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ISSN 0030-400X, Optics and Spectroscopy, 2020, Vol. 128, No. 6, pp. 855–866. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Optika i Spektroskopiya, 2020, Vol. 128, No. 6, pp. 852–864.
Effects of Terahertz Radiation on Living Cells: a Review
O. P. Cherkasovaa, b, c, *, D. S. Serdyukova, d, A. S. Ratushnyake, E. F. Nemovaa, E. N. Kozlovf,
Yu. V. Shidlovskii f, g, K. I. Zaytsevb, h, and V. V. Tuchi ni, j
a Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia
b Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 117942 Russia
c Institute of Laser and Information Technologies, Russian Academy of Sciences,
Federal Research Center “Crystallography and Photonics,” Shatura, 140700 Russia
d Institute of Cytology and Genetics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia
e Institute of Computational Technologies, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia
f Institute of Gene Biology, Russian Academy of Sciences, Moscow, 119334 Russia
g Sechenov First Moscow State Medical University (Sechenov University), Moscow, 119146 Russia
h Bauman Moscow State Technical University, Moscow, 195251 Russia
i Saratov State University, Saratov, 410012 Russia
j Institute of Precision Mechanics and Control, Russian Academy of Sciences, Saratov, 410028 Russia
*е-mail: o.p.cherkasova@gmail.com
Received December 23, 2019; revised February 5, 2020; accepted April 28, 2020
Abstract—This review discusses the current state of research concerning the effects of terahertz (THz) radia-
tion on living cells in the context of biosafety of THz radiation for the human organism.
Keywords: terahertz radiation, biological effects, cells, nonthermal exposure, safe radiation dose
DOI: 10.1134/S0030400X20060041
ELECTROMAGNETIC RADIATION
OF THE TERAHERTZ RANGE
The terahertz (THz) range of the electromagnetic
spectrum lies between the infrared (IR) and micro-
wave ranges and covers the frequency range from 0.1 to
10.0 THz, which corresponds to the wavelength band
from 30 μm to 3 mm [1–4]. In the recent decades, the
use of the THz range has been extensively increasing
thanks to advances in the methods of generating and
detecting THz radiation [5–7]. Currently, THz tech-
nologies are a rapidly developing field, as illustrated by
the growing number of patent applications analyzed in
[8]. The number of patents granted between 2006 and
2015 increased threefold. A certain decline in the pat-
ent activity that occurred in 2016 was not discussed by
the authors. In our opinion, this may be related to a
temporary saturation with technological solutions in
the THz range of frequencies (Fig. 1).
The interest to THz radiation and its growing
implementation in different areas of human activity
are due to several of its features important for practical
applications [9–13]:
—THz radiation is nonionizing and does not dam-
age biological molecules, because the photon energy
(0.04–0.00 4 eV) is low in comparison to the energy of
ionization that could cause atomic or molecular disso-
ciation;
—it penetrates well through many dielectric mate-
rials, such as wood, paper, textiles, plastics, and
ceramics;
—it is efficiently absorbed by polar compounds,
including water;
—it is absorbed differently by various biological tis-
sues;
—its energy corresponds to the energy of hydrogen
bonds and Van-der-Waals forces of intermolecular
interactions;
—in the form of molecular crystals, different mol-
ecules exhibit THz absorption spectra with character-
istic sets of resonance frequencies;
—in contrast to visible and IR light, THz radiation
exhibits Rayleigh scattering (1/λ4) in opaque and
finely dispersed media, in nano- and microporous
systems, as well as in biological tissues, so that the
effects of Mie scattering can be disregarded; interac-
tions of THz radiation with these media can be
described in terms of effective medium theory and dif-
ferent models of dielectric response.
These properties of THz radiation make it a prom-
ising tool that might be applied for the purposes of
security surveillance [14, 15], new-generation com-
BIOPHOTONICS
856
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
CHERKASOVA et al.
munications [16], as well as medical diagnostics and
therapy [17, 18].
PROBLEMS OF USING THz RADIATION
IN MEDICAL DIAGNOSIS: THz DOSIMETRY
Biomedical applications of THz radiation largely
concern the field of noninvasive, minimally invasive,
and intraoperational diagnostics. Thanks to the sim-
plicity of measurements, human skin tissues, as well as
different types of malignant skin tumors, were one of
the first biological tissues to be investigated in the THz
spectral range in vivo and in vitro [19–26]. The use of
THz imaging was described for assessment of tissue
damage after burns [27], for control of wound healing
without removing the bandages and plaster [28], for
detection of caries [29], and for noninvasive diagnosis
of diabetes [30, 31]. It was found that healthy tissues
and malignant neoplasms of different nosological
forms and location exhibit different optical features in
the THz range [2, 13, 32, 33]. Diagnostic use of THz
radiation was also described in ophthalmology [34].
