Measurements of hair temperature avalanche effect with alexandrite and Nd:YAG hair removal lasers

Abstract

Background and Objectives

In this study, we investigate the photothermal response of human hair using a pulsed laser source employed in the hair removal treatment. The purpose is to understand the dynamics behind the most common clinical practice to better define the salient features that may contribute to the efficiency of the process.

Study Design/Materials and Methods

Temperature changes of hair samples (dark brown color) from a human scalp (skin type Fitpatrick II) were measured by a thermal camera following irradiation with single and multiple neodymium: yttrium‐aluminum‐garnet (Nd:YAG) (1064 nm) and alexandrite (755 nm) laser pulses. Particularly, the hair was treated with an individual laser pulse of a sufficiently high fluence, or with a series of lower fluence laser pulses. We investigated the temperature increase in a broad range of fluence and number of pulses. From the data analysis we extrapolated important parameters such as thermal gain and threshold fluence that can be used for determining optimal parameters for the hair removal procedure. Our experimental investigations and hypothesis were supported by a numerical simulation of the light‐matter interaction in a skin‐hair model, and by optical transmittance measurements of the irradiated hair.

Results

An enhancement of the temperature response of the irradiated hair, that deviates from the linear behavior, is observed when hair is subjected to an individual laser pulse of a sufficiently high fluence or to a series of lower fluence laser pulses. Here, we defined the nonlinear and rapid temperature built‐up as an avalanche effect. We estimated the threshold fluence at which this process takes place to be at 10 and 2.5 J/cm² for 1064 and 755 nm laser wavelengths, respectively. The thermal gain expressed by the degree of the deviation from the linear behavior can be higher than 2 when low laser fluence and multiple laser pulses are applied (n = 50). The comparison of the calculated gain for the two different laser wavelengths and the number of pulses reveals a much higher efficiency when low fluence and multiple pulses are delivered. The avalanche effect manifests when the hair temperature exceeds 45°C. The enhanced temperature increase during the subsequent delivery of laser pulses could be ascribed to the temperature‐induced changes in the hair's structural properties. Simulations of the hair temperature under Nd:YAG and alexandrite irradiation indicate that the avalanche phenomenon observed in the hair suspended in air may apply also to the hair located within the skin matrix. Namely, for the same fluence, similar temperature increase was obtained also for the hair located within the skin.

Conclusion

The observed “avalanche” effect may contribute to the reported clinical efficacy of laser hair removal and may at least partially explain the observed efficacy of the brushing hair removal procedures where laser fluence is usually low. The repeated irradiation during the brushing procedure may lead to an avalanche‐like gradual increase of the hair's thermal response resulting in sufficiently high final hair temperatures as required for effective hair reduction.

INTRODUCTION

Demand for laser removal of unwanted or excess hair has grown significantly since first introduced in the mid‐1990s. ¹ Compared to traditional methods of hair removal such as threading, plucking and waxing, laser hair removal has been found to be superior in achieving long term hair reduction. ² , ³ Lasers most commonly in use for hair removal include alexandrite, neodymium: yttrium‐aluminum‐garnet (Nd:YAG) and diode. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹

Currently, laser hair removal is being performed using either a stamping or a brushing (“in‐motion”) technique. ⁶ With the “stamping” technique the laser handpiece is positioned over the treated skin from spot to spot without any overlapping, and single high fluence pulses are delivered at a relatively low repetition rate to each of the spots. ⁵ This technique is most commonly being used with Nd:YAG and alexandrite lasers since these lasers are able to deliver high energy laser pulses with sufficiently short pulse durations (~0.3–25 ms). ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ Using this technique the hair follicles are targeted directly by selectively over‐heating the hair with individual pulses with durations shorter than the thermal relaxation (i.e., cooling) time of the hair.

On the other hand, the brushing technique involves lower fluence laser pulses delivered at higher pulse rates (3–10 Hz) with the handpiece in a constant movement forwards and backwards at a speed of 2–3 cm/s until a sufficiently high cumulative energy is delivered to the whole treated area. ⁶ This in‐motion continuous technique is more often used with diode lasers since they are typically not capable of delivering high energy pulses with short enough pulse durations.

