Boiling nucleation on melanosomes and microbeads transiently heated by nanosecond and microsecond laser pulses

Selective tissue damage on the cellular level can be achieved by microbubble formation around laser-heated intracellular pigments. To acquire a more detailed understanding of the laser tissue interaction in the highly pigmented retinal pigment epithelium (RPE), we irradiate aqueous suspensions of absorbing microparticles by short pulsed laser irradiation (12 ns, 240 ns, and 1.8 micros). Porcine retinal pigment epithelial melanosomes, gold beads, and magnetic silica beads are used as absorbers. Pulsed laser heating of the particles leads to vaporization of the surrounding liquid. The resulting transient microbubbles on the particle surface are imaged directly on a microscopic level by fast flash light photography. Furthermore, the bubble dynamics is probed by a low power laser. Threshold radiant exposures for bubble formation and nucleation temperatures are experimentally determined. Superheating of the surrounding water to 150 degrees C for melanosomes and to more than 200 degrees C for magnetic silica beads (psi = 3 microm) and gold beads (psi = 500 nm) is achieved. With these data, the absorption coefficient of a melanosome is calculated by thermal modeling of the experimental thresholds for bubble formation.