Quantitative phase measurements of tendon collagen fibres

Collagen is the main component of structural mammalian tissues. In tendons, collagen is arranged into fibrils with diameters ranging from 30 nm to 500 nm. These fibrils are further assembled into fibres several micrometers in diameter. Upon excessive thermal or mechanical stress, damage may occur in tendons at all levels of the structural hierarchy. At the fibril level, reported damage includes swelling and the appearance of discrete sites of plastic deformation that are best observed at the nanometer-scale using, for example, scanning electron microscopy. In this paper, digital in-line holographic microscopy is used for quantitative phase imaging to measure both the refractive index and diameter of collagen fibres in a water suspension in the native state, after thermal treatments, and after mechanical overload. Fibres extracted from tendons and subsequently exposed to 70 °C for 5, 15, or 30 minutes show a significant decrease in refractive index and an increase in diameter. A significant increase in refractive index is also observed for fibres extracted from tendons that were subjected to five tensile overload cycles.