The Mechanical Properties of Individual Crosslinked and Uncrosslinked Fibrin Fibers

Abstract

Blood clots have the mechanical task of stemming blood flow; yet despite decades of work our understanding of the mechanical properties of blood clots is still poor. In fact, currently there is no good mechanical model of a blood clot available. One of the key missing elements in constructing a model of a blood clot is knowledge of the mechanical properties of the clots constituents, fibrin fibers.We have used a combined atomic force/fluorescence microscopy technique to determine numerous mechanical properties of single fibrin fibers both crosslinked by clotting factor XIIIa and uncrosslinked. In this technique, fibers are suspended across grooves and the atomic force microscope is used to stretch the fibers and record force data. Meanwhile, the fluorescence microscope records images of the stretching process.We have determined uncrosslinked fibers are very soft, with a modulus of 3.9 MPa, but increase in modulus (stiffness) at ∼100% strain by a factor of 3. They also have a sigmoid shape to their energy loss, at low strain fibers display little energy loss while at high strains 70% of the energy used to stretch the fiber is lost. In comparison, crosslinked fibers were stiffer, with a modulus of 8.0 MPa, less extensible, deformed at strains as low as 10%, strain hardened by a factor of 1.9 and showed large energy loss beginning at low strains. The viscoelastic properties of fibrin fibers were determined and it was shown crosslinking has the effect of decreasing extensibility and increasing modulus and dissipated energy.