We investigated the mechanisms of material ejection in Q-switched Er:YAG laser tissue ablation. Q-switched laser ablation at moderate and high radiant exposrues is associated with very high volumetric energy densities in the target material. For water, an initial phase of nonequilibrium surface 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 radiation 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, nosecondayr expulsion was observed even though the recoil stress largely exceeeds 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.
We investigated the mechanisms of material ejection in IR laser tissue ablation using free-running Er:YAG laser pulses. We found that for water and soft tissues such as liver the primary mechanism for material ejection is a phase explosion within the target material. The recoil induced by the primary material ejection causes a secondary material expulsion that largely increases the ablation efficiency. For mechanically stronger tissues such as skin, the material ejection is driven by confined boiling, and recoil-induced expulsion plays no role. The high efficiency of recoil-induced material expulsion is associated with a loss in ablation precision and an increase of mechanical side effects that needs to be considered for an appropriate choice of laser parameters.