Active development and implementation of
THz-range technologies in everyday practice leads to
a growing exposure of human population to this type
of radiation, which raises concerns about potential
associated health risks, considering our insufficient
understanding of the induced biological effects. In this
context, it is important to find out how the response of
living objects depends on physical parameters of THz
radiation and to determine the safety limits of its use
[35]. The currently existing sanitary standards concern
only the spectrum range from 300 kHz to 300 GHz
[36, 37]. In Russia, the safety limit for power flux den-
sity (PFD), or intensity, of electromagnetic radiation
with frequencies below 300 GHz is set at 200 μW/cm2
per hour of exposure for the working personnel [36]
and 10 μW/cm2 for the general population (round-
the-clock exposure) [38]. According to the guidelines
of the International Commission on Non-Ionizing
Radiation Protection (ICNIRP), the general safety
limit for the intensity of radiation in the frequency
range of 2 to 300 GHz is 1 mW/cm2 for 6 min of expo-
sure [39]. This safety limit is based on verified thermal
effects, which were shown to be caused by exposure in
this frequency range. For frequencies higher than
300 GHz, there exist no conventional limits for expo-
sure of the general population. Limitations are used
only for laser radiation, where the safety limits lie
between 1 to 100 mW/cm2 depending on the laser radi-
ation type [40]. Although there exist extrapolations of
the data for the neighboring ranges of the electromag-
netic spectrum, they are inappropriate to set evidence-
based norms [35, 39].
It is important to notice that permissible exposure
limits cannot be determined without understanding
the mechanisms by which THz radiation interacts with
biological objects. To date, there exist two most popu-
lar hypotheses:
—the first one assumes that the effects of THz radi-
ation are due to heating of exposed objects as a result
of its strong absorption by water, which is mainly
observed for continuous-wave radiation sources [10,
41–43];
—the second hypothesis considers nonthermal
mechanisms of interactions between THz waves and
biological systems. For instance, in spite of its low
energy and therefore a low probability of breaking
chemical bonds, THz radiation can induce linear or
nonlinear resonance effects in DNA. Under certain
conditions, this can significantly alter the molecular
dynamics and locally disrupt hydrogen bonds between
DNA strands, modifying gene expression [44, 45].
This notion is particularly relevant for exposure to
high-energy pulsed THz radiation [46]. Whereas the
average power of picosecond THz pulses is usually
rather low (of the μW or mW order), its peak values
may be as high as 1 MW and more [47], which is
enough for THz radiation to penetrate the cytoplasmic
and nuclear membranes [48, 49].
The nonthermal character of radiation effects on
biological systems is explained using the theory pro-
posed by H. Fröhlich: in the 1970–1980s, he hypoth-
esized that THz radiation contributes significantly to
formation of special coherent states, so-called Fröh-
lich condensates, in biological matter [50, 51]. Their
presence does not necessarily imply that they can
interact with the radiation as free oscillators in case of
frequency resonance. According to Fröhlich, radiation
acts as a trigger that switches the kinetics of biochem-
ical processes in living organisms [51].To explain the
effect of radiation using this approach, it is insufficient
to identify oscillations with frequencies close to the
frequencies of radiation that is being studied. It is
essential to investigate the kinetics of the entire net-
work of processes involved in the interaction between
Fig. 1. Dynamics of annual numbers of patent applications
from 2006 to 2016 [8].
2006
2007
2008
2010
2012
2014
Number of patents
200
400
600
800
1000
2016
2009
2011
2013
2015
300
500
700
900
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
EFFECTS OF TERAHERTZ RADIATION ON LIVING CELLS: A REVIEW 857
electromagnetic radiation and biological objects [52,
53].
To date, several review articles have described
effects of THz radiation on all levels of biological orga-
nization, from affecting the conformation of biopoly-
mers (proteins and DNA) [2, 4, 10, 54–57] to induc-
ing organism-level responses [10, 55, 56]. The latter
can be illustrated by the following examples. Exposure
of mice to THz laser radiation with a frequency of
3.68 THz and intensity of 40 mW/cm2 for 30 min had
a negative effect on the behavior of the animals, caus-
ing the avoidance response, displaced motor activity,
and anxiety, which persisted at least 24 h after irradia-
tion [58]. In drosophila flies, exposure to THz radia-
tion (frequency range, 0.1–2.2 THz; pulse duration,
1 ps; peak power, 8.5 mW; pulse frequency, 76 MHz)
for 30 min had an effect on the following parameters:
—the lifespan of adult individuals and the first-
generation offspring;
—the period of reaching maturity in the first-gen-
eration offspring;
—the proportion between the numbers of male and
female individuals.
