The Aber Biomass Monitor, based on a patented RF Impedance technique, is now used extensively for measuring the live cell concentration during cell culture and microbial fermentations. Versions of the instrument are used in both process development and cGMP manufacturing. The most recent options are the Aber Biomass Monitor 220, introduced in 2003 and the latest dedicated production instrument, the Model 230, was launched at ESACT in June 2005.

Recent applications in cGMP include:

• Checking conformance. Many of the existing cGMP licensed processes have a qualified off-line method for estimating viable cell density. The off-line method is normally based on microscopic counting after the addition of a Trypan Blue stain and there are a variety of automated cell counting devices that can replace the laborious manual cell count. Operator errors can be made during the sampling, analysis and data processing and so a number of companies now realise that there are significant cost implications with potential errors. For example, an error in the cell count can lead to incorrect changes to the process, i.e.change of feed rates, or concentrate addition based on a cell count. These changes to feed rates or concentrate additions may cause protein titers or other process indices to be out of specification and harvested material to be discarded. The cost implications for batch failure are obvious, and there are the additional significant costs to investigate non-conformance events.

• On-line Process Control. Many cGMP cell culture processes are based on a perfusion or fed-batch bioreactor system. Control of the feed or addition rates to maintain pseudo-steady-state conditions in these bioreactors can be especially challenging. Tight control of the perfusion or concentrate addition rate allows the bioreactor to be operated under the optimum conditions for maximum recombinant protein production. In the fed-batch system, concentrate additions based on cell numbers is a straightforward control scheme. In a manner analogous to perfusion feed rate control, concentrate additions can be based on "a per cell per day" rate.

Simple control of perfusion bioreactors, based on infrequent daily sampling and estimation of the live cell concentration, will lead to large process deviations. In the initial growth phase, prior to steady-state, the cell numbers may be doubling every day. A robust automatic perfusion rate control system based on the on-line Aber Biomass Monitor probe is now being used in cell culture manufacturing processes. The system operates in a completely closed loop ie no samples need to be taken to obtain process information. On-line optical sensors have been used in some cases to maintain the process but these will only measure the combination of total cells and debris that can collect during the process. Moreover the optical sensors are prone to fouling over the extensive production runs. The Biomass Monitor only measures the viable cell mass and is therefore ideal for this application and it has been applied for process control in sono-perfused cytostats, spin-filter perfused bioreactors and for maintaining steady-state, continuous culture of bioreactors with external loop filters for monoclonal antibody and recombinant protein production.

• Monitoring micro-carrier culture or aggregated cells. Measuring the cell density of animal cells grown on micro-carriers can only be achieved by sampling and then estimating a total cell count based on nuclear counts. Apart from the errors of sampling micro-carrier culture, the off-line method only provides a total cell count. For cell culture systems based on the packed bed system, it is very difficult to have access to a sample of the carriers in a sterile way and biomass can only be estimated by indirect methods such as oxygen uptake. In both cases the Biomass Monitor provides a unique (orthogonal) on-line method for estimating the live cell mass. RF Impedance is also being assessed for cGMP off-line with highly aggregated Chicken Embryo Fibroblast (CEF) cells. The method provides high precision and accuracy during CEF cell preparation steps. The latest Live Cell Analyser Model 920 that can be used either off-line or online, is ideal for this application.