Extended field-of-view microendoscopy through aberration corrected GRIN lenses

Graded index (GRIN) lens-based microendoscopes are widely used to perform two-photon fluorescence microscopy in deep (> 1 mm) regions of highly scattering biological tissue, such as the mammalian brain. However, GRIN microendoscopes are limited by intrinsic aberrations which severely restrict the usable field-of-view (FOV). The effect of aberrations is particularly relevant in ultrathin (diameter < 500 μm) microendoscopes which allow a less invasive insertion of the optical probe into the brain tissue but which are characterized by relatively small imaging FOV. Currently, there are limited commercially available solutions to correct aberrations in these ultrathin microendoscopes because of the difficulty in fabricating corrective optics at the small spatial scale corresponding to the microendoscope diameter. Here, we report the development and application of a new approach to correct aberrations in GRIN microendoscopes using microfabricated polymeric lenses. Corrective optical elements were first designed using optical simulation software, then fabricated by two-photon lithography, and finally coupled with the GRIN lens to generate aberration-corrected microendoscopic probes. The method that we developed was applied to several types of GRIN lenses that differed in length and diameter, and corrected microendoscopes had up to 9 folds larger FOV compared to uncorrected probes. We put corrected microendoscopes to the test by performing high-resolution functional imaging of hundreds of hippocampal or thalamic cells expressing genetically encoded fluorescent indicators in the mouse brain in vivo.