It is a general understanding that selective photothermolysis is the mechanism of how laser hair removal works. ¹ , ⁵ The “selective” means that the goal is to use a laser wavelength which is more highly absorbed in the melanin in hair than in the surrounding skin. ¹² , ¹³ However, although hair removal lasers have been in use for over 20 years the underlying mechanism of hair reduction is still not completely understood. ¹⁴ , ¹⁵ This applies especially to the brushing hair removal procedures where the hair temperature elevations resulting from low fluence laser pulses are expected to be relatively low.

To increase the selective absorption of hair at the treatment laser wavelength, early hair removal techniques were based on infiltrating black carbon into hair ducts. ¹⁶ More recently, an “avalanche” effect was proposed for Nd:YAG (1064 nm) laser hair removal after the absorption of the hair suspended in air was observed to get enhanced by the treatment laser light itself. ¹⁷ The observation was tentatively attributed to an avalanche “hair darkening” effect when hair is subjected to a series of Nd:YAG pulses. It was also suggested that due to back scattering of laser light within the skin matrix, the fluences to which the hair located within the skin would be exposed to is comparable to the fluences used on hair suspended in air, and that therefore, the observed avalanche effect applied also to clinical conditions.

In this paper, we explore in more detail the initially reported “avalanche” effect as observed for the Nd:YAG laser light by measuring the temperature response of hair samples following irradiation by an alexandrite (755 nm) hair removal laser, in addition to the Nd:YAG (1064 nm) laser. A goal of the study was to determine whether the avalanche effect is predominantly a temperature‐dependent phenomena, independent of the hair removal laser wavelength. Simulations of the hair temperature under Nd:YAG and alexandrite irradiation were also made to verify whether the avalanche phenomenon observed for both wavelength in the hair suspended in air apply also to the hair located within the skin matrix. Additionally, a limited series of measurements of the hair transmission spectra before and after the irradiation was made to confirm the proposed hair darkening effect.

MATERIALS AND METHODS

Laser system

The laser system used in the study was an AvalancheLase® (manufactured by Fotona d.o.o., Slovenia) consisting of two solid crystal laser sources, Alexandrite (755 nm) and Nd:YAG (1064 nm), with adjustable pulse durations t _p  = 0.3–300 ms. The system was fitted with an R35 manual handpiece (manufactured by Fotona d.o.o., Slovenia) set to a spotsize of 10 mm.

Hair temperature measurements

The experimental set‐up is shown in Figure 1A. The human hair samples (dark brown color) were cut from a human scalp (skin type Fitzpatrick II), and fixed in air in front of the laser handpiece. To minimize the influence of the hair diameter and color on the results, measurements were performed on limited sections of long hair samples cut from the same area of the same subject. The experimental protocol was carried out in accordance with the Helsinki Declaration.

Figure 1.

Experimental set‐up for hair temperature measurement during laser illumination. A human hair was cut from a human scalp, and fixed in air in a straight horizontal position. The Nd:YAG or alexandrite laser beam (d = 10 mm) was directed onto the hair, and the resulting hair temperature increase was measured by a thermal video camera, set to record at every time instant the maximal temperature of the hair sample. (B) Typical thermal image of illuminated hair just after the laser pulse. (C) Typical evolution of hair maximal temperature during consecutive laser pulses. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

The Nd:YAG or alexandrite individual laser pulses (t _p  = 2 ms) were directed onto the hair, and the resulting hair temperature increase following each laser pulse was measured for different fluences F (in J/cm²) with a thermal video camera (Flir ThermaCAM P45), set to record the maximal temperature increase (ΔT) of the hair sample following a pulsed irradiation. Figure 1B shows typical thermal image of the hair just after the laser pulse. The pulses were delivered at sufficiently long separation times (t _s  ≥ 2 s) to allow the hair to cool down to the ambient temperature in‐between pulses. A room temperature air blower was used to shorten the hair cooling time following pulsed irradiation. Figure 1C shows typical evolution of hair temperature during pulsed illumination, where graduate increase of maximal temperature and the cooling back to the room temperature can be seen.

