Page 118 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Figure 69 - As the cell temperatures increases, their rate of denaturation (damage) increases – exponentially!
For example, when considering bulk dermis, at a temperature of 75C, it will take those cells around 6.75 milliseconds to fully denature. But at 85C, it only takes 0.29 ms. So, a 10C rise in temperature, reduces the cooking time by a factor of just over 23 (or, the denaturation rate is 23 times faster!).
Now, this is very important because it means that applying a higher fluence to targets like hair or blood vessels, can induce the desired reaction in much shorter timescales. And this is why the pulsewidth becomes less and less important as the fluence increases. With higher cell temperatures, the rate of cooking is so fast, that the pulsewidth becomes irrelevant – particularly at higher temperatures (higher fluences).
This is why we always advocate higher fluences (with corresponding skin cooling) for the treatment of hair and vessels. Then we don’t need to worry about the pulsewidth at all.
Conclusion
The exponential increase in damage to cells is directly due to the temperature increase, which is linear with an increasing fluence. This indicates that we must always be very careful when increasing the fluence in skin, and must compensate for the higher epidermal temperatures with more surface cooling.
Just how important is pulsewidth?
Again, this topic is covered in detail in Chapter 1 – here is a short review.
________________________________________________________________________ 118 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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