Dynamics of laser induced transient micro bubble clusters

Selective retina treatment (SRT) is a laser based therapy of retinal diseases associated with disorders of the retinal pigment epithelium (RPE) while preserving photoreceptors and choroid. Microsecond laser pulses applied to the 100-200 strongly absorbing melanin granules inside the RPE cells induce transient micro bubbles which disrupt the cells. Aim of this work is to understand bubble dynamics in clusters. Investigations were carried out on porcine RPE explants and on a floppy disc based model system. Laser pulse durations of 3 ns (532 nm, Nd:YAG) and 1.7 μs (527 nm, Nd:YLF) were used. Bubble dynamics was explored with a fiber interferometer (830 nm) and fast flash photography (25 ns). Bubble sizes, velocities and lifetimes were measured. Single nucleation sites, which coalesce after some μs, are observed with ns pulses. Using μs pulses, fewer but larger cluster sites are observed which become two times bigger at the same factor above threshold. A linear increase of the bubble hight with radiant exposure is found for both pulse durations. RPE Bubble formation thresholds of 85 mJ/cm² and 255 mJ/cm² for ns and μs pulses are found, respectively. Typical expansion velocities are 5 m/s while collapsing bubbles can reach 30 m/s. Bubble heights are up to 5 μm. Earlier investigations on single melanosomes showed a bubble size limitation to 4 μm with μs pulses. The melanosome clusters do not show this size limit. It has to be investigated whether the results are transferable to whole eye globes.