Michael R. Murphy

Current state and implications of research on biological effects of millimeter waves: A review of the literature

In recent years, research into biological and medical effects of millimeter waves (MMW) has expanded greatly. This paper analyzes general trends in the area and briefly reviews the most significant publications, proceeding from cell-free systems, dosimetry, and spectroscopy issues through cultured cells and isolated organs to animals and humans. The studies reviewed demonstrate effects of low-intensity MMW (10 mW/cm2 and less) on cell growth and proliferation, activity of enzymes, state of cell genetic apparatus, function of excitable membranes, peripheral receptors, and other biological systems. In animals and humans, local MMW exposure stimulated tissue repair and regeneration, alleviated stress reactions, and facilitated recovery in a wide range of diseases (MMW therapy). Many reported MMW effects could not be readily explained by temperature changes during irradiation. The paper outlines some problems and uncertainties in the MMW research area, identifies tasks for future studies, and discusses possible implications for development of exposure safety criteria and guidelines.

Plasma membrane permeabilization by 60‐ and 600‐ns electric pulses is determined by the absorbed dose

We explored how the effect of plasma membrane permeabilization by nanosecond-duration electric pulses (nsEP) depends on the physical characteristics of exposure. The resting membrane resistance (R(m)) and membrane potential (MP) were measured in cultured GH3 and CHO cells by conventional whole-cell patch-clamp technique. Intact cells were exposed to a single nsEP (60 or 600 ns duration, 0-22 kV/cm), followed by patch-clamp measurements after a 2-3 min delay. Consistent with earlier findings, nsEP caused long-lasting R(m) decrease, accompanied by the loss of MP. The threshold for these effects was about 6 kV/cm for 60 ns pulses, and about 1 kV/cm for 600 ns pulses. Further analysis established that it was neither pulse duration nor the E-field amplitude per se, but the absorbed dose that determined the magnitude of the biological effect. In other words, exposure to nsEP at either pulse duration caused equal effects if the absorbed doses were equal. The threshold absorbed dose to produce plasma membrane effects in either GH3 or CHO cells at either pulse duration was found to be at or below 10 mJ/g. Despite being determined by the dose, the nsEP effect clearly is not thermal, as the maximum heating at the threshold dose is less than 0.01 degrees C. The use of the absorbed dose as a universal exposure metric may help to compare and quantify nsEP sensitivity of different cell types and of cells in different physiological conditions. The absorbed dose may also prove to be a more useful metric than the incident E-field in determining safety limits for high peak, low average power EMF emissions.

Use of cholinesterases as pretreatment drugs for the protection of rhesus monkeys against soman toxicity

Purified fetal bovine serum acetylcholinesterase (FBS AChE) and horse serum butyrylcholinesterase (BChE) were successfully used as single pretreatment drugs for the prevention of pinacolyl methylphosphonofluoridate (soman) toxicity in nonhuman primates. Eight rhesus monkeys, trained to perform Primate Equilibrium Platform (PEP) tasks, were pretreated with FBS AChE or BChE and challenged with a cumulative level of five median lethal doses (LD50) of soman. All ChE-pretreated monkeys survived the soman challenge and showed no symptoms of soman toxicity. A quantitative linear relation was observed between the soman dose and the neutralization of blood ChE. None of the four AChE-pretreated animals showed PEP task decrements, even though administration of soman irreversibly inhibited nearly all of the exogenously administered AChE. In two of four BChE-pretreated animals, a small transient PEP performance decrement occurred when the cumulative soman dose exceeded 4 LD50. Performance decrements observed under BChE protection were modest by the usual standards of organophosphorus compound toxicity. No residual or delayed performance decrements or other untoward effects were observed during 6 weeks of post-exposure testing with either ChE.

Characterization of the cytotoxic effect of high-intensity, 10-ns duration electrical pulses

