Abstract
Purpose :
To elucidate the hierarchical deformation mechanisms of corneal collagen, and quantify the contribution of a subfibrillar “spring-like” mechanism associated with the straightening of tropocollagen molecules.
Methods :
Strips were cut from human donor corneas and mounted on to a bespoke extensometer compatible with X-ray scattering apparatus at Diamond Light Source. Small and wide angle scatter patterns were acquired from the strips at rest and at known strains. Images were analysed using bespoke scripts that extracted data including collagen fibrillar and molecular spacing, orientation and coherence, as well as D-period. A scattering model was developed based upon a helical assumption of molecular morphology, which calculates molecular tilt/pitch through comparison of azimuthal distributions of fibrils and molecules.
Results :
At strain increments of 0%, 1.4%, 2.8% and 5% the average molecular tilt was calculated to be 16°, 14°, 12° and 11°. The tilt in the rest configuration matches observations from electron microscopy studies, and the changes in tilt imply fibrillar elongations of 0%, 0.9%, 1.8%, 2.1%. The D-period for fibrils aligned approximately parallel to the applied strain increased under strain, supporting the molecular tilt model, while those aligned orthogonally exhibited a decrease in D-period.
Conclusions :
The calculated change in molecular tilt supports the hypothesis that there is a significant subfibrillar deformation mechanism in corneal collagen, which can be approximated as being “spring-like”. This mechanism allows fibrils to elongate under relatively small loads, and appears to be the primary source of compliance over all hierarchies at specimen strains of up to 2.8%, which more than covers the physiological and pathological range. This deformation mechanism is likely to be more energetically efficient than straightening of crimp over larger length scales, as it will displace negligible amounts of interstitial fluid and proteoglycan.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.