Mechanism of precise tissue ablation with minimal side effects (under confined stress conditions of irradiation)

Pulsed laser ablation of biological tissues under confined stress conditions (PACS) is demonstrated to be precise tissue removal with minimal thermal and mechanical damage to adjacent tissue layers. PACS was described based on results obtained with four complementary techniques: (1) laser-flash photography, (2) time-resolved stress detection (TRSD), (3) laser Michelson interferometry, and (4) pendulum recoil measurements. Comparison of ablation process was made between aqueous solutions and collagen gels. Experiments demonstrated two distinct ablation stages with different ablative forces that drive material ejection at temperatures substantially below 100 degree(s)C. The first stage is associated with the ejection of thin subsurface layer of the irradiated volume. The material ejection from a superficial layer (d << 1/(mu) _eff) is caused by a rapid growth of cavitation bubbles produced by laser-induced thermoelastic expansion in irradiated volume. The second, delayed stage of material removal occurs due to the collapse of coalesced cavitation bubbles in the depth (d <EQ 1/(mu) _eff) of irradiated volume. Collapse of large cavities with subsequent generation of turbulent motion toward medium surface can stimulate jets in liquids and plumes of debris in gels. Depending on water content and mechanical properties of a medium, the second stage may be either very pronounced (aqueous solutions) or practically absent (stiff gels). Estimates predict the possibility to remove precisely only a monolayer of cells employing PACS with an appropriate combination of laser wavelength and pulse duration.