These observations indicate changes in system-
level traits. The underlying processes may involve epi-
genetic regulation and different intercellular signaling
pathways [59–61].
In the problem of investigating THz radiation and
implementing it for the purposes of diagnostics and
therapy, several aspects can be identified:
—safety of using THz radiation in diagnostic pro-
cedures, which requires development of the standards
to determine the safety limits for using THz radiation
sources in medical diagnostic systems for an appropri-
ate period of time (e.g., 10–20 min); this research can
be conveniently performed in cells and cell cultures;
—possible involvement of THz radiation in the
programming of cell growth and development, its
effects on functional state and proliferation of cells,
and cell–cell interactions, which may be significant
for creation of engineered tissue constructs; these
experiments should rule out any damaging effects on
the genetic apparatus of the cells;
—the problem of influence on the functioning of
complex multicellular organisms, investigation of
long-term consequences of this exposure.
In the present review, we focus mainly on the bio-
logical effects of THz radiation on the cell level. This
is an issue of central importance aimed at targeted
identification of specific cell responses, which might
be masked within a systemic response of a multicellu-
lar organism.
EFFECTS OF THz RADIATION
ON BLOOD CELLS
To date, a substantial body of data has been accu-
mulated concerning the optical properties of blood
and its components in the THz range [13, 62, 63].
It should be noted that no changes in the spectral and
morphological characteristics of blood cells were
observed in the course of spectroscopy on standard
pulsed THz units (frequency range, 0.1–3.2 THz;
average power, 100 nW; exposure, 1–5 min) [5, 62,
63].
At the same time, there have been publications
describing effect of THz radiation on blood cells. It
was found that exposure of erythrocyte suspension to
monochromatic radiation of a backward-wave tube
with adjustable output frequency (frequency, 0.18 to
0.33 THz; intensity, 3 mW/cm2; duration, 180 min)
decreased their osmotic resistance, which was assessed
by hemoglobin release from erythrocytes [64]. In
erythrocyte suspension exposed to continuous-wave
THz radiation (frequency, 3.68 THz; intensity,
40 mW/cm2; duration, 60 min), hemoglobin release
after addition of water (1 : 2) increased 24 times in
comparison to unexposed erythrocytes [65]. Viability
of erythrocytes exposed to broad-band THz radiation
(frequency range, 0.1–1.75 THz; duration, 60 min)
decreased more than in control cells placed in solu-
tions containing 0.54 to 0.48% NaCl [66]. Cell viabil-
ity was assessed by staining with tryptan blue, a dye
that penetrates damaged cell membranes. A study
using the Novosibirsk free-electron laser (FEL) as a
THz source (peak power, up to 1 MW; pulse fre-
quency, 5.6 MHz) showed that exposure of erythro-
cytes to radiation with wavelengths of 130–146 μm and
a mean intensity of 10 W/cm2 for 5 s did not induce
detectable changes in cell morphology or number of
aggregated erythrocytes. When the period of exposure
was increased to 10–15 s, the number of erythrocytes
within aggregates decreased. The authors supposed
that the observed effect was due to strong ultrasound
waves passing the exposed medium at the frequency of
laser pulses, 5.6 MHz [67]. In control experiments,
erythrocytes were heated to body temperature and
exposed to ultrasound, which did not cause any simi-
lar effects. Exposure times of longer than 25 s caused
cell lysis. All these facts indicate that exposure to THz
radiation can affect permeability of erythrocyte mem-
branes.
Study [68] was performed to determine biologically
safe limits for the energy THz radiation in a number of
pulsed THz systems, which involved evaluation of
damaging effects on DNA of blood leukocytes using
the DNA comet assay [69]. Temperature variations
were calculated for the selected irradiation modes. It
was shown that exposure of blood leukocytes to pulse
THz radiation (frequency range, 0.1–6.5 THz; dura-
tion, 20 min) did not cause DNA damage for intensi-
ties of up to 200 μW/cm2, while the temperature of the
858
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
CHERKASOVA et al.
exposed specimen increased by less than 1°C. For
comparison, other studies [70, 71] showed that expo-
sure of blood specimens obtained from healthy donors
to pulsed THz radiation (frequency range, 0.12–
0.13 THz; mean intensity, 30–250 μW/cm2; duration,
20 min) did not induce genetic changes in blood leu-
kocytes and did not affect cell cycle kinetics, whereas
THz radiation with higher intensity of up to
2mW/cm
2 could induce DNA damage [72]. Exposure
to continuous-wave THz radiation (with a frequency
of 3.68 THz and intensity of 40 mW/cm2, which sig-
nificantly exceeded the permissible limits) for 30 min
decreased the number of viable cells, and after 90 min
of exposure their number dropped nearly twofold [65].
