  Dielectric spectra of human blood reveal a rich variety of dynamic processes.
Achieving a better characterization and understanding of these processes not
only is of academic interest but also of high relevance for medical
applications as, e.g., the determination of absorption rates of electromagnetic
radiation by the human body. The dielectric properties of human blood are
studied using broadband dielectric spectroscopy, systematically investigating
the dependence on temperature and hematocrit value. By covering a frequency
range from 1 Hz to 40 GHz, information on all the typical dispersion regions of
biological matter is obtained. We find no evidence for a low-frequency
relaxation (alpha-relaxation) caused, e.g., by counterion diffusion effects as
reported for some types of biological matter. The analysis of a strong
Maxwell-Wagner relaxation arising from the polarization of the cell membranes
in the 1-100 MHz region (beta-relaxation) allows for the test of model
predictions and the determination of various intrinsic cell properties. In the
microwave region beyond 1 GHz, the reorientational motion of water molecules in
the blood plasma leads to another relaxation feature (gamma-relaxation).
Between beta- and gamma-relaxation, significant dispersion is observed, which,
however, can be explained by a superposition of these relaxation processes and
is not due to an additional delta-relaxation often found in biological matter.
Our measurements provide dielectric data on human blood of so far unsurpassed
precision for a broad parameter range. All data are provided in electronic form
to serve as basis for the calculation of the absorption rate of electromagnetic
radiation and other medical purposes. Moreover, by investigating an
exceptionally broad frequency range, valuable new information on the dynamic
processes in blood is obtained.