Cytotoxic effects of 10-ns electrical pulses (EP, 50-380 kV/cm) were analyzed in cultured U937 cells (human lymphoma). Densities of live and dead cells were compared in over 500 samples at intervals from 0.5 to 48 h post exposure. EP trains of 1-20 pulses caused a minor, if any, decrease in cell survival: 24 h post exposure, the density of live cells typically dropped just 10-20% compared with unexposed parallel control. Within studied limits, this effect did not significantly depend on the EP number, voltage, or repetition rate. However, much longer EP trains could cause a sharp survival decline. The transition from plateau to profound cell killing occurred at about 150 pulses at 150 kV/cm, and at over 1000 pulses at 50 kV/cm. Artifact-free thermometry using a fiber optic microprobe established unequivocally that cell killing by extra-long EP trains was not a result of sample heating and has to be explained by other mechanisms. Testing for specific apoptotic cleavage of poly(ADP-ribose) polymerase at scattered time intervals (1-24 h) after EP exposure produced mostly negative results. Overall, 10-ns EP caused far weaker cytotoxic effect than it was reported earlier from experiments in other cell lines and mostly with longer pulses (60 and 300 ns). The survival curve shape (i.e., the shoulder followed by exponential decline) is also characteristic for other cytotoxic factors, such as low-LET ionizing radiation, thereby possibly pointing to common mechanisms or targets.

Strength-duration curve for an electrically excitable tissue extended down to near 1 nanosecond

As part of a health and safety assessment of ultrawideband sources, it was useful to determine stimulation thresholds for an electrically excitable tissue down into the low nanosecond range. Stimulation thresholds were measured using gastrocnemius muscles isolated from 16 frogs (Rana sp.). Single pulses were delivered with a pair of surface electrodes, and muscle twitch was measured with an isotonic transducer. Pulse durations of 100, 10, and 1 ms; 100, 10 and 1 /spl mu/s; and 100 and /spl ap/1 ns were used. Tissue voltage and current strength-duration (S-D) curves on log-log plots had a classic appearance, with thresholds for ultrashort pulses being linear. For a pulse of /spl ap/1 ns, the mean threshold voltage in the muscle was 4.5 kV and the mean threshold peak current was 35 A. When delivered by direct contact, a single ultrawideband pulse of /spl ap/1 ns could reliably produce a biological effect, stimulation of an electrically excitable tissue. The observation that the S-D curves extended downward to /spl ap/1 ns in a linear manner suggested that classical ion channel mechanisms regulated excitation and that other processes, such as electroporation, did not occur. Although a single nanosecond pulse delivered by direct contact can elicit a biological response, such a stimulus in air is unlikely to produce an effect.

Lack of Behavioral Effects in Non-Human Primates after Exposure to Ultrawideband Electromagnetic Radiation in the Microwave Frequency Range

The effect of acute exposure to ultrawideband (UWB) electromagnetic radiation on the Primate Equilibrium Platform (PEP) task, where the monkeys task is to manipulate a joystick control to compensate for the random perturbations in the pitch plane that are generated by a computer at unpredictable intervals, was examined. The duration of the UWB exposure was 2 min at a pulse repetition rate of 60 Hz (total of 7200 pulses). The bandwidth of the pulse was 100 MHz to 1.5 GHz (peak power between 250-500 MHz) with a peak E-field strength of 250 kV/m. Each monkey was exposed twice. The interval between exposures was 6 days. The exposure to UWB electromagnetic radiation had no effect on PEP performance when tested immediately after exposure.

Neural and behavioral teratological evaluation of rats exposed to ultra‐wideband electromagnetic fields

Several investigators have reported teratologic effects of electromagnetic field exposure. The majority of these studies have been performed at levels of exposure that could produce substantial heating of the animals. New and unique sources of ultra-wideband (UWB) electromagnetic fields are currently being developed and tested that are capable of generating nonthermalizing, high-peak-power, microwave (MW) pulses with nanosecond (ns) pulse widths, picosecond (ps) rise times, and an UWB of frequencies. Our study was performed to determine if teratological changes occur in rat pups as a result of (i) daily UWB exposures during gestation days 3-18, or (ii) as a result of both prenatal and postnatal (10 days) exposures. Dams were exposed either to (i) UWB irradiation from a Kentech system that emitted a 55 kV/m-peak E field, 300 ps rise time, and a 1.8 ns pulse width, average whole-body specific absorption rate 45 mW/kg; (ii) sham irradiation; or (iii) a positive control, lead (Pb) acetate solution (2000 microg/ml) continuously available in the drinking water. Offspring were examined for ontogeny (litter size, sex-ratios, weights, coat appearance, tooth-eruption, eye-opening, air-righting, and ultrasonic stress vocalizations). Male pups were tested on various performance measures (locomotor, water-maze learning, and fertilization capabilities). The pups postnatally exposed were examined for hippocampal morphology and operant behavior. Behavioral, functional, and morphological effects of UWB exposure were unremarkable with these exceptions: (i) The UWB-exposed pups emitted significantly more stress vocalizations than the sham-exposed pups; (ii) the medial-to-lateral length of the hippocampus was significantly longer in the UWB-exposed pups than in the sham-exposed animals; (iii) male offspring exposed in utero to UWB mated significantly less frequently than sham-exposed males, but when they did mate there was no difference in fertilization and offspring numbers from the sham group. There does not appear to be a unifying physiological or behavioral relationship among the significant differences observed, and our findings could be due to the expected spurious results derived when a large number of statistical comparisons are made. Significant effects found between our positive-controls and other groups on numerous measures indicates that the techniques used were sensitive enough to detect teratological effects. Bioelectromagnetics 21:524-537, 2000. Published 2000 Wiley-Liss, Inc.

