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In those hair follicles, this may be enough to damage them sufficiently. But, the other 99%+ of the light energy has to go somewhere...
Most of it will be absorbed in the ‘bulk’ dermis – that is, throughout the rest of the dermal tissues including the collagen, blood vessels, nerve tissues and tissue water etc.
This is very interesting – the vast majority of the laser/IPL energy used for hair removal merely heats up the skin, not the target hairs!! This fact also indicates that cooling is required.
Pre-Cooling
So, we essentially have three hot sites – the basal layer, the hair follicles and the bulk dermis.
Yet, what we really want are the hair follicles to be heated. But, at present, there is no way around the fact that we have a helluva lot of unwanted heating too.
The answer to this issue is cooling, in particular, ‘pre-cooling’. In fact, pre-cooling is absolutely essential to reduce the pain sensations significantly and ensure good clinical results.
Pre-cooling is when we apply cooling to the skin surface BEFORE the light energy is fired into the skin. The purpose of pre-cooling is to significantly reduce the temperature of the basal layer to prevent tissue damage around that location.
By applying a cooling effect at the skin surface for an appropriate time, the temperature of the epidermis and basal layer may be reduced so that the temperature rise, caused by the light energy absorption in the melanin, will not reach excessive levels.
If pre-cooling is applied correctly and sufficiently, then the patient should feel very little pain during the procedure, even when high fluences are applied, simply because the temperatures in the basal layer never gets too high.
Techniques
How do we apply ‘pre-cooling’?
Many IPL systems have cooled glass tips. If they are made from sapphire then they will cool very efficiently. Sapphire has a very high heat conductivity. If they are quartz they will not be so efficient.
The trick is to ensure that the glass tip is cold when applied to the skin – preferably near 0°C. The tip must be applied for an appropriate time to ensure proper pre-cooling. This may
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