Page 65 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd 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 – the “thermal” bit, but it does not discuss the denaturation phase – the “chemical” bit. This is the 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 (“cooking”) 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, at the “correct” temperature. (Cells will denature at much lower temperatures – as low as 55C – but they would take minutes to fully denature at that temperature).
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, to the base of the follicle, where many stem cells reside. 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 stem cells. This is where many treatments go wrong.
Figure 28 - These eggs have all been boiled at the same temperature, but for different times. In other words, the fluence was the same while the pulsewidths were different. We can see the difference in ‘denaturation’ from left to right. As the pulsewidth increased, the amount of denaturation also increased.
The critical part of laser hair removal is the irreversible denaturation the follicular stem cells. Ross et.al. suggest that the bulge and the bulb, along with the entire follicle epithelium need to be targeted, and denatured sufficiently, to ensure complete follicle death. Most treatments
________________________________________________________________________ 65 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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