This post describes the 13 main features behind laser/IPL hair removal:
1. Light, usually red, is fired at dark hair (red light can penetrate far into the dermis);
2. The aim is to heat up the melanin in the hair shaft so that the germ cells can be ‘cooked’;
3. This only happens in the actively growing anagen phase;
4. Some of the light energy is absorbed by epidermal melanin (darker skins absorb more than lighter skins, and so, become ‘hotter’);
5. This generates temperature above 45C which triggers the thermal pain receptors, just below the dermal-epidermal junction;
6. Pre-cooling the skin lowers the temperature of the pain receptors, thereby reducing pain;
7. If sufficient heat is generated in the germ cells, over a sufficient length of time, they will cook irreversibly;
8. Post-cooling the area after the treatment will remove any excess heat energy;
9. Less than 10% of the heat energy is used to kill the germ cells;
10. The follicles cannot regenerate if all the germ cells are cooked;
11. Only around 50 to 60% of the anagen follicles will be killed in any one session;
12. The longer the gap between repeat sessions, the more anagen follicles will be presented for the next treatment – this means fewer sessions overall!
13. The thickness and density of the hairs are not important, as long as sufficient fluence is applied.
In a nutshell, light is converted into heat which ‘cooks’ the target germ cells – but only if the correct does of light energy is applied over the correct length of time.
The wavelength is not important. Melanin absorbs across the full visible spectrum, plus a chunk of the near infrared too.
It turns out that the pulsewidth is not so important either, as long as it’s between around 5 and 50ms. But even this restriction disappears as the fluence is increased.
The type of light doesn’t really matter either. It’s merely a method for transferring energy into the follicles. Laser – IPL? Not important!
There is no real evidence that the blood supply is important. Some people think that we need to coagulate the vessels supplying the follicle. The physics doesn’t agree. Red light is mostly reflected by blood, yet we always use red light to kill follicles. Plus, those vessels are relatively deep – we don’t typically use sufficient fluence to get enough energy at that depth, even if they did absorb strongly. But, in fact, melanin blood absorbs about 10 times as much as blood at the Nd:YAG wavelength (1064nm).
Hope this helps,
Mike.
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