Dose-response of human follicles during laser-based hair removal: Ex vivo photoepilation model with classification system embracing morphological and histological features

Objectives: Photoepilation is a commonly used technology in home-use devices (HUDs) and in professional systems to remove unwanted body hair using pulses of laser or intense pulsed light (IPL). Albeit HUDs and professional systems operate at different fluences and treatment regimes, both demonstrate high hair reduction. The underlying mechanisms, however, remain unknown partly due to high divergence of the existing literature data. The objective of this study was to develop an ex vivo photoepilation model with a set of criteria evaluating response to light pulses; and to investigate dose-response behavior of hair follicles (HFs) subjected to a range of fluences. Methods: After ex vivo treatment (single pulse, 810 nm, 1.7–26.4 J/cm², 4–64 ms pulse) human anagen HFs were isolated and maintained in culture for 7–10 days. Response to light was evaluated based on gross-morphology and histological examination (H&E and TUNEL stainings). Results: HFs treated ex vivo demonstrated a dose-dependent response to light with five distinct classes defined by macroscopic and microscopic criteria. Fluences below 13.2 J/cm² provoked catagen-like transition, higher fluences resulted in coagulation in HF compartments. Conclusion: Observed changes in the HF organ culture model were reflected by clinical efficacy. The developed photoepilation model provides an easy and fast method to predict clinical efficacy and permanency of light-based hair removal devices. Lasers Surg. Med.