A key benefit of all modern diode lasers is the inclusion of sapphire tip contact cooling.  Typically, these are cooled to 3 – 5C and effectively cool the epidermis and basal layer i.e. the top 0.1mm of the skin.  The melanocytes and melanin granules are very small targets, in effect floating in a transparent water bath, (Skin typically 60- 65% water) which acts as a heat sink for the incident energy passing through the epidermis.  Below the basal layer the difference in skin types disappears and the relative absorption of the laser energy by the hair/hair follicle relative to the surrounding tissue is the most important factor.

 

With older diode lasers the limitation is the amount of energy that can be delivered within the target’s Thermal Relaxation Time (TRT). 

For Example:

Assume medium thickness hair with a TRT = 40ms

Low peak power density laser

Typically, these lasers could only deliver around 8J/cm² in 40ms which is insufficient.  Therefore, in order to deliver sufficient energy into the target is takes longer than the TRT and at the TRT, by definition, 50% of the energy has leaked out of the target into the surrounding tissue.  This leakage needs to be replaced and typically these earlier style lasers will require something like 20-25J/cm² and this will take in the region of 100ms.  The result is that more than 3/4 of the energy delivered is leaked into the surrounding tissue – heating it. 

This heating effect both:

1. a) Adds to the incident laser heating.
2. b) It is delayed heating, which can occur after the cooling has been removed.  i.e. If there is significant hair density – hairs/cm²then there is also a significant bulk heating effect of the skin, which while the cooling is in place is held back from the skin’s heat sensors but once this cooling is removed as the head moves on this heat can rise to the surface and potentially cause pain.

Modern contact tip cooling used correctly is so much more efficient than cooled air cooling. Note: Imagine standing in a bathing costume outside in 5C with a stiff breeze blowing – not nice.  Then contemplate what it would feel like jumping into a swimming pool at the same 5C temperature.  For most people it would be breath takingly cold.

H0wever, with so much of the energy being leaked into the surrounding tissue these lasers run out of steam and start to cause discomfort as the hair shafts become lighter and finer. 

High Peak Power Density Lasers

Recent developments in diode technology are enabling far more powerful diodes.  As a result, all of the necessary energy can be delivered well within the TRT and so much less energy/heat is lost to the surrounding tissue. 

A typical comparison is shown below:

Assumption is hair with TRT of approximately 40ms	Low Peak Power Density Laser	High Peak Power Density Laser
Total Required fluence (energy)	22 J/cm2	13 J/cm2
Pulse width	100ms	15ms
Approximate Energy/ heat lost into surrounding skin	1st TRT = 50% 2ndTRT = 25% ½ 3rd TRT =6% Total 81%	3/8 1st TRT = 19%     Total 19%
Heating lost to skin tissue	18 J/cm2	2.5 J/cm2
Percentage reduction in the heating of the surrounding tissue by using the High Peak Power Density Laser		86%

This massive reduction in heat leakage into the surrounding skin makes the treatment far more comfortable and greatly reduces the risk of damage to the surrounding tissue.

When treating deeper rooted hairs, the pulse duration can be extended but remain well within the TRT of the hair.

The reduced loss of heat into the surrounding tissue both increases the safety margin and enables the peak power and fluence to be increased to effectively treat finer and lighter hair.

Therefore, the new technology of high peak power density lasers for hair removal is far more effective and provides better patient comfort and an increase in safety margin.

Find out more about the Cambridge Stratum Quantum 1 and Quantum 2 high peak power density lasers. Contact us for more information or to arrange a demonstration using our online form.