At the same time, exposure of human blood lympho-
cytes to continuous THz radiation (frequency,
0.1 THz; intensity, 31 μW/cm2; duration, 120 and
1440 min (24 h)) caused an increase in aneuploidy of
chromosomes 11 and 17 during cell division, which
implies genomic instability and can lead to malignant
transformation [73].
Exposure of a human T-cell culture to THz radia-
tion (frequency, 2.52 THz; intensity, 636 mW/cm2;
duration, 30–50 min) was accompanied with an
increase in temperature of specimens by 3°C [74].
Therefore, the effects of THz radiation were compared
to those of heating to the corresponding temperature.
It was found that THz radiation activated 75% of
genes, including those that encode proteins of the cell
membrane and components of intracellular signal
transmission pathways, whereas heating alone acti-
vated only 55% of genes. The same authors observed
that the expression of genes encoding heat shock pro-
teins, transcriptional regulators, cell growth factors,
and proinflammatory cytokines was elevated 240 min
after irradiation [75]. It was concluded that THz radi-
ation can affect gene expression, and this effect was
independent from the increase in temperature induced
by irradiation [76].
Thus, experiments on exposure of blood cells to
THz radiation showed that it increased permeability of
the cell membrane, affected cell morphology, prolifer-
ation, and aggregation, and had genotoxic and cyto-
toxic effects (Table 1). As it follows from Table 1, these
effects of THz radiation were observed for PFD levels
that exceed the accepted permissible limits or for sig-
nificant exposure times.
EFFECTS OF THz RADIATION
ON SKIN CELLS
The report of the committee on potential health
effects of exposure electromagnetic fields pointed out
that, considering the expected increase in the use of
THz technologies in the 21st century, effects of THz
radiation on the skin (low-intensity long-term expo-
sure) and the cornea (high-intensity short-term expo-
sure) should be a priority research direction [35]. In
compliance with these recommendations, a large por-
tion of the corresponding research is performed in
fibroblast cells. For instance, in study [77], cultures of
human skin fibroblasts were exposed to continuous-
wave THz radiation (frequency, 2.52 THz; intensity,
84.8 mW/cm2; duration, 5–80 min), while cells
heated to 40°C and cells exposed to UV radiation
(wavelength, 254 nm; power, 38 W; duration, 3 min)
served as controls. It was found that irradiation for 5,
10, or 20 min did not diminish the numbers of viable
cells. For exposure periods of 40 to 80 min, their num-
bers slightly decreased (by less than 10%); at the same
time, cell proliferation was enhanced, and DNA tran-
scription levels remained unchanged. Exposure of
240 min upregulated the expression of heat shock pro-
tein genes, with a dynamics that was different from the
pattern observed for the thermal control [78]. At the
same time, THz radiation did not affect the expression
of marker genes of DNA damage caused by exposure
to UV radiation [77, 78]. It was shown that increasing
the intensity of THz radiation to 227 mW/cm2 induced
activation of stress response genes. The study also
included theoretical estimation and experimental
measurements of temperature in the course of irradia-
tion. The authors related the observed effects to the
increase in cell temperature by 3°C [79]. Exposure of
human fibroblasts to continuous-wave THz radiation
(frequency, 0.14 THz; intensity, 20–200 mW/cm2;
duration, 20 min) did not cause significant differences
in cell proliferation, migratory activity, and NO pro-
duction [80]. This work did not investigate the markers
of DNA damage.
A whole range of genetic and cytological tech-
niques was employed to investigate the effects of
pulsed THz radiation (frequency range, 0.1–
0.15 THz; mean intensity, 0.4 mW/cm2; duration,
20 min) on a culture of HFFF2 human fibroblasts;
experiments were performed at 16°C. According to
theoretical estimates, the temperature increase during
irradiation was 0.3°C. The authors did not observe
induced DNA damage as assessed using the DNA
comet assay, histone H2AX phosphorylation, or telo-
mere length modulation, nor did they detect apoptosis
or changes in specific signaling proteins. These results
suggest that effects induced by THz radiation may
rather be aneugenic than clastogenic, and could prob-
ably result in chromosome loss. Furthermore,
enhanced actin polymerization, which was observed
by ultrastructural analysis after THz irradiation, sup-
ports the hypothesis that abnormal assembly of the
mitotic spindle may lead to the observed impairment
of chromosome disjunction during cell division. Tak-
ing into account that chromosomal rearrangements
and aneuploidy are well-established signs of malignant
transformation, the authors underline that, under
conditions of growing use of THz radiation, under-
standing of its effects on the genome will be of central
significance [81].