Ultraviolet‐visible spectroscopy

The optical spectra in transmission geometry were taken by using a modified commercial microscope (Dialux Trinocular; Leitz Wetzlar) coupled with an optical fiber to a spectrometer (LR1‐T; Aseq Instruments), equipped with a cooled silicon charged coupled device. The sample was kept on a glass slide and back illuminated by a tungsten‐halogen lamp. The transmitted light was collected through a 50× microscope objective (NPL Fluotar 50x/0.85; Leitz Wetzlar). The collimated light passed through a mechanical slit to cut away the unwanted light transmitted through the glass, and collect only the light transmitted through the hair. An aspherical lens was positioned after the slit to focus onto the entrance of the spectrometer's optical fiber. The transmission through the sample was normalized by the light transmitted through the empty glass besides the sample, and displayed as transmittance.

Simulation of hair and skin laser heating

Realistic 3D mesh model of three‐layers skin with single hair was built using freely accessible Salome software environment (see Figure 2A,B). We took into account the real dimensions of the epidermis (70 μm), dermis (3.43 mm) and subcutis (5 mm), as well as the dimensions of the hair (diameter 60 μm, length under the skin 2.5 mm, and follicle diameter 200 μm). ¹⁸ , ¹⁹

Figure 2.

3D model of laser heating of hair and skin. (A) The model of the skin includes layers of epidermis (70 μm), dermis (3.43 mm), and subcutis (5 mm). (B) Magnified region (dashed rectangle on A) of the hair with shaft with diameter 60 μm, length 2.5 mm, and with follicle diameter 200 μm. (C) Calculated temperature field using F _p  = 50 J/cm²

In the second step, using the meshed Monte Carlo method, ²⁰ the irradiation of tissue with a laser beam of the same properties as in the experimental environment was simulated to calculate distribution of absorbed laser energy into different regions of tissue. Absorption and scattering coefficient (μ_a, μ_s), anisotropy factor (g) and refractive index (n) were used for each tissue component as shown in Table 1. As a result of optical simulation, we use absorbance and proceed to the last step, where we simulated tissue heating and heat transport by numerically solving heat transport equation in the Matlab PDE software environment. Thermal properties, such as specific heat (c_p), thermal conductivity (k), and density (ρ) are listed in the Table 1. We considered the number of pulses, pulse duration, fluence, initial temperature profile, convective boundary conditions on the skin surface and adiabatic boundary conditions at other surfaces. The final result is the temperature evolution over time at every mesh node.

Table 1.

Optical and thermal properties of different tissue components

	Epidermis 21 , 22 , 23 , 24	Dermis 21 , 22 , 23 , 24 , 25	Subcutis 22 , 26 , 27	Hair 28 , 29
μa [mm−1]	0.07	0.038	0.008	0.56
μs [mm−1]	26.1	5.25	2.27	79.7
g	0.89	0.72	0.78	0.9
n	1.42	1.37	1.44	1.37
cp [J/kgK]	3200	3500	2870	2512
k [W/mK]	0.34	0.41	0.3	0.63
ρ [kg/m3]	1120	1090	860	1300

RESULTS

Hair temperature results

The first set of measurements was carried out to determine whether there is a memory effect associated with irreversible changes in the optical properties of a hair, resulting from the hair's exposure to past laser irradiations. Figure 3 shows the observed hysteresis in the evolution of hair temperature elevations (ΔT _p ) following individual laser pulses as observed by applying a series of consecutive Nd:YAG laser pulses to the same hair section. In the initial phase of the experiment the pulse fluence (F _p ) of individual pulses in the pulse sequence was increased from pulse to pulse, starting with the first pulse at F _p  = 5 J/cm², and ending with the final 14th pulse at F _p  = 25 J/cm². This phase was followed by a second series of pulses with the pulse fluence being decreased from pulse to pulse, starting with the first pulse with F _p  = 25 J/cm², and ending with the final 13th pulse with F _p  = 6 J/cm². The pulses were delivered at sufficiently long separation times to allow the hair to cool down in‐between pulses.

Figure 3.

