Nonlinear thermal response of water-saturated tissue phantoms under IR laser heating: experiment and modelling

Abstract

The thermal response of model media exposed to laser heating at a wavelength of 1.56 μm to 90 °C is studied using infrared thermography and numerical modeling. The test samples included epoxy resin with scattering polystyrene particles, a cover glass, and a polyacrylamide hydrogel with a water content of 90%. The kinetics of the central temperature T₀(t) and the effective radius of the thermal field on the sample surface, obtained from measured thermograms and the calculated spatiotemporal temperature distribution assuming a Gaussian profile, were compared. A linear thermal response is observed for the epoxy resin and glass, well described by a model with a constant effective absorption coefficient. A pronounced discrepancy is detected for the hydrogel: the measured and calculated kinetic curves of the temperature fields demonstrated a significant difference. Differential scanning calorimetry does not reveal any structural or phase transitions in the studied temperature range. Experiments with a hydrogel coated with a glass plate (evaporation suppression) show that the main cause of nonlinearity is water evaporation from the near-surface layer. A thermal model has been developed that takes into account temperature-dependent absorption of water and heat loss due to evaporation. It is shown that updated model qualitatively reproduces the T₀(t) kinetics, but quite differ on periphery of irradiated zone. It means that additional factors must be taken into account, such as changes in the optical properties of the near-surface layer and thermocapillary effects. The practical implications of the identified effects for laser processing of biological tissues with thermographic control are discussed.

Speaker

Alexander Sviridov
NIC "Kurchatov Instituter"
Russia

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