Numerical analysis of thermal response and phase transition in deformable tissues during laser therapy

This dissertation develops a comprehensive Multiphysics numerical framework to systematically investigate the coupled thermal, phase change, and mechanical responses of biological tissue subjected to laser irradiation. The proposed model integrates light transport, advanced bioheat formulations including the Pennes bioheat (PBH), dual phase lag (DPL), and local thermal non equilibrium (LTNE) models, together with water mass transport and nonlinear tissue deformation. Particular emphasis is placed on the explicit incorporation of water evaporation, which is a dominant mechanism governing the thermal and mechanical behavior of soft tissue under high energy exposure conditions, such as laser therapy and hyperthermia treatments. The numerical results demonstrate that incorporating water evaporation significantly mitigates temperature overestimation and induces a transition from thermal expansion to tissue shrinkage, thereby yielding smaller predicted ablation zones and improved agreement with experimental observations. This shrinkage effect becomes increasingly pronounced at elevated temperatures, highlighting the strong coupling between thermal, mass transport, and mechanical responses. A systematic comparative assessment of bioheat models reveals that the LTNE formulation provides the closest agreement with experimental measurements; however, inherent limitations of the classical LTNE model are identified. To overcome these limitations, an extended LTNE-δ model is proposed, which enhances predictive accuracy and numerical stability, particularly under high power laser irradiation. The developed framework is subsequently applied to several representative laser therapy applications, including tumor ablation, tattoo removal, and hair removal. These case studies demonstrate the capability of the proposed model to accurately predict the spatial and temporal evolution of temperature, water content reduction, and stress generation during treatment. Overall, this work advances the fundamental understanding of laser tissue interactions and establishes a robust numerical foundation for the optimization of thermotherapy procedures, improvement of treatment safety, and support of future clinical and biomedical applications.