Determining the mechanical properties of individual fibrin fibers

Fibrin fibers are the major structural component of blood clots, which perform the mechanical task of stemming the flow of blood. Determining the mechanical properties of individual fibrin fiber will advance our understanding of the clotting process and the underlying causes of some fibrin-related disease. Moreover it will allow us to develop a good mechanical model of blood clots. In this research, we combined an Atomic Force Microscope with an inverted Fluorescent Microscope and also developed an accurate and direct calibration method for lateral force measurements. With these techniques, we were able to collect stress-strain curves on single fibrin fibers in aqueous conditions, which allowed us to extract numerous mechanical properties of fibrin fibers.; Fibrin fibers show extraordinary extensibility and elasticity. Thrombin-induced crosslinked fibers can be stretched 332%+/-71% of their original length before rupture and stretched up to 180%(2.8 times of their original length) without permanent lengthening. Moreover we also determined the following mechanical properties: (1) Fibrin fibers show clear viscoelastic properties; (2) the ratio of lost to stored energy increases with increasing strain; up to 85% energy loss was observed; (3) The Young's modulus increases with increasing strain (strain hardening), on average about 7.1 MPa and 5.5 MPa for crosslinked and uncrosslinked fiber, respectively; (4) by taking incremental stress-strain curve, we determined that the stress in fibrin fibers relaxes with two different relaxation rates; a fast rate of about 2.0 seconds, and a slow rate of about 55 seconds; (5) the preliminarily results show that the strength and toughness of the fibrin monomer during stretch is about 330 pN and 5200 KJ/mol, respectively.