On-Line Cell Counting

Yesterday I wrote about automated cell counters for off-line determination of cell density and viability in bioreactor cultures and today I thought I would briefly expand on the topic by discussing instruments for on-line biomass determination.

Without an on-line biomass probe one would typically sample a bioreactor once or twice a day, which during a two week culture would mean having anywhere from 14 to 28 data points for cell density and viability. While this has been sufficient to allow for the development of numerous successful processes, having a continuous stream of cell density data from an on-line probe could provide a greater level of insight into how a process is running without having to remove a large number of samples from the reactor and thereby increasing the risk of contamination.

In a process development setting, an on-line probe could provide real-time data about how a given process change impacts cell density and viability. It could be feasible to incorporate the data into a feedback control loop that could make process changes (e.g. feed rate, pH setpoint, temperature, etc.) based on changes in cell density or viability.

In a manufacturing setting, having a on-line cell density probe could help mitigate contamination risk by reducing excess sampling for cell density counts. Additionally on-line biomass probes could help a biotech company in their efforts to implement their process analytical technology (PAT) initiatives.

There are two main classes of instruments that can be used for on-line biomass determination: capacitance-based probes and optical-based probes. Aber Instruments and Fogale Biotech both manufacture capacitance-based biomass probes and Optek manufactures an optical-based probe. (There may be other manufacturers of these types of instruments but these are the first ones that come to mind.) The primary difference between the two classes of probes is that the capacitance probes measure only the number of viable cells in a culture while the optical probes measure the total cell density.

When culture viability is close to 100% both of these classes of probes should be suitable for biomass determination but when the culture viability begins to decline, a capacitance probe should be better able to detect the decrease in viable cells than an optical probe.