Unwanted body hair removal is a very common and widespread practice around the globe, rapidly growing In the last few years, addressing both men and women.
Since the late 90’s, light-assisted hair removal (LHR) techniques have shown unsurpassed effectiveness compared to traditional methods, allowing an almost permanent hair loss. Moreover, an increasing demand followed by an intense market competition has driven a continuous pursuit towards technology improvement and cost reduction. As a consequence, this is nowadays a highly accessible and affordable treatment, even in the form of compact devices for domestic use.
The underlying principle of LHR is that a suitable light source can induce damage on the stem cells surrounding the hair follicle, with minimum or no damage caused to the skin or adjacent tissues. As a result the germination of new hair is prevented for long periods of time (months or years), or even permanently .
A “suitable light source” must be understood as a proper combination of wavelength, fluence and pulse duration. When it comes to wavelength, the range 600?1100nm is preferred because Melanin contained in hair (and skin, indeed) absorbs light within this range . Moreover, the longer the wavelength, the deeper the penetration into the skin tissue.
Figure 1. Relative absorption (logarithmic scale) of light by skin cromophores (top). Depth of penetration of light radiation according to its wavelength (bottom).
At certain fluence levels (in the range of 10-100J/cm2), the absorbed light turns into heat that n fact burns the hair. Pulse duration (typically 10-100ms, but shorter and larger pulsed can be chosen) must be set in order to achieve a threshold temperature at the hair follicle, and keep it long enough to damage the stem cells, but not too long to extend the affected skin volume, which would cause unnecessary tissue damage and pain to the patient. In addition, we must bear in mind the relative Melanin concentration in the skin and hair, which relates to the skin phototype (Fitzpatrick scale).
Always burn, never tans (pale white skin)
Always burn easily, tans minimally (white skin)
Burns moderately, tans uniformly (lightbrown skin)
Burns minimally, always tans well (moderate brown skin)
Rarely burns, tans profusely (dark brown skin)
Never burns (deeply pigmented dark brown to black skin)
Figure 2. Skin phototype classification according to Fitzpatrick scale.
Darker skins are so because of higher Melanin concentration, and the same applies to the different types of hair. Hence LHR effectiveness is maximized on white to moderate brown skins and with dark hair (phototypes II to IV). Extreme cases (phototypes I and VI) are more critical, but LHR can be still reasonably effective if the right wavelength, fluence and pulse duration are chosen .
There are basically two groups of light sources that are able to generate intense enough light pulses for LHR. One is known as intense pulsed light (IPL), which is based on Xenon flash lamps. The other type is laser.
Within the laser option, two main typologies are used: solid state lasers (SSLs), and high power diode lasers (HPDLs). As usual, the perfect light source does not exist, and for this reason, all the mentioned types are widely used nowadays. The main advantage of IPL is its lower technical complexity, which translates into being the cheapest option. Nevertheless, it shows also the lowest effectiveness. IPL heads emit radiation in a wide spectrum (600?1200nm), which means that other molecules apart from Melanin can absorb light (water and Oxyhemoglobine).
Figure 3. Different types of light source show different emission wavelengt
This would lead to an extra heating of the irradiated area, and subsequently more pain. Additionally, IPL applicability is limited to phototypes I and II . Concerning SSL lasers, the Alexandrite laser was the first to be extensively used for LHR. Its architecture is the most complex and bulky among the mentioned light sources (thus showing the highest cost), but its effectiveness is still the best for phototypes I and II. Alexandrite laser emits laser light at 755nm. Besides, Nd:YAG lasers emit at 1064nm, and for this they are well suited for darker skins (phototypes V and VI). In the middle we find HPDLs as the best trade-off between versatility (applicable mostly to phototypes II to IV), effectiveness and cost. Traditionally, their emitting wavelength is in the range of 800-810nm for LHR applications, but the fact is that it is possible to manufacture commercial HPDLs systems emitting in the range from 760nm to 1064nm (and beyond). Therefore, HDPL technology could be applied to any skin phototype and replace any other existing LHR system.
At this point the reader may ask why are we still using Alexandrite or Nd:YAG lasers. The answer is that SSLs are brighter light sources than HPDLs. This simply means that light beams out-coming from SSLs can be concentrated into smaller spots, compared to HPDLs, which turns into a higher power density, and the possibility to be fibre-coupled. Fibre coupling is the essential reason why the hand piece of LHR systems based on SSLs are simpler and lighter, thus more ergonomic. The bulky laser components are kept within the main body of the equipment, while at the hand piece there is only the fibre end plus a simple lens set, sensors and minor electronic components. Additional features can be added as liquid-cooled tips for high fluence values, but then the hand piece would gain some weight. In contrast, the light source is located at the hand piece itself in HPDL (and IPL) systems. This comprises the diode laser stacks (which are mostly copper), the cooling pipes for the stacks and the tip (if it is the case) and the wiring (copper again!), which usually show a large cross?section in the range of 3 to 6mm. In addition, there can be more or less bulky optics in front of the laser stacks, like homogenizing rods or prisms. Altogether can yield a hand piece heavier than 500g, plus the water and power supply cable. Certainly, this does not contribute to ergonomics and comfort from the operator’s perspective.
So what if HPDLs could be fibre-coupled in a similar way as in SSL systems, keeping all the bulky stuff in the main body of the equipment and leaving just an ergonomic and compact hand piece? What if different wavelengths were combined in the same laser systems, so the full range from 760nm to 1064nm could be covered by a single HPDL source? This is now possible at Monocrom thanks to THEA, a smart combination of re-shaping & combining optics along with the use of a liquid optical fibre.
This new type of fibres allow to transmit light from laser sources not as bright as SSLs, due to its larger diameter (Ø 3 to 5 mm) and high angle of acceptance (NA<0.3), but in the end, similar laser spot sizes and fluence values will be applied to the patient’s skin, with an even more flat-top intensity distribution. Additionally, liquid fibres are less sensitive to excessive bending radii, with no risk of breaking it (there is liquid inside!), and they are more efficient in light transmission than traditional solid fibre bundles (packing factor is 1). Furthermore, Monocrom’s unique clamping technology enables the best diode performance, leading to higher lifetimes (108 shots) and higher peak power values.
Figure 4. Intensity distribution of the laser spot homogenised through the liquid fibre
As a conclusion, HPDLs systems are gaining terrain in the LHR application arena against their SSL competitors, thanks to the deployment of new solutions brought to market by Monocrom.
. Stephanie D. Gan and Emmy M. Graber, Laser Hair Removal: A Review, Dermatologic Surgery 39 (6), 823-838 (2013)
. Stratigos A.J. and Dover J. S., Overview of lasers and their properties, Dermatologic Therapy 13 (1), 2-16 (2000)