Pulsed laser ablation of soft tissues, gels, and aqueous solutions at temperatures below 100°C

Background and Objective: It is desirable for laser microsurgical procedures to remove tissue accurately and with minimal thermal and mechanical damage to adjacent non-irradiated tissues. Pulsed laser ablation can potentially remove biological tissue with microprecision if appropriate irradiation conditions are applied. The major goal of this study was to determine whether laser ablation is possible at temperatures below 100°C. Another aim was to test thermoelastic and recoil stress magnitudes and to estimate their effects on phantom and biological tissue. Study Design/Materials and Methods: Pulsed laser ablation of water (aqueous solution of potassium chromate) and water containing soft tissues (collagen gel and pig liver) irradiated under confined stress conditions was studied. The ablation mechanism and stages of the ablation process were determined based on time-resolved measurements of laser-induced acoustic waves with simultaneous imaging of the ablation process by laser-flash photography. Results: This study reveals the important role of tensile thermoelastic stress, which produces efficient cavitation that drives material ejection at temperatures substantially below 100°C. Ablation thresholds for the aqueous solution, collagen gel, and liver were 20, 38, and 55 J/cm³, respectively, which correspond to temperature jumps of 5, 10, and 15°C. Two distinct stages of material ejection were observed: (1) initial removal of small volumes of material due to the rupture of single subsurface bubbles, (2) bulk material ablation in the form of jets produced by intense hydrodynamic motions formed upon collapse of large bubbles after coalescence of smaller bubbles. The duration of material ejection upon short-pulse ablation may vary from microseconds to submilliseconds, and depended on the mechanical properties of materials and the incident laser fluence. Conclusion: Nanosecond laser ablation of water, gels, and soft tissue under confined- stress conditions of irradiation may occur at temperatures below 100°C. This ablation regime minimizes thermal injury to adjacent tissues and involves thermoelastic stress and recoil pressure magnitudes, which may be tolerated by tissues adjacent to an ablated crater.