Laser-activated bubbles mitigate toil and troubles
A number of biomedical
applications take advantage of laser light interactions with tissue that are accompanied
by absorption of light by tissue. As the absorbed energy converts into heat, it
produces transient nonuniform thermal fields in cells, and this can be exploited
for a variety of diagnostic and therapeutic purposes.
At the same time, laser light can create local
vapor bubbles in single cells in the areas of absorption. Although these bubbles
can damage the cells in which they appear, they also offer a highly visible nonfluorescent
target for optical monitoring. They could be particularly helpful in cytometry and
cell-level therapy, said Dmitri O. Lapotko, head of the Laser Cytotechnology Lab
at the Lykov Heat and Mass Transfer Institute in Minsk, Belarus. However, this and
other applications of the bubbles have yet to be fully realized.
Lapotko cited several reasons for this.
First, there are no widely available instruments for analysis of such short-lived
thermal events in single cells and at nanometer and micron levels. Also, the extent
to which basic properties of laser-activated bubbles — including size, lifetime
generation probability and threshold — are dependent on cell properties is
not well understood. For this reason, he and colleagues recently explored and characterized
these events. This ultimately led them to propose new biomedical applications for
the bubbles as cell-level photothermal phenomena.
Researchers have developed
a photothermal microscope that takes advantage of vapor bubbles created by laser
light in the area of light absorption. This could help advance optical monitoring
by allowing nonfluorescent imaging at the cellular level.
A laser photothermal microscope that
the researchers developed at the institute uses nanosecond pulses of visible laser
light to excite thermal phenomena and induce bubbles in individual living cells.
Two collinear probe laser beams register the phenomena as a time-resolved photothermal
image and as an integrated photothermal response.
Lotis TII, also of Minsk, developed
the device’s pulsed triple-laser system, based on a solid-state Nd:YAG laser.
A standard optical microscope images the bubbles and measures their generation probability
and threshold at a specific wavelength and fluence.
The experiments showed that the size and lifetime
of the laser-activated bubbles ranged from 0.44 to 100 μm and 0.02 to 10 ms,
respectively. The bubbles can be generated in vivo or in vitro in living cells of
any type. The probability and threshold depended on the physiological state of the
Application of the nano- and microbubbles
requires control of these and other parameters, however; for example, if investigators
wanted to generate 3- to 10-μm bubbles in cells of type A but not in cells
of type B. To address this, the researchers collaborated with Alexander Oraevsky
of Fairway Medical Technologies Inc. in Houston, to develop exogenous nanostructures
with clusters of light-absorbing gold nanoparticles, which attach to cell membranes
using cell-specific antibodies and then concentrate into clusters.
The team controlled the bubbles’
parameters by using clusters of light-absorbing gold nanoparticles attached to cell
membranes with cell-specific antibodies. Because the nanoparticles offer much higher
absorption than natural components of cells, the scientists could target specific
cells and use lower laser fluences, allowing a greater degree of control.
Light absorption is several orders
of magnitude higher with gold nanoparticles than with any natural component of cells,
Lapotko explained. Using the nanoclusters therefore allowed them to reduce the
laser fluence to levels at which bubbles were generated only in cells to which they
were attached, providing the necessary degree of control. At the same time, no bubbles
emerged in the surrounding cells and tissues (those without nanoclusters), suggesting
that the mechanism has single-cell precision and provides a safety level that matches
The technique could be used for a variety
of biomedical applications. Lapotko describes two of these: laser-activated bubble
cytometry and laser-activated nanothermolysis as cell elimination technology, or
Lantcet. The former would allow studies of physiological processes in single, intact
living cells, he said, as bubble generation probability and threshold characterize
parameters and processes such as oxygenation in red blood cells, and apoptosis/necrosis
and redox state in various cells. No fluorescence is needed, and a standard optical
microscope or flow cytometer is sufficient for these measurements.
Lantcet could enable users to selectively
target and destroy specific cells, such as tumor cells, without damaging the surrounding
normal cells or tissue. With the laser-activated bubble technique, a single pulse
would be enough to damage the cells. Thus, the method may be more efficacious (and
safer) than photodynamic therapy, in which laser-induced chemical reactions in dyes
destroy the cells, and photothermolysis, in which laser-induced heating destroys
Lapotko is developing a Lantcet-based
technique specifically for cleaning bone marrow and blood transplants used for treatment
of leukemia. To this end, he is collaborating with researchers from Fair-way Medical
Technologies as well as from M.D. Anderson Cancer Center and Rice University, both
also in Houston.
Finally, he noted that the bubbles
may help to “see” nano objects that often cannot be seen with optical
devices because of the diffraction limit. “We use a pulsed laser to generate
the bubble around such nano objects,” he said. “This bubble expands
to micrometer size and, thus, [the nano object] becomes visible.”
Lasers in Surgery and Medicine, March 2006, pp. 240-248.
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