Thermal-feedback-controlled coagulation of egg white by the CO₂ laser

Temperature feedback control during laser-assisted tissue coagulation was investigated and demonstrated with the egg-white model. We observed the dynamics of photothermal denaturation during CO2 laser irradiation by simultaneously controlling surface temperature and monitoring He–Ne laser transmission of egg-white samples. Once a quasi-constant surface temperature was established, transmission of egg white tended to decrease linearly with time. Analysis of experimental data strongly suggested a first-order rate process. Since transmission was primarily affected by heat-induced increase in the scattering coefficient and depth of coagulation, we speculated that changes in transmission were reliable indicators of accumulating photothermal damage. Our experiments demonstrated that thermal feedback can effectively control or limit photothermal damage.