Mechanical index-guided low intensity ultrasound therapy for hair regeneration: Insights from finite element modeling and in vivo experiments

This study investigates low-intensity ultrasound (LIUS) as a noninvasive method for stimulating and regenerating hair follicles in androgenetic alopecia (AGA), with a focus on optimizing parameters for both safety and efficacy. Finite element method (FEM) simulations in COMSOL Multiphysics® modeled acoustic pressure fields and calculated the mechanical index (MI) in a multilayer skin-follicle structure. Simulations were conducted at frequencies from 20 kHz to 1 MHz with a constant intensity of 0.1 W/cm2. MI profiles helped identify parameters for stable and inertial cavitation. In vivo experiments were conducted using C57BL/6 male mice with DHT-induced alopecia, which were subjected to ultrasound sonication at 150 kHz and 1 MHz at various intensities. Skin temperature was monitored during treatment, and hair regrowth was assessed. Simulations revealed frequency-dependent beam narrowing and pressure changes, with peak pressures increasing from 81 kPa at 20 kHz to 145 kPa at 1 MHz. The 150 kHz frequency showed intrafollicular RMS pressures nearing cavitation thresholds. Dermal MI profiles at 150 kHz peaked at ∼0.95 at a depth of ∼0.3 cm, while at 1 MHz, MI increased significantly with increasing intensity. In vivo treatments under optimized conditions yielded safe temperature increases and enhanced hair regrowth compared to controls. Selecting appropriate frequency and intensity can optimize MI for beneficial mechanical effects while preventing unsafe cavitation. Future studies will follow up on in vivo results and conduct clinical trials to confirm therapeutic efficacy and safety.