Temperature increase ΔT _p of the same hair section following consecutive irradiation by individual Nd:YAG laser pulses. In the initial phase (represented by open circles), the pulse fluence of individual pulses (F _p ) was increased from pulse to pulse, starting with the first pulse with F _p  = 5J/cm², and ending with the final pulse with F _p  = 25J/cm². In the subsequent phase (represented by closed circles), the individual pulse fluences (F _p ) were decreased from pulse to pulse, starting with the first pulse with F _p  = 25J/cm², and ending with the final 13th pulse with F _p  = 6J/cm². Nd:YAG, neodymium: yttrium‐aluminum‐garnet

Temperature elevation after each laser pulse can be approximated by the equation ³⁰ :

$$\Delta T_p=\frac{{\mu }_a}{\rho c_p}{F}_p$$ (1)

where μ _a , ρ, and c _p are absorption coefficient, density, and specific heat. The equation shows linear relation with the laser fluence F _p under the assumption of constant material parameters. However, Figure 3 shows that the increase of ΔT _p is linear only up to approximately 10 J/cm². Above this level, the temperature elevation is growing progressively. We attribute the observed deviation from the linear dependence of Equation 1 to the “avalanche” effect, i.e., to the increased thermal response of the human hair after being irradiated (i.e., “pumped”) by a laser pump pulse with F _p  > F _aval , where F _aval is the threshold fluence required for the avalanche effect. For the used Nd:YAG laser parameters and the measured hair the avalanche threshold was found to be at about F _aval  ≈ 10 J/cm². The observed hysteresis in Figure 2 demonstrates this dependence of the hair's thermal response on the irradiation history.

As it is illustrated in Figure 3, we can divide the temperature elevation ΔT _p into two contributions:

$$\Delta T_p=\Delta T_{lin}+\Delta T_{aval}$$ (2)

where ΔT _lin represents the “no‐avalanche” and ΔT _aval represents the additional temperature increase due to the avalanche effect. By combining all material parameters into a single temperature coefficient (K), we can rewrite the Equation (1) accordingly:

$$\Delta T_{lin}=K{F}_p.$$ (3)

The temperature coefficient K (with K _Nd  ≈ 1.56°C·cm²/J as measured for Nd:YAG laser pulses) defines the linear growth of ΔT _p with F _p , as observed at low fluences. And further, we can define the avalanche gain as:

$$G_{aval}=1+\frac{\Delta T_{aval}}{\Delta T_{lin}}.$$ (4)

It is important to note that G _aval depends mainly on the hair absorption cross‐section and the absorbed laser fluence at each consecutive laser pulse.

Similar “avalanche” effect was observed also for the alexandrite laser which is characterized by a higher absorption in the measured human hair (K _Alex  = 5.8°C·cm²/J). The avalanche threshold was observed at a lower fluence of F _aval  ≈ 2.5 J/cm².

The next set of measurements was performed to determine whether it is possible to reach large avalanche gains G _aval by delivering a series of relatively low fluence pump pulses. In this order, up to N = 50 consecutive pump pulses with a fixed pump pulse fluence (F _p ) was delivered to the same hair section at a sufficiently slow repetition rate to prevent hair temperature build‐up from pulse to pulse. As an example, Figure 4 shows the evolution of hair temperatures during the delivery of a series of Nd:YAG or alexandrite pump laser pulses with the pump fluences of 14.4 J/cm² (for Nd:YAG) and 5 J/cm² (for alexandrite), set to be only slightly above the corresponding avalanche thresholds.

Figure 4.

Hair temperature evolution during the delivery of N = 50 alexandrite (A) or Nd:YAG (B) pump pulses with F _p  = 5 J/cm² (alexandrite) and F _p  = 14.4 J/cm² (Nd:YAG). The pulse repetition rate was 0.5 Hz. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

Figure 5 shows the evolution of the avalanche gain for the Nd:YAG pulse series of Figure 4B. The initial avalanche gain as observed for the first pulse in the series gets significantly additionally enhanced during the delivery of the subsequent 20–30 pulses.

Figure 5.

Gradual increase of the avalanche gain G _aval during the delivery of Nd:YAG pulses shown in Figure 4B. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

Figure 6 represents an overview of the measured dependences of the avalanche gain (G _aval ) on the pump fluence (F _p ) and the number of delivered pulses N, for the Nd:YAG and alexandrite laser wavelength.

Figure 6.

