Cryogen spray cooling in laser hair removal
Laser therapy is an effective method
for hair removal. Lasers work by transferring energy in the form
of light into the skin.
Pigmented (colored) areas of the skin, in this case the pigmented
hair follicles, pick up this light energy and transforms it into
heat. The pigmented areas get so hot that they are effectively “cooked”.
This cooking destroys the hair follicle. However, people who have
fairly dark skin can find that lasers result in damage to their
skin as well as the follicles. The absorption of laser energy
by epidermal melanin causes localized heating and increases the
risk of non-specific thermal damage to the skin outside of the
hair follicles. One way to get around this problem is to use a
cryogen cooling spray. This cools the surface of the skin, but
the cooling effect does not penetrate deep into the skin to the
depth of the hair follicles. So a cryogen cooling spray can be
used to counteract the heating effects of the laser on the skin
epidermis, but still allow the laser to heat up the hair follicle
and cook it.
Skin indentation is a condition that occurs in patients when
dermatologists use a cryogen spray to cool the skin of the patient
prior to a laser treatment. During this process, the patients’ skin
becomes cold and it contracts such that an indentation is formed
on the surface of the skin. The skin indentation due to the cryogenic
cooling process has a subsequent effect on the effectiveness of
the laser treatment. The indentation in the skin changes the properties
of the heat transfer in the skin and, because of this, the indentation
also affects the safety and effectiveness of the laser procedure
in the patient. Due to the critical nature of this process and
the importance of safety in laser surgery, scientists have taken
it upon themselves to do an intensive study to determine the precise
manner in which these skin indentations affect the heat transfer
through the skin. With this research they hoped to find the best
way to perform laser surgery on patients that would result in
minimal damage to the skin epithelium but allow maximum effect
of the laser on hair removal.
The study that was done to determine the effect of skin indentations
on heat transfer was performed at the Beckman Laser Institute
in Riverside, California. In the study, scientists made videos
of the cryogenic process on different skin samples. Special attention
was placed on the distance of the cryogenic nozzle from the skin
and the particular anatomical location on the patients’ body
as well as the specific shape of the skin indentations. All of
these factors were examined in order to see how the formation
of indentations affected the heat transfer in the skin.
In the study, liquid cryogen was delivered to the skin through
a high pressure rubber hose. The hose was fitted with a stainless
steel nozzle. The nozzles that were used were very similar to
the commercial nozzles "GentleLASE" or "Vbeam" nozzles
used in many laser clinics. The results of this experiment were
observed with a camera in order to obtain a better perspective.
A high speed Photron fastcam was used to make images of the
cryogenic cooling process as it took place. This was done to avoid
the problems that ensued when measuring the results directly from
the experiment. It was previously found that direct measurements
often interfered with the natural process of the skin indentation.
The locations on the human body that were tested included the
front and back sides of the hand as well as the volar forearm.
The camera was positioned near these locations such that the image
of the indentation could be seen and measured from a convenient
angle.
In addition to the camera recording of the actual skin, heat
sensors were also used to make a measurement of temperature change
when sprayed with cryogen. The heat sensors were made of a thin
silver foil that was embedded in epoxy. The epoxy had been chosen
because of its similar heat transfer qualities to human skin.
The two kinds of sensors that were used either had a flat surface
or a curvature to them similar to the curvature of the skin indentations
themselves. In this way, scientists hoped to estimate the exact
manner in which heat transfer might take place on real skin.
The immediate results in terms of physical indentation on the
skin were that the indentations reached their deepest depression
after about 10 milliseconds of cryogenic spraying. After that,
the indentations remained at the same depth throughout the duration
of the spray. The indentations would begin to subside immediately
after the spray was terminated and would return to their normal
state usually after a similar length of time as had initially
taken to create them. The locations on the body also had an effect
on the indentations such that the more pliable parts of the skin
would create larger indentations. After watching these cryogenic
effects on the skin, the heat sensors were designed to look exactly
like the indentations that had been formed.
Further results on the heat transfer were derived from the sensor
tests when they were sprayed with the cryogen cooling spray. When
the cryogen nozzles were placed only 20 mm away from the sensors
and a spray was done, the temperature of the sensors dropped the
fastest. It would also reach the lowest temperature point of all
the tests. For a 40 mm distance, however, the temperature drop
was much slower and less pronounced overall. Specifically, in
the 20 mm trials, the temperature of the flat sensors reached
a low of -30 degrees celsius in only 20 milliseconds. In the 40
mm trials, it reached only -25 degrees celsius and took twice
as long to reach its lowest temperature.
The flat surface sensors recorded the greatest change in temperature
over the shortest amount of time. The curved sensors that had
large cross sectional surfaces recorded the second greatest change
in temperature and finally, the curved sensors with smaller cross
sections showed the least amount of temperature change. The results
of the test seemed counterintuitive to scientists and therefore
required further analysis.
It was expected that the flat sensors would record the greatest
change in temperature because the contact of the cryogen was direct
on a flat surface. This expectation proved to be correct. The
curved sensors, however, were expected to record a greater temperature
when the cross sections were smaller rather than when they were
larger. Scientists expected that the results would be explainable
in terms of the collection of cryogen that built up in the curved
areas of the sensors. Cryogen, they thought, would have collected
and thickened in the concave areas during the spraying period
and they believed that this would adversely affect the temperature
change. This did not occur in the manner that scientists had first
predicted.
The collection of cryogen did indeed happen in the larger curved
cross sections of the sensors but it did not slow down the effects
of the cryogen as expected. On the contrary, an increase in the
cross sectional size of the sensors and therefore in the indentations
themselves increased the heat transfer properties rather than
decreasing them. Ultimately, the temperature change in the sensors
was greater on the larger cross sections.
Explanations for this heat transfer phenomena are not entirely
understood. On a concave indentation with a small cross section,
the cryogen collects and an upper layer of cryogen remains. This
layer is especially notable on the small cross sections because
of the shape of the indentation and the manner in which the cryogen
layer eventually alters the heat transfer. Regardless of the explanation
for this, it is clear that a larger indentation aids in heat transfer.
It would be the ideal situation if scientists could avoid indentations
altogether. A flat surface proved to be the best circumstance
for improving the heat transfer of the cryogen. However, if an
indentation occurs, it is best that this indentation be a larger
rather than a smaller indentation.
Even though it was significant to discover that the smaller
cross sections showed less of a change in temperature than the
larger ones, these results were of relative insignificance overall
because the distance of the cryogenic spray nozzle from their
targets was much more significant to the ultimate temperature
of the skin. This distance could be used to offset the other effects
of the indentations in the skin later on when actual laser treatments
were being performed. Similarly, nozzles that produced higher
momentum spurts could create larger indentations if needed and
thereby affect the ultimate temperature in a laser treatment.
The results of this research should help to define the most appropriate
skin cooling technique to use in dark skinned individuals undergoing
laser hair removal and other laser skin treatments.
Cryogen spray cooling in laser hair removal references
- Basinger B, Aguilar G, Nelson JS. Effect of
skin indentation on heat transfer during cryogen spray cooling.
Lasers Surg Med. 2004 Feb; 34(2):155-63. PMID: 15004828
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