Page 49 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
However, this process is not well understood by many practitioners, usually due to a lack of knowledge of the denaturation dynamics. Selective Photothermolysis only covers the heating and cooling of the target tissues, but does not discuss the denaturation phase. This is the chemical process which determines whether the outcome is successful, or not. Without a proper understanding of the denaturation of the target proteins, it will always be difficult to achieve good results.
A successful destruction of unwanted cells requires at least two stages – applying heat to raise the local temperature to the required levels, followed by a sustained denaturation of the tissue proteins. Selective Photothermolysis deals with the first of these aims, while completely ignoring the second. Many treatments fail to achieve their goal simply because the targets are not heated for a sufficiently long period.
Applying light energy to hair follicles results in a localised temperature rise. Proper selection of the wavelength, fluence and pulsewidth will generate the required temperature-time history in the targets. In general, red and infra-red wavelengths are ‘best’ since these leave blood vessels relatively untouched, while allowing for deeper penetration into the dermis. With proper skin cooling at the surface, safe and effective treatments are easily achievable. However, the desired process depends entirely on the duration of the elevated temperatures within the germ cells. This is where many treatments go wrong.
The purpose of laser hair removal is to irreversibly denature the follicular germ cells. Most treatments result in a partial denaturation of some, or all, of the germ cells. 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.
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 (see Figure 18). 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.
________________________________________________________________________ 49 Chapter 3 Laser/IPL Hair Removal
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