Dependence of the avalanche gain (G _aval ) on the pump fluence (F _p ) and the number of delivered pulses (N), for the Nd:YAG and alexandrite laser wavelength as obtained from the experimental data. The lines are to guide the eye only. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

Finally, Figure 7 shows the measured relationship between the elevated hair temperature following a single pump pulse, and the resulting avalanche gain. Above the avalanche threshold temperature (T _aval ) of about 45°C the avalanche gain grows approximately linearly with the elevated hair temperature regardless of the type of laser.

Figure 7.

Dependence of the avalanche gain (G _aval ) on the elevated hair temperature (T _p ) following a single Nd:YAG or alexandrite pump laser pulse. The threshold temperature (T _aval ) for the avalanche effect is observed to be at approximately 45°C. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

A potential dependence of the avalanche effect on the repetition rate of laser pulses was also tested. Figure 8 shows the evolution of temperatures when a hair was irradiated by alexandrite laser pulses resulting in a single pulse temperature of T _p  = 55 ± 2.5°C, and delivered at different time intervals of 60, 30, 15, and 2 s. Measurements revealed that for low T _p the induced changes in the hair's photothermal response were very short‐lived, with a life‐time of about 5 s. However, when the laser fluence was increased to result in T _p above approximately 70°C the change in the photothermal response was observed to persist for longer than 60 s.

Figure 8.

Dependence of the evolution of the elevated hair temperature (T _p ) following alexandrite pump laser pulses at different pulse periods of 60, 30, 15, and 2 s

Hair transmittance measurements

The avalanche effect was observed by the irradiation of a hair with either a single high fluence pulse or a sequence of lower fluence pulses exceeding the avalanche threshold and compared to assess the origin of the effect the optical properties of hairs were measured before and after irradiation by the alexandrite laser with either a single high fluence pulse (F _p  = 12.5 J/cm²), or a series of 50 low fluence pulses (F _p  = 5 J/cm²).

The measured changes in the transmittance of the hair after laser pump irradiation are depicted in Figure 9A. The spectrum of a nonirradiated hair is characterized by the increasing transmittance (i.e., decreasing absorbance) towards higher wavelengths. The broad absorption spectrum is typical for melanin, and results from the superimposition of a number of distinct chromophore absorption bands over the visible and near infrared range. ³¹

Figure 9.

(A) Transmittance spectra of the hair following a single high fluence (F_p  = 12.5 J/cm²) alexandrite laser pulse (black solid line), and following 50 low fluence (F_p  = 5 J/cm²) alexandrite laser pulses (red solid line). The corresponding reference measurements are represented by the dashed lines. (B) Optical images of the hair shaft, after irradiation with a single high fluence alexandrite pulse (left panel), before irradiation (central panel) and after irradiation by 50 low fluence alexandrite laser pulses (right panel)

A difference between the two reference measurements can be observed below 550 nm. The discrepancy is most likely caused by an inadequate thermalization of the spectrometer light source before performing the alexandrite reference transmittance measurement (black dashed line). Since a halogen light source was used, that is characterized by a low intensity in the blue part of the spectrum, any slight deviation in the emission intensity significantly affected the measured transmittance in this part of the spectrum. However, the spectra above 550 nm agree very well, demonstrating that optical properties of the two hairs in the infrared region were almost identical.

The irradiation with a single pulse of alexandrite at high fluence (F _p  = 12.5 J/cm²) leads to the avalanche gain of G _a  > 3 (see Figure 6), resulting in T _p  > 200°C, which is in the temperature range that has been found to result in a total desiccation and consequent degradation of the hair. ³² As a result, the transmittance of the hair gets drastically reduced over a broad spectral range. Optical inspection under the microscope in Figure 9B (left panel) showed a visible change of the hair structure (surface and shape) and a consequent darkening (charcoaling) of the hair. It is to be noted that in the absence of the avalanche gain the final temperature would reach only about 100°C where no significant damage to the cortex has been observed. ³³

A less pronounced decrease in transmittance was observed following the irradiation by 50 consecutively delivered low fluence (F _p  = 5 J/cm²) pulses. The corresponding final elevated temperature of about 90°C (see Figure 4B) is in the range for which only damage to the cuticle, without damage to the cortex is expected. ³³ Accordingly, Figure 9B shows a slightly deteriorated hair cuticle exhibiting irregular surface geometry.

