In Situ Oxygen Gradient Generation and Control inside a Microfluidic Habitat

Microfluidic devices are a useful tool in simulating complex microhabitats where organisms experience steep concentration gradients of oxygen, nutrients, or other chemical stimulants within a very small space and time scale.  These applications of microfluidic cell culture require an environment with a controlled oxygen concentration. An oxygen gradient can be achieved by flowing an oxygenated fluid stream through a microchannel and allowing the oxygen to diffuse through either a polymer membrane or hydrogel to the cell culture area. However, in this type of system it can be difficult to closely control the oxygen concentration either spatially or temporally.  The diverse microbial community of the eastern subterranean termite gut is an example of a microhabitat found in nature that experiences very steep oxygen gradients. These oxygen gradients result in the formation of spatially-distinct microniches for the protists, bacteria, and archea inhabiting the gut. These microniches allow microbial symbionts to co-exist with each other and the termite host while converting woody feedstocks into useful products such as acetate. Here, we describe a method to generate, control, and model oxygen in situ within a microfluidic habitat designed for culture of termite gut microbes. Novel microfabrication methods were developed to produce oxygen at gold electrodes photopatterned on the vertical side walls of the microchannel. Finite element simulations were used to model oxygen production rates under varied geometries, operating currents, and flow rates.  Additionally, in-plane hydrogel barriers to flow and motility were constructed between the channel containing the electrodes and the cell culture chamber. These barriers allowed maintenance of a stable oxygen gradient and also dampened any sudden changes to the system that could harm cells. With these and other recent advances in microfabrication and modeling techniques, increasingly complex natural micro-scale habitats can be synthetically recreated in microfluidic devices.