LASER LIGHT INTERACTION WITH BIOMATTER AND BIOMEDICAL
Institufe of Spectroscopy
Russian Academy of Sciences,
142092 Troitsk, Moscow Region,
Any new achievement in science and technology was always finding some useful
application in biology and medicine. And naturally the invention of the laser
was not an exception to this rule.
All the special properties of the
laser light, namely its (1) spatial
coherence (directivity and focusability), (2) high intensity in both
continuous-wave and pulsed modes of operation, (3) monochromaticity (temporal
coherence) and wavelength tenability, and (4) short (controllable) pulse
duration proved valuable in biomedical applications. Progress in laser
technology leads to subsequent progress in biomedical laser applications, both
experimental and clinical. Speaking of the progress made in recent years, one
should single out the following trends in laser and photonic technologies: (1)
femtosecond lasers, (2) solid-state tunable lasers, (3) laser diode arrays and
their application to the pumping of solid-state lasers, (4) soft X-ray lasers,
and (5) IR fiber optics.
At the root of all biomedical laser applications is the interaction of
laser light with matter at various biological organization levels: (1) molecules, (2)
cells, (3) biotissues, (4) organs and (5) the whole organism.
It is convenient to subdivide the biomedical laser applications as
1. Laser diagnostics based on resonant (spectrally selective) and
nonresonant laser-matter interaction.
Laser therapy based on
laser-induced molecular processes consequent upon resonant excitation of
3. Laser surgery based on laser induced destructive
When a biomolecule absorbs laser light, the expenditure of the excitation
energy can follow various pathways, depending on the species of the molecule,
its surroundings, and radiation intensity. Some
absorbed energy degrades to heat in a radiationless manner, some may be
transferred to the neighboring molecules.
The excited molecules may take part in chemical reactions, and finally, they
may be raised to higher quantum states, provided the light intensity is high
enough (ultraviolet pulses), and then be involved in photochemical processes.
Almost all these excitation pathways are now being successfully utilized in
laser biomedicine: laser thermal surgery, laser fluorescence diagnostics, laser
photodynamic therapy, and laser phototherapy. The sum of the quantum yields
attained in the particular channels is equal to unity. Depending on the
irradiation conditions and the state of the medium, the quantum yield in any
desired channel can be varied with a view to improving the efficiency of its
One can vary the laser radiation parameters (intensity, duration) over very
wide limits, thus making it possible to implement various types of
light-biomatter interaction (linear and nonlinear, single- and multiple-photon,
coherent and noncoherent) and thereby induce various effects in biotissues
(photochemical modification, thermal destruction, explosive ablation, optical
breakdown, shock pressure waves, etc.) as qualitatively.
When considering the interaction between laser radiation and biotissues, one
should take into account (and potentially utilize to advantage) the
inhomogeneous character of their various properties: biological, chemical, and
physical (optical, acoustical, electrical etc.). The characteristic sizes of
these inhomogeneities varies over very wide limits: from the size of individual
molecules to that of blood vessels and organs. As an illustration, presents a
qualitative view of the spatial spectrum of these inhomogeneities. The spatial
characteristics the inhomogeneities probably have a fractal structure  which
has yet to be studied quantitatively (the fractal dimension in
The spatial inhomogeneity of the optical characteristics of biotissue
(specifically its absorption characteristics) leads to a spatially nonuniform
distribution therein of heat liberated as a result of absorption of pulsed laser
radiation. The following three characteristic times are of importance in this
case: tp the laser pulse duration, ts=a/C0 -
the time it takes for sound to propagate with a velocity of С0 beyond
an inhomogeneity of size a, i.e., the relaxation time of the pressure resulting
from the pulsed local heating, and thd=a2/kc - the time it
takes heat to propagate by diffusion beyond the inhomogeneity (k- is a numerical
coefficient depending on the shape of the inhomogeneity and с- is the thermal
diffusitivity), i.e., the relaxation time of pulsed heating. Shows regions
of enhanced pressure DP (pressure confinement) and enhanced temperature DT
(temperature confinement) in the case of microsphere of radius R (R=a, k=27) for
pulses differing in duration. Both these effects are important in the pulsed
irradiation of real biotissues and should be not only taken into consideration,
but also used to advantage.
