Author(s): Vogel, Alfred; Venugopalan, Vasan | Abstract: The mechanisms of pulsed laser ablation of biological tissues were studied. The transiently empty space created between the fiber tip and the tissue surface improved the optical transmission to the target and thus increased the ablation efficiency. It was found that the structure and morphology also affect the energy transport among tissue constituents.

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Mechanisms of pulsed laser ablation of biological tissues.

A single diagram, encompassing most medical applications for all types of laser in current use, forms the basis of this review of recent medical developments. Emphasis is placed on the physical processes that govern different microscopic mechanisms of laser-tissue interaction. Four distinct photophysical groups are considered in a general classification of these specific modes of interaction: for continuous wave exposure, the photothermal and the photochemical transformations; and, for pulsed irradiations, the electromechanical and the photoablative processes.

Photophysical processes in recent medical laser developments: A review

When pulsed, ultraviolet laser radiation falls on the surface of an organic polymer or biological tissue, the material at the surface is spontaneously etched away to a depth of 0.1 to several micrometers. In the process, the depth of etching is controlled by the width of the pulse and the fluence of the laser, and there is no detectable thermal damage to the substrate. The material that is removed by etching consists of products ranging from atoms to small fragments of the polymer. They are ejected at supersonic velocities. This dry photoetching technique is useful in patterning polymer films. It is also under serious investigation in several areas in surgery.

https://science.sciencemag.org/content/sci/234/4776/559.full.pdf

Ablation of Polymers and Biological Tissue by Ultraviolet Lasers

The thermal damage caused by 2.94-micron Er:YAG laser ablation of skin, cornea, aorta, and bone was quantified. The zone of residual thermal damage produced by normal-spiking-mode pulses (pulse duration approximately 200 microseconds) and Q-switched pulses (pulse duration approximately 90 ns) was compared. Normal-spiking-mode pulses typically leave 10-50 microns of collagen damage at the smooth wall of the incisions; however, at the highest fluences (approximately 80J/cm2) tears were produced in cornea and aorta and as much as 100 microns of damaged collagen is found at the incision edge. Q-switched pulses caused less thermal damage, typically 5-10 microns of damage in all tissues.

/pdf/er-yag-laser-ablation-of-tissue-effect-of-pulse-duration-and-1hw9w7oo3e.pdf

Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage.

Laser-tissue interactions

Radiation of 193‐nm (ArF excimer laser) wavelength can cause the etching of surfaces of poly(ethylene terephthalate) (PET) films in a controlled manner without any subsequent processing. Etch rates of 1200 A/pulse of 370 mJ/cm2 were realized in air. The reaction can be attributed to (ijk) the high absorption cross section of the films for the radiation which results in the energy being trapped in the first 2700 A, (ii) the high efficiency for bond breaking at these photon energies, and (iii) the formation of numerous small fragment molecules which promotes their volatization. The process is observed in a vacuum but is considerably modified in the presence of air.

R. Srinivasan

Papers

Self-developing photoetching of poly(ethylene terephthalate) films by far-ultraviolet excimer laser radiation