I’ve just been confirmed for two talks at this year’s British Medical Laser Association meeting in London in May. I’ll be presenting OP18 and OP20 on Friday May 10th, 2019 at 8.15am and 8.45am respectively, in the Endeavour Room.
1st Presentation – OP18
The first presentation is entitled “Laser Tattoo Removal – a deeper analysis of the various processes.”
In this talk I will be presenting my recent work on the range of physical processes which occur during the laser heating processes which can occur including micro-cavitation, explosive boiling, the carbon steam reaction, fragmentation, the photo-acoustic reaction and photospallation.
All of these processes are temperature dependent (which, in turn, depends on the incident intensity and wavelength). As the absorbing ink particles rapidly rise in temperature, different physical processes occur with increasing ‘ferocity’.
At the ‘spinodal’ temperature of water – 277°C – the water boils rapidly resulting in the formation of steam bubbles around the ink particles. As these bubbles collapse they release energy back into the ink particles resulting in micro-cavitation. At around 299°C the boiling of water becomes explosive which generates rapidly expanding steam vacuoles.
Ink particles will begin to physically fragment at these temperature with some particles leaving the skin at very high speeds (photospallation) – some exceeding the speed of sound!
At 705°C the carbon-steam reaction begins; where carbon molecules, in a water medium, undergo a chemical reaction resulting in the production of hydrogen and carbon monoxide gases. This is particularly noticeable with black pigment since they are usually composed of carbon.
Photoacoustic effects occur when the absorbed energy induces temperatures over one thousand degrees Celsius on the surface of the ink particles. If the pulsewidth is on the order of the stress confinement time (related to the speed of sound in the ink particles) then acoustic waves are formed which propagate within the ink particles. These waves produce resonating forces within the ink which can exceed the molecular bond strength of the particles, leading to fragmentation.
Finally, at around 3685°C carbon particles will undergo sublimation – the phase change from solid directly into a gas (it does not pass through a liquid phase under this process). At this stage the solid carbon particles are no longer present.
Plasmas are very unlikely to form under laser tattoo removal conditions. There is simply no where near sufficient energy applied to induce either multi-photon optical breakdown or thermally initiated plasma formation. For example, a 350 picosecond pulsewidth would require a fluence of around 35 J/cm2 applied at the ink surfaces to induce multi-photon optical breakdown. Whereas a 10 nanosecond pulse requires nearer 1000 J/cm2 to do the same!
2nd Presentation – OP20
The second presentation is “Longer pulsewidths are clinically better when treating hair/blood vessels with laser/IPL systems.”
It is generally accepted that shorter pulses are ‘better’ for smaller, thinner targets, while longer pulses are more suited for larger targets. This is incorrect. The aim of photothermal treatments is to denature a sufficient volume of target proteins such that the tissue is irreversibly destroyed.
The amount of cell denaturation is a complex function of temperature and time, using the Arrhenius rate equation (above). The final denaturation state depends exponentially on temperature, while it is a linear dependency on time.
Achieving the desired goal requires that the target cells attain a suitable temperature for a sufficient period of time. Thin objects lose heat rapidly through conduction. Hence such targets need to have a high temperature maintained to ensure sufficient denaturation. If they cool down too quickly then they will not achieve the required level of denaturation.
This can be ensured by applying a longer energy pulse, thereby maintaining the required temperature for the required time.
Similarly, a larger target must also achieve the necessary denaturation level. However, larger targets have greater mass than thin targets. This results in a lower temperature rise for the same incident energy/fluence. In addition, due to the thermodynamics, larger targets lose heat slower than thinner targets. Consequently, larger targets are cooler than thin targets, but they maintain their elevated temperature for a longer period.
However, to ensure irreversible denaturation, larger targets will need longer energy pulses so that the lower temperature will have sufficient time to generate the required amount of cell denaturation.
In summary, to achieve the desired clinical result in hair or blood vessels, the cells/proteins need to be heated for a required length of time corresponding to the local temperature. For small or thin targets, which cool quickly, this can be achieved by extending the heating time. For larger targets, which cool more slowly but achieve a lower peak temperature, a longer heating time also result in the required denaturation.
Clinically, this means that a higher degree of success should occur with longer pulses compared with short pulses. Of particular importance is the understanding that increasing the applied fluence will rapidly increase the amount of tissue damage due to the exponential increase in protein denaturation with temperature.
In reality, this means that it is always better to extend the pulsewidth rather than increase the fluence, if you want to use more energy in any particular situation! This is a more controllable method of increasing the tissue damage.
Please note: pulse duration is the same as pulsewidth is the same as the pulse length!!
If you have any queries about this stuff please leave me a message.
Ciao,
Mike.
Mike Murphy's Blog 