laser biomedical diagnostic can conveniently be subdivided into two
classes: (1) macrodiagnostics of various objects (topography, detection of
pulsation, blood flow measurements, etc.) and (2) micro (spectro) diagnostics at
various biological organization levels: organs, cells, organelles and
The central problem is the development of the methods for the laser
diagnostics of inhomogeneities (tomography of biotissue which is an absorbing
and highly scattering medium, i.e., a very "awkward" object for optical
techniques. The tomography of biotissue is important for medical diagnostics,
laser therapy and surgery. To choose the optimal regime and correct dose of
laser irradiation it is necessary to know the values of light absorption and
scattering of tissue Moreover, it's important to know the spatial distribution
of light absorption and scattering nonhomogeneities over irradiated area. For
example, the determination of sizes and the depth of location of tissue injures,
blood vessels, inclusions. pigments etc. is of real importance. The various
methods of diagnostics of light absorption inhomogeneities in turbid media are
widely discussed: photon migration , time-resolved photon migration ,
optical coherence tomography [4-6] and others. Therefore they are the most
effective for transparent media and are sensitive to the refraction coefficient
inhomogeneities mainly. The most exciting results are reached by using these
methods for diagnostics of eyes injuries . The achieved spatial resolution is
tens of microns both in lateral and z-axial directions (in the depth of
The light propagation in inhomogeneous medium is determined by scattering and
absorption processes. It is the case for biological tissues the scattering
coefficient is 10-100 times higher than the absorption coefficient . However,
for medical use of lasers the value of light absorption coefficient ma and it's
distribution over the irradiated region is of great importance. Therefore, the
problem of working out accessible and simple methods of diagnostics of light
absorption inhomogeneities in biological tissues is still actual.
One of the most perspective methods of measurement of absorbing
inhomogeneities in turbid media is opto-acoustic method. It is based on
excitation of thermal or acoustic waves (thermooptical excitation) by absorbing
of time-modulated or moving laser radiation (see, for example, [9-11]). The
excitation of acoustic waves (opto-acoustic signals) takes place due to
nonsteady and uniform thermal expansion of the heated area. The advantage of
opto-acoustic method is the direct dependence of amplitude of excited wave on
the light absorption coefficient, that is, there is no opto-acoustic signal is
the turbid medium without absorption.
Time-resolved laser opto-acoustic tomography is powerful and perspective
method of investigation of light absorption in inhomogeneous media . At the
same time this equipment is based on widely-used laser sources and electronic
devices. The realized experimental methods allow to get spatial resolution of
10-15 mm by the probing depth up to several millimeters. The value of light
absorption coefficient varies from unites to several hundreds of reversed
centimeters. The probing depth is determined by the light penetration depth in
the medium. To achieve high lateral resolution is still actual problem. At the
same time the ways of it's solution are quite clear and don't require
qualitative complication of signal processing. The measurements of light
absorption by means of time-resolved laser opto-acoustic tomography may be
carried out in real time. Therefore, this method is useful in medical
Spectral diagnostics on molecular level makes use of a wide
variety of laser spectroscopy methods which enable us to obtain extremely high
parameters inaccessible without using laser light. Here they are with some of
their biomedical applications.
1. Spectral resolution may reach any desired values (from nm to
MHz). A high spectral resolution in combination with a high sensitivity is used,
for example, to detect gas-phase isotopically-labeled products of metabolism and
to diagnose, on this basis, certain diseases (see ).
