The reliable, noninvasive measurement of dissolved gases in microfluidic devices is an important and difficult activity for biological applications. The PreSens pCO2 mini and chemical optical CO2 sensors have been designed to noninvasively measure carbon dioxide in the recirculating fluid in these devices.
The small CO2 sensor spot placed in contact with the solution takes up a small volume and has a thin profile, at approximately 5 mm diameter and 0.1 mm thickness. Microfluidic devices for biological applications limit the culture media to small volumes and small flow rates, creating the advantage of minimizing the use of reagents.
However, the small volumes create challenges for measuring concentrations of various chemical species within the media. For microfluidic devices for biological applications, the response time and ability to detect small changes in gas concentration are very important.
The dissolved gas levels in the recirculating media must be within a physiological range to maintain the proper environment for the cellular components. A rapid response time is critical for the creation of a feedback mechanism to enable adjustment of gas concentrations quickly. The small volumes of media used in the microfluidic devices preclude the sampling of media for analysis, so the ability of the sensing system to monitor noninvasively is crucial. With the small volumes and low flow rates used in these microfluidic devices, the application limits the consumption rate of gases by the sensor to small rates.
The pCO2 mini system was evaluated for its ability to monitor changing dissolved CO2 concentrations inside a microfluidic device to be used in a microfluidic biological culture device. Two configurations were evaluated: a flow-through cell located external to the microfluidic device and a sensor spot housed inside the device. The repeatability and response time were measured and compared to a commercially available Severinghaus-type sensor (a generic expresson for a CO2 measuring system based on a pH electrode in contact with a bicarbonate buffer).