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
EFFECTS OF TERAHERTZ RADIATION ON LIVING CELLS: A REVIEW 859
Yaekashiva et al. investigated effects of THz radia-
tion on a culture of human skin NB1RBG fibroblasts
using a continuous-wave source tunable in the fre-
quency range of 70–300 GHz [82]. Irradiation was
performed in a special temperature-controlled cham-
ber (37 ± 0.2°C); the radiation intensity was
1.27 μW/cm2 at 0.1 THz and 0.38 μW/cm2 at 0.3 THz;
the time of exposure ranged from 180 to 5640 min
(94 h). No changes in cell proliferation and activity,
nor any signs of cytotoxicity were detected in these
experiments.
In another study, human keratinocyte cultures
were exposed to continuous-wave THz radiation at the
frequencies of 1.4, 2.52, and 3.11 THz for 20 min; the
radiation intensity was constant and constituted
44.2 mW/cm2 [83]. mRNA analysis performed after
240 min showed that the patterns of gene expression
were specific for each frequency. The authors con-
cluded that exposure of cell cultures to different THz
frequencies can induce unique biochemical and cellu-
lar responses. This suggests that the use of THz radia-
tion as a tool stimulating specific cell properties
requires careful selection of frequency [83].
In studies [84–86], human epidermal keratino-
cytes and dermal fibroblasts within artificial multilayer
human skin tissue were exposed to pulsed THz radia-
tion (frequency range, 0.1–2.5 THz; intensity, 5.7 and
57 mW/cm2; duration, 10 min). For comparison, cells
were also exposed to pulsed UV light (wavelength,
400 nm; duration, 2 min). It was found that THz radi-
ation selectively suppressed the expression of genes
associated with psoriasis, atopic dermatitis, and other
inflammatory skin diseases, as well the genes of pro-
teins involved in apoptosis [84]. For the genes associ-
ated with carcinogenesis, it was shown that THz radi-
ation downregulated the expression of genes promot-
ing proliferation of tumor cells, tumor growth, and
metastasis, and at the same time stimulated the
expression of tumor suppressor proteins [84–86].
Importantly, activity of the same proteins had differ-
ent dynamics in UV-irradiated cells and in those
exposed to THz radiation. Based on these results, the
Table 1. Effects of THz radiation on blood cells
Cells Frequency,
THz (type)
Intensity,
mW/cm2Exposure, min Effects Reference
Jurkat cells
(human T-cell line)
2.52 (cw) 636 30–50 Activation of genes of intra-
cellular signal transduction
pathways;
[74]
240 elevated expression of genes
of heat shock proteins, cell
growth factors, antiinflam-
matory cytokines
[75]
Jurkat cells 2.52 (cw) 227 20–40 Cell death [76]
Erythrocytes 2.3 (p) 10000 0.42 Lysis [67]
Human blood lymphocytes 3.68 (cw) 40 30 Decreased cell viability [65]
90
Erythrocytes 3.68 (cw) 40 60 Decreased osmotic resistance [65]
Human blood lymphocytes 0.1(cw) 0.031 120 Increased aneuploidy
of chromosomes 11 and 17
during cell division
[73]
144 0
Human blood lymphocytes 0.13 (p) 2 20 DNA damage [72]
5
Erythrocytes 0.18 (cw) 3 180 Decreased osmotic resistance [64]
0.33 (cw)
Standard 0.002–0.3 1 30 None [39]
Standard ≤0.3 0.2 60 None [36]
Human blood leukocytes 0.5–6.5 (p) 0.008 20 None [68]
Human blood leukocytes 0.1–2.0 (p) 0.125 20 None [68]
Human blood leukocytes 0.1–1.0 (p) 0.2 20 None [68]
Human blood lymphocytes 0.12–0.13 (p) 0.03–0.25 20 None [70]
Human blood lymphocytes 0.1 (cw) 0.031 60 None [73]
860
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
CHERKASOVA et al.
authors suggested that THz radiation may be used in
therapeutic purposes to normalize the functioning of
genes involved in skin neoplasms and inflammatory
diseases.
Keratinocytes of mouse dorsal skin were exposed in
vivo to pulsed THz radiation (frequency range, 0.1–
2.6 THz; intensity, 0.32 mW/cm2; duration, 60 min)
[87]. In 1440 min (24 h) after irradiation, 149 genes
were found to be activated; they were involved, in par-
ticular, in tissue growth and regeneration, organogen-
esis, and cell migration. Furthermore, the pattern of
gene expression in cell exposed to THz radiation was
different from the patterns characteristic for exposure
to UV or neutron radiation [87]. The authors sup-
posed that THz irradiation decreased hydration of the
skin, which modified the activity of intracellular sig-
naling pathways.
