Page 175 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Using PA’s Monte Carlo results for fluence in the skin (Figure 62), which includes the back- scattered fluence, we generated the temperature profile for a hair down to 5mm deep, for a fluence of 10 J/cm2 at the skin surface. The model was set such that there was only one timestep – which is equivalent to an instantaneous temperature rise (no heat loss is allowed).
Figure 98 shows these temperature profiles, immediately after irradiation at 10 J/cm2.
Figure 98 - The temperature profiles in a hair shaft irradiated with four devices at 10 J/cm2
It is clear that the Alexandrite wavelength, with is high absorption coefficient in melanin (see Table 10) generates the highest temperatures through the hair shaft. The profiles generated by diode lasers and IPLs devices are very similar, whereas the profile from an Nd:YAG laser is the lowest, due to the relatively lower absorption coefficient at 1064nm.
Now, we should make it clear that these are the ‘instantaneous’ temperature profiles – they assume virtually no heat loss from the hair shaft. In the real world, there is a flow of heat from the melanosomes almost as soon as they begin to heat up. Unfortunately, we cannot model this in a one-dimensional model, as ours. So, the only useful information we can take from this model are the instantaneous profiles above. We cannot extrapolate their development over time.
What is particularly interesting are the temperatures at a depth of 4 to 5 mm – they are quite significant (apart from the Nd:YAG). If they were maintained at these temperatures for a suitable time, the stem cells would easily denature. Likewise, if a higher fluence is applied, then their likelihood of denaturing increases exponentially!
________________________________________________________________________ 175 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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