cryogen spray cooling in laser hair removal
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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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