On the audio- and radio-frequency dielectric behaviour of anchorage-independent, mouse L929-derived LS fibroblasts

Development of multi-frequency impedance scanning electron microscopy

Nanometre-scale observation of specimens in water is indispensable in many scientific fields like biology, chemistry, material science and nanotechnology. Scanning electron microscopy (SEM) allows high-resolution images of biological samples to be obtained under high vacuum conditions but requires specific sample-preparation protocols. Therefore, there is a need for convenient and minimally invasive methods of observing samples in solution. We have developed a new type of impedance microscopy, namely multi-frequency impedance SEM (IP-SEM), which allows nanoscale imaging of various specimens in water while minimising radiation damage. By varying the frequency of the input voltage signal of the sine wave, the present system can detect dielectric properties of the sample’s composition at nanometre resolution. It also enables examination of unstained biological specimens and material samples in water. Furthermore, it can be used for diverse samples in liquids across a broad range of scientific subjects such as nanoparticles, nanotubes and organic and catalytic materials.

Direct observation of unstained biological samples in water using newly developed impedance scanning electron microscopy

Nanometre-scale observation of specimens in water is indispensable in several scientific fields, such as biology, chemistry, materials science and nanotechnology. Scanning electron microscopy (SEM) obtains high-resolution images of biological samples under high vacuum conditions but requires specific sample-preparation protocols. Observations of unstained biological samples in water require more convenient and less invasive methods. Herein, we have developed a new type of impedance microscopy, namely impedance SEM (IP-SEM), which allows the imaging and sub-micrometer scale examination of various specimens in water. By varying the frequency of the input signal, the proposed system can detect the impedance properties of the sample’s composition at sub-micrometer scale resolution. Besides examining various unstained biological specimens and material samples in water. Furthermore, the proposed system can be used for diverse liquid samples across a broad range of scientific fields, such as nanoparticles, nanotubes and organic and catalytic materials.

On-line monitoring of hybridoma cell growth using a laser turbidity sensor

A high-sensitivity turbidity probe was used for on-line monitoring of the cell concentration in batch hybridoma cultivation. Good correlation between off-line cell counts and the linearized sensor signal was found. The quality of the signal was sufficiently high to provide for on-line estimation of the specific growth rate using an efficient filtering procedure. These positive results suggest that such laser turbidity sensors will facilitate development of systems for on-line monitoring and control of animal cell cultivations.

On-line biomass monitoring of CHO perfusion culture with scanning dielectric spectroscopy

In this work, dielectric spectroscopy was used to monitor two CHO perfusion culture experiments (B14 and B16). The capacitance of the cell suspension was recorded every 20 minutes over an excitation frequency range of 0.2 MHz to 10.0 MHz. A phase plot of the capacitance at a low excitation frequency vs. the value at a higher frequency proved to be an accurate indicator of the major transition points of the culture, i.e., maximum cell viability, end of lactate consumption, point of zero viability. For both experiments, the capacitance signal correlated very well (R(2) >0.98) with viable cell number up to concentrations of 1 x 10(7) cells/mL. Visual observation of the capacitance spectra indicated that changes in the capacitance relative to frequency were related to the cellular morphology. A multivariate model was developed using off-line data that could predict the median cell diameter within a single experiment (B14) with an error of 0.34 microm (2%). Upon extension to a subsequent experiment (B16), the predicted error was 1.18 microm (9%).

On-line determination of animal cell concentration in two industrial high-density culture processes by dielectric spectroscopy

Dielectric spectroscopy was applied to two industrial high cell density culture processes and used to determine on-line the concentration of CHO cells immobilized on macroporous microcarriers in a stirred bioreactor and in a packed-bed of disk carriers. The cell concentration predicted from the spectroscopic data was in excellent agreement with off-line cell counting data for both processes. Deviations between the two counting methods only occurred in the case of a significant decrease of the cell viability, from 93% to 64%, which induced a change of the average cell size in the culture. Results for the packed-bed process were further confirmed by the application of indirect yield models based on the measurement of glucose, lactate, and the protein of interest. Moreover, dielectric spectroscopy was used as a tool to characterize the packed-bed process. It was possible to determine both the maximum cell concentration that could be reached in the culture system, 2.0 x 10(11) cell per kg of disk carrier, and to quantify the increase of specific protein productivity induced by the production phase, from 5.14 x 10(-8) microg x cell(-1) x h(-1) to 4.24 x 10(-7) microg x cell(-1) x h(-1).

