I read an interesting paper on this topic, last week. It was a clinical study looking into the application of ‘super hair removal’ using a low fluence output from a diode laser.
As you may already know, I’m not a big fan of this technique. I feel that it is ‘tricky’ to administer properly and makes it too easy to burn the skin.
The study used an 810nm diode laser with a fluence range between 5 to 10 J/cm2, in a pulsewidth range between 10 and 20ms, with a repetition rate of 10 Hz. They divided the treatment area into 10×10 cm squares and applied a ‘lateral movement of the handpiece in a constant sweeping mode’ at a speed of approximately 10 cm/second.
They delivered a maximum of 9.6 kiloJoules into each square area, resulting in an average of 96 J/cm2. Of course, this was over an extended period of time, so cannot be directly compared with a ‘stamping’ technique fluence, which typically uses between 20 and 40 J/cm2 per pulse. They didn’t say how many passes were needed to achieve this target.
Their diode laser had a water-chilled window which cooled the skin. In addition, they used a Zimmer air cooling device too.
They found that many patients reported ‘low’ pain using this technique, with a relatively small number saying it was high or very high. The vast majority reported a ‘good’ result after six months after six sessions. A small number reported ‘burns’ but these were resolved after six months.
It appears that this technique was very successful, but they appear to have been quite precise about its application.
This differs from the conventional ‘stamping’ technique which fires just one shot each area of the skin, with a greater fluence.
And this is very interesting…
There is clearly a significant difference between these two techniques. The SHR technique builds up a substantial amount of heat energy in the full treatment area, while the stamping technique delivers just one, large pulse of energy – sufficient to generate enough heat to cook the germ cells in the follicles.
The SHR technique cannot do that – the fluence is too low. So, how does it work? Well, the researchers suggest that the surrounding dermis is slowly heating up due to the high number of passes. As a result, the follicles, which do get much hotter than the dermis, cannot cool so quickly. They postulate that this means that the follicles stay hotter for longer, resulting in cooking of the germ cells.
I thought I’d test this idea in my computer model – which I had created to study the effects of heat generation in the skin and hair follicles.
To my surprise, they were correct!!! Even raising the background dermal temperature by a couple of degrees makes a significant difference to the outcome.
And then I realised why – the cooking (denaturation) process is exponentially dependent on the local temperature. This means that even a small increase in temperature will induce much more denaturation.
So, if the surrounding dermis is just a few degrees warmer than normal (37ºC) then the follicles will cook much more easily. However, the thermal pain nerves kick in at 45ºC – meaning that the dermis should be kept below that temperature. This leaves only a small 8ºC ‘window’ to work in, to prevent great pain.
This is why the cooling is so important – without proper cooling, the skin will become too hot and the treatment very uncomfortable.
So, it appears that SHR can be used successfully to remove hair, but it must be applied correctly, with sufficient cooling.
By ‘correctly’ I mean that the ‘in motion’ (or sliding/sweeping) technique must be tied to the repetition rate (Hz). This will minimise overlapping or “under-lapping”, where areas are missed, or not sufficiently heated.
I think of these techniques as follows:
The ‘stamping’ technique is a bit like when a steam press is used to iron hotel sheets – one, quick blast of steam is usually enough;
whereas the ‘SHR’ technique is more like a standard ironing of a sheet or clothes – repeated passes of an iron over the same area.
They both achieve the same goal, but in slightly different ways.
Hope this helps,
Ciao for now,
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