Page 66 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
result in a partial denaturation of some, or all, of the stem cells, which are mostly found in the bulge and the bulb. If the follicle is well developed (late A3 to A6), then the bulb may be too deep for the fluence to have any significant effect. This leads to regeneration of the partially damaged follicles, usually resulting in finer, thinner hair growing back. These are more difficult to remove, due to the physics.
It is generally understood that ‘denaturation’ of cells starts at around 60C. This is incorrect. In fact, human collagen will denature at room temperature but at an exceedingly slow rate. The rate of denaturation is exponentially dependent on the local temperature. A small increase in temperature results in a large increase in denaturation rate. For example, a unit volume of human collagen will denature completely in around 6.6 seconds at 55C, but it will achieve exactly the same result in 0.2 seconds at 65C. The reason why “60C” is often quoted as the ‘threshold’ temperature of denaturation is simply because the denaturation process takes around 1 second to occur – this is easily visible to the human eye. It is merely within our observable timescale!
This is true for all cells – higher temperatures require much less time to achieve denaturation. To attain ‘irreversible denaturation’ a quantity, known as ‘Omega’, must equal or exceed 63.2% of the target cells, by volume, of fully denatured proteins (seeFigure 29). Anything less than this can allow the cells to regenerate.
To ensure irreversible denaturation of the target tissues at least two stages must occur:
Stage 1 - a thermodynamic process - local temperature rise due to the absorbed light energy by the tissue chromophore(s) followed by (when the ‘correct’ conditions have been achieved)
Stage 2 – a chemical denaturation process – this is determined by the tissue temperature and denaturation time which results in a loss of protein integrity.
In hair follicles, the aim is to fully denature more than 63.2% of the target stem cells. This can be achieved by targeting the melanin in the hair shaft and allowing sufficient time and heat energy to flow to the surrounding stem cells.
The critical issue here is that the local temperature MUST be maintained for a particular time, to achieve irreversible denaturation. If that local temperature is 90C then a local heating time of around 13 ms will induce irreversible denaturation. A higher temperature will require less time to achieve the same result.
Hence, the laser/IPL parameters must be chosen carefully so that the correct temporal-thermal conditions may exist in the target cells, to ensure irreversible denaturation.
This, therefore, requires a careful examination of the hair – its colour and depth are the two important factors in determining the correct laser/IPL parameters.
________________________________________________________________________ 66 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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