Mike Murphy's Blog

I occasionally see comments such as ‘the target needs to be heated to 70°C to be properly killed’.

This is only half the story. As with all cooking, a target may need to be heated to 70°C to be properly cooked, but it must be maintained at that temperature for a certain amount of time to achieve thorough cooking. When we ‘cook’ tissue proteins, we are actually ‘denaturing’ those proteins – this is when the amino acid chains unravel. Once unravelled, they are effectively dead (they cannot ‘ravel’ back…)

The fact is, a temperature on its own is quite meaningless. It must be linked to a time to make sense of the situation. This may be represented at (T,t) where ‘T’ is the temperature and ‘t’ is the time. As ‘T’ increases, ‘t’ decreases – and vice versa. This combination of temperature and time also depends on the target’s intrinsic properties.

The relationship between ‘T’ and ‘t’ can be found in the Arrhenius Equation (see below). We published a paper on this in the Lasers in Medical Science journal back in 2013 (you can find it here).

The Arrhenius Equation

In collagen proteins (in the skin), a 70°C temperature must be maintained for at least 35 milliseconds (ms) to thermally destroy the proteins to a point where they cannot regenerate.

However, if the temperature is raised to 75°C, then the time required drops to only 6.75 ms. This is simply because the proteins break down at a faster rate at that higher temperature.

If we raise the temperature to 80°C then the minimum time required is only 1.4 ms. The Arrhenius Equation tells us that the rate of protein denaturation is exponentially dependent on the temperature, while only linearly dependent on time. (Obviously!!)

This is the case for all proteins and other cells. The higher the temperature, the shorter the time required to cook them properly.

The temperatures achieved in those proteins depends on the light energy absorbed by the proteins. A higher fluence will result in a higher temperature. Likewise, shorter pulsewidths will generate higher temperatures than longer pulsewidths (since less heat energy can conduct from the target during the pulse).

The properties of different proteins can vary quite widely. For example, if a blood vessel is heated to 90°C, then the collagen proteins will cook (denature) in around 0.1 ms, much faster than the blood, which requires around 39 ms – simply because of the different thermal reactions in those proteins. This, obviously, can have a major impact on any light-based treatment.

So, if you see a comment like ‘a temperature of 70°C must be achieved…’ then be aware that this is only true if the temperature is maintained at that level for the corresponding time.

Just to confuse the issue a little, the time needed to cook the proteins is usually not equal to the pulsewidth of the light energy! Most of the ‘cooking’ process actually occurs after the pulse has ended…

Conclusion

To effectively kill the target proteins (in hair or blood or collagen etc) we must apply the right combination of fluence and pulsewidth. Otherwise, the right conditions to kill the targets may not occur.

This applies to all photothermal treatments – hair, blood vessels, retina, pigmentation etc.

Hope this helps,

Mike.

PS We are holding our next MasterClass just outside London on November 3^rd and 4^th. We also plan to hold another one in New York in March of next year. Contact us on DermaLaseMasterClass@gmail.com for further information on either of those events.