The structure of collagen is very interesting. It is the most abundant protein in mammals and is the main component of connective tissue. For this reason, most of our tissues’ mechanical properties are dictated by the mechanical properties of a collagen network.
Collagen is assembled into banded fibrils. Due to the crosslinking pattern between collagen molecules there are overlap and gap regions along the fibril. When imaged by AFM the overlap regions have a larger z height than the gap regions. The overlap and gap region period is around 67nm and is called the D-spacing.
A tendon is composed of many of these fibrils. In this study they loaded rat tendons with 0%, 5% and 10% strain for 8h then fixed them. They then imaged the samples with AFM, measuring the D-spacing in the fibrils that made up the tendon. They found that the D-spacing increases with strain and that the variability of the spacing decreases with strain.
It is not surprising that the D-spacing is dependant on strain. What I found the most interesting is that the variability of spacing decreased with strain. They suggest that the D-spacing is a measurement of a fibrils individual strain and that each of the fibrils inside the tendon is under a different strain. If that is correct, it would make it easy to visualize load distributions in collagen networks and understand our tissue structure to a greater degree.