Simulation results

Since the experimental results were obtained in a hair suspended in air, a numerical study simulating hair irradiation in a skin was performed to study the relevance of the observed avalanche effect in clinical settings. Figure 10 shows the dependence of the calculated temperature increase of the hair at a depth of z = 1 mm, and at the follicle located at z = 2.5 mm, for different values of laser types and pulse fluence impinging on the skin surface. For both lasers, the temperatures of the hair and follicle within the skin are at clinically used fluences above the avalanche threshold observed for the hair suspended in air.

Figure 10.

Calculated elevated temperature of the hair (at z = 1 mm) and follicle (at z = 2.5 mm) as a function of the Nd:YAG (A) or alexandrite (B) single pulse laser fluence. The dotted lines represent the temperature threshold for the avalanche effect. Nd:YAG, neodymium: yttrium‐aluminum‐garnet

The simulations also show that despite of the beam being progressively absorbed by the skin chromophores the temperature increase of the hair located within the skin is similar to that of the hair suspended in air due to the scattering of the laser light within the skin matrix, effectively enhancing the number of photons that get trapped within the highly absorbing hair.

DISCUSSION

Our measurements show an enhancement of the temperature response of the irradiated hair when hair is subjected to an individual laser pulse of a sufficiently high fluence, or to a series of lower fluence laser pulses. This “avalanche” effect may contribute to the clinical efficacy of laser hair removal procedures. From Figure 6 it can be seen that similar gains as with the stamping technique (N = 1) can be achieved by using low fluence and multiple pulses. Moreover, from Figure 7 we can see that the correlation between the gain G _aval and T _p is the same for both laser wavelengths. However, the avalanche fluence for Nd:YAG laser is much higher due to the lower absorption of the hair (compared to the alexandrite laser). It is worth to mention that both graphs are person‐related due to the natural variation of hair and skin absorption (i.e., color), which will lead to different values of F _aval and G _aval . In principle, our observations based on thermographic measurement could result in development of a clinical protocol consisting of setting the correct F _aval and G _aval before the treatment of each individual patient.

Further studies are needed to determine the underlying mechanism of the observed avalanche gain, however it is expected that a complex combination of several processes is being involved, depending on the evolution of the hair temperature during laser irradiation. One of the most important processes involved in laser heating of hair is the loss of water content with the increase of hair temperature. Thermogravimetric analysis ³⁴ showed that the amount of external (free) and internal (bound) water in untreated hair were, respectively, of approximately 11% and 3.5%, with the evaporation taking place at two different temperature ranges. The lower temperature range (<180°C) affects only the external water while the bound water gets evaporated at temperatures above about 180°C. Here, it is important to note that at low temperatures the process of dehydration is relatively slow. For example, when hair is exposed to a temperature of 65°C the free water gets evaporated only after about 40 min. ³⁴ The reabsorption of water following heat‐drying is similarly slow, with the time of reabsorption of about 40 min at room temperature and 40% of relative humidity. ³⁵ Since during the delivery of the series of low fluence laser pulses in our experiments the temperatures were below or not significantly higher than 65°C, this suggests that for low fluence laser irradiations the role of water evaporation in the observed avalanche effect (Figure 4) may not be significant. Namely, during the delivery of 50 pulses the hair was submitted to the elevated temperature for less than 1 s during each pulse, and during the pulse sequence for less than 1 min altogether. Similarly, the observed dependence of the avalanche effect shown in Figure 8 does not appear to be due to rehydration of the hair during the time in‐between pulses since the shortest pulse period of 15 s where no avalanche effect was observed was significantly shorter that the time required for rehydration.

