Material ejection in Q-switched Er:YAG laser ablation of water, liver, and skin

We investigated the mechanisms of material ejection in Q-switched Er:YAG laser tissue ablation. Q-switched laser ablation at moderate and high radiant exposures is associated with very high volumetric energy densities in the target material. For water, an initial phase of nonequilibrium surface vaporization is thus followed by a vapor explosion of the superficial liquid layer that is characterized by complete vaporization of the entire liquid volume. The ablation of deeper layers with lower peak temperatures proceeds as phase explosion. For mechanically strong tissues, the nonequilibrium surface vaporization is followed by a vapor explosion coupled with thermal dissociation of the biomolecules into volatile products. In deeper layers, ablation proceeds as confined boiling with mechanical tearing of the tissue matrix by the vapor pressure. The recoil stress induced by the primary material ejection at a radiant exposure of 5 J/cm² is in the order of 500-900 MPa. For water and soft tissues such as liver, the recoil causes a powerful secondary material expulsion. For mechanically stronger tissues such as skin, no secondary expulsion was observed even though the recoil stress largely exceeds the static tensile strength of the tissue. Recoil-induced material expulsion results in an increase of both ablation efficiency and mechanical side effects of ablation that becomes ever more pronounced with decreasing pulse duration. Neither the succession of phases in nanosecond-laser tissue ablation nor recoil-induced material expulsion have yet been modeled theoretically even though they are of great importance for the efficiency and precision of ablation. Their consideration remains a major challenge for future work.