Abstract
Purpose :
Discomfort remains the primary cause of contact lens drop-out despite decades of advancements in materials design. While the precise etiology of contact lens discomfort remains unknown, researchers have implicated inadequate lubrication in several ocular disorders, including lid wiper epitheliopathy, conjunctivochalasis, and superior limbic keratoconjunctivitis, where increased frictional forces at the eyelid-ocular surface interface result in mechanical trauma and inflammation during blinking. Contact lens wear may induce discomfort through a similar mechanism if contact pressures and shear stresses exceed physiological norms. The objective of this study was to impose a range of shear stresses and contact pressures on human corneal epithelial (hTCEpi) cell layers in vitro using hydrogel probes and determine the critical conditions beyond which the hallmarks of apoptosis and necrosis manifest in cell death detection assays.
Methods :
Friction experiments were performed against hTCEpi monolayers using a custom microtribometer mounted on an epifluorescent microscope. Solid hydrogel probes (17.5 wt.% acrylamide and 0.5 wt.% methylenbisacrylamide) were used to apply normal loads from 0.3 - 3 mN at 1 mm/s sliding speeds. Cell death pathways were detected using fluorescent probes (apoptosis: annexin-V and caspase 3/7; necrosis: propidium iodide). Indentation experiments were performed over the same range of normal forces to determine the contributions of contact pressure versus shear stress on cell death in vitro.
Results :
Cells subjected to shear stresses above ~80 Pa exhibited apoptosis within the sliding path of the probe, while shear stresses below ~80 Pa did not result in significant differences in cell death compared to a control. Gene expression analyses revealed increased expression of pro-apoptotic genes DDIT3 and FAS. Indentation experiments revealed negligible increases in cell death despite increased contact pressure, suggesting that frictional shear stress in excess of physiological norms is the dominant driver of cell death.
Conclusions :
This study revealed that frictional shear stresses in excess of 80 Pa are sufficient to trigger apoptotic pathways in human corneal epithelial cell monolayers. Understanding the extent to which frictional shear stresses can cause pro-inflammatory or cell death responses in vitro may aid in the informed design of soft contact lenses in the future.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.