THE THERMODYNAMIC RESPONSE OF SOFT BIOLOGICAL TISSUES TO PULSED ULTRAVIOLET-LASER IRRADIATION

The physical mechanisms that enable short pulses of high-intensity ult raviolet laser radiation to remove tissue, in a process known as laser ablation, remain obscure. The thermodynamic response of biological ti ssue to pulsed laser irradiation was investigated by measuring and sub sequently analyzing the stress transients generated by pulsed argon fl uorine (ArF, lambda = 193 nm) and krypton fluorine (KrF, lambda = 248 nm) excimer laser irradiation of porcine dermis using thin-film piezoe lectric transducers. For radiant exposures that do not cause material removal, the stress transients are consistent with rapid thermal expan sion of the tissue. At the threshold radiant exposure for ablation, th e peak stress amplitude generated by 248 nm irradiation is more than a n order of magnitude larger than that produced by 193 nm irradiation. For radiant exposures where material removal is achieved, the temporal structure of the stress transient indicates that the onset of materia l removal occurs during irradiation. In this regime, the variation of the peak compressive stress with radiant exposure is consistent with l aser-induced rapid surface vaporization. For 193 nm irradiation, ioniz ation of the ablated material occurs at even greater radiant exposures and is accompanied by a change in the variation of peak stress with r adiant exposure consistent with a plasma-mediated ablation process. Th ese results suggest that absorption of ultraviolet laser radiation by the extracellular matrix of tissue leads to decomposition of tissue on the time scale of the laser pulse. The difference in volumetric energ y density at ablation threshold between the two wavelengths indicates that the larger stresses generated by 248 nm irradiation may facilitat e the onset of material removal. However, once material removal is ach ieved, the stress measurements demonstrate that energy not directly re sponsible for target decomposition contributes to increasing the speci fic energy of the plume (and plasma, when present), which drives the g as dynamic expansion of ablated material. This provides direct evidenc e that ultraviolet laser ablation of soft biological tissues is a surf ace-mediated process and not explosive in nature.