Do Microwave Level Sensors Affect Cell Cultures?

May 20, 2013
Microwaves are being used to kill cancer cells and sterilize food, so it begs the question: Do microwave radar level sensors kill or alter live cells?
For pharmaceutical and biopharmaceutical processes, it’s important that unintentional negative impacts on cell and bacteria structures be avoided. This hot topic invariably sparks a discussion and exchange between level sensor manufacturers and drug and biopharmaceutical manufacturers — especially with cell line specialists and process developers. 
With such a critical point of contention, it became virtually mandatory for Endress+Hauser to quantify the influence, if any, of microwave level measurement devices on living cell applications, where microwaves could be suspected of influencing cell growth or altering DNA.
To settle the issue, Endress+Hauser conducted a survey with the independent Rheinisch-WestfälischeTechnischeHochschule (RWTH) in Aachen, Germany, to evaluate the compatibility of two of its microwave level devices used in bioprocesses and cell culture: Levelflex and Micropilot radar level measurement instruments. Professor Dr. J. Silny, from the Research Center for Electromagnetic Environmental Compatibility, based the survey on careful study of available scientific papers, combined with analysis specific to intended applications.
Endress+Hauser’s level measuring instruments emit microwaves directed to the surface of the liquid to be measured. While the microwave penetrates the liquid, one part of the energy of the wave is reflected at the surface of the fluid and is received by the antenna of the instrument. This reflected wave is used for measuring level, as the time spent by the wave to be emitted and reflected is sensed and converted into distance. 
The other part of the energy of the microwave penetrates the fluid and is absorbed by it, including by the cells and microorganisms. Ideally, the properties of the medium and the living cells (e.g., cell growth and DNA) would not be significantly affected by the energy of the wave dispersed in the medium.
The largest part of the emitted energy is reflected at the surface of the liquid medium. The main questions, however, are posed around the part of the energy which may be absorbed by the liquid medium, for example in a cell culture. Could the microwaves from instruments produce an adverse, negative or destructive effect on cells during level measurement?
Potential Effects of Microwaves in Cell Cultures 
Microwaves are part of the electromagnetic spectrum (Figure 1) where the radiation frequency ranges between 300MHz and 300GHz (wavelength from 1m down to 1mm). The emitted radiation has to be absorbed somewhere at the molecular scale, and this absorption can induce biological effects.
Part of the energy from microwaves emitted from the sensors enters and penetrates the process medium or cell culture and is dispersed according to the dielectric constant of the different components of the medium. The propagation, plane of polarization and direction are affected by different phenomena (diffraction, scattering, interference, polarization). 
The depth penetration in a physiological solution is:
  • 66mm for a frequency of 0.1GHz
  • 29mm for a frequency of 1GHz
  • 3.4mm for a frequency of 10GHz
  • 0.3mm for a frequency of 100GHz
The penetration depth of microwaves in a cell culture is limited to less than 1mm at 100GHz, and in that layer 2/3 of the energy is absorbed. 
The second step consists of examining how the energy is absorbed. As in any system, the absorbed energy is transformed into heat. But in this case, the process can be affected if the temperature increases and if there is a potential risk of modifying the cell, the cell growth and/or the DNA. 
Literature offers different hypotheses about possible working mechanisms of microwaves in the cells, but heating is the only scientifically recognized way that microwaves can act on biological molecules. Temperature is significant for all chemical and physiological processes. The scientific literature as a whole provides no evidence for any non-thermal effect of microwaves in the frequency range between 0.1 GHz and 150 GHz in cell cultures.
Exposure conditions and the warming resulting from microwaves are classified into non-thermal, athermal and thermal.
  • Non-thermal: The temperature in any part of the cell culture is unchanged. The specific absorption rate (SAR) is smaller than 0.1W/kg.
  • Athermal: The temperature can theoretically be affected, but a large volume keeps the culture cell at a constant temperature. The SAR can be in a range of 0.1 to 50W/kg.
  • Thermal: A change of 0.2°C occurs in less than six minutes. The quality of the biological reaction can be altered. The value of the SAR is far beyond the above ranges.
Experiments have shown that a warming up to 1 °C can be reached with an applied SAR of about 1 W/kg in a small experimental volume. However, in the relatively large volume of cell cultures in typical process vessels in which radar level sensors are installed, the diffusion rate causes a fast propagation of the heating into adjacent regions of the cell culture. The result of this is a negligible increase in temperature of the cell cultures. 
Microwaves and Absorption Rates
The Levelflex sensor shown in Figure 2 uses guided radar — that is, the radar “beam” is focused on a wave guide, which keeps it from spreading out — while E+H’s Micropilot sensor uses an antenna and emits free waves. Both are process measuring instruments used in pharmaceutical and biotechnological productions for measuring levels in tanks, vessels and bioreactors. The sensors calculate the distance between the antenna and the surface of the fluid medium, which can be cell cultures, colloids, suspensions — all of which are usually highly conductive solutions. 
The measurement is the transit time between the emission of a “short package” of microwave oscillations emitted from the antenna and the reflected signal received by the same antenna. The emission is transmitted via either a coaxial or horn antenna. 
Generally the cell cultures produce a strong reflection signal because of high conductivity mediums from the high concentration of free ions. Part of the signal is reflected from the surface and travels back to the transmitter. The frequency range extends between 0.1GHz and 82GHz, allowing a very fine tuning of the performance of the measurement. The values of the SAR are respectively:
  • 0.025W/kg for K-Band/26GHz Levelflex
  • 0.013W/kg for C-Band/6GHz Micropilot
  • 3.601W/kg for K-Band/26GHz Micropilot
Evaluation and Conclusion 
The complete evaluation was conducted at the research center for electromagnetic interaction (FEMU) in Aachen, which reported the following conclusion: The microwaves from Levelflex and Micropilot are not able to generate athermal or thermal conditions in the cell culture. This statement can be substantiated with the following three reasons:
  1. Power densities applied by the systems tested are below values that would produce destructive SARs.
  2. Volumes of cell culture in which the emitted microwaves are absorbed are small in relation to the entire volume of the medium contained in the vessels.
  3. Potential low heating that may occur in the small absorption volume will be simultaneously distributed in the whole volume of the cell culture in the vessel. As a consequence, only an insignificant warming of the cell culture can be expected.
The study concludes that microwaves emitted from both of E+H’s radar level measurement instruments do not negatively influence the process and the biological reactions in the cell culture.
Endress+Hauser radar level sensors are typical of the kinds of level sensors used in the pharmaceutical industry, so the results reported here likely support a similar conclusion for other radar sensors operating within the same GHz wavelengths. However, E+H only evaluated its own sensing technologies. If one is considering using a different radar level sensor, it would be prudent to ask that supplier for specific information concerning the wavelength and SAR of the instrument. 
Published in the May 2013 edition of Pharmaceutical Manufacturing magazine
About the Author

Francois Prautois | Global Development Manager Life Sciences