  Load-bearing tissues are typically fortified by networks of protein fibers,
often with preferential orientations. This fiber structure imparts the tissues
with direction-dependent mechanical properties optimized to support specific
external loads. To accurately model and predict tissues' mechanical response,
it is essential to characterize the anisotropy on a microstructural scale.
Previously, it has been difficult to measure the mechanical properties of
intact tissues noninvasively. Here, we use Brillouin optical microscopy to
visualize and quantify the anisotropic mechanical properties of corneal tissues
at different length scales. We derive the stiffness tensor for a lamellar
network of collagen fibrils and use angle-resolved Brillouin measurements to
determine the longitudinal stiffness coefficients (longitudinal moduli)
describing the ex vivo porcine cornea as a transverse isotropic material.
Lastly, we observe significant mechanical anisotropy of the human cornea in
vivo, highlighting the potential for clinical applications of off-axis
Brillouin microscopy.
