Measuring mechanical anisotropy of the cornea with Brillouin microscopy

Amira M Eltony,Peng Shao,Seok-Hyun Yun

doi:10.1038/s41467-022-29038-5

Abstract

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.

Highlights

Load-bearing tissues are typically fortified by networks of protein fibers, often with preferential orientations
We measure the corresponding mechanical anisotropy in longitudinal modulus of the bulk porcine cornea using Brillouin microscopy which we find to be consistent with the microstructural-scale imaging
X-ray diffraction measurements suggest that collagen fibrils do not form a perfect crystalline lattice, but instead exhibit shortrange order[24]

Summary

Introduction

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. We use Brillouin optical microscopy to visualize and quantify the anisotropic mechanical properties of corneal tissues at different length scales. There has been sustained interest in characterizing the mechanical anisotropy of soft tissues, at the microstructural scale, in order to build accurate constitutive models which can be used to predict the tissues’ response to mechanical loading[10]. Brillouin scattering has revealed anisotropic properties of solid-state materials[14], silks[15], and plant stems[16], but to our knowledge, has not yet been used to analyze soft biological tissue such as the cornea. Measuring the degree of anisotropy may be a useful indicator of changes in the cornea’s collagen organization due to disease or following surgical intervention[17–19]

Methods

Results

Conclusion