Thermal fluctuations of fibres at short time scales.

Fibrin clots are important adaptive structural elements during haemostasis and owe their physiological performance largely to their unique elastic properties (see ref. 1). The fibrin fibres in clots are self-assembled aggregates of fibrinogen monomers catalysed by the interaction with the enzyme thrombin. We investigated the dynamics of both individual fibrin fibres and whole networks with the passive particle tracking technique using an ultra-fast digital CCD camera, high quality differential interference contrast microscopy and careful control of the ambient vibrational noise. The dynamics of fibrin fibres observed at short time scales (down to 10-4 s) agrees with predictions from semi-flexible polymer theory. The time dependence of the mean-square displacement (MSD) of transverse fluctuations was observed to follow a power-law behaviour, Δr⊥2(t) ∼ t3/4, except in the case of a fibrin clot exposed to shear stress that contained regions with a scaling exponent close to 1/2. We measured the persistence length of individual fibrin fibres and revealed a dependence of the saturation value of the MSD on the coordination number of network branch points previously unobserved for fibrin networks. Monitoring the motions of individual semi-flexible polymers at very short time scales provides a critical test for prevailing theories of polymer dynamics and the nature of the hydrodynamic interaction at short time scales.