Page 109 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
If the target follicles are deeper, such as with arm pits and bikini lines, then more fluence is required to ensure sufficient heating of the stem cells. But this then means that more cooling must also be applied to counter the effects of the excess heat energy.
Quite simply:
Higher fluence = more cooling!
Our Computer Models – What are the ‘optimum’ fluences?
We created two computer models to look at the effects of light energy in the skin. We wanted to figure out the optimum fluences and pulsewidths required for hair follicle destruction, for different devices.
PA’s optical model is a Monte Carlo numerical simulation which ‘tracks’ individual photons as they propagate through the skin’s layers. Their trajectories depend on the absorption and scattering coefficients, plus the anisotropy (as discussed in Chapter 1). The photons are followed until they are finally absorbed or scattered out of the skin. This is repeated many millions of times to simulate a pulse of light energy, where the wavelengths can be changed, as required.
Mike’s thermal model then took this data and used a 1-dimensional solution to the heat conduction equation. Hence, the model studies the temperature profile along a radius of hairs of various sizes, following absorption of light energy from a diode, alexandrite or Nd:YAG laser, or an IPL. He can vary the fluence and pulsewidths of these systems.
The thermal model was designed so that the depth was not considered. We did this by assuming a certain incident fluence on the surface of the hair, regardless of the depth. That information can be added later, knowing the fluence distribution within the dermis, taken from PA’s Monte Carlo model.
The initial temperature profile is determined by the absorption coefficient of melanin for the corresponding wavelength, plus the amount of energy absorbed and the pulsewidth. Using timesteps of only 0.2 milliseconds, we can observe how the temperatures change over time and how they heat up the surrounding tissues for any desired time period – in particular, the stem cells.
Using the Arrhenius Equation (discussed in Appendix 7), it is possible to calculate how much cellular damage occurs for each situation. By doing this we can find out how much energy is required to ensure complete destruction of the follicle, along that radius.
________________________________________________________________________ 109 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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