Page 133 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Cellular denaturation time (milliseconds)
Hair shaft diameter
(microns)
TRT
(milliseconds)
70C
80C
8
0.029
100
4.496
33
1.3
Table 24 – The TRT varies greatly according to the physical size of the hair shaft. But the cellular denaturation time depends only on the cells’ intrinsic chemical properties.
Interestingly, the TRT does not depend on the local temperature – it is the time taken to drop by a certain amount (usually 50%). The actual values of those temperatures don’t make any difference to the TRT!
However, the denaturation times do depend greatly on the local temperatures. Plus, the actual time needed to ‘cook’ the cells varies significantly on the local temperature. As a consequence, the hotter cells (at 80C) only require less than one millisecond to fully denature while the cooler cells (at 70C) requires around 33 ms – a massive difference.
As we can see, the cooking times required to kill the cells depends strongly on their temperatures, which depends on the fluence applied. These times do not depend on the size of the targets, as with the TRTs.
Conclusion
The ‘cooking’ times and the ‘cooling’ times (TRT) of cells are independent of each other. There is no direct relation between them. Using the cooling times of targets to determine the most appropriate cooking times is simply wrong.
Further, the cooking times are hugely dependent on the local temperatures. So, using very short, high fluence pulses can achieve exactly the same end result as a longer, lower fluence pulse. It is the combination of fluence and pulse duration that is important – however, these two scenarios may have a significant difference in terms of unwanted, collateral damage.
So, we should not use the TRTs of tissue targets to determine the appropriate pulse duration.
After all, nobody ever chose the baking time for a cake based on how quickly it cools (its relaxation time)!!
Summary
This topic is quite complex, as you can see. In essence, the success rate of any treatments depends on the required fluence hitting the desired target, at various depths in the skin.
________________________________________________________________________ 133 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
© The Laser-IPL Guys, 2025