Correction of the influence of baseline artefacts and electrode polarisation on dielectric spectra

The deconvolution of biological dielectric spectra can be difficult enough with artefact-free spectra but is more problematic when machine baseline artefacts and electrode polarisation are present as well. In addition, these two sources of anomalies can be responsible for significant interference with dielectric biomass measurements made using one- or two-spot frequencies. The aim of this paper is to develop mathematical models of baseline artefacts and electrode polarisation which can be used to remove these anomalies from dielectric spectra in a way that can be easily implemented on-line and in real-time on the Biomass Monitor (BM). We show that both artefacts can be successfully removed in solutions of organic and inorganic ions; in animal cell and microbial culture media; and in yeast suspensions of varying biomass. The high quality of the compensations achieved were independent of whether gold and platinum electrodes were used; the electrode geometry; electrode fouling; current density; the type of BM; and of whether electrolytic cleaning pulses had been applied. In addition, the calibration experiments required could be done off-line using a simple aqueous KCl dilution series with the calibration constants being automatically calculated by a computer without the need for user intervention. The calibration values remained valid for a minimum of 3 months for the baseline model and indefinitely for the electrode polarisation one. Importantly, application of baseline correction prior to polarisation correction allowed the latter's application to the whole conductance range of the BM. These techniques are therefore exceptionally convenient to use under practical conditions.

From concept to market in industrial impedance applications

This paper discusses some of the technical aspects of converting a laboratory idea into a commercial product. The example used is the development of the Aber Instruments Biomass Monitor, which is now used worldwide in industry for the pitching of yeast in breweries and for biomass measurements in the pharmaceutical industry. Although the issues raised will relate to instrumentation in a production environment, many of the themes will be equally applicable to medical instruments.

Specific heat flow rate: an on-line monitor and potential control variable of specific metabolic rate in animal cell culture that combines microcalorimetry with dielectric spectroscopy

One of the requirements for enhanced productivity by the animal culture systems used in biotechnology is the direct assessment of the metabolic rate by on-line biosensors. Based on the fact that cell growth is associated with an enthalpy change, it is shown that the specific heat flow rate is stoichiometrically related to the net specific rates of substrates, products, and indeed to specific growth rate, and therefore a direct reflection of metabolic rate. Heat flow rate measured by conduction calorimetry has a technical advantage over estimates for many material flows which require assays at a minimum of two discrete times to give the rate. In order to make heat flow rate specific to the amount of the living cellular system, it would be advantageous to divide it by viable biomass. This requirement has been fulfilled by combining a continuous flow microcalorimeter ex situ with a dielectric spectroscope in situ, the latter measuring the viable cell mass volume fraction. The quality of the resulting biosensor for specific heat flow rate was illustrated using batch cultures of Chinese hamster ovary cells (CHO 320) producing recombinant human interferon-gamma (IFN-gamma) during growth in a stirred tank bioreactor under fully aerobic conditions. The measuring scatter of the probe was decreased significantly by applying the moving average technique to the two participant signals. It was demonstrated that the total metabolic rate of the cells, as indicated by the specific heat flow rate sensor, decreased with increasing time in batch culture, coincident with the decline in the two major substrates, glucose and glutamine, and the accumulation of the by-products, ammonia and lactate. Furthermore, the specific heat flow rate was an earlier indicator of substrate depletion than the flow rate alone. The calorimetric-respirometric ratio showed the intensive participation of anaerobic processes during growth and the related IFN-gamma production. Specific heat flow rate was monotonically related to specific cell growth rate and associated with specific IFN-gamma production. Specific heat flow rate is potentially a valid control variable for the growth of genetically engineered cell lines producing target proteins.

