Dynamics of laser induced micro bubble clusters on tissue phantoms

Selective retina treatment (SRT) is a laser based method to treat retinal diseases associated with disorders of the retinal pigment epithelium (RPE) while preserving photoreceptors and choroid. Applying microsecond laser pulses to the 100- 200 strongly absorbing melanin granules inside the RPE cells induces transient micro bubbles which disrupt the cells. Aim of this work is to understand bubble dynamics in clusters with respect to the influence of the adjacent retina. Bubble dynamics were investigated in vitro on porcine RPE. An about 200 μm thick layer of agarose gel was applied to the RPE layer in order to simulate the mechanical properties of retina. Different laser pulse durations from 1 ns (532 nm, Nd:YAG) to 1.7 μs (527 nm, Nd:YLF) were used. The bubbles were investigated interferometrically (fiber interferometer @ 830 nm) and with fast flash photography (25 ns flash duration). Bubble lifetimes were measured. The results show that with retina phantoms the bubble formation threshold was reached at 2.5 times higher irradiation than without retina phantom for 1.7 μs laser pulses. The microbubbles generated with 1 ns laser pulses were almost not influenced by the agarose layer. Irradiation twofold over bubble formation threshold resulted in 3.5 times longer bubble lifetimes for μs and 2 times longer for ns pulse durations, respectively.