Page 176 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Note that these temperatures are all way too high - they don’t relate to the real world simply because there is no allowance for heat diffusion during a real pulsewidth. For example, if we look at the Alex maximum temperature in a hair, with a 10 J/cm2 over a 10 ms pulse, then it is only around 346C – much lower than the instantaneous temperature! And this high temperature doesn’t remain there for long.
To observe how this temperature profile changes with time would require a two-dimensional model – this will take up too much of our time! (If anyone fancies making one, please let us know...)
Increasing the fluence – what happens?
All the fluences in the previous section are based on achieving an Omega of 1 in the stem cells – this is the Arrhenius parameter which tells us if those cells are irreversibly denatured – dead! Technically, an Omega of 1 means that 63% of the volume of those cells are dead. Those fluences are the absolute minimum required to achieve this state.
What happens to the cells if we increase the fluence? The most obvious answer is that more of the cells will be irreversibly cooked – a greater volume that 63%. So, what it this relationship between fluence and the volume of dead cells?
Using our original thermal transfer model we calculated the new values of Omega for the above thresholds plus 1 J/cm2, and then plus 2 J/cm2. These are modest increases in fluence, but they reveal a very interesting outcome.
Device / fluence
Final Omega
% volume dead cells
IPL:
Threshold – 11.5 J/cm2
1.0
63.2
12.5 J/cm2
5.3
99.5
13.5 J/cm2
26.5
100
Diode:
Threshold – 11.15 J/cm2
1.0
63.2
12.15 J/cm2
5.4
99.5
13.15 J/cm2
27.9
100
Nd:YAG:
Threshold – 24.5 J/cm2
1.0
63.2
25.5 J/cm2
2.1
87.8
26.5 J/cm2
4.6
99.0
________________________________________________________________________ 176 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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