Dielectric properties of yeast cells as simulated by the two-shell model

The paper reports a re-evaluation of the previous studies on yeast by considering the influence of vacuole upon the dielectric properties of the cell. In this respect, relative permittivity and conductivity of yeast cells dispersed in KCI solutions of various concentrations were measured in the frequency range from 0.1 to 100 MHz. The analysis of data revealed that the beta-dielectric dispersion of yeast cell suspensions is a composite of three (or probably four) distinct sub-dispersions. Since the dielectric response of the cell wall was experimentally avoided (according to Asami et al. (1976) J. Membr. Biol. 28, 169-180), the two-shell model, related to the plasma membrane and the vacuolar membrane, respectively, appeared to be the best approximation for yeast cells. The most relevant parameters obtained with the aid of the two-shell model were as follows. Specific capacitance of the plasma membrane and the vacuolar membrane were 0.703 +/- 0.011 microF/cm2 and 0.483 +/- 0.029 microF/cm2, respectively; electrical conductivity of the cytoplasm and the vacuole interior were 0.515 +/- 0.028 S/m and 3.22 +/- 0.48 S/m; finally, the permittivity of the cytoplasm was 50.6 +/- 2.

Electropermeabilization and fluorescent tracer exchange: the role of whole-cell capacitance

Transmembrane crossing of charged fluorescent tracers such as propidium iodide (PI) and carboxyfluorescein+ (CF) can be used to quantitate membrane permeabilization. Murine myeloma Sp2/0-Ag14 cells were loaded with CF (0.1 fmol/cell) before electropulsation (0.5-3.0 kV/cm, 40 microseconds) in medium containing 25-50 micrograms/ml PI at 21-23 degrees C. Cytograms of PI vs. CF fluorescence showed three readily distinguishable subpopulations: 1) intact living cells with CF but without PI (these form > 95% of the prepulsed population), 2) transiently electropermeabilized but resealed cells showing both CF and low-level PI fluorescence, and 3) permanently permeabilized cells without CF but with very high PI fluorescence. Despite the ready influx of PI, the efflux of CF from transiently permeabilized cells was negligible and was insensitive to pulse parameters; however, electrically killed cells (subpopulation 3) lost all CF fluorescence and probably lost their cytoplasm. This difference in transmembrane passage of the dyes is best explained by binding of intracellular CF to macromolecules (and/or organelles). In isotonic "pulse medium," the membranes resealed after electropulsing with a time constant (tau R) of about 2 min. In 150 mOsm medium, resealing was faster (typically tau R approximately 0.5 min). The population distribution of PI uptake [coefficient of variation (CV) > 40%] was very broad and could not be accounted for by the radius dependence of pulse-induced voltage (CVradius approximately 10%). The variability in PI uptake could be explained if the electrical energy of the charged membrane, which depends on the whole-cell capacitance (Cc), was taken into account. Evaluation of the Cc values with single-cell resolution was based on measurement of the electrical charging time constant of the plasma membrane by electrorotation.

Real-time biomass-concentration monitoring in animal-cell cultures

The accurate, on-line measurement of cell concentration in animal-cell cultures is an on-going problem in bioprocess engineering, and the development of new monitoring techniques is an area of intensive and fruitful research. This article summarizes the recent advances, trends and problems in this field and focuses, in particular, on optical sensors, including the latest laser and infrared probes. Alternative methods, such as multiple-extinction fluorimetry, real-time imaging and particle-size analysis, are also discussed. Although many of these techniques are still at an experimental stage, we believe that some of them have been developed sufficiently that we advocate their routine use in bioprocess monitoring and control.

