Page 80 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
there. Note that the follicle walls heat up first, then the heat conducts into the centre of the hair. This is due to the hair shaft surface melanin absorbing preferentially.
After some time, the hair becomes very hot (see Figure 28). The red cells show the highest temperatures – all located within the hair shaft. Only the melanin inside the hair can absorb the incoming light energy – that’s why the hair shaft is much hotter than the surrounding regions. But the yellow cells next to the hair shows that heat is beginning to spread from the shaft.
One of the aims of this model was to determine how much heat is ‘lost’ to conduction during the pulses. This determines how hot the hair itself becomes – longer pulses will allow for more conduction while the light energy is still arriving, leading to a lower peak (maximum) temperature at the end of the pulse.
Figure 28 – The hair shaft melanin absorbs light energy and become hot
But, we found that the most important time is not the pulsewidth. Rather, it is the time which the germ cells, on the follicle wall, are denaturing (cooking). To determine this, we needed to look closely at the temperature-time history of the germ cells during the whole process.
We found that it can take anywhere between 90 and 150 milliseconds for the germ cells to reach their peak temperature, due to the time it takes for the heat energy to transit across the follicle to the wall (see Figure 29).
We used a well-known method to determine the amount of cellular damage in germ cells. There is an equation known as the Arrhenius Damage Integral which allows us to calculate the volume of denatured cells. With this equation we were able to find if the applied fluences, wavelengths and pulsewidths would ‘cook’ the target cells irreversibly – in other words, they were dead!
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