Page 128 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
This is one of the main differences between nano- and picosecond lasers.
Reactions in Tattoos – Photo-Mechanical or Acoustic
Nano- and picosecond lasers deliver their energy to the ink particles so rapidly that there is very little time for the heat to dissipate. As a consequence, the peak temperatures attained can be very high, but only for a very short time.
Photo-mechanical or Photo-acoustic
If the laser energy is absorbed by a target chromophore is a very short time (typically billionths of a second), then the temperatures may reach more than one thousand degrees Celsius. This causes a ‘mechanical’ reaction in the targets which usually results in a physical disruption of some kind. This occurs in laser tattoo removal where the ink particles become extremely hot (for a very short time) and cause a physical rupturing of the fibroblast cells which contain them.
Such huge temperature rises can induce a number of reactions. Firstly, it can boil tissue water to generate steam bubbles (which we observe as ‘frosting’). Secondly, it can generate cavitation bubbles near the particle surfaces. Thirdly, the high surface temperatures can cause the particles to physically expand (thermoelastic expansion), which can lead to a shattering of the aggregates, into smaller aggregates or individual particles.
It is, however, unlikely that particles are ‘fragmented’ into smaller particles! The temperatures needed to achieve this transformation are not likely to be attained.
There is also a possibility that an acoustic shockwave is created, due to the very high temperature gradients across aggregates. Given that the speed of sound is much greater in ink than in water (more than 12 times faster in carbon ink!!), and the thermal conductivity in ink is greater than in water, most of the thermal energy is essentially ‘trapped’ inside the ink, until it shatters into multiple, smaller fragments, which lose heat much more rapidly.
This is the reason why these reactions are called ‘photoacoustic’ or ‘photomechanical’! There are physical processes occurring in the tattoo ink, resulting in fragmentation of the aggregates.
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Chapter 2 LEVEL A Laser Tattoo Removal
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