Microfluidic gas sensing with living microbial cells confined in a microaquarium

We investigated on-chip cytotoxicity gas sensing using the bacterial chemotaxis of Euglena confined in a microaquarium. The sensor chip made from PDMS had one microaquarium and two microfluidic channels passing aside of the microaquarium. The chemotactic microbial cells were confined in the microaquarium, whereas two gases (one sample and one reference) flowed in the two isolated microchannels. Gas molecules move from the microchannels into the microaquarium by permeation through porous PDMS wall, and dissolve into the water in the microaquarium, where Euglena cells are swimming. The chemotactic movements of Euglena were observed with an optical microscope and measured as traces in real time. By injecting CO₂ and air into each microchannel separately, the Euglena cells in the microaquarium moved to air side, escaping from CO₂. This observation showed that the concentration gradient of CO₂ was produced in the water in the microaquarium. The CO ₂-avoiding movement of Euglena was increased largely at a CO ₂ concentration of 40%, and then moderately increased above 60%. Some Euglena cells stopped swimming at the air side of the microaquarium and remained there even after CO₂ has been removed, which can be used as the indicator of CO₂ history.