2. Temporal resolution, with development in short and
ultrashort pulse lasers, was improved from ns to ps and then from ps to fs. The
ultrafast laser spectroscopy methods contribute greatly to understanding
fundamental processes in biological systems (see ). The list of
intrinsically fast processes in biological systems is very long: energy and
electron transport in photosynthesis and reaction centers, proton transfer and
isomerization in vision pigment, bacterial membranes, etc.
3. Sensitivity of detection of atoms or molecules  can
reach its limiting values, i.e. single-particle level. Highly-sensitive methods
of detecting traces of microelements are important for understanding their
pathways in environment, food products and human body. Highly-sensitive
fluorescence methods of detecting single molecules are used to diagnose
diseases, starting from cellular level.
4. Selectivity of laser spectroscopy methods in detecting atoms
can be very high that, in fact, enables detecting traces of radioactive isotopes
and their pathways in environment and human body. The selectivity of detection
of biomolecules usually characterized by wide absorption bands is rather limited
though. Nevertheless, quite good results have been obtained in some cases, for
instance, in measuring the blood and tissue oxygenation level . There is a
vast field here for new methods and their biomedical applications.
5. Spatial resolution on light wavelength level is standard in
biomedicine, in particular with the use of confocal microscopy. Now methods are
being searched to increase the spatial resolution up to the nanometer range:
near-field laser microscopy , laser photoelectron spectromicroscopy ,
etc. This will make it possible to use laser radiation for mapping of
biomolecules, DNA sequencing  which is of great importance for genetic
It seems quite realistic to develop optical spectroscopy methods for
noninvasive measurement of concentration of such molecules as glucose,
cholesterol, etc. in living biotissue "in situ" including the use of portable
low-cost instruments accessible to every patient.
All the methods of
laser therapy can be conventionally
subdivided into two classes: photodynamic therapy and photothermotherapy.
Photodynamic therapy is class of phototherapeutical methods is
based on photochemical reactions of exogenous and endogeneous chromophores. It
comprises well-elaborated laser photodynamic therapy  and rapidly developing
laser low-power biostimulation .
Photothermotherapy is class of the methods of this class are
based on the effect of heating of biotissue due to nonradiative relaxation of
laser excitation. This means nonderstructive local transident heating under
short-pulse laser radiation .
There are at least three mechanisms that are responsible for the transient
local heating of biotissue exposed to
First, even a CW laser radiation is absorbed as separate quant by individual
chromophores in the tissue. In the laser intensity range typical of biomedical
applications, the excited chromophores are distributed in the tissue thinly
enough, and so there must inevitably occur a slight overheating of the molecular
chromophores for a very short period of time.
Secondly, the absorptivity of the tissue at a certain radiation wavelength
varies over sufficiently wide limits throughout the tissue volume. A most simple
example is the enhanced absorption of hemoglobin in blood which is distributed
all over the bulk of the tissue by vessels in a great variety of diameters and
lengths (fractal structure with a dimension of D=2.7 ). The spatial
variations of the tissue absorptivity are especially great in the visible and
near IR regions of the spectrum, but are smoothed out in the UV and IR regions
because of the strong absorption of all organic molecules in the UV region and
of water in the IR region.
Thirdly, account should be taken of the nonuniform spatial distribution of
the laser radiation intensity. For example, when coherent light propagates
through an optically inhomogeneous, scattering biotissue, random interferences
there inevitably give rise to speckle structures  with irregularities
characteristically measuring up to 1/2. If the given effect, for example, local
heating, lasts for a time shorter than the correlation time of the speckle
pattern fluctuations (which depends on the laser bandwidth and internal motions
in the exposed medium), this speckle-like laser intensity distribution will be
The spatial spectrum of absorption inhomogeneities of biotissues being very
one can cause, by properly selecting the duration and wavelength of the laser
pulses used, the local transient heating of the desired micro- and
macrostructures in the exposed biotissue.
