Abstract
Intra-tissue refractive index shaping (IRIS) is a novel, non-ablative form of vision correction by which femtosecond laser pulses are tightly focused into ocular tissues to induce localized refractive index (RI) change via nonlinear absorption. Here, we examined the effects of Blue-IRIS on corneal microstructure to gain insights into underlying mechanisms. Three-layer grating patterns were inscribed with IRIS ~180 µm below the epithelial surface of ex vivo rabbit globes using a 400 nm femtosecond laser. Keeping laser power constant at 82 mW in the focal volume, multiple patterns were written at different scan speeds. The largest RI change induced in this study was + 0.011 at 20 mm/s. After measuring the phase change profile of each inscribed pattern, two-photon excited autofluorescence (TPEF) and second harmonic generation (SHG) microscopy were used to quantify changes in stromal structure. While TPEF increased significantly with induced RI change, there was a noticeable suppression of SHG signal in IRIS treated regions. We posit that enhancement of TPEF was due to the formation of new fluorophores, while decreases in SHG were most likely due to degradation of collagen triple helices. All in all, the changes observed suggest that IRIS works by inducing a localized, photochemical change in collagen structure.
Highlights
Following the success of inducing phase changes in ophthalmic hydrogels in 2006 [1,2], we proposed a new paradigm for refractive vision correction, in which corneal refractive properties can be modified without flap cutting or tissue ablation
Because the fringes inside the Intra-tissue Refractive Index Shaping (IRIS) regions shifted towards the direction of decreasing optical path length, we concluded that IRIS induced a positive refractive index (RI) change in the rabbit cornea
When writing at 90 mm/s and 100 mm/s, Blue-IRIS patterns could be recognized on the Mach-Zehnder Interferometer (MZI), but the phase change was so low that signal from the patterns was below the noise floor of the MZI
Summary
Following the success of inducing phase changes in ophthalmic hydrogels in 2006 [1,2], we proposed a new paradigm for refractive vision correction, in which corneal refractive properties can be modified without flap cutting or tissue ablation This technique, termed Intra-tissue Refractive Index Shaping (IRIS), relies on tightly focused, low-energy femtosecond laser pulses to locally induce phase and refractive index (RI) changes in the corneal stroma via multiphoton absorption. While dopants were needed for 800 nm IRIS to enhance two-photon absorption and attain meaningful RI changes [3], Xu et al [4] demonstrated that Blue-IRIS can more readily induce larger RI changes because of the significant, native two-photon absorption of cornea at 400 nm. Our work in hydrogel materials demonstrated that 400 nm laser micromachining can locally decrease RI by increasing the water content of written regions, and it was assumed that this hydrophilicity was associated with depolymerization [7]
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