Frequency domain electrical conductivity measurements of the passive electrical properties of human lymphocytes

An extensive set of electrical conductivity measurements of human lymphocyte suspensions has been carried out in the frequency range from 1 kHz to 100 MHz, where the surface polarization due to the Maxwell-Wagner effect occurs. The data have been analyzed according to well-established heterogeneous system theories and the passive electrical parameters of both the cytoplasmic and nuclear membranes have been obtained. Moreover, a further analysis to take into account the roughness of the membrane surface on the basis of a fractal model yields new estimates for the membrane conductivity and the membrane permittivity, independently of the surface architecture of the cell. These findings are confirmed by measurements carried out at higher frequencies, in the range from 1 MHz to 1 GHz, on lymphocytes dispersed in both hyperosmotic and hypoosmotic media, that influence the surface complexity of the membrane due to the microvillous protrusions. The surface roughness of the cell is described by a macroscopic parameter (the fractal dimension) whose variations are associated to the progressive swelling of the cell, as the osmolality of the solution is changed.

Cytotoxicity Studies of Anchorage-independent LS-L-929 Mouse Fibroblasts Using Membrane Integrity, ATP Content and ATP/ADP Ratio as Determinants: Including the Xenobiotic Effects of Components in an Experimental Shampoo Formulation

Cell death (CD50), as measured by loss of membrane integrity using fluorescein diacetate and ethidium bromide, and reduction in ATP content (ATP50), were used as endpoints in a comparative study of the cytotoxicity of 40 chemicals (supplied by FRAME) to anchorage-independent LS-L-929 mouse fibroblasts. The rank order correlation of the two tests was strong (r = 0.98), but the ATP50 values only showed a weak correlation with rat acute lethality (LD50) data (r = 0.43). More-mechanistic endpoints were sought in order to characterise cellular energy status, and the effects of acetaminophen, acetylsalicylic acid, diazepam, digoxin and ethanol on ATP/ADP ratio, phosphorylation potential and adenylate energy charge were examined in detail. The first two ratios were very sensitive to toxic insult to the cells and gave results which more closely mimicked differences in human acute oral lethality than did the ATP50 and CD50. In addition, there was evidence that digoxin and diazepam affected oxidative phosphorylation, which suggested an alteration to mitochondrial function. Energy charge appeared less sensitive to the xenobiotics, probably owing to the dampening effect of the denominator in the ratio.

The effects of individual components of an experimental detergent-based formulation on cell death were examined. It was found that one detergent gave different results, depending on the presence or absence of a second detergent. A commercial cream containing varying amounts of glyceryl monostearate contaminated with varying levels of sodium dodecyl sulphate gave results which reinforced the point that cytotoxic assays are valuable in quality assurance.

Dielectric spectroscopy of mammalian cells. 1. Evaluation of the biomass of HeLa- and CHO cells in suspension by low-frequency dielectric spectroscopy

The capacitance of suspensions of CHO and HeLa cells (0.5-3 x 10(6) cells/ml) has been measured between 0.2 and 10 MHz. As frequencies decrease, there is a continuous increase in capacitance of both the cell suspension and the spent growth medium free of cells, a phenomenon which is partially attributed to an increased polarisation of the electrodes. At a given frequency, subtraction of the capacitance of the spent medium from that of the cell suspension allows one to determine the capacitance of the cells only. The intensity of this signal varies linearly with the biomass and cell size. At low frequencies such as those used in this study (0.25 MHz), where sensitivity is the highest, concentrations as low as 0.5 x 10(6) cells/ml can be accurately measured. Suggestions are made how to make these measures on-line, non-invasive and in real time.

Dielectric spectroscopy as a novel and convenient tool for the study of the shear sensitivity of plant cells in suspension culture

Plant cell suspensions of different species and different age were subjected to hydrodynamic stress while following the decline in the volume fraction of intact cells by measuring the permittivity of the cell suspension at radio frequencies. Results were compared with the fresh weight, dry weight, packed cell volume and cell number of the suspensions. At first a rapid decline is seen as the most shear-sensitive cells are broken up, followed by a slower decline as less sensitive cells are broken up. The sensitivity of the cells to shear stress depended strongly on the cell line used but only slightly on their age, older cells being more sensitive. The dependence of the shear sensitivity on the cell line might be an effect of the species investigated, the culturing conditions of the cell line, or both. It was found that cells that grow in a finely dispersed suspension are much less prone to shear stress than is often assumed.