Broadband dielectric spectroscopy on blood


Blood is a highly functional body fluid, it delivers oxygen to the vital parts, it transports nutrients, vitamins, and metabolites and it also is a fundamental part of the immune system. Therefore the precise knowledge of its constituents, its physical, biological, and chemical properties and its dynamics is of great importance. Especially its dielectric parameters are of relevance for various medical applications like cell separation (e.g., cancer cells from normal blood cells), checking the deterioration of preserved blood, and dielectric coagulometry. In addition, the precise knowledge of the dielectric properties of blood is prerequisite for fixing limiting values for electromagnetic pollution (via the conductivity in the specific absorption rate (SAR), see also electromagnetic pollution). 

In our group, we have investigated the dielectric constant, loss and conductivity (ε', ε'' and σ', respectively) of blood as function of frequency, temperature and hematocrit value (hct, volume fraction of red blood cells) (see figures 2 and 3). Our measurements (see, for example, fig. 1) cover an exceptionally broad frequency range from 1 Hz to 40 GHz.



Fig. 1. One of the setups used for broadband dielectric spectroscopy on human blood: open ended coaxial reflection measurement.  

The obtained broadband dielectric spectra of blood show three major dispersions (fig. 2/3). Those are termed β-, γ-, and δ-dispersion and are revealed by steps in the frequency dependent dielectric constant and conductivity. The first process, located around 10 MHz, is due to a Maxwell-Wagner relaxation caused by the inhomogeneities introduced by the presence of the red blood cells. The analysis of this relaxation-like process allows for the test of model predictions and the determination of various intrinsic cell properties as, e.g., the capacitance of the cell membrane. Around 18 GHz, a further relaxation process arises from the tumbling of the dipolar water molecules in the blood sample. Between β- and γ-relaxations, significant dispersion is observed, which, however, can be explained by a superposition of these relaxation processes and is not due to an additional “δ-relaxation” often found in biological matter. The huge dispersion effect observed at the lowest frequencies (< 10kHz) is due to electrode polarization arising from the strong ionic conductivity in blood. We find no evidence for a low-frequency relaxation (“α-relaxation”) caused, e.g., by counterion diffusion effects as reported for some types of biological matter. 


Fig. 2. Dielectric constant and real part of the conductivity of blood samples with different hematocrit values as function of frequency, measured at body temperature (310 K). The lines are fits assuming different models to account for the electrode-polarization effect and the β- and γ-relaxations. (from M. Wolf et al., Biochim. Biophys. Acta. 1810, 727 (2011))  

Our measurements provide dielectric data on human blood of so far unsurpassed precision for a broad parameter range. By investigating an exceptionally broad frequency range and by analyzing the hematocrit (Fig. 2) and temperature dependence (Fig. 3), valuable new information on the dynamic processes in blood were obtained. All data are available in electronic form (see links below) to serve as basis for the calculation of the absorption rate of electromagnetic radiation and other medical purposes.  


Fig. 3. Frequency and temperature dependence of the dielectric constant of whole blood. (from M. Wolf et al., Biochim. Biophys. Acta. 1810, 727 (2011))  


For futher details, see: 

Broadband dielectric spectroscopy on human blood

M. Wolf, R. Gulich, P. Lunkenheimer und A. Loidl, Biochim. Biophys. Acta. 1810, 727 (2011) DOI-Link

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