Evaluation of Corneal Biomechanics Using Silk Biomaterials and Optical Coherence Elastography

The work throughout my PhD encompassed multiple different fields of bioengineering which is reflected in this thesis. The thesis work presented here is divided into three separate chapters, and while all aim to study the role of corneal biomechanics in ocular health, different approaches are taken. The first chapter involves the use of biomaterials, specifically silk fibroin films, in assessing the stiffness mechanosensing behavior of corneal epithelial cells. In light of recent publications in the field of mechanobiology demonstrating the importance of matrix stiffness on limbal stem cell maintenance and corneal epithelial wound regeneration, we believed that developing a biomaterial that could control the stiffness microenvironment of corneal cells would be useful for corneal tissue regeneration. For this purpose, silk fibroin films were chosen as they have already demonstrated considerable promise as a suturable biomaterial that can act as a corneal scaffold during wound regeneration. In this work, methods were developed to both control the mechanical stiffness of the silk films and also for their mechanical characterization using atomic force microscopy (AFM). Culturing corneal epithelial cells onto silk films of varying stiffnesses demonstrated a strong correlation of mechanosensing markers with the stiffness of the films. This work helped demonstrate that the mechanical properties silk films could be tuned to control the stiffness of the cellular microenvironment, and thus, could help control corneal cell behavior for promoting wound healing of the ocular surface. The second chapter of this thesis involves the exploration of optical coherence elastography (OCE) as an imaging method to evaluate corneal tissue stiffness in situ. We felt that a significant limitation to the field of mechanobiology was the lack of imaging methods that could provide quantitative information of tissue stiffness non-destructively. Most commonly used methods of tissue mechanical characterization, including AFM, require extraction of the tissue from its organ and are therefore destructive by nature. While valuable information can still be obtained using these techniques, the mechanical environment of the tissue can change dramatically once extracted and prepared for testing. OCE was explored as a clinically promising, non-destructive method that could quantify stiffness directly in situ. OCE is based on the principles of elastography where mechanical shear waves are generated and tracked within a tissue to measure properties such as tissue stiffness. This chapter focuses on the establishment and validation of our first institutional OCE system. A proof of concept is demonstrated in ex vivo porcine corneal experiments. The following and final chapter is dedicated to examining the mathematical theories used to estimate corneal tissue stiffness from OCE-acquired wave data. These theories are important for any potential clinical and scientific applications of OCE as they ultimately determine the accuracy of OCE-based measurements. Here we explore our hypothesis that intraocular pressure (IOP) can cause significant overestimations in tissue stiffness estimates due to its effects on wave behavior. Using a theoretical approach and through analysis of published OCE data, we examine the potential effects of IOP on wave behavior in the cornea. We find that IOP is able to alter wave velocities and that its effects are dependent on elastic wave frequency. We incorporated our findings into the mathematical models governing wave behavior in corneal-like materials.