Data concerning the effects of THz radiation on
skin cells are summarized in Table 2. It can be noted
that, in contrast to what was observed for blood cells,
there is no definite relationship between the effects of
THz radiation and the power of the THz source. This
may be partially due to the difference of approaches
used to assess these effects. In particular, methods
evaluating proliferation, migration, and functional
activity of the cells [80, 82] are less sensitive than
molecular biological techniques that can be used to
detect cellular DNA damage, as well as to analyze the
expression of individual genes and the synthesis of the
corresponding proteins [78, 79, 83–87]. At the same
time, in most studies that descried some effects of THz
radiation, the power of sources was significantly above
safety limits.
EFFECTS OF THz RADIATION
ON NERVE CELLS
Effects of continuous-wave laser THz radiation (fre-
quencies, 0.71, 1.63, 2.45, 2.56, 3.68, and 4.28 THz;
intensity, 2–20 mW/cm2; duration, 60 min) were
investigated in isolated neurons of the supraesopha-
geal ganglion of great pond snail Lymnaea stagnalis
[88]. Significant effect were observed at 3.68 and
0.71 THz (Table 3). Exposure to 0.71 THz radiation
changed adhesive properties of cell membranes and
disrupted contacts between the neurons and the sup-
port. Exposure to 3.68-THz radiation caused struc-
tural changes in the somatic membranes, axons, and
growth cones. This was a delayed effect that developed
within 2400–3000 min (40–50 h) after irradiation.
During this period, pigment granules redistributed,
and the membrane became heterogeneous. Next,
there appeared abnormal randomly directed sprout-
Table 2. Effects of THz radiation on skin cells
Cells Frequency,
THz (type)
Intensity,
mW/cm2Exposure, min Effects Reference
Human skin fibroblasts 2.52 (cw) 84.8 40 Number of viable cells
decreased by 10%;
[78]
80 enhanced proliferation;
no DNA damage
227 1, 2 Activation of proinflamma-
tory cytokines and stress
response
[79]
Human keratinocytes 1.4 44.2 20 Effects on gene expression
specific for each frequency
[83]
2.52
3.11 (cw)
Artificial human skin 0.1–2.5 (p) 5.7 10 Decreased expression
of genes associated with pso-
riasis and atopic dermatitis;
enhanced expression of
tumor suppressor proteins
[84]
57 [85]
[86]
Mouse keratinocytes 0.1–2.6 (p) 0.32 60 Activation of genes involved
in tissue healing and other
[87]
Standard 0.002–0.3 1 30 None [39]
Human fibroblast culture 0.1–0.15 (p) 0.4 20 No DNA damage; enhanced
actin polymerization
[81]
Human skin fibroblasts 0.07–0.3 (cw) <0.001 180, 4200, 5760 None [82]
Human skin fibroblasts 0.14 (cw) 20–200 20 None [80]
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
EFFECTS OF TERAHERTZ RADIATION ON LIVING CELLS: A REVIEW 861
like structures, whereas classical neurite growth was
not observed [88]. The same experiment showed that
radiation-induced responses of cell membranes were
not the same at different stages of cell growth. The
effect described above was observed in cells at the ini-
tial stage of regeneration of neural network (before the
formation of neural sprouts). In neurons with devel-
oped sprouts that were in the course of network forma-
tion, the observed effects were different: damages to
neurite growth cones and arrest of their growth, which
impaired the formation of neuronal connections [88,
89] (Fig. 2).
Table 3. Effects of THz radiation on nerve cells
Cells Frequency,
THz (type)
PFD,
mW/cm2
Exposure,
min Effects Reference
Glial cells 0.12–0.18 (cw) 3.2 1 Number of apoptotic cells increased
1.5 -fo ld
[92]
5 Number of apoptotic cells increased
2.4-fold
Neurons of L. stagnalis 0.71 (cw) 10–20 60 Changes in adhesive properties of
membranes
[88, 89]
1.63 (cw) 2–20 60 None [88, 89]
2.0 (p) 2–20 60 None [90]
2.14 (p) 0.3 1 Cell death after 60 min [89]
2.3 (p) 30 1 Cell death (120 min) [89]
3 1 Cell death (180 min)
0.3 1 Membrane changes
2.3 (p) 2–20 0.6 Reversible membrane permeability [90, 91]
2.45 (cw) 2–20 60 None [88, 89]
2.56 (cw) 2–20 60 None [88, 89]
3.68 (cw) 2–5 60 None [88, 89]
10–20 Structural changes in the somatic
membrane, axons, and the growth
cone
4.28 (cw) 60 None [88, 89]
Fig. 2. Effect of continuous-wave radiation with a frequency of 3.68 THz on isolated neurons at the stage of neural network for-
mation: (a) an example of network formation; (b) thickenings in the neurite growth zone and arrest of their growth after exposure
to THz radiation. Exposure caused damages to neurite growth cones and arrest of their further growth, impairing formation of
neuronal connections. Scale bar, 25 μm.
