Page 143 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Ice vs air-cooling vs cryogens
There are a number of techniques which may be employed to cool the skin – air cooling, ice- cooling or cryogenic cooling.
Ice-cooling is, by far, the most efficient method of cooling the skin, of these three. It uses conduction of heat from the skin into a water-based substance. Combined with a gel on the skin surface, this method will rapidly cool the skin down to the reticular dermis.
Ice
The thermal conductivity of water is around 0.54 W/mK, while that of ice is 2.22 W/mK (both at -10 C). Hence ice can conduct heat away from an object around 4 times faster than water. So, putting an ice-pack onto the skin’s surface will draw heat from it much faster than any water-based gel on its own.
If ice (taken from a freezer, not a refrigerator!) is put onto the skin surface, the top layers of the skin will eventually reach 0C after a fairly short time. That 0C level will essentially move deeper into the skin, the longer the ice is in contact with the surface.
However, it must be remembered that one of the functions of the blood supply is to keep the skin at the correct temperature (of around 35C). So, there is a contact ‘battle’ between the cooling effect of the ice, and the skin’s natural response to it. For this reason, we often see erythema after ice has been applied for more than a few seconds – the blood vessels will have dilated to bring in more warm blood than usual to compensate for the ice cooling.
Air
The thermal conductivity of air is only around 0.023 W/mK (at -10C ). This is why air is seen as a thermal insulator - its heat conductivity is very low. Water conducts heat around 23 times faster than air (which is why the people form the Titanic, in the water, died of hypothermia before rescue arrived – while the people in the boats (in the air) all survived). Ice (at -10C) conducts heat about 96 times faster than air!!
Air-cooling utilises an air blower to force cold air over the skin surface. This uses convection to extract heat from the skin, but is quite limited in its effectiveness. Unless the cold air is kept at the same region for a length of time, the cooling effect, with respect to depth into the dermis, is minimal.
Cryogen
Cryogenic cooling uses a spurt of cryogen at a temperature of around minus 10C, or thereabouts, which makes contact with the skin just before a laser pulse of energy is fired. The main problem with this technique is that is doesn’t cool the skin for a sufficiently long period of time, meaning that only the most superficial region is cooled, briefly. While this may reduce the pain sensation during the application of the light energy, it does not cool the dermis for any
________________________________________________________________________ 143 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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