The Role of Chromophore on Pulsed Laser Ablation of Biological Tissue

Despite the widespread use of pulsed lasers in biomedical applications, the physicochemical mechanisms and dynamics of ablation processes remain unclear. The role of chromophore on pulsed infrared laser ablation of porcine dermis was investigated by performing mass removal and thermal injury experiments. A tunable Transversely Excited Atmospheric (TEA) CO 2 laser was selected for this study because it is capable of targeting both the primary chromophores in porcine dermis; tissue water and tissue structural matrix. At a wavelength of 10.6gm, the dominant chromophore is tissue water. The ablation event was consistent with explosive vaporization (spinodal decomposition) yielding a heat of ablation of 3.74 x 103 kJ/kg, an ablation threshold of 1.15 J/cm 2, and a zone of thermal injury of 53itm. At a wavelength of 9.5[tm, the dominant chromophores are both tissue water and tissue structural matrix (SM). The ablation event was also consistent with explosive vaporization yielding a heat of ablation of 3.33 x 103 kJ/kg, an ablation threshold of 1.47 J/cm 2, and a zone of thermal damage of 34p1m. Under the conditions examined in this study, chromophore appears to influence ablation dynamics, but not ablation mechanism. At 9.5[tm, the SM is compromised or weakened during irradiation thereby facilitating material removal, and consequently yielding a more efficient explosive ablation event with lower thermal damage. However, at 10.6[tm, the SM remains intact during irradiation, inhibiting material removal, and hence causing a less efficient explosive ablation event with higher thermal damage. Ablation threshold does not seem to have any dependence on chromophore and scales with the volumetric energy density or superheat temperature within the tissue water. Since chromophore governs the partition of energy deposition within the tissue, we hypothesize that tissue chromophore may influence ablation mechanism under different conditions. Thesis Supervisor: B.B. Mikic Title: Professor and Associate Head of Mechanical Engineering