Thermal modeling and experimental validation of human hair and skin heated by broadband light

In recent years, selective photothermolysis (PT) based on broadband light and assisted by skin vacuum (Photopneumatic Technology or PPx, by Aesthera) was introduced into the aesthetic market for epilation (hair removal) procedures. The present study aims to: (a) determine the overall effect of PPx on skin humidity and associated skin optical properties, and; (b) develop a PT numerical model to study the spatial and temporal hair and skin temperature variations during PPx epilation.

A customized cup was used to apply moderate vacuum for 10 seconds on the forearms and faces of five volunteers. Skin humidity was measured on test sites immediately after vacuum application. A selective PT model was developed and used to simulate transport of photons from a broadband light source into human skin. The PT model is experimentally validated using agar gel tissue phantoms. Systematic numerical simulations are carried out for fluences varying from 5.0 to 7.0 J/cm(2) and pulse durations varying from 2.5 to 25 milliseconds.

Results of in vivo skin surface humidity measurements reveal that while absolute humidity varies between 30% and 75% between different volunteers and different locations, vacuum has a minimal effect on it. Experimental measurements of temperature variation during and after broadband light treatment agree well with model predictions. Numerical results indicate that average hair follicle temperature increases as a linear function of fluence. Longer pulse duration results in lower follicle temperature. However, immediately after each pulse, the temperature distribution within the hair follicle is highly non-uniform: the minimum temperature occurs at the follicle edge while the maximum is located between the center and edge of the follicle.

Vacuum appears to have minimal effect on skin humidity. The PT model developed and validated is expected to assist clinicians in the selection of broadband light treatment parameters to achieve optimal therapeutic outcomes during epilation procedures.