Tendon Force Transmission at the Nanoscale

The subject of this study is connective tissues, in particular tendon. Connective tissues consist of an extracellular matrix made of the protein collagen, which is responsible for the mechanical integrity of the body. While connective tissues may appear simple, there are still many aspects of connective tissue function that are poorly understood. One such aspect is the microscopic mechanisms of force transmission through tendons over macroscopic distances. Force transmission is at the heart of tendon function, but the large range of scales in the hierarchical structure of tendons has made it difficult to tackle. The tendon hierarchy ranges from molecules (2 nm) over fibrils (200 nm), fibers (2 μm) and fascicles (200 μm) to tendons (10 mm), and to derive the mechanisms of force transmission it is necessary to know the mechanical behavior at each hierarchical level.

The aim of the present work was to elucidate the mechanisms of force transmission in tendons primarily by investigating the mechanical behavior at the hierarchical level of collagen fibrils. To do so we have developed an atomic force microscopy (AFM) method for tensile testing of native collagen fibrils. The thesis contains five papers; the first two deal with the basic mechanical behavior and properties of collagen fibrils since existing knowledge is scarce and provide varying results. The first report “Viscoelastic behavior of discrete human collagen fibrils” investigated the behavior of collagen fibrils under dynamic loading and found that fibrils behave viscoelastically, which means that the stiffness depends on the rate of loading. The second paper “Tensile properties of human collagen fibrils and fascicles are insensitive to environmental salts” looked at differences in saline environment, and found no effect on the mechanical properties, in contrast to a previous report. The next two papers deal more directly with tendon force transmission. The one called “Mechanical properties of human patellar tendon at the hierarchical levels of tendon and fibril” investigated the relation between mechanical properties at the fibril and tendon hierarchical levels. The results suggested that there is little fibril slippage within a tendon. The other “Tensile force transmission in human patellar tendon fascicles is not mediated by glycosaminoglycans” looked at the involvement of glycosaminoglycans in tendon force transmission, finding no evidence for such a role. In the final (unpublished) paper, the failure mechanics of collagen fibrils were related to differences in cross-linking and found that human collagen, containing mature cross-links, behaves fundamentally different from rat-tail collagen, containing only immature cross-links.