Light transmittance dynamics and spectral absorption characteristics during auxiliary cryogen spray cooling in laser dermatology

Cryogen spray cooling (CSC) is widely applied in laser dermatologic surgery to minimize the risk of non-specific epidermal thermal damage caused by the competitive laser energy absorption of epidermal melanin. However, the light absorption and attenuation by cryogen film and subsequent frost formation on the skin surface during CSC are needed to be investigated by using R-134a, R-404A, and R-32. A spray system equipped with an integrating sphere-based light collection apparatus was constructed to evaluate the time-resolved laser transmittance and spectral absorption characteristics induced by R-134a, R-404A, and R-32 sprays, under the clinical-used 755-nm and 1064-nm laser irradiations. No obvious light absorption peaks exist in the wavelengths of 370-1400 nm. R-404A produces the largest average light absorbance (0.089), as compared to those of R-134a (0.066) and R-32 (0.068) in the near-infrared range (780-1400 nm). Given the lowest boiling point and highest latent heat of R-32, the evaporation of liquid film and melting of subsequent frost are promoted, leading to smallest light attenuation. R-32 spray shows great potential in clinics owing to its high light transmittance, small light absorption, and high cooling capability. For R-32 spray, the durations between spurt termination and laser irradiation are recommended to be 8-100 ms and 13-100 ms with average light transmittances of 86% and 95% under 755-nm and 1064-nm laser irradiations.