Radiofrequency-Radiation Exposure Does Not Induce Detectable Leakage of Albumin Across the Blood-Brain Barrier

Abstract McQuade, J. M., Merritt, J. H., Miller, S. A., Scholin, T., Cook, M. C., Salazar, A., Rahimi, O. B., Murphy, M. R. and Mason, P. A. Radiofrequency-Radiation Exposure Does Not Induce Detectable Leakage of Albumin Across the Blood-Brain Barrier. Radiat. Res. 171, 615–621 (2009). The blood-brain barrier (BBB) consists of tight junctions between the endothelial cells that line the capillaries in the central nervous system. This structure protects the brain, and neurological damage could occur if it is compromised. Several publications by researchers at Lund University have reported alterations in the BBB after exposure to low-power 915 MHz energy. These publications increased the level of concern regarding the safety of wireless communication devices such as mobile phones. We performed a confirmation study designed to determine whether the BBB is altered in rats exposed in a transverse electromagnetic (TEM) transmission line cell to 915 MHz energy at parameters similar to those in the Lund University studies. Unanesthetized rats were exposed for 30 min to either continuous-wave or modulated (16 or 217 Hz) 915 MHz energy at power levels resulting in whole-body specific absorption rates (SARs) of 0.0018–20 W/kg. Albumin immunohistochemistry was performed on perfused brain tissue sections to determine the integrity of the BBB. Chi-square analysis revealed no significant increase in albumin extravasation in any of the exposed animals compared to the sham-exposed or home cage control animals.

Lack of effects on heart rate and blood pressure in ketamine-anesthetized rats briefly exposed to ultra-wideband electromagnetic pulses

Fourteen Sprague-Dawley rats were exposed to pulses produced by a Bournlea ultra-wideband (UWB) pulse generator (rise time, 318-337 ps; maximum E field, 19-21 kV/m). Exposures at a repetition frequency of 1 kHz for 0.5 s or to repetitive pulse trains (2-s exposure periods alternating with 2 s of no exposure, for a total of 2 min) resulted in no significant changes in heart rate or mean arterial blood pressure. These results suggest that acute whole-body exposure to UWB pulses does not have a detrimental effect on the cardiovascular system.

Comparative effects of extremely high power microwave pulses and a brief CW irradiation on pacemaker function in isolated frog heart slices.

The existence of specific bioeffects due to high peak power microwaves and their potential health hazards are among the most debated but least explored problems in microwave biology. The present study attempted to reveal such effects by comparing the bioeffects of short trains of extremely high power microwave pulses (EHPP, 1 micros width, 250-350 kW/g, 9.2 GHz) with those of relatively low power pulses (LPP, 0.5-10 s width, 3-30 W/g, 9.2 GHz). EHPP train duration and average power were made equal to those of an LPP; therefore both exposure modalities produced the same temperature rise. Bioeffects were studied in isolated, spontaneously beating slices of the frog heart. In most cases, a single EHPP train or LPP immediately decreased the inter-beat interval (IBI). The effect was proportional to microwave heating, fully reversible, and easily reproducible. The magnitude and time course of EHPP- and LPP-induced changes always were the same. No delayed or irreversible effects of irradiation were observed. The same effect could be repeated in a single preparation numerous times with no signs of adaptation, sensitization, lasting functional alteration, or damage. A qualitatively different effect, namely, a temporary arrest of preparation beats, could be observed when microwave heating exceeded physiologically tolerable limits. This effect also did not depend on whether the critical temperature rise was produced by LPP or EHPP exposure. Within the studied limits, we found no indications of EHPP-specific bioeffects. EHPP- and LPP-induced changes in the pacemaker rhythm of isolated frog heart preparation were identical and could be entirely attributed to microwave heating.