Fibrils

Fibrils (from the Latin fibra) are structural biological materials found in nearly all living organisms. Not to be confused with Fibers or Filaments, fibrils tend to have diameters ranging from 10-100 nm (whereas fibers are micro to milli-scale structures and filaments are having diameters approximately 10-50 nanometers in size). Fibrils not usually found alone, but rather are part of greater hierarchical structures commonly found in biological systems. Due to the prevalence of fibrils in biological systems, their study is of great importance in the fields of microbiology, biomechanics, and materials science.

Fibrils are composed of linear biopolymers, and are characterized by rod-like structures with high length to diameter ratios. Often times, they spontaneously arrange into helical structures. In biomechanics problems, fibrils can be characterized as classical beams with a roughly circular cross-sectional area on the nanometer scale. As such, simple beam bending equations can be applied to calculate flexural strength of fibrils under ultra-low loading conditions. Like most biopolymers, stress-strain relationships of fibrils tend to show a characteristic toe-heel region before a linear, elastic region. Unlike biopolymers, Fibrils do not behave like homogeneous materials, as yield strength has been shown to vary with volume, indicating structural dependencies.

Differences in structure between fibrils of different origin is typically determined by x-ray diffraction. SEM can be used to observe specific details on larger fibril species such as the characteristic 67nm bands in collagen, but often is not fine enough to determine the full structure.

Hydration has been shown to produce a noticeable effect in the mechanical properties of fibrillar materials. The presence of water has been shown to decrease the stiffness of collagen fibrils, as well as increase their rate of stress relaxation and strength. From a biological standpoint, water content acts as a toughening mechanism for fibril structures, allowing for higher energy absorption and greater straining capabilities.

...
Wikipedia