LIRIC (laser induced refractive index change) employs femtosecond laser pulses to inscribe local refractive index (RI) changes in corneal tissue via multi-photon absorption. Using a near-infrared wavelength (810 nm) offers a decreased risk of photochemical damage to the retina for clinical applications of LIRIC for refractive vision correction compared to shorter wavelengths that have been used, however the multi-photon order is higher, limiting the amount of RI change achievable before damage. Excised rabbit corneal tissue samples were soaked in 0.25% and 0.50% solutions of either riboflavin or sodium fluorescein, photosensitizers with large two-photon absorption cross-sections at 810 nm that are biologically compatible and safe for use in the cornea. Near-infrared LIRIC was performed on these doped cornea samples as well as on undoped samples using two different laser sources: a Ytterbium fiber laser at 5 MHz and a Ti:Sapphire oscillator at 80 MHz, with average powers in the laser focal volume up to 140 mW and 950 mW, respectively. For the native cornea, similar RI change was induced at both repetition rates for a scanning speed of 10 mm/s (5 MHz) and 20 mm/s (80 MHz). Doping the cornea with either sodium fluorescein or riboflavin allowed for a 10x increase in scanning speed at 5 MHz and a 5x increase in scanning speed at 80 MHz while producing a similar magnitude of RI change to that of the native cornea. With all samples, using the lower repetition rate allowed for a large reduction in the average power needed to induce a similar amount of RI change.
Blue intratissue refractive index shaping (blue-IRIS) is a method with potential to correct ocular refraction noninvasively in humans. To date, blue-IRIS has only ever been applied to cat corneas and hydrogels. To test the comparability of refractive index change achievable in cat and human tissues, we used blue-IRIS to write identical phase gratings in ex vivo feline and human corneas. Femtosecond pulses (400 nm) were focused ∼300 μm below the epithelial surface of excised cat and human corneas and scanned to write phase gratings with lines ∼1 μm wide, spaced 5 μm apart, using a scan speed of 5 mm/s. Additional cat corneas were used to test writing at 3 and 7 mm/s in order to document speed dependence of the refractive index change magnitude. The first-order diffraction efficiency was immediately measured and used to calculate the refractive index change attained. Our data show that blue-IRIS induces comparable refractive index changes in feline and human corneas, an essential requirement for further developing its use as a clinical vision correction technique.
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