Biomedical Engineering Reference
In-Depth Information
fibers without much attenuation. NIR light is also used to remotely control
televisions, air conditioners, and many other electric products due to its lo-
wer absorption by the surrounding materials. All these facts tell us that NIR
light is tender and friendly to living bodies.
In this chapter, applications that utilize transparency of NIR light, such as
multi-photon microscopy, coherent anti-Stokes Raman scattering microscopy,
light-power transmission into a living body, optical communication through
skin, laser nanosurgery, and laser trapping of cells, are presented. All of this
work uses the “transparency”, in other words less scattering and less absorp-
tion, properties of NIR light.
We have described four ways of applying NIR to living bodies. The first is
to use the high transmittance of NIR light on living bodies. In Sect. 1.4, we
report on transcutaneous optical power supply and optical communication
methods using NIR light. A cardiac pacemaker implanted in a rat is driven
by NIR light sent from outside the body. The second method is to sense the
infrared vibration at a focused spot by using a set of NIR lasers. This is
called CARS microscopy and is discussed in Sect. 1.3. The third way is to
send and focus NIR short-pulse light into a deep portion of a living organ,
and induce multi-photon absorption, whose energy corresponds to a photon
in the visible range, by the resultant spatially and temporally condensed
intense spot. This method is discussed in Sects. 1.2 and 1.5 as multi-photon
microscopy and multi-photon surgery, respectively. The fourth way is to use
the momentum of the NIR photons to move and control small particles such
as cells, organellas, and latex beads attached to protein and DNA molecules.
To move these kinds of particles, a force of the order of at least a piconewton
is required. This means that the total irradiation power of the light is about
1 watt or more. Thanks to the transparency of NIR light, one can feed focused
laser beams of some watts into biological specimens without thermally or
photochemically destroying them.
1.2
Imaging Cells through a Multi-Photon Process
1.2.1
Nonlinear Optics in Cells
In the observation of a living specimen, it is important to reduce physical
or physiological damage in the specimen during or even before the observa-
tion. For this reason, a confocal fluorescence microscope has been often used
to observe those specimens, where the optically sectioned three-dimensional
images can be obtained without slicing the specimen mechanically. However,
even with a confocal microscope, the observable depth in such specimens is
usually given as 50
m, and this observable depth is still not enough to see
the functions of whole parts of organs or other apparatuses in bodies.
Microscopic techniques have been advanced by the introduction of the
multi-photon fluorescence microscope, which has been realized with the re-
cent development of ultra-fast high-power laser systems [1,2]. Since the proba-
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