Photodestruction of tattoo dye

laser tattoo removal The procedure for removing tattoos, despite its apparent simplicity, is the most difficult to realise in laser cosmetology and requires a lot of experience from a doctor. The patient thinks they have a choice – to remove the tattoo quickly, but with scars or to undergo laser procedures and remove only the dye without harm to the skin. Unfortunately, it is not always that this result can be practically achieved.
The non-selective methods for tattoo removal, in which the dye is removed along with the skin that contains it, do not produce an aesthetic effect commensurate with the effect of selective destruction of the dye. Theoretically, with a selective photo destruction, the dye should disappear and the skin should not get damaged. The mechanism of influence is based on the fragmentation of dye particles by acoustic waves, which are generated within the dye particles when the short laser pulses are absorbed. In this case, the waves destroy only the dye, and they subside in the skin, without causing it any harm. But this is only in theory. It is always different in practice. In the vast majority of cases, especially when deep tattoos are being removed, skin gets scarred. That is, despite using a selective method of influence, the aesthetic result is often the same as for non-selective methods.

For the first time, the effect discolouration of tattoo dye by the irradiation of short laser impulses was discovered accidentally in the 70s of the 20th century. Then it seemed that the problem of non-traumatic removal of tattoos had been solved. Unfortunately, it was an illusion. Even in theory, the process of tattoo dye destruction is very complex and unique. There are a few difficulties. The first difficulty is the unknown chromophore that we are trying to destroy. Usually, by acting on a particular bio-object (hair, blood vessel, etc.), we know exactly the optical and thermal properties of the chromophores, which are used as “targets” for the laser beam. In the case of tattoos, the chromophore can be anything. The size of the dye particles can vary from hundreds of nanometres to hundreds of micrometres. The dye absorption spectrum is just as unpredictable. All we know about tattoos is colour. Based on the colour of tattoos a lot of specialists deduce how long the laser radiation wave should be to destroy it. It is very common to read and hear such reasoning, “blue tattoos are removed by this laser, and the green ones are removed by that one.” What are such statements based on? It’s just that someone was working on some particular tattoos, and it turned out that for that particular dye the absorption spectrum fell into the generation range of a specific laser. But, as practice has shown, this is an isolated occasion.

It is often found that two externally indistinguishable tattoos are removed by completely different lasers. And if one of them can be perfectly discolourated by the surface exposure to radiation of one wavelength, then the second one might not even react at all. Here is a practical example: we are removing a green tattoo. Use Alexandrite laser and nothing can be done. The dye does not get changed, even when the density of radiation is increased until the skin starts to get destroyed. We change the laser to Ruby and we get effective dicolouration at the minimum energy density. Then comes the next patient who seems to have tattoos of exactly the same colour, and everything happens the other way around – only Alexandrite is appropriate for fragmentation.

Unfortunately, the dye colour does not tell us anything about the range of its absorption, which is important for effective fragmentation. The colour of the tattoo is a mixture of scattered and reradiated light that has hit the tattoo dye. We know that the absorption range should be placed in a more shortwave part in relation to the radiation range. That is, if you follow the classical approach, it would be easier to destroy the dye of red colour, because it must be well absorbed in a wide range of colours from purple to orange, and the most complicated is the blue, because it could only be destroyed by violet or ultraviolet radiation, which is poorly penetrated into the skin. But in practice it turns out to be the opposite. It is the red dye that creates most of the problems, and the blue dye, in most cases, is easily destroyed. And the destruction of blue dye can be achieved by green, red or infra-red radiation, that is, effective absorption occurs for longer wave radiation than short-wave.

The reason is that we don’t see the whole radiation spectrum of the dye. The visible range is just a small part of it. In actual fact, any dye may have very efficient ranges of both absorption and radiation. Therefore, in order to determine the optimal wavelength of radiation that can effectively fragment the dye particles, it is necessary either to define the dye absorption spectrum or to test all available wavelengths. It is very difficult to determine the actual range of absorption of the tattoo dye inside the skin, and the equipment needed to do so will cost much more than any laser. Therefore, only one real way out of this problem is to test all available lasers to determine whether a dye can be influenced. Testing itself involves determining the length of the wave that will allow dye discolouration with minimum energy parameters of the pulse.

However, this is only the first problem. Let us assume we have been able to pick the necessary wavelength of laser radiation. That is, the dye can be effectively destroyed. Does this guarantee the absence of skin? Unfortunately not. To understand the cause, you must remember the description of the process of the photo destruction of tattoo dye: a painted area is subject to a Q-Sw pulse with the optimum wavelength of radiation, which is effectively absorbed by this chromophore. As a result of short pulse absorption within a dye particle, an acoustic wave is generated and, further, by the effect of the cavitation, the dye particle is shattered to the smallest components. There are macrophages that destroy and remove the fragmented particles.
Now let’s look at this process in more detail: if some chromophore absorbs the Q-Sw pulse (i.e. the pulse of nanosecond duration), then the laws of non-linear optics begin to operate, at a certain value of radiation density, and more precisely, a multiphotonic absorption mechanism is launched, which results in the ionization of the chromophore substance (in this state the substance stays for a nanosecond). The ionized substance has one feature. In this state, the absorption factor rises to 100%, so that all of the laser pulse energy that falls on the dye particles is absorbed by them. The ionized state of the substance is also interesting because of the fact that the molecules of the absorbent substance cannot maintain their stability. In the normal state, it is a set of neutral atoms and molecules that keep together. In the case of ionization, we get a collection of positively charged particles that begin to push apart. This is the beginning of the acoustic wave generation process. In fact, this is a micro-explosion and its power will depend directly on the power of the laser pulse. The spread of the longitudinal wave means that the molecules are beginning to approach each other and then push back. And if the molecules have strayed sufficiently far from each other, the intermolecular links get destroyed and the substance is fragmented. The distance the molecules should depart from each other to disrupt intermolecular links differs for various substances. And if the power of the laser pulse is properly picked, then the dye particles only can be destroyed, without fragmentation of the skin cells. If we start to increase the power of pulses, there is a significant increase in the risk of generating acoustic waves that can destroy not only dye but also the skin, meaning that the process will become non-selective.

