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Research Papers

# Fluid and Thermal Dynamics of Cryogen Sprays Impinging on a Human Tissue Phantom

[+] Author and Article Information
Walfre Franco

Beckman Laser Institute, University of California, Irvine, CA 92612; Department of Mechanical Engineering, University of California, Riverside, CA 92521wfranco@uci.edu

Henry Vu, Guillermo Aguilar

Department of Mechanical Engineering, University of California, Riverside, CA 92521

Wangcun Jia, J. Stuart Nelson

Beckman Laser Institute, University of California, Irvine, CA 92612

J Biomech Eng 130(5), 051005 (Jul 11, 2008) (8 pages) doi:10.1115/1.2948404 History: Received July 20, 2007; Revised April 16, 2008; Published July 11, 2008

## Abstract

Cryogen spray cooling (CSC) protects the epidermis from unintended heating during cutaneous laser surgery. The present work investigated the time-dependent flow characteristics of cryogen sprays and correspondent thermal dynamics at the surface of a human tissue phantom. First, a numerical analysis was carried out to evaluate an epoxy block substrate as a human tissue phantom. Next, the velocity and diameter of cryogen droplets were measured simultaneously and correlated with surface temperature of the human tissue phantom during CSC. Finally, velocity and diameter measurements were used to compute the spray number, mass, and kinetic energy fluxes, and temperature measurements were used to compute the surface heat flux. Numerical modeling showed that the thermal response of our phantom was qualitatively similar to that of human stratum corneum and epidermis; quantitatively, thermal responses differed. A simple transformation to map the temperature response of the phantom to that of tissue was derived. Despite the relatively short spurt durations ($10ms$, $30ms$, and $50ms$), cryogen delivery is mostly a steady state process with initial and final fluid transients mainly due to the valve dynamics. Thermal transients $(16ms)$ are longer than fluid transients $(4ms)$ due to the low thermal diffusivity of human tissues; steady states are comparable in duration ($≈10ms$, $30ms$, and $50ms$) although there is an inherent thermal delay $(≈12ms)$. Steady state temperatures are the lowest surface temperatures experienced by the substrate, independent of spurt duration; hence, longer spurt durations result in larger exposures of the tissue surface to the same lower, steady state temperature as in shorter spurts. Temperatures in human tissue during CSC for the spray system and parameters used herein are estimated to be $≈−19°C$ at the stratum corneum surface and $>0°C$ across the epidermis.

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## Figures

Figure 1

Schematic of tight-seal acrylic chamber, spray system, tissue phantom, and PDPA components. (a) Spray system and tissue phantom were placed inside to conduct experiments at reduced, constant relative humidity levels (16–18%). (b) A thin film thermocouple embedded in epoxy was used to measure surface temperatures. (c) Chamber walls were designed to be perpendicular to PDPA components to minimize refraction.

Figure 2

Count of coincident and noncoincident measurements in 1ms time bins during CSC

Figure 3

(a) Experimental surface temperature (left scale) and estimated surface heat flux q″ (right scale). (b) Tissue and phantom surface temperature response, θt and θp, to q″, and mapped temperature, θp′, matching tissue response.

Figure 4

(a) Velocity and (b) diameter distributions for the cone center of a cryogen spray in steady state 32.5mm away from the nozzle tip

Figure 5

Average cryogen droplet (a) velocity and (b) diameter as a function of time during 10ms, 30ms, and 50ms cryogen spurts. The vertical dashed lines represent the beginning and end, t¯o and t¯f, of the spray steady state.

Figure 6

Average tissue phantom surface (a) temperature and (b) heat flux as a function of time during 10ms, 30ms, and 50ms spurts. The vertical dashed lines represent the beginning and end, t¯o and t¯f, of the spray steady state.

Figure 7

Spray number, (a)–(c), mass, (d)–(f), and kinetic energy, (g)–(i), fluxes during 10ms, 30ms, and 50ms spurts. The vertical dashed lines represent the beginning and end (left and right lines, respectively) of the spray steady state.

Figure 8

Estimated human tissues surface thermal responses during CSC with 10ms, 30ms, and 50ms spurts for tissues with 0.3 and 0.6 water contents

## Errata

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