Laboratory Rat Thrombi Lose One-Third of Their Stiffness When Exposed to Large Oscillating Shear Stress Amplitudes: Contrasting Behavior to Human Clots

Abstract

Rats impress by their high platelet count resulting in hypercoagulability, which protects the animals from severe bleeding. However, platelets also import numerous stiff junction points into the fibrous system of a clot, also enhancing the pre-stress of the fibrin fibers, which lowers their deformability. Clot deformation is clinically important since large strains are present in the arterial tree (caused by the propagation of pressure and pulse waves), and a clot is considered “safe” when it can deform over a long range of strain amplitudes. We tested clot formation and the behavior of fully formed blood clots of laboratory rats at large sinusoidal shear stress amplitudes by rheometry and compared outcomes to human reference data. We found that fiber density (by scanning electron microscopy) and clot stiffness was pronounced compared to humans and differed with sexual dimorphism and with rat strain. Using our large amplitude oscillation (LAOS) protocol, we detected that rat clots yielded with a frustrated attempt to stiffen instead of showing the macroscopic stiffening response that is typical for human clots. We attribute this behavior to the appearance of multiple microfractures until, finally, a few leading fibers uptake the load. Rat clots also failed to align fibers in shear direction to initiate affine deformation. The rat clot phenotype differs substantially from the human one, which must be considered in research and toxicological testing. If microfractures in the fiber meshwork are concentrated in vivo, parts of a clot may break off and be washed away. However, homogenously distributed microfractures may open pores and allow the penetration of plasminogen activators. What occurs in the rat vasculature depends on the on-site clot composition.

Clots reveal anomalous elastic behavior of fiber networks

“…We also show that this arises from the bending-to-buckling transition, which occurs at low strains, and is an anomalous elastic effect associated with a different, additional power law regime. This effect may be a broader phenomenon: Recent rheological measurements of clots from rats ( 75 ) reported a decrease in shear modulus with increasing shear deformation, although the mechanistic or mechanical basis of such softening response was not investigated. Here, by comparing the mechanics of uncross-linked and cross-linked PPP networks, we show that the anomalous softening-stiffening behavior of uncross-linked fibers is due to a transition from bending to buckling-dominated response, which softens a fraction of the fibers originally under compression.…”

Clots reveal anomalous elastic behavior of fiber networks

“…Although softening of fiber networks has been reported in simulations ( 47 ),( 48 ) in this work we provide the first experimental proof of this effect in crosslinking-inhibited plasma clots and also show that this arises from the bending-to-buckling transition which occurs at low strains. This effect may be a broader phenomenon: recent rheological measurements of clots from rats ( 65 ) reported a significant decrease in shear modulus with increasing shear deformation, although the mechanistic or mechanical basis of such softening response was not investigated. Here, by comparing the mechanics of uncrosslinked and crosslinked PPP networks, we show that the atypical softening-stiffening behavior of uncrosslinked fibers originates from a bending to buckling-dominated response, which softens a fraction of the fibers originally under compression.…”