Enhancement of high power pulsed laser ablation and biological hard tissue applications

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Kang, Hyun Wook

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Pulsed lasers are used therapeutically to selectively remove tissue with minimal thermal and mechanical damage to peripheral tissue. The aims of this research are to understand the governing mechanisms of pulsed infrared laser tissue ablation, to obtain optimal laser parameters for precise laser treatment without collateral damage, and to develop methods for efficient tissue removal with the aid of liquid confinement. The effect of a liquid layer on laser metal ablation was examined. Acoustic pressure and optical reflectance provided information for determining the dominant mechanisms during metal ablation. The liquid-assisted ablation improved ablation efficiency and reduced the ablation threshold by as much as 40 % with respect to ablation in air. The degree of ablation was contingent on thermal and mechanical properties of the type of metal. The dynamics of laser osteotomy in a liquid environment was explored. The underlying mechanisms during liquid-assisted ablation included rapid vaporization, plasma confinement, and cavitation with jet formation. Compared to direct ablation, the liquid-confined ablation with higher ablation volume and augmented acoustic excitation demonstrated the feasibility of liquid-enhanced laser osteotomy. Acoustic transient measurement with a piezoelectric microphone, Schlieren flash photography, and temperature measurement with an IR camera were employed to study ablation mechanisms and the effect of water spray during long-pulsed laser ablation of dental tissues. Spray-assisted ablation created larger pressure transients and enhanced ablation efficiency. Water cooling by the spray provided a safe and efficient modality for dental treatment. Finally, optimal laser parameters for laser lithotripsy such as wavelength and pulse duration were studied. Higher light absorption of the Er:YAG (λ = 2.94 µm) laser produced more material removal than the Ho:YAG laser. In addition, during Ho:YAG laser lithotripsy, a smaller fiber diameter with a shorter pulse duration reduced the operative time and cumbersome process due to retropulsive kidney stone movement, providing efficient laser lithotripsy.