Abstract:

Pulsed cryogen spray cooling (CSC) has become an effective method to protect the epidermis from non-specific thermal injuries in laser treatments of vascular malformation such as port wine stains. Cooling experiments with a skin phantom are usually conducted to quantify the heat transfer of CSC from a skin surface. Due to the high cooling ability of CSC and poor thermal conductivity of the skin phantom, the skin surface usually experiences a fast variation of the surface temperatures during CSC. It is therefore desirable to have a fast response surface thermal sensor to measure such temperature change. In traditional methods, commercial thermocouples of round or plate-shaped joints are buried on the surface or within the skin phantom in the experiments and usually give a poor response of the surface temperature. In this study, a fast response, thin film thermocouple (TFTC) of type T on a skin phantom made of epoxy or plastic glass is developed. The thin film thermocouple is manufactured by the Magnetron Spurting technique with a precise control of the compositions of Cu and Ni as well as the film thickness from 0.5 to 2 microns. A 0.05 micron thick of SiO₂ protection layer is deposited on the TFTC as protection. Careful calibrations of TFTCs indicate that the thermoelectric property of the TFTCs agrees well with that of standard T-type thermocouples. The time constant of the TFTCs is about 1. 2 ms, determined by pulsed laser heating experiments. Due to its small thermal capacity,the TFTC is sensitive to environmental fluctuations and the original temperature data show large noises. A low pass Butterworth (BW) filter is then introduced in the data acquisition system to remove such noises. Effect of the cut-off frequency and sample rate on the filtered temperature data has also been investigated and optimal cut-off frequencies and sample rates are recommended. A typical result for the temperature variation of the epoxy phantom surface during CSC is presented to illustrate the effectiveness of our TFTCs.