Hierarchical mechanics of collagen fibril networks

Collagen is the main structural protein of the mammalian body. It makes up 25% of the total protein mass, and is responsible for the resilience and integrity of most tissues like skin, tendon, cartilage and bone.
Most collagenous tissues consist chiefly of collagen I: a rod-shaped, triple helical protein with a mass of 300 kDa. These proteins self-organise into fibrils with a diameter ranging from several to hundreds of nanometers. The fibrils then connect to form a network with remarkable mechanical properties. For example, it exhibits a behavior known as strain stiffening, which means its resistance to deformation increases sharply if it is deformed to a certain extent. This prevents the material from changing its shape much further (which in the in vivo case might interfere with biological functioning of the tissue).
It is our goal to determine the physical cause of the mechanical properties. We wish to understand the underlying mechanisms of the mechanics at each level of complexity: the monomer level, the fibril level and the network level. This requires us to look at the structure of each of these levels as well as the mechanical properties.
We use reconstituted collagen networks. This has two major advantages: it eliminates much of the biochemical complexity that clouds true fundamental understanding in vivo, and it allows us to also study the kinetics of fibril formation, which should provide additional insight. The structures of the fibrils and networks are visualised by atomic force microscopy and fluorescence confocal microscopy, respectively. Static light scattering is used to assay fibril properties as a function of time. The mechanical properties at the network level are measured with cone-plate rheology, while at the fibril level we use active and passive laser tweezer microrheology. In order to study the effect of the intermolecular interactions at the lowest (monomer) level on the mechanics of the fibrils one level higher, we do microrheology on samples in a custom-designed microfluidic dialysis cell. By directly varying the conditions during the experiment, we can tune the intermolecular interactions inside fibrils that have already formed.

More information: Martijn de Wild

Collaborators: Kees Storm