Theory of Dielectric Measurements
The limited ionic permeability of the plasma membrane gives living cells specific dielectric properties. At high frequencies, above 10 MHz, the ions’ movement amplitude is limited, and not significantly affected by the presence of the cytoplasmic membrane. A cell suspension has nearly the same overall electric properties as the suspending medium.
Permittivity at 10 MHz:
10 MHz = ε suspending medium
At lower frequencies, the displacement amplitude is higher, ions accumulate at the membrane, and the cells behave as tiny capacitors. The global charge accumulation is dependent on the viable cell density.
At low frequencies, when the cells are fully polarized, there is no further permittivity increase related to charge accumulation. The increase in permittivity from high to low frequencies has a sigmoidal shape and is known as the β-dispersion (Figure 1).
Viable cell density monitoring: measuring biovolume
The overall permittivity amplitude of the β-dispersion is proportional to biovolume, cell size, and membrane capacitance. In order to achieve high accuracy cell-density measurement, FOGALE Nanotech(www.fogale.com) is using a particular property at the critical frequency of the b-dispersionthe permittivity at fc is independent of cell size.
Permittivity at fc:
εfc = εbiomass + εsuspending medium
The permittivity difference measured between the critical frequency (fc) and 10 MHz results in high accuracy and very good linearity with off-line biovolume, even in permittivity changing media.
Δε(fc-10MHz) = εbiomass = k*biovolume
Biovolume is defined as the volume enclosed by intact cytoplasmic membranes (viable cell density); Biovolume=VCC*Vc; VCC: Viable cell count, and Vc: Average cell volume.
Using its dielectric-measurement technology FOGALE nanotech recently launched a new generation of on-line biomass-monitoring tools (Figure 2). The patented Biomass System is an affordable sensor for viable cell-density monitoring, and it is directly inserted into the bioreactor with in situ sterilizable standard 12-mm and 25-mm diameter probes to fit any bioreactor port. The probes’ design and components are ideal for working in a cGMP environment as the wetted material conform to FDA requirements (PEEK, 316-L stainless steel).
The Biomass System is useful for monitoring animal cells in suspension (including clumping cell lines) or attached to micro-carriers. It is also routinely used for yeast or bacteria batch and fed-batch processes with high biomass concentration up to 200g/L dry weight.