Elsevier

Biophysical Chemistry

Volume 112, Issues 2–3, 20 December 2004, Pages 267-276
Biophysical Chemistry

The mechanical properties of fibrin for basic scientists and clinicians

https://doi.org/10.1016/j.bpc.2004.07.029Get rights and content

Abstract

In this review, I set forth some basic information about the mechanical properties of fibrin clots and attempt to identify the big questions remaining. The intent is to make this topic understandable to both basic scientists who are interested in blood clotting and to hematologists or cardiologists, since I believe that this is something everyone working in these fields should know. The viscoelastic properties of fibrin are remarkable and unique among polymers. Moreover, these properties are essential to the physiology of blood clotting and are important for understanding and therefore preventing and treating thrombosis.

Introduction

In my experience, most biologists, biochemists, hematologists