Page 121 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
As it does, the temperature of its locality increases, including the stem cells. And the weird thing is that the maximum temperature of the stem cells is virtually identical, for all three pulsewidths!! This is not intuitive, but it does make sense when considering the physics.
In each of the above cases, for each pulsewidth, the total amount of energy delivered to the hair was the same. Only the delivery time was altered (the pulsewidth). As the heat spreads out from the hair shaft, it becomes more ‘homogeneous’. The ‘spreading’ motion is the same, regardless of the pulsewidth. That makes perfect sense since the heat conduction process in the dermis does not depend on the pulsewidth. Therefore, the final temperature distributions will all tend towards the same profile, after some time – as we see here.
Then Mike ‘fired’ 30 J/cm2 at the same hair and generated the above distributions (only for a 1ms pulsewidth this time – Figure 71). The upper blue dotted line shows the temperature profile at the end of the 1 ms pulse – it is quite similar to the 10 J/cm2 pulse, except at higher temperatures, as we should expect.
Likewise, the temperature distribution settles into a roughly Gaussian shape after around 282ms. Only this time the maximum temperature at the follicle walls, where the stem cells are located, is now significantly higher than with the 10 J/cm2 pulses.
He repeated the above test using 10ms and 100ms and obtained the same results as before – the final distributions (at peak stem cell temperatures) were almost identical.
These calculations show that the pulsewidth is not important when considering laser/IPL hair removal. It is the total energy (fluence) which is the important factor. The temperature of the hairs is also trivial. The temperature-time history of the stem cells in the bulge and the bulb is what will determine whether a sufficient quantity of them die or not.
In Figure 71 we can see how the temperatures of the hair shaft and follicle wall change as we vary the pulsewidth – this is with a diode laser, but that doesn’t matter – it’s the same for all the technologies we use today.
The hair shaft wall temperature changes quite significantly from around 228C to 139C as the pulsewidth increases from 10 to 300ms (all at the same fluence of 10 J/cm2). This is to be expected. As the pulsewidth increases, there is more time for the heat energy to ‘leak’ out of the hair shaft, thereby preventing a higher temperatures in longer pulses.
But, the temperature of the follicle wall hardly changes at all – from 81.7 to 81C over the same pulsewidth range. And this is very important! The heat energy diffuses out towards the follicle wall during and after the pulse. In fact, most of the diffusion occurs after the end of the pulse. The resulting temperature in the follicle wall is, therefore, mainly determined by the total energy which reaches it. How long it takes the energy to reach it, is not so important.
________________________________________________________________________ 121 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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