Page 50 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
In hair follicles, the aim is to fully denature more than 63.2% of the target germ 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 germ 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.
Selective Photothermolysis
Chemical denaturation
laser wavelength
tissue absorption coefficients tissue thermal properties
absorber and target size geometry
activation energy (temperature) denaturation time
tissue’s intrinsic Arrhenius parameters
Table 7 - The parameters required for stage 1 (SP) and stage 2 (chemical denaturation).
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, thickness and depth are all important factors in determining the correct laser/IPL parameters.
Hair thickness is particularly important. Thin hair loses heat more rapidly compared with thicker hair. But, due to their lower mass, thin hairs will achieve higher temperatures than thicker hairs, for the same incident fluence. This makes the parameter selection process a little ‘trickier’.
But if those parameters are chosen properly, then all hair can be safely and effectively removed permanently, by completely destroying the germ cells.
Figure 18 - The amount of cell damage, represented by Omega, determines whether a tissue can regenerate. If more than 63.2% of the cells are irreversible denatured, then that tissue has very little chance of recovering
________________________________________________________________________ 50 Chapter 3 Laser/IPL Hair Removal
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