Instead, the avalanche effect as observed at low laser fluences appears to be a consequence of processes other than hair desiccation. For example, exposing the hair to 50°C resulted in the hair's stiffness to decrease, with the fibers stiffness quickly returning to their normal value after re‐equilibration at room conditions, despite of the hair's hydration level not returning fully back to the original value. ³⁶ This observation indicates that heat may affect some of the hair's properties at least partially independently from the effects of dehydration. Similarly, transmission electron microscopy showed progressively increased number of longitudinal cracks in the cuticle when the hair was exposed to temperatures of 47°C, 61°C, or 95°C. ³³

In terms of cortex damage, there have been no signs of damage reported at any of temperatures below 100°C. ³³ On the other hand, a total evaporation of the bound water followed by a total degradation of the hair shaft has been observed around 200°C. ³²

The above reports of hair changes upon exposure to elevated temperatures match well with what was observed in our experiments for low and high fluence irradiations (Figure 9). This suggests that the low fluence, i.e., low temperature avalanche effect is mainly a consequence of structural and morphological changes of the hair's surface while the high fluence avalanche effect results also from the hair's desiccation and damage to the cortex.

An important limitation of the study is that temperature measurements were carried out on a hair suspended in air, while in a clinical situation the hair is embedded within the skin matrix. In the clinical setting the incoming laser light gets absorbed and scattered by the skin, affecting the laser fluences reaching the deeper lying hair sections and hair follicles.

However, simulations of the hair temperature under Nd:YAG and alexandrite irradiation indicate that the observed avalanche phenomenon may apply also to hair located within the skin matrix. As can be concluded from Figure 10, the temperatures of the hair located within the skin matrix during hair removal treatments are similar or higher than those obtained for the hair suspended in air during our measurements. This is due to the scattering of the laser light within the skin matrix, effectively enhancing the number of photons that get trapped within the highly absorbing hair, in spite of the beam being progressively absorbed by the skin chromophores.

Therefore, the hair temperatures under the clinical setting are expected to be above the avalanche threshold of about 45°C when using relatively low fluence values above F _p  ≈ 10 J/cm² and F _p  ≈ 2.5 J/cm² for the Nd:YAG and alexandrite laser epilation, correspondingly. These results agree with the results reported for Nd:YAG laser in, ¹⁷ where the hair damage threshold was found at about 10 J/cm² for short laser pulses (0.3 ms).

It is to be noted that the enhancement of the hair's thermal response (“the avalanche gain”) depends on the laser parameters such as the laser wavelength, pulse duration, fluence and number of delivered pulses, and as well on the hair color and thickness.

CONCLUSIONS

We measured the photothermal response of human hair using two commercially available laser sources Nd:YAG and Alexandrite employed in the medical treatment of hair removal. Hair temperature was monitored for a broad range of laser fluences and different number of pulses. We find a progressively increasing temperature which deviates from the linear behavior and leads to a rapid increase in the hair temperature.

This avalanche effect is observed when hair is subjected to an individual laser pulse of a sufficiently high fluence, or to a series of lower fluence laser pulses. The threshold fluence and the thermal gain for both laser wavelengths were determined for the measured type of hair. The threshold fluence depends on the laser wavelength and defines the region where the temperature of the hair increases nonlinearly with the laser fluence. The avalanche gain at low fluence (but >F _aval ) and 50 laser pulses was around 2. This value scales up with the laser fluence. To achieve the same G _aval the required fluence is approximately two‐times higher when a single pulse is applied. We estimated the avalanche increasing temperature which occurs at T > 45°C.

During multiple laser pulses excitation at low fluence the thermal changes of the hair, then the enhanced temperature increases, most probably encompass for different processes: energy dissipation through water evaporation, and hair's structural changes once the water contents is fully evaporated. This process has been observed depending of the pulse separation time interval.

Moreover, simulations on the complex skin‐hair model showed that the avalanche fluence could be similar in clinical application. We believe that the avalanche effect may play a significant role in laser hair removal and may at least partially explain the observed efficacy of the brushing hair removal procedures where laser fluences much lower than for the stamping procedures are being used. The repeated irradiation during the brushing procedure may lead to an avalanche‐like gradual increase of the hair's thermal response resulting in sufficiently high final hair temperatures as required for effective hair reduction.

CONFLICTS OF INTEREST

Some of the authors (Matjaž Lukač, Urban Jernejčič, and Nejc Lukač) are affiliated also with Fotona, d.o.o.

Vella D, Lukač M, Jernejčič U, Lukač N, Klaneček Ž, Milanič M, et al. Measurements of hair temperature avalanche effect with alexandrite and Nd:YAG hair removal lasers. Lasers Surg Med. 2023;55:89–98. 10.1002/lsm.23622