I often read about human tissue ‘denaturing’ or ‘coagulating’ at some temperature – usually 60 or 70°C. However, these claims are inaccurate. They rarely state the time over which the denaturation process occurs.
Denaturation is when the proteins in cells or tissues ‘unravel’. For example, collagen is made up of three amino acid chains, intertwined and linked by hydrogen bonds, to form a strong molecule. When sufficient heat energy is applied for a sufficient time, the hydrogen bonds break down which causes the amino acid chains to ‘slip’. With enough energy and time, these acid chains will disassociate completely, leaving the collagen molecules non-viable.
Once the tissue denatures past a certain point, it cannot recover – it is dead! Before this point, there is a possibility the acid chains may reform correctly (this is a function of the heat shock proteins, whose job is to fix heat-damaged tissue, when possible). As more heat is applied, the chances of passing this point of no return, increases. If a certain heat is applied for a long enough time, the same will occur.
So, the probability of any tissue denaturing to a point where it becomes non-viable depends on the combination of temperature and time.
This can be calculated, in a first approximation, using the Arrhenius Damage Integral. Svante Arrhenius (1859-1927) was a Swedish physicist/chemist who won the Nobel Prize for Chemistry in 1903. He is credited with first working out how species of chemicals react at various temperatures, and their corresponding time-dependence.
Using his famous equation, we can calculate when a tissue will reach 100% denaturation for any given combination of temperature and time. The table below shows some of these calculations.
|70||35||258 sec||54.7 sec|
|60||1.1 sec||8.6 hr||1.4 hr|
Time required to reach 100% denaturation points depending on temperature
(all times are in milliseconds unless otherwise stated)
The data in the table clearly shows how the temperature of a tissue strongly determines the time required to reach 100% cellular denaturation. Clearly, ‘skin’ reacts to heat mush more quickly than collagen, which itself reacts more quickly than blood.
For example, ‘skin’ will become fully denatured (this means it is irreversible – the tissue is ‘dead’) after about 1.1 seconds if it is maintained at a temperature of 60°C. But, the same skin will fully denature after only 1.4 ms if the temperature is 80°C, and only after 0.06 milliseconds at 90°C.
Clearly, there is a huge difference in how long it takes the various tissues to reach 100% denaturation, which is highly dependent on the temperature. This is because the time required to denature the proteins is exponentially dependent on the local temperature. So, even a small rise in temperature can have a significant effect on the time required to induce denaturation.
This can be applied to laser/IPL treatments of skin and its components. The temperature rise in the skin depends on the applied energy at the skin surface and the amount of energy that is ultimately absorbed by the target.
Knowing the above, it is now clear that an increase in the skin surface energy density (fluence) can have a hugely significant effect on the timing of the denaturation process inside the skin.
This is why we must always be very careful when increasing the fluence applied, for all treatments. The time required to induce the desired reaction is very strongly linked to the applied fluence (and subsequent temperature rise).
Human beings are quite simple creatures. We live in a linear world. We don’t cope well with non-linear things, like exponential growth etc. (As an example, I keep hearing on TV that the spread of Covid infection has become ‘exponential’. It has never been exponential, anywhere on the planet!! This is merely ill-informed journalists and politicians who don’t understand what an exponential process really is!).
So, trying to ‘judge’ the above temperature-time combination, using only human intuition, is very tricky. Our brains are just not wired up to do that very well!!
Just always keep in mind that a small increase in applied fluence, may result in a LARGE jump in temperature rise in tissues – with consequently, much more damage!
Hope this helps,