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April 01, 2009 (Vol. 29, No. 7)

Strategies for Measuring Viable Biomass

Novel Technique Aims to Extend Application of RF Impedance to Single-Use Bioreactors

  • Process Validation

    Click Image To Enlarge +
    Figure 1. Capacitance Standard Reference Port checks probe conformance.

    Aber Instruments’ Biomass Monitor, which is based on RF impedance, is widely used for monitoring and controlling the viable biomass in cell culture manufacturing processes. It measures the live cell biovolume in a bioreactor, which is often expressed as live cells/mL or g/L of live biomass. These readings are derived from the real-time capacitance value in pF/cm. The concept is quite similar to an on-line dissolved oxygen probe where the value is derived from the sensor current measured in nA and normalized to a set temperature.

    In a cGMP cell culture process, it is essential to verify the performance of capacitance-based viable biomass probes if they are used on-line for critical decisions or process control. Capacitance probes, however, cannot be simply verified against known standards of viable biomass. There are also no suitable known national standard capacitance solutions or suspensions. 

    Readily available standard liquids such as water or ethanol have specific capacitance values of 7 pF/cm or less and are not representative of the often much larger capacitance values  recorded during a cell culture process. In some high density processes where the cells are grown on carriers or in a fixed bed at over 108 cells/mL, values in excess of 100 pF/cm have been recorded.

    A device for validating the Aber Biomass Monitor probe has now been developed and is set for launch later this year. The Capacitance Standard Reference Port (patent pending) is an electrical method used to generate capacitance that does not require the handling of biological material. When the biomass probe is installed within this new device (Figure 1) and connected to the head amplifier, a capacitance (in pF/cm) within the same range as encountered during the cell culture or fermentation process is generated and recorded by the Biomass Monitor. This method will allow a rigid protocol to be developed for thoroughly testing the Biomass Monitor probe prior to use in a cGMP process.