Mechanical Strain Alters the Surface Charge of Collagen Fibrils

Collagen fibrils act like nanoscale cables in the extracellular matrix of vertebrate tissues and provide a scaffold for cells to attach to. However, beyond this mechanical function, the surface charge of collagen fibrils is also likely to play an important role. Here, we show that native, type I collagen fibrils from a mammal tendon exhibit a particular dependence of surface charge on longitudinal strain. Fibrils first become more positive with strain of up to 10% and then become more negative again with strain between 10 and 17%. The effect correlates with the stiffness of fibrils and can be explained by structural rearrangements, which expose hidden, ionizable residues. Fibrils treated with glutaraldehyde did not show any change in surface charge when strained. The electrical surface potential, which is directly related to the number ratio of exposed amine and carboxy groups on the surface, was determined by Kelvin-probe force microscopy of fibrils attached on an extensible, thin polymer film. By stretching the film, a large number of individual fibrils could be strained simultaneously without resorting to sophisticated nanomechanical devices. It is conceivable that cells react to such changes of the fibril charge and that this effect is an additional contributor, besides mechanics, to a number of physiological processes. It may also need to be considered in the design of tissue-engineering scaffolds.