25 Pm25 Pm25 Pm
(a)
(a)(a) (b)(b)(b)
25 Pm25 Pm25 Pm
862
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
CHERKASOVA et al.
The same biological object, isolated neurons of
L. stagnalis, was used to investigate the effects of high-
intensity pulsed THz radiation generated by FEL
(pulse duration, 30–100 ps; pulse frequency, 5.6–
11.2 MHz). Exposure to 2.3-THz radiation with a
mean intensity of 30 mW/cm2 for 60 s caused a gradual
decrease of the membrane potential accompanied
with morphological damage to the membrane and
intracellular structures and followed by cell death
within 120 min (2 h) after exposure. When the mean
intensity was decreased 10-fold to 3 mW/cm2, cell
death occurred within 180 min (3 h) after exposure.
With a further decrease in the mean intensity (to
0.3 mW/cm2), the number of viable cells stabilized in
120 min (2 h) after exposure. However, no viable cells
could be detected in 60 min (1 h) after 60-s exposure
to radiation with a frequency of 2.14 THz, even when
the mean intensity was low (0.3 mW/cm2) [89].
Modifications of barrier properties of neural mem-
branes were investigated using exposure to FEL radia-
tion (frequency, 2.3 THz; mean intensity, from 0.5 to
20 mW/cm2). It was found that irradiation caused
dose-dependent damage to nonspecif ic permeability
of the cell membrane, as observed by appearance of
the vital dye trypan blue in the cytoplasm. The dye was
present only in some parts of the cells and unevenly
distributed in the cytoplasm. The authors supposed
that THz radiation induced formation of pass-through
hydrophilic pores, which could be the only possible
way for the dye to penetrate the cell. The effect was
reversible: in 24 h after irradiation, the membrane
potential and functional responses of these cells did
not differ from normal [90]. Irradiation at the 2.0-THz
frequency with the same exposure parameters did not
induce significant changes in most exposed cells. Only
a few individual neurons were uniformly stained and
had a decreased or zero membrane potential, but their
numbers did not differ from those observed in control
experiments. For all irradiation regimens used, the
temperature of the medium in the working chamber
with neurons was monitored, and no significant varia-
tions were detected during the exposure [90].
The restoration of damaged membranes was
assessed using the BCECF-AM dye (7′-Bis(2-car-
boxyethyl)-5(6)-carboxyfluorescein acetoxymethyl
ester), which can penetrate undamaged cell mem-
branes and is converted into the fluorescent form,
BCECF, by intracellular esterases of live neurons.
Fluorescence was detected in unstained cells and in
some cells that had captured trypan blue, suggesting
that their membrane can be restored after damage to
keep negatively charged f luorescent probes within the
cell [90].
To test the hypothesis that irradiation induces for-
mation of hydrophilic lipid pores and identify the
mechanisms of this process, prior to laser irradiation,
the saline surrounding the neurons was supplemented
with lucifer yellow, a dye that does not penetrate intact
membranes, and with antioxidants. It was found that
the dye uptake by the cells signif icantly decreased in
the presence of histochrome, a phenolic antioxidant.
This may indicate that hydrophilic pores in the cell
membrane are generated in free-radical processes that
can be blocked by antioxidants [91]. It was found that
THz radiation can reversibly damage the barrier prop-
erties of the cell membrane and induce targeted deliv-
ery of biologically active compounds, while antioxi-
dants can modulate this process and provide protec-
tion from adverse effects of electromagnetic radiation
in the THz range [91].
After exposure of a glial cell culture to continuous-
wave radiation (frequency, 0.12–0.18 THz; intensity,
3.2 mW/cm2; duration, 1 min), the number of cells at
the early stage of apoptosis increased 1.5 times; when
the exposure was prolonged to 5 min, the number of
such cells increased 2.4-fold [92]. The estimated
increase in the temperature of the specimens was no
more than 0.1°C. This results apparently supports the
notion that THz radiation can represent a biological
hazard [93].
