Biomedical Engineering Reference
In-Depth Information
expression [(in addition to tissue ultrastructure, i.e., electro-
magnetic (EM)] might be an excellent tool to examine low-
intensity high-volume heat injury.
keratoses (AKs) with one treatment (36). However, pulse light
sources can theoretically also be used and have proved to be
useful in a range of skin disorders (37,38). There is little
endog-
enous
photochemistry beyond 319 nm. Some exceptions are
photoisomerization of bilirubin (450 nm) and singlet O
2
pro-
duction by
Propionibacterium acnes
porphyrins (similar
absorption peaks as PpIX). Beyond 800 nm, photochemistry,
even with exogenous photosensitizers or prodrugs, is unlikely.
Vaporization
At a certain threshold power density, coagulation gives way to
vaporization. Often, this process is referred to as ablation and
is an important component of laser skin resurfacing (LSR). At
this higher power density (and higher fl uence), water expands
as it is converted to steam. Vaporization is benefi cial in that
much of the heat is carried away from the skin during the pro-
cess (4).The vaporization energy for water is approximately
2.4 kJ/cm
3
. When there is vaporization, there is also increasing
pressure as the water tries to expand in volume. The expansion
leads to localized microexplosions. At temperatures beyond
100°C (without further vaporization), carbonization takes
place, which is obvious by blackening of adjacent tissue and the
escape of smoke. Carbon is the ultimate end product of all liv-
ing tissues being heated and carbon temperatures often reach
up to 300°C (34). Normally, this should be avoided, because
the depth of tissue injury will extend well beyond the black-
ened skin surface. This is particularly true, for example, when
treating something like a rhinophyma or performing LSR.
Pulsed Light Sources and PDT
It has been shown that lower irradiances demonstrate a more
marked PDT effect for equivalent total light doses (39).
Although pulsed light sources have been shown to be as effec-
tive as CW sources in some experiments, the PDT effect was
only equivalent
if the light doses and average irradiances were
similar
. Most pulsed light sources in dermatology do not meet
the theoretical PDT saturation threshold (4 × 10
8
W/m
2
) (40).
All of this suggests that an optimal pulsed light PDT confi gu-
ration might require multiple passes with blue, green, yellow,
or red light. The interval between passes should be designed,
such that the average irradiance is similar to that of CW devices
with similar emission spectra.
Excimer Laser (308 nm)
The mechanism of action for the excimer laser (XeCl) is thought
to be the same as narrow band UV therapy. There is an ery-
thema action spectrum determined by plotting a reciprocal of
the minimal erythema dose against wavelength. It appears a
reduction in cellular proliferation and most likely plays a part in
UV radiation shorter than 320 nm. This range has a profound
infl uence on epidermal cellular DNA synthesis and mitosis.
In the original study of Parrish in 1981, he showed that wave-
lengths between 300 and 313 nm were most effective (41).
It appears that the excimer laser works through in an immuno-
modulatory way much like UVB. The precise chromophore
is unknown. There may be a thermal component as well at
fl uences >800 mJ/cm
2
.
Photomechanical
With even more rapid heating, there is insuffi cient time for
pressure relaxation. In this scenario, there will be disassembly
and microcracks in the tissue. On the other hand, with
slow energy deposition, there will be no pressure increase.
Mechanical damage plays an important role in SPT with high-
energy, submicrosecond lasers for tattoo and pigmented lesion
removal. Inertially confi ned ablation occurs when there is
high pressure at constant volume. In a very short pulse, the
energy is invested so quickly one that there is no time for the
pressure to be relieved. Under these conditions of inertial con-
fi nement, there is not enough time for material to move—this
can lead to the generation of tremendous pressures. New stud-
ies support a role for femtosecond ablation of skin where
beam can be focused from the surface to create small “holes”
300-500
Biostimulation
Biostimulation is thought to belong to the group of photo-
chemical interactions. However, the term biostimulation has
not been scientifi cally very well defi ned. Most biostimulation
studies involve low-power lasers and have been a subject of
controversy for decades. Typical fl uences are in the range of
1-10 J/cm
2
, and normally there is no acute temperature eleva-
tion. Some scientists defi ne biostimulation by the absence of
any thermal mechanism (4). The most vexing problem about
many biostimulation studies is the diffi culty in assessing what is
occurring at a clinical level (beyond the cell culture model). It is
unknown if any features of laser light, coherence, monochro-
maticity, polarization, are really relevant for biostimulation (4).
One example of a “biostimulation” device in dermatology is the
Gentlewaves LED Photomodulation unit (Light BioScience,
LLC, Virginia Beach, Virginia, USA). This device uses 590-nm
light in a high repetition rate and low-power density to pur-
portedly increase collagen synthesis and enhance facial tone.
Cell culture work supports this concept (42). However, more
work clearly needs to be done in this arena of biostimulation.
Is it really plausible to unlock the code for subcellular processes
through low-level irradiation?
m below the stratum corneum. In this scenario, the
overlying skin is undamaged (35).
μ
Photochemical
Photochemical reactions are governed by specifi c reaction
pathways and are becoming increasingly important in derma-
tology (4). The most common type of photodynamic action in
dermatology is one where singlet oxygen is created. In the
reaction, a photosensitizer is excited by a certain wavelength of
light. In the presence of oxygen, oxygen is transformed from
its triplet state, which is its normal ground state, to an excited
singlet state. The excited singlet state oxygen reacts with bio-
logical molecules and often attacks plasma and intracellular
membranes. The most common photosensitizer (PS) in der-
matology is PpIX. This PS is formed by skin cells by the pro-
drug, aminolevulinic acid (ALA). The proper combination of
wavelength and power density will achieve the best results.
Overall, most photochemical reactions proceed more effi -
ciently with lower power densities, such that, for example, the
Blu-U light (DUSA, Vahalla, New York, USA) will normally
outperform a pulsed source (IPL, KTP, or PDL) for actinic
