Slices of cortical tissue were used to evaluate the effects of millimeter wave exposure on individual pyramidal cells under conditions mimicking their in vivo environment.
The brains were isolated from three neonatal rats. A total of eight neurons were patched in eight slices from three rats. Several millimeter wave power flux densities were applied to the slice in randomized order to remove any possible effects of cumulative exposure.
For evaluation of the neuronal activity in the absence of intracellular calcium signaling, the intracellular calcium stores were buffered by adding 10 mM EGTA.
| Exposure | Parameters |
|---|---|
|
Exposure 1:
60.125 GHz
Modulation type:
CW
Exposure duration:
continuous for 60 s
|
|
Several millimeter wave power flux densities were applied to the slice in randomized order to remove any possible effects of cumulative exposure.
| Frequency | 60.125 GHz |
|---|---|
| Type | |
| Polarization | |
| Exposure duration | continuous for 60 s |
| Modulation type | CW |
|---|
| Exposure source | |
|---|---|
| Setup | open-ended rectangular waveguide with a 3.8 mm x 1.9 mm aperture positioned above the tissue chamber; microwave power perpendicular to the chamber's plane; tip of the waveguide placed with an air gap of 4.8 mm to the surface of the aCSF (artifical cerebrospinal fluid) solution and 7 mm on top of the brain slice; the microwave beam expands at the aCSF surface forming a half-power ellipse of 5.5 mm² (major and minor axis 14.2 mm and 4.9 mm respectively); it is refracted upon entering the solution and forms a half-power ellipse of 6.5 mm² (major and minor axis 15.2 mm and 5.4 mm respectively) at the top of the 300 µm thick brain slice |
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| power | 185 W | - | measured | - | - |
| power density | 90 mW/cm² | - | measured | - | at the surface of the aCSF solution |
| power density | 800 nW/cm² | maximum | measured | - | 30-50 nW/cm² , 70-90 nW/cm² , 140-200 nW/cm² , 250-330 nW/cm² , 490-600 nW/cm², 700-800 nW/cm² |
Even at these low power levels (three orders of magnitude below the existing limit value for human exposure of 1 mW/cm²), millimeter waves were able to produce considerable changes in neuronal firing rate and plasma membrane properties. At the power flux density approaching 1 µW/cm², 1 minute of exposure reduced the firing rate to one third of the pre-exposure level in four out of eight examined neurons. The width of the action potential amplitudes was narrowed by millimeter wave exposure to 17 % of the baseline value and the membrane input resistance decreased to 54 % of the baseline value across all neurons. These effects were short lasting (2 min or less) and were accompanied by millimeter wave-induced heating of the bath solution at 3°C.
Comparison of these data with previously published data* on the effects of bath heating of 10°C indicated that millimeter wave-induced effects cannot be fully attributed to heating and may involve specific millimeter wave absorption by the tissue.
Blocking of the intracellular Ca2+-mediated signaling did not significantly alter the millimeter wave-induced neuronal responses suggesting that millimeter waves interacted directly with the neuronal plasma membrane.
*Lee JCF et al. (2005): Effects of temperature on calcium transients and Ca2+-dependent after hyperpolarizations in neocortical pyramidal neurons. J. Neurophysiol. (93) 2012-20.
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