Now let’s turn our attention to another nuance in the classical explanation of the fragmentation of tattoos dye particles. All particles of the chromophore absorb laser radiation and are therefore destroyed. How is that possible? The radiation falls on the surface of the skin and is absorbed by the dye particles that are located closer to the surface. And since this process implies the ionization of the absorbing substance, that is, the increase in its absorption capacity to 100%, what will the underlying dye particles absorb if all the radiation is absorbed in the superficial layer itself? In other words, it is not possible to fragment deeper dye particles by generating acoustic waves in them because these particles do not consume anything.
So why do tattoos fade then? The particles closest to the surface of the skin absorb all the laser radiation. The acoustic waves generated in them spread deep into the skin, causing the mechanical destruction of the underlying dye particles. It would all be fine, but the acoustic waves fade out very quickly in the skin, as in a viscous environment, and to be able to fragment dye particles in deep skin layers, their capacity on the surface must exceed the safe capacity for the skin. As a result, not only the dye but also part of the skin is mechanically destroyed

VIDEO: Typical procedure for removing a tattoo with a Q-Sw laser

This can only be avoided by reducing the power of laser pulses, thereby reducing the power of acoustic waves. Although It’s not going to fragment the dye all over the skin depth. Only the most superficial tattoo layers of will be destroyed, and many sessions will have to be held for full fragmentation.
There’s another way to get around this problem. If we’re going to get the tattooed surface exposed with more than one pulse and their sequence will be as a train. At the same time, the power of each pulse of a train should be sufficient to fragment the dye particles in the narrow layer, and the intervals between the pulses must be equal to or greater than the thermal relaxation (TRT) of the dermis cells.
In this case, the dynamics of the tattoo photo destruction process will be as follows.
The first pulse of the train will be absorbed by the topmost layer of dye. It will generate small acoustic waves that will lead to the mechanical destruction of those particles only. Then there is a period of time during which the heat from the chromophore is distributed to a large volume and the surrounding skin is getting cooler. Then the second pulse falls on the skin surface. It passes the first layer of the tattoo without absorption because the chromophore is already destroyed. As a result, the pulse is absorbed into the next layer, to which the first pulse could not be reached. The process is then repeated. Absorption takes place a well as acoustic wave generation and fragmentation of the absorbing dye particles. As a result, we have the dye destruction in the next layer. Then everything is repeated, and pulse after pulse the tattoo dye gets destructed layer by layer. The total duration of such an impact does not exceed a few milliseconds, so both the user and the patient perceive it as one pulse effect.

VIDEO: Procedure for removing tattoos by the LINLINE method

This method has a much higher degree of selectivity than classical ones, as evidenced by its lesser pain and a much lower risk of scarring.

The flaws in the method include the following:
The method does not allow anesthesia to be used in the form of cream or injections. This is due to the fact that the anesthetic can begin to absorb laser radiation and, consequently, the risk of skin injury will increase. On the other hand, even if the anesthetic is resistant to Q-Sw pulses, it will nevertheless change the physical properties of the skin, which will primarily affect the TRT and may lead to thermal damage. In fact, with any selective method, the use of local anesthesia is unacceptable.

There is another disadvantage of the method -large intervals between procedures. Fragmented dye is partially rebuilt after impact and the procedure needs to be repeated. Due to the inevitable being of the capillaries located in the immediate vicinity of the dye particles, a haematoma is formed in the processed area. If we use classical methods, the haematoma has an irregular density that decreases with depth. But in our case, the sources of acoustic waves do not lie on the surface of the skin, they are distributed throughout. Therefore, an “non-specific” haematoma is equally dense in the whole volume of tattooed area.
This “non-specific” haematoma may cause skin scarring if the doctor interpreted it incorrectly during a follow-up session and accepted it as the restored dye. To avoid this you must check: Whether it is dye or a haematoma. To do this, you can select the same settings as in the first session and make a test pulse. If the colouring is gone – it’s alright, if it’s not – it’s a haematoma and the second session should be postponed.

This method may not seem very convenient for patients and doctors because it does not allow anesthesia, and it requires multiple sessions with quite large intervals. The only, but unique, advantage is that we can remove a tattoo completely, regardless of its depth.
This method is implemented in the Multiline platform with the use of Nd:YAP/Q-Sw/KTP, RUBY, ALEX, Nd:YAP/Q-Sw. Any of these lasers can be used to fragment the tattoo dye, but it is better to have all these emitters in order to have more choices of wavelengths

   Tattoo removal results Before-After


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