Page 79 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
It is a ‘balancing act’ – the more fluence applied at the skin surface, the more cooling must also be applied – before, during and after the treatment. See the sections on ‘Skin Cooling’ for a more detailed discussion on this very important topic.
Heat transfer in the Hair Follicle – A Computer Model
Many people keep asking “when is the best time to treat hair in different parts of the body?” The answer to this depends on the growth cycles across the body and so, the answer is not obvious.
One way to tackle this question is to build a computer model and test it with various inputs to see what it yields. We did this and found some very interesting results.
Figure 27 – Our 1-dimensional thermal transfer model of a hair follicle
We used MS Excel to construct this model (see Figure 27). Each row represents a section of skin containing a hair shaft (100 microns diameter) in a follicle (350 microns diameter) at a particular time. The top row is ‘time = 0’. So, time progresses in a downward direction (in 0.01 ms steps).
The aim of this model is to heat up the melanin inside the hair shaft and observe how the temperatures are generated and how they conduct from the hair towards the follicle walls and the adjacent dermis.
The green areas represent the normal background temperature of 37oC. A fluence, pulsewidth and wavelength are selected and the resulting temperature rise in each region is calculated, for each time step. In Figure 27 we can see the yellow cells which indicate a rise in temperatures
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