Subsurface femtosecond tissue alteration: selectively photobleaching macular degeneration pigments in near retinal contact.

This paper uses advances in the ultrafast manipulation of light to address a general need in medicine for a clinical approach that can provide a solution to a variety of disorders requiring subsurface tissue manipulation with ultralow collateral damage. Examples are age-related macular degeneration (AMD), fungal infections, tumors surrounded by overlying tissue, cataracts, etc. Although lasers have revolutionized the use of light in clinical settings, most lasers employed in medicine cannot address such problems of depth-selective tissue manipulation. This arises from the fact that they are mostly based on one photon based laser tissue interactions that provide a cone of excitation where the energy density is sufficiently high to excite heat or fluorescence in the entire cone. Thus, it is difficult to excite a specific depth of a tissue without affecting the overlying surface. However, the advent of femtosecond (fs) lasers has caused a revolution in multiphoton microscopy (Zipfel et al. Nat. Biotechnol. 2003, 21, 1369-1377; Denk et al. Science 1990, 248, 73-76) and fabrication (Kawata et al. Nature 2001, 412, 697-698). With such lasers, the photon energy density is only high enough for multiphoton processes in the focal volume, and this opens a new direction to address subsurface tissue manipulation. Here we show in an AMD animal model, Ccr2 KO knockout mutant mice, noninvasive, selective fs two-photon photobleaching of pigments associated with AMD that accumulate under and in ultraclose proximity to the overlying retina. Pathological evidence is presented that indicates the lack of collateral damage to the overlying retina or other surrounding tissue.