Biophotonics_Concepts_to_Applications

basement membrane (a thin connective tissue). The photons involved in photo-
biomodulation usually are attenuated before they reach the deeper fatty layer of the
skin.
In contrast to other laser-tissue interactions that use higher optical power levels
to generate ablation, heating, or cell destruction effects, photobiomodulation acts by
inducing a photochemical therapeutic reaction in the cell. Thefirst law of photo-
biologystates that in order for low-power visible light to generate any reaction in
biological tissue, the electronic absorption bands associated with molecular pho-
toacceptors or chromophores in the tissue must absorb the photons [ 33 – 38 ]. Thus
photobiomodulation is based on the principle that, when light impinges on certain
chromophore molecules, the photon energy causes electrons to jump from
low-energy levels to higher energy levels. In nature, the energy that is thereby
stored in the higher energy levels in the chromophore molecules can be used by the
biological system to carry out various cellular tasks, such as photosynthesis and
photomorphogenesis (the control of plant development by light). Examples of
chromophores include chlorophyll in plants, bacteriochlorophyll in bluegreen algae,
flavoproteins or respiratory enzymes in cells, and hemoglobin in red blood cells.
A general concept of the perceived underlying mechanism for photobiomodu-
lation is illustrated in Fig.6.20. Because mitochondria play an important role in
energy generation and metabolism, any proposed mechanism of photobiomodula-
tion involves some type of interaction of light with mitochondria. It has been found
that mitochondrial components absorb red to near-infrared light, which results in
the transformation of light energy to metabolic energy thereby producing a mod-
ulation of the biological function of cells [ 36 ]. Investigations have shown that
illumination by laser light increases both the intracellular reactive-oxygen species
(ROS) and the synthesis of adenosine triphosphate (ATP) in mouse embryonic
fibroblasts (cells that maintain the structural integrity of connective tissue) and other
cell types in culture. In addition, the release of nitric oxide (NO) has been observed.

Near-infrared

light beam

Mitochondria

AT P

NO

ROS

Cell proliferation

Growth factor production

Cell motility enhancement

Fig. 6.20 A general concept
of the perceived underlying
mechanism for
photobiomodulation

176 6 Light-Tissue Interactions