The new sensor, described in an early view article in Analytical Chemistry , measures the viable biomass concentration, dissolved oxygen concentration, pH and temperature while also being able to withstand steam-sterilisation. Fermentation in industrial scale bioprocess systems for the production of active pharmaceutical ingredients (APIs) is both money- and time-consuming, making the selection of the correct cell lines and process conditions very important. In order to increase the efficiency of cell and process condition screening experiments there is a need to conduct experiments in a high-throughput manner similar to that conducted in drug discovery. However, for this to be possible small, integrated sensors suitable for monitoring microfermentors in real-time need to be developed.
According the authors from the University of Twente in the Netherlands, most approaches to sensor miniaturisation use small fibre-optic sensors that use an immobilised dye that cannot operate below pH 6 which hinders their use as many yeast cell cultures are fermented at pH 5 in industrial processes. Optical sensors do not survive well during the steam sterilisation processes that are used to clean industrial reactors and so the Dutch researchers used electrochemical sensors that could be integrated into silicon wafers. This not only had the advantage of enabling them to withstand multiple steam sterilisation cycles in between fermentation runs of several days but they could also be designed to be small enough to fit in the bottom of a well plate. "We studied the sensor arrays in both micro-well and lab-scale fermentors to calibrate the response of our array in real yeast fermentations with respect to the conventional measurement techniques used in industrial scale bioprocessors," said Erik Krommenhoek, of the University of Twente and lead author of the article.
The arrays measures dissolved oxygen concentrations amperometrically, with the pH being monitored using an ion-sensitive effect transistor and the temperature with a platinum thermistor. The viable biomass concentration was determined using conductivity impedance measurements to an accuracy of 1g per L.
While the researchers have designed the sensor array such that they can be integrated into the bottom of a 96 well plate for process optimisation, "they could also be used as stand alone sensors in industrial bioreactors," said Krommenhoek.