Thus, it was found that THz radiation had selective
effects on nerve cells depending on the frequency
(Table 3). For instance, exposure to low-frequency
continuous-wave THz radiation modified the adhe-
sive properties of cell membranes and induced apop-
tosis. Radiation with a frequency of 3.68 THz affected
the formation of neural networks; in this case, its
intensity exceeded the maximal permissible level [35–
39]. At the same time, FEL radiation with a high peak
intensity and low mean intensity (0.3 mW/cm2)
caused either reversible membrane permeability at the
frequency 2.3 THz or cell death at the frequency of
2THz.
EFFECTS OF THz RADIATION
ON STEM CELLS
It was shown that exposure to ultrabroad-band
pulsed THz radiation (frequency range, 1–30 THz
with a maximum at 10 THz; mean intensity, 1–
3mW/cm
2; duration, 540 min, or 9 h) promoted dif-
ferentiation of mouse mesenchymal stem cells to adi-
pocytes. In contrast, short-term exposure to continu-
ous-wave THz radiation (frequency, 2.52 THz; inten-
sity, 1–3 mW/cm2; duration, 120 min, or 2 h) helped
to maintain pluripotency [94–96]. Irradiation was
performed at 26–27°C, and expression of heat shock
proteins did not increase, which implied a nonthermal
nature of the observed effects. The authors supposed
that upregulation of gene expression was determined
by the structure of their promoters that can form spe-
cific structures facilitating transcription when exposed
to THz radiation. The authors concluded that the use
of THz radiation as a stimulus capable of modulating
gene expression and cell programming may be of great
practical significance for regenerative medicine [96].
OPTICS AND SPECTROSCOPY Vol. 128 No. 6 2020
EFFECTS OF TERAHERTZ RADIATION ON LIVING CELLS: A REVIEW 863
Exposure of a human embryonic stem cell culture
to a pulsed THz source (central frequency, 2.3 THz;
mean intensity, 0.4 W/cm2; duration, 60 min (1 h);
temperature, 36.5–37.5°C) enhanced the transcrip-
tion of 1% of genes involved in the functioning of
mitochondria [97]. These results indirectly support
the conclusions of [96]: sensitivity of genes to THz
radiation depends on the properties of their promot-
ers. At the same time, the authors did not observe
genotoxic effects of THz radiation or any effects on
cell morphology and the mitotic index.
MECHANISMS THAT UNDERLIE
THE EFFECTS OF THz RADIATION
To sum up, the data discussed above indicate that
THz radiation can have a multitude of effects on ani-
mal cells, which involves modification of the proper-
ties of cell membranes, pore formation, modulation of
cell viability and proliferation. The following mecha-
nisms might be involved in cell responses to THz radi-
ation:
—changes in the conformation of membrane-
bound proteins triggering intracellular regulatory cas-
cades that affect the genetic apparatus of the cell and
its enzyme system, as well as permeability of the cell
membrane for different compounds;
—changes in the conformation of membrane-
bound proteins responding to external regulatory sig-
nals;
—changes in the conformation of membrane-
bound proteins acting as pumps or transport channels
for uptake or release of various compounds;
—redistribution of the electric charge on the cell
membrane;
—induction of resonance oscillations of macro-
molecules that make up the cell membrane, and the
cytoskeleton as a whole.
Some authors observed cytotoxicity of THz radia-
tion and its effect on the genetic apparatus of the cell,
which were of a rather specific character and
depended on the parameters of the radiation source
and on the design of the experiment [98, 99]. How-
ever, there currently exists no scientif ic consensus as to
whether THz radiation has a damaging effect on bio-
logical objects of different complexity [100–102]. This
is primarily related to the fact that there exist no stan-
dardized experimental procedures, the diversity of
available THz sources is insufficient, and there are not
enough tools to enable accurate assessment of the ini-
tial functional state of the biological object in ques-
tion. Accordingly, for adequate evaluation of specific
biological effects of THz radiation, each of the above
aspects should be taken into account. Variations in
these parameters may be of crucial importance, to the
extent that opposite effects may be observed in exper-
iments with a minor difference of a certain parameter.
In the analysis of biological responses, the use of par-
ticular techniques with different sensitivity can also
directly affect the outcome of the experiment (for
instance, the method may produce a negative result
because of insufficient sensitivity). As a result of lim-
itations arising because of these factors, specific
responses to THz radiation are often difficult to
detect, the experiments themselves are sometimes
insufficiently reproducible, and comparison of data
from different experiments may be incorrect.
FUNDING
This work was supported by the Russian Science Foun-
dation, grant no. 18-12-00328.
COMPLIANCE WITH ETHICAL STANDARDS
Statement on the Welfare of Animals
This article does not contain any studies involving ani-
mals or human participants performed by any of the
authors.
Conf lict of Interests
The authors declare that they have no conflict of interest.
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