Port-wine stain (PWS), a prevalent skin disease affecting the appearance of patients, arises from vascular malformations within the dermis of skin tissue. Laser surgery is a frequently used method for treating PWS. Based on the principle of selective photothermolysis, lasers with a specific wavelength can cause permanent thermal injury to vascular lesions without causing damage to the overlying normal epidermis. However, melanin in the epidermis absorbs laser energy, potentially leading to unnecessary epidermal heating and resulting in complications such as necrosis or undesired pigmentation. Pre-cooling the epidermis with cryogen spray can effectively mitigate or even eliminate thermal injury. Although fluoride-based refrigerants are commonly used, they have limitations in terms of their cooling efficacy and environmental compatibility. Carbon dioxide (CO₂), a nontoxic, environmentally friendly, nonflammable, and readily available natural cryogen, is a promising alternative. Existing research on CO₂ spraying has primarily focused on its cleaning and steady-state heat dissipation capacities, diverging from the context of cutaneous laser surgery. Thus, this study aims to investigate transient CO₂ spray cooling by analyzing the effects of spurt duration (40?100 ms) and spray height (20?40 mm) on the cooling efficacy and radial variations in cooling. Furthermore, a modified Nusselt number is employed to extrapolate the acquired cooling capacity to the surfaces of the other materials. Subsequently, the study integrates spray cooling with laser thermal effects to calculate the temperature distribution within the skin tissue, thereby evaluating potential thermal injury. The findings of the CO₂ spray cooling and thermal injury estimation are expected to provide valuable insights for clinical applications.

In this study, an experimental system for open-loop transient spray cooling is developed. Liquid CO₂ is supplied from a CO₂ cylinder, and nitrogen gas is used to regulate the pressure of liquid CO₂, which compensates for the flow pressure losses along the pipe to avoid cavitation within the pipe. The customized solenoid valve, data acquisition (DAQ) board, and corresponding LabVIEW program work together to control the spray duration. The spray height is adjusted by using a three-dimensional positioner. An epoxy resin board is employed as the skin phantom because its thermal and physical properties are similar to those of human skin. A high-speed camera is used to record the complete spray process, and four thermocouples are used to measure the surface temperatures of the epoxy resin board. Surface temperature data are used to calculate heat fluxes, heat transfer coefficients, and modified Nusselt number correlations. Based on the simplified Pennes equation describing bioheat transfer, a numerical simulation is conducted to model skin tissue undergoing sequential spray cooling and laser irradiation. The temperature distribution within the skin tissue is calculated, and thermal injury is estimated using Arrhenius integral analysis.

As illustrated in Fig. 5, the transient CO₂ spray process can be categorized into three stages: developing, stable, and decay regimes. Extending the spurt duration of CO₂ spray cooling leads to a lower surface temperature. However, the maximum heat flux is constrained by the mass of dry ice particles participating in the surface heat exchange per unit time. Continuously increasing the spurt duration after exceeding the point at which the spray reaches a stable regime does not further enhance the maximum heat flux (Fig. 6). Spray height primarily influences the mass loss of dry ice particles owing to evaporation and sublimation during flight. Reducing the spray height results in a larger mass of dry ice particles participating in the surface heat transfer, which usually means a lower surface temperature and higher maximum heat flux (Fig. 7). Below a certain spray height, further reductions do not increase the maximum heat flux. The limiting factor shifts from the mass of dry ice particles participating in the heat exchange per unit time to the low thermal conductivity of the epoxy resin. Furthermore, CO₂ spray cooling exhibits significant radial decay owing to the lower concentration of dry ice particles away from the spray center (Fig. 8). Thermal injury estimation (Figs. 9?11) reveals that CO₂ spray cooling effectively protects the epidermis. With a spray height below 40 mm and duration of 100 ms, epidermal thermal injury is avoided. However, shorter durations and higher spray heights may lead to thermal injuries at the periphery of the cooled area. This outcome can be further optimized through additional measures, such as employing a multiorifice nozzle and decreasing the radius of the laser spot.

This study experimentally determines the cooling capacity of transient CO₂ spray and uses simulations to estimate its protective effect on the epidermis during laser therapy. A lower surface temperature is realized by extending the spray duration and decreasing spray height. The maximum heat flux is primarily influenced by the mass of dry ice particles participating in the surface heat exchange per unit time, although this influence diminishes with decreasing spray height. A radial decline in the dry ice particle concentration from the spray center results in a significant radial attenuation of CO₂ spray cooling capacity. Additionally, with appropriate spray and laser parameters, CO₂ spray cooling can protect the epidermis from thermal injury, and there is room for further optimization.