NOTE – In the first draft of this post, I made an error on my ratio calculations. This has now been rectified.
As with all light-based laser/IPL treatments, we require a target (a chromophore) which will absorb the light energy preferentially. In hair, with the visible and near infrared wavelengths that we typically use today, the only two chromophores available are melanin and/or blood.
How do these compare as absorbers?
First:
Well, melanin is found in the hair shaft and bulb of each follicle. It is produced during the anagen phase of the growth cycle, with the production stopping during catagen. In ‘dark’ hairs, it is found from the skin surface all the way down to the bulb.
The blood supply is located beneath the bulb, supplying the follicle with oxygen and nutrients throughout the anagen phase. Again, during the catagen phase, this supply stops as the blood vessels detach from the bulb.
Second:
In most laser/IPL treatments we fire red light energy at the hairs, usually with wavelengths longer than 600nm. These wavelengths are absorbed by melanin reasonably well, and can penetrate deep into the dermis.
Oxy-haemoglobin and deoxy-haemoglobin absorb light energy strongly in the yellow/green part of the visible spectrum. But, we must also consider met-haemoglobin (MetHb). This form of haemoglobin can be produced when ‘normal’ haemoglobin is heated, as when exposed to light energy1. The heme group iron then oxidises creating this new form of blood. Met-haemoglobin absorbs strongly around 680nm in the visible part of the spectrum.
However, these wavelengths do not penetrate so far into the dermis, compared with red or infrared light. This suggests that targeting hair follicles with yellow/green light energy might only be effective when the bulb (and hence, blood vessels) are close to the skin surface, during the early part of the anagen phase. But, as the bulb descends into the dermis in later anagen, those blood vessels become increasingly more difficult to reach, with yellow and green light energy.
MetHb also absorbs more strongly than ‘normal’ haemoglobin at 1064nm – around 4 times more strongly. This explains why this wavelength is useful for the treatment of deeper leg veins, for example. So, there is a possibility that infrared light energy (1064nm typically) may damage the blood vessels supplying the follicle, when the bulb is still relatively close to the skin surface.
But this become more difficult as the bulb descends, and can only occur if the fluence is sufficiently high, since the fluence drops rapidly towards the deeper parts of the dermis.
Third:
There is little histological evidence that blood vessels are destroyed or coagulated during these treatments. There is ample evidence of the follicles being denatured by laser/IPL energy – this is mainly due to the melanin absorbing much of the light energy and inducing a thermal reaction.
Around 20 years ago there was a lot of clinical research into this treatment. A review of those published studies shows no clear evidence of follicle destruction due to destruction of the blood vessels (unless you know otherwise!!).
If we consider the ratio of light absorption in melanin and blood we find the following:
| Wavelengths | Ratio of absorption by melanin to oxyhemoglobin |
| 755 nm (alexandrite) | 59 |
| 810 nm (diode) | 29 |
| 1064 nm (Nd:YAG) | 10 |
When oxyhemoglobin is converted into metHb, its absorption increases by a factor of four2, at 1064nm. Hence, the ratio of absorption by melanin to metHb, at that wavelength, is around ‘2.5’.
It is more than obvious that the melanin chromophore absorbs more strongly than normal blood, at all wavelengths! Only when metHb is formed does the ratio drop to a more equitable level.
Conclusion
It appears that the absorption by melanin is significantly stronger than that by blood, at all wavelengths typically used today, except at 1064nm. There is considerably more of a target with melanin in the follicles, in terms of volume, compared with blood, especially in darker hair.
From this evidence it would seem obvious that melanin is most likely to be the main target chromophore in hair, with blood being a distant second (except for 1064nm).
Hope this helps,
Mike.
PS We have our next MasterClass in Liverpool on June 2nd and 3rd. Contact us on DermaLaseMasterClass@gmail.com for more info.
References
1: Rubin IK, Farinelli WA, Doukas A, Anderson RR. Optimal wavelengths for vein-selective photothermolysis. Lasers Surg Med. 2012 Feb;44(2):152-7. doi: 10.1002/lsm.21161. Epub 2012 Jan 12. PMID: 22241659.
2 : Cannarozzo G, Fusco I, Zingoni T. The effects of 1064 nm laser on red telangiectasias using pulse shape modulation. Skin Res Technol. 2023 Jun;29(6):e13379. doi: 10.1111/srt.13379. PMID: 37357667; PMCID: PMC10235999.
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