It can be expected that the pulsed heating of the desired kind of
microvolumes in biotissue with laser pulses of a certain wavelength, pulse
duration, and intensity opens up entirely new possibilities for
photothermotherapy. This is due to the fact that the local transient overheating
of microvolumes may be substantial (DT= (1, 100) deg), the time- and
space-averaged macroscopic heating of the entire irradiated region being much
lower. Secondly, by varying the irradiation wavelength, one can effect the
pulsed heating of the desired biologically important microvolumes, the
relationship between the locally absorbing regions and the wavelength being
studied beforehand by a similar method. It can be expected that the locally
measured (with reference to DT) pulsed heating of microvolumes of a certain kind
will have a material effect on the course of biochemical processes, and so will
form the basis for new therapeutic techniques based in principle on the use of
tunable ultrashort laser pulses. But their actual types can only be revealed
through experimental investigations into local absorption regions, their nature
and size, and the characteristics of the pulsed overheating and temperature
gradients produced. All this should be the aim of special systematic studies in
the near future.
Laser surgery use well established fact that the heating of biotissue causes
its irreversible damage. As the tissue temperature grows higher, there takes
place the denaturation of enzymes and loosing of membranes (40-45°C),
coagulation and necrosis (60°C), drying out (100°C), carbonization (150°C), and
finally, pyrolysis and vaporization (above 300°C).
Pulsed laser radiation allows developing finer surgical techniques involving
minimal thermal injuries . First, by using pulsed laser radiation of certain
wavelength, one can selectively heat local, restricted areas featuring an
elevated absorption (this is achieved when operating in region 2. It is this
selective photothermolysis technique that is at the root of precise
microsurgery. The effect of local laser heating of spatially nonuniform
(granulated) biotissue has rather long been discussed in connection with the
problem of pulsed-laser-induced retinal injuries . This brings up following
important question. What is essential is that the maximum permissible
temperature to which biotissue can be heated without running the risk of
destruction depends on the irradiation time? With the traditional heating
methods. short-term heating conditions are difficult to realize, but laser
radiation makes it possible to deposit energy in a small. tissue volume which
cools rapidly as a result of thermal diffusion. It therefore becomes possible to
study the limits of permissible tissue overheating during time periods from
10-3 to 10-12 s. This is obviously the object of future
Secondly, high-power pulsed laser radiation can help effect the pulsed
ablation (expositive evaporation) of the surface of soft and hard biotissues
. This process occurs in region 3 of Fig. 4, where heating is strong enough
and is accompanied by a substantial pulsed pressure rise. The mechanism and
conditions of the laser ablation of various materials, including biotissues,
were discussed in .
Concluding this fragmentary analysis of laser light interaction with
biomatter with applications which is a supplement to my old review often years
standing , I clearly see the rapidly of development and fruitfulness of this
field of application of lasers. As far as these this field can probably only be
compared with the laser fiber science and technology for telecommunications.
Both these areas will have a great effect of the further development of the
human society. Incidentally, when the laser just made its appearance, neither
one not the other field was considered a potentially serious area of its
application. This is one more evidence of how subjective and unreliable all
sorts of predictions can be even in science and technology.
1. Mandelbrot, B. (1983) The Fractal Geometry of Nature, Freeman, New
2. Nossal, R., Kiefer, J., Weiss, G.H., Bonner, R., Taitelbaum, H.,
and Havlin, S. (1988) "Photon migration in layered media", Appl.Opt. 27,
3. Patterson, M.S., Chance, B., and Wilson, B.C. (1989)
"Time-resolved reflectance and transmitance for the noninvasive measurement of
tissue optical properties", Appl.Opt. 28, 2331-2336.
4. Patterson, M.S.,
Moulton, J.D., Wilson, B.C., Berndt, K.W. and Lakowicz, J.R. (1991)
"Frequencydomain reflectance for the determination of the scattering and
absorption properties of tissue", Appl.Opt. 30, 4474-4476.
