Elastic Behavior and Platelet Retraction in Low- and High-Density Fibrin Gels

Abstract

Fibrin is a biopolymer that gives thrombi the mechanical strength to withstand the forces imparted on them by blood flow. Importantly, fibrin is highly extensible, but strain hardens at low deformation rates. The density of fibrin in clots, especially arterial clots, is higher than that in gels made at plasma concentrations of fibrinogen (3-10 mg/mL), where most rheology studies have been conducted. Our objective in this study was to measure and characterize the elastic regimes of low (3-10 mg/mL) and high (30-100 mg/mL) density fibrin gels using shear and extensional rheology. Confocal microscopy of the gels shows that fiber density increases with fibrinogen concentration. At low strains, fibrin gels act as thermal networks independent of fibrinogen concentration. Within the low-strain regime, one can predict the mesh size of fibrin gels by the elastic modulus using semiflexible polymer theory. Significantly, this provides a link between gel mechanics and interstitial fluid flow. At moderate strains, we find that low-density fibrin gels act as nonaffine mechanical networks and transition to affine mechanical networks with increasing strains within the moderate regime, whereas high-density fibrin gels only act as affine mechanical networks. At high strains, the backbone of individual fibrin fibers stretches for all fibrin gels. Platelets can retract low-density gels by >80% of their initial volumes, but retraction is attenuated in high-density fibrin gels and with decreasing platelet density. Taken together, these results show that the nature of fibrin deformation is a strong function of fibrin fiber density, which has ramifications for the growth, embolization, and lysis of thrombi.

Interplay of Platelet Contractility and Elasticity of Fibrin/Erythrocytes in Blood Clot Retraction

“…Although fibrin and RBCs are viscoelastic in nature, they are modeled as elastic materials due to the elastic nature of their mechanical response at low levels of strain rate (Supporting Materials and Methods, section H). As fibrin has been shown to behave as a semiflexible polymer (20,21), it is possible to determine the theoretical bulk and shear moduli for fibrin alone (Supporting Materials and Methods, section B), which are in agreement with reported experimental mechanical properties of fibrin (11,30). This can be used transitively to determine the development of the fibrin network (19).…”

“…Cross-linking results in the stabilization of the viscoelastic fibrin network, which has many fibers originating from platelet aggregates (6)(7)(8)(9). The activated platelets are able to generate contractile forces that are propagated through the cross-linked fibrin fibers (10,11), effectively resulting in a reduction of the clot volume (12,13). The volume reduction of the clot, or clot contraction, has been suggested to play a critical role in hemostasis (14), wound healing, and the restoration of blood flow past otherwise obstructive blood clots (15).…”

Fibrin structural and diffusional analysis suggests that fibers are permeable to solute transport

“…The storage modulus in the linear elastic regime is well-described by entropic models valid for semiflexible polymers [36], being governed by the total fiber length and the fiber persistence length, which in turn is dependent on the number of protofibrils per fiber. Dense fibrin networks with small fiber segment lengths between junctions and networks of thick fibers behave as athermal fibrous networks even at low strain [37,38].…”

Flow chamber and microfluidic approaches for measuring thrombus formation in genetic bleeding disorders

“…61 Despite these challenges, the motivation for measuring thrombus formation in hemophilia blood under flow stems from the well-documented regulation of coagulation by blood flow and the role of fibrin in establishing the mechanical properties of thrombi. 62–64 …”