthogonal regulation of cellular behavior. However, light is absorbed and scattered as it travels through media and cells, and the applicability of optogenetics in larger mammalian bioreactors has not been determined. In this work, we computationally explore the size limit to which optogenetics can be applied in cylindrical bioreactors at relevant height-to-diameter ratios. We model the propagation of light using the radiative transfer equation and consider changes in reactor volume, absorption coefficient, scattering coefficient, and scattering anisotropy. We observe sufficient light penetration for activation in simulated bioreactors with sizes of up to 80,000 L at maximal cell densities. We performed supporting experiments and found that significant attenuation occurs at the boundaries of the system, but the relative change in intensity distribution within the reactor was consistent with simulation results. We conclude that optogenetics can be applied to bioreactors at an industrial scale and may be a valuable tool for specific biomanufacturing applications.