J.C. and Wong, K.S. (1993) "Time-resolved optical tomography", Appl. Opt. 32,
6. Нее, M.R., lzatt, J.A., Jacobson, J.M., Fujimoto, J.G. and
Swanson, E.A. (1993) "Femtosecond transillumination optical coherence
tomography". Opt. Lett. 18, 950-952.
7. lzatt, J.A., Нее, M.R., Huang, D„
Fujimoto, J.G., Swanson, E.A., Lin, C.P., Schuman, J.S. and Puliafito, C.A.
(1993) "Ophthalmic diagnostics using optical coherence tomography", Ophtalmic
Technologies III, Spie Proc. 1877, 136-144.
8. Cheong,W.F.,Prahl, S.A.
andWelch,A.J. (1990) "A review of the optical prоperties of biological
tissue",IEEEJ. Quant. Elec. 26, 2166-2185.
9. Zharov, V.P., Letokhov,
V.S. (1986) Laser Opto-acoustic Spectroscopy, Springer, Berlin.
A.C., (1986) "Applications of photoacoustic sensing techniques". Rev. Mod. Phys.
11. Gusev, V.E. and Karabutov, A.A. (1993) Laser
Optoacoustic, AIP, New York.
12. Karabutov, A.A., Podymova, N.B.,
Letokhov, V.S. (1996) "Time-resolved laser optoacoustical tomography of
inhomogeneous media", Appl. Phys. B63, 545-563.
13. Murnick, D.E., and
Peer, B.J. (1994) "Laser-Based Analysis of Carbon Isotope Ration", Science 263,
14. Hochstrasser, R.M. (1989) "Biological applications of
ultrafast laser methods", Ber. Bunsenqes. Phys. Chem. 93, 239-245.
Kliger, D.S. (1983) (ed.). Ultrasensitive Laser Spectroscopy, Academic Press,
New York. Letokhov, V.S. (1986) (ed.), Laser Analytical Spectrochemistry, Adam
Hilger, Bristol and Boston.
16. Wilson, B.C., Sevick, E.M., Patterson,
MS. and Chance, B. (1992) "Time-dependent optical spectroscopy and imaging for
biomedical applications", Prос. IEEE 80, 918-930.
17. Betzig, E.,
Chichester, R.J. (1993) Single Molecules Observed by Near-Field Scanning Optical
Microscopy, Science 262,1422-1425.
18. Letokhov, V.S, (1989) "Prospects
of Laser Spectroscopy of Biomolecules with Nanometer Spatial Resolution", in
Feld, M.S., Thomas, J.E., Mooradian, A. (eds.). Laser Spectroscopy IX, Academic
Press, Boston, 494-499.
19. Keller, R.A., Katzir, A., (1993) (eds,).
Proceedings of Advances in DNA Sequencing Technology, SPIE Рubl.
20. Jori, G., and Peria, C. (1985) (eds). Photodynamic Therapy of
Tumors and other Deseases, Libreria Progetto, Padova.
21. Каru, Т. (1989)
Photobiology of Low-Power Laser Therapy, Harwood Acad. Publ., Chur.
Letokhov, V.S. (1991) "Effects of transient local heating of spatially and
spectrally heterogeneous biotissue by short laser pulses:, Nuovo Cimento 13D,
23. Dainty, J.C. (1984) (ed.) Laser Speckle and Related
Phenomena, Springer, Berlin.
24. Anderson, R.R. and Parrish, J.A. (1983)
"Selective photothermolysis: precise microsurgery by selective absorption of
pulsed radiation", Science 220, 524-527.
25. Hansen, W.P., and Fine, S.
(1968) "Melanin granule models for pulsed laser induced retinal injury", Appl.
Optics 7, 155-159.
26. Srinivasan, R. (1986) "Ablation of polymers and
biological tissue". Science 234, 559-563.
27. Letokhov, V.S. (1985)
"Laser Biology and Medicine", Nature 316, 325-330.