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

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/cm(2) 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.