Characterization of Cryogen Spray Cooling With Thin-Film Thermocouple

Abstract

Cryogen spray cooling (CSC) has been used effectively to protect the epidermis during laser dermatologic surgery. However, the temperature reduction in human skin induced by CSC has not been reliably determined due to the short cooling period and significant temperature gradient in the skin. Therefore, CSC has not been optimized for different laser dermatologic surgery procedures. Although it would be desirable to measure in situ human skin temperature, embedding a sensor within 100 μm beneath the skin surface is not feasible. In addition, infrared skin temperature measurement is also not workable because the skin is covered with a cryogen layer of unknown thickness. In this study, we selected an epoxy which has similar thermal properties to that of human skin as our cooling target. Thin-film thermocouples (TFTC) were deposited directly onto the epoxy substrate using micro-fabrication techniques to minimize the thermal contact resistance between TFTC and the substrate. The negligible mass of TFTC also creates a minimal disturbance to heat flow across the surface. Due to the difference in thermoelectric property between thin film and leading wire, special sensor design and calibration procedures were developed. TFTC were calibrated from −46 to 50°C. The thermoelectric sensitivity is around 50–60% of that of bulk material. Skin phantom temperature reductions produced by a commercial medical laser nozzle at different spray durations were measured using the TFTC. The results not only help to elucidate the mechanisms involved in interaction between cryogen spray and human skin but also provide a thermal boundary condition for numerical modeling of laser dermatologic surgery.