The mechanism of laser hair removal

Laser hair removal is a very simple procedure offered in pretty much all beauty salons. The advertisement brochures promise an excellent result and a comfortable procedure ensured by anesthesia.
True, there are some restrictions such as the level of natural pigmentation, ban on tanning (visits to solariums) before and after the procedure. It seems like there is nothing left to desire. However, the result often does not appear very impressive, hairs re-grows, and the consequences of the simple, painless procedure turn into a serious problem. Burns, changes in pigmentation, and occasional scar tissue forming in the skin – these are all side effects which can be felt by any patient who has opted for laser hair removal.

Despite its seeming simplicity, the mechanism of laser hair removal is very complicated, and it is very challenging to actually destroy hairs (i.e. to ensure the complete and total removal of unwanted hairs) without inflicting trauma on the skin. In order to destroy a hair in its entirety and not simply traumatise it, the hair follicle, or more specifically, the hair papilla, need to be heated up to the temperature of coagulation. To do that, it is necessary to choose some kind of a light-absorbing substance (a chromophore) which will become a “target” for the laser beam. The chromophore will absorb the laser pulse, transform the light energy into heat energy, and transmit the heat to the hair follicle. The chosen chromophore should be primarily located in the specific entity we want to destroy, and the absorption of the radiation with the desired wavelength by the chromophore should be more effective than that of other chromophores located in the zone of laser treatment. Otherwise, the radiation will be absorbed not only by the selected entities but also by other bio-entities, will be transformed into heat, and will overheat and destroy them. Thus, the treatment will not be selective, and, consequently, will be traumatic.

For laser hair removal, the role of chromophore is best fulfilled by melanin – a chromophore responsible for the pigmentation of the skin and hair. In connection with this, there is a dilemma related to hair removal from strongly pigmented skin as it is very difficult to avoid significantly overheating it. In regard to this, the classic hair removal methods impose certain restrictions on tanning and the natural pigmentation level.

There is still no definitive answer on what wavelength is best to use for hair removal. Melanin’s absorption spectrum is practically linear; the absorption effectiveness decreases smoothly along with the increase of the wavelength. It would seem that it is best to use those wavelengths which are absorbed by the melanin more effectively, i.e. the short waves. But this is not so. The thing is, radiation with wavelength of up to 600 nm is absorbed well by the haemoglobin contained in all blood vessels. As a result of this, the radiation will be absorbed primarily by them, causing the skin to overheat and burn without any effect whatsoever on the deep hair levels. Therefore, wavelengths of 600 to 1,200 nm are used in hair removal.

Laser hair removal was first performed with the help of a ruby laser (694 nm); later on, neodymium lasers (1064 nm), alexandrite lasers (755 nm), diode lasers (600 to 1,200 nm), and even broadband pulsed light sources – lamps (photo epilators). The unprepared user will find it very hard to sort through the diverse advertising information provided by the laser equipment manufacturers and select a grain of truth from it.

What is the classical explanation of laser and photo epilation mechanism? There are two explanations: the first one – radiation goes down the hair as it would a light guide and, when reaching the hair follicle, it causes it to overheat and coagulate. The second one – radiation is absorbed by the melanin-containing hair parts (the cortex), gets transformed into heat, and this heat spreads down the hair, reaches the hair follicle, and causes either its complete coagulation or the coagulation of the blood vessels feeding the hair.

The first explanation of laser hair removal is inapplicable in principle. It’s known that melanin is chosen as a chromophore specifically because it absorbs radiation well; therefore, the laser beam has no way of “going down the hair to the hair follicle” because radiation is incapable of penetrating the absorbing medium freely! If this were really possible, why wouldn’t we save money by not putting windows in our homes? Seeing as radiation is capable of penetrating absorbing media, our homes should be illuminated by the light coming through the walls. In that case, we can save money by not putting in windows. Our homes should be illuminated without them. The whole nightmare is not the fact that someone is providing an incorrect explanation. It is terrible that such explanations are given by those who confidently recommend their devices for treating people. They have the audacity to offer a procedure whose consequences are unpredictable.

The second explanation is way more logical. It shows understanding of the process of interaction between radiation and biological tissue but, in my opinion, it is not entirely accurate. Let’s look once again at the mechanism of thermal damage of the hair follicle. As we know, radiation is absorbed by the melanin which is basically located quite a distance away from the hair follicle. There is no melanin in that part of the hair follicle where the hair papilla is, and the hair papilla is what we need to coagulate. Melanin is produced by the melanocytes located in the upper part of the hair follicle. That is why the hair above the hair follicle has a defined color.

It is also necessary to bear in mind that hair has a low thermal conductivity which determines its high TRT (thermal relaxation time) while the surrounding tissues, conversely, possess good thermal conductivity and, consequently, have a low TRT. If the hairs were shaped like balls, these nuances would be of no consequence; however, they are elongated objects with a very small cross section and a large area which is in contact with the surrounding tissues. So what will happen when the melanin granules located in the hairs absorb the radiation? With its low TRT (around 1 microsecond), melanin quickly transforms the light energy into heat energy and transmits it to the hair structures. It is also necessary to note that melanin is located in the middle section of the hairs (in the cortex), and the distance to the hair sheath and, consequently, to the epidermis is just a few layers of cells (the hair cuticle) resembling thin scales.

Melanin has two ways of transmitting heat – either deep into the hairs through a narrow channel with poor thermal conductivity or in all directions in the tissues which can easily collect the excess heat. In order to understand the path which the heat will take, we can picture a nail sticking out of a piece of ice. If we start heating up the nail head, would we manage to achieve heating the nail without first melting the ice and then evaporating the resulting water? Of course not. But the nail has a very good thermal conductivity, right? That is, heat moves along the metal much faster than it does along the hair. So then how could we manage to overheat all the hair structures without first overheating the skin? This would contradict all the laws of physics.

Of course, the heat accumulated in the melanin-containing hair layers disperses in the skin. If the intervention time was limited to the hairs’ TRT, we would achieve nothing but trauma to the hairs and heating up the skin. This is exactly the mechanism of first laser epilators worked which made it possible to achieve only a temporary effect – slowing down hair growth. Later on, the pulse length was increased, and the effectiveness of hair removal began increasing. The explanation was found quickly – the hairs’ TRT is measured not in single milliseconds but in tens of milliseconds.

I, however, consider a slightly different explanation more logical. Let’s see what will happen if we increase the impulse length to values significantly exceed a certain value equal to the hairs’ TRT. As it was described above, the heated up hair sections transmit heat to the surrounding tissues. This process continues until the temperature of the surrounding tissues is much lower than that of the hair. However, if the treatment time considerably exceeds the heated site’s TRT, the tissues near the site will inevitably heat up as well which will hamper further heat outflow in that direction. As a result, the conditions for thermal transfer deep inside the hair may turn out to be better than those in the skin. In this way, the effect of hair removal will be achieved. It all looks good, but at what price? In order to make heat move deep inside the hair, we need to overheat the skin around it to pretty much the same temperature as we do with the melanin-containing hair layers, that is, to a temperature notably higher than the temperature of coagulation. As a result, it is impossible to avoid the thermal destruction of the skin near the hair. If additional cooling systems are used on the skin surface, coagulation will affect only the deep skin layers. If cooling is not used, skin coagulation will be visually noticeable, and we will end up with scar tissue around each hair. Moreover, scarring often takes place earlier than hair destruction. That is, even such a simple (from the client’s standpoint) procedure as hair removal might cause irreversible damage to the skin, and it would be hard to classify the obtained aesthetic effect as good.
Can such a method be called selective? The notion of selectivity is based on the principle that we subject to intervention only those biological sites which we plan to destroy. If this is accompanied by trauma to the surrounding tissues, this becomes pseudo-selective intervention, and we are, at a minimum, using the term incorrectly, thus misleading the patients.

The skin overheating described above will be observed even without taking into consideration the patient’s natural skin pigmentation. If we also take into account the absorption of radiation into the skin’s melanin, the hair removal process becomes completely nonselective. As it was noted above, melanin’s TRT is about 1 microsecond. That is, when using an impulse with a length of tens of milliseconds, practically the entire energy of the laser impulse will be used to overheat the skin. It is accepted thinking that by increasing the radiation wavelength, we decrease the level of intervention on the natural pigmentation and, consequently, cause less trauma to the skin. However, the hair melanin will begin absorb less, but we need to charge the hair with a certain amount of energy. That is, the decrease in the coefficient of the melanin’s laser radiation absorption needs to be compensated by something. The only way, as it seems at first glance, is to increase the impulses’ energy. But is it such a good one? Due to their small size, the hairs located in the laser radiation range collect only a very small portion of the laser impulse’s energy, and the rest goes to the skin. That is, by increasing the laser impulses’ energy, we will inevitably increase the burden on the skin; and, having in mind that as we increase the wavelength to the closest infrared range, we are also increasing the depth of radiation penetration into the soft tissues, the deep skin layers will be subjected to overheating. This means that by decreasing the intervention on the melanin, we are increasing the risk of deep burns and scarring.

How can we avoid trauma to the skin during hair removal without sacrificing the intervention’s effectiveness? The best approach would be to choose a new chromophore which would be located only in the hair follicle, but, unfortunately, such a chromophore hasn’t been discovered yet. Maybe, in the future, the methods of the photodynamic therapy will assist us in doing this but it is just a fantasy for the time being. That is, we are left with the same chromophore. Someone else might find a better way, but I saw only one and want to present it to you.

In order to achieve our goal, we need to create a number of conditions:

  • with uniform absorption into the melanin of the skin and hair, the same impulse would cause irreversible thermal changes in the hair while heating the skin just a little bit;
  • the area of thermal damage during hair removal should not exceed the hair’s geometric dimensions, i.e. the heat from the hair should not be redistributed into the skin;li>
  • it is desirable to decrease the overall impulse energy considerably in order to lower the level of thermal impact on the skin as a whole.li>

How this is resolved in practice: instead of one long pulse, we need to use a train of short nanosecond pulses. In this case, we can create such conditions that the chromophore coefficient will increase to 100% (multi-photon absorption). At the same time, part of the laser pulse energy will be expended for heat, and another part will be used to generate an acoustic wave (a microexplosion). Moreover, the higher the energy density is, the bigger the energy part which is expended toward the acoustic wave. Let’s imagine the following picture. A laser pulse falls on the skin surface; This energy density is sufficient for the processes of multi-photon (non-linear) absorption, i.e. for increasing the melanin’s absorption coefficient to 100%. In this case, the laser pulse will be effectively absorbed by the melanin in the hair and in the skin. As a result of his absorption, on the one hand, the melanin will be heated up insignificantly, and, on the other hand, an acoustic wave, i.e. a microexplosion, will be generated. The microexplosion in the skin will not bring about irreversible consequences because the skin is a very elastic medium, and the acoustic wave will simply die away in it. This same explosion, if it occurs on the line between the cortex and the cuticle, will yield completely different consequences. As hair is a considerably harder structure than skin, the explosion will cause cuticle cells to shoot off from the hair shaft. This will generate an air gap between the hair and the skin. The microexplosions occurring inside the cortex will lead to a fragmentation of the latter only in immediate proximity to the skin because the radiation energy density is considerably higher there. The rest of the melanin-containing section of the hair will be heated up insignificantly. When the next pulse from the train gets absorbed, the area of the fragmented cortex will increase, and its remaining part will be heated up even more. In this way, from pulse to pulse, the melanin-containing part of the hair will become more and more isolated from the surrounding tissues, and its temperature will keep rising. As a result, we see that the overheated part of the hair ends up in something similar to a thermos, and the only tissue it is directly in contact with is the hair follicle. This is precisely where the excess heat will be discharged; if sufficiently high temperature accumulates inside the cortex, it may lead to the thermal destruction of the hair follicle along with the papilla.

Someone might object that, actually, the melanin distributed inside the skin will absorb the radiation as well, so it is improper to talk about a lack of overheating, and there is still a risk we could damage the natural skin pigmentation. This is not exactly so. The radiation absorption in the skin will proceed in a different manner. After stopping the laser intervention, melanin begins spreading heat freely onto the surrounding tissues whose TRT is significantly lower than that of the hair; therefore, the heat then disperses over a large area, and the melanin cools off. If a new pulse is sent to the skin surface after a period of time close to the dermis cells’ thermal relaxation time, the melanin will be heated up anew, starting from its initial temperature, i.e. between pulses, the temperature of the tissues near the melanin granules will vacillate in a highly safe range. The heat accumulation cannot occur due to the presence of temporary intervals between the pulses close to the TRT of the melanin-containing cells.

This method is different in that the area of thermal damage is restricted by the hair dimensions, and there is no thermal destruction of the skin independent of the patient’s skin pigmentation level.

In addition, as the melanin’s radiation absorption coefficient increases, an opportunity arises to decrease the laser pulses’ energy, thus reducing the degree of the skin’s thermal damage to safe levels. The procedure of this laser hair removal is easily tolerated but is not painless, especially on the sensitive areas of the body. The painfulness is explained by the fact that after the coagulation, the excess heat is redistributed in the skin anyway. The fewer hairs remain, the less painful the procedure is. If we work in this manner on hairless skin, the patient may feel nothing at all. The ban on tanning is removed as well.

It is necessary to point out that this method comes with one restriction common to all truly selective methods – there must not be any outside mixes. This refers to the use of any cooling products or anesthetics as well. The reason is, that as soon as we introduce outside substances into the intervention zone, we face the risk of receiving new chromophores which will effectively absorb the nanosecond pulses. This may lead to thermal damage as well as to the generation of more powerful acoustic waves capable of causing mechanical destruction of the skin. Therefore, in order to increase the comfort of the procedure, I recommend that a stream of cool air is blown over the intervention area.

The above described method of laser hair removal is protected by domestic and international patents and is implemented in the Multiline™ in a set with an Nd:YAP/Q-Sw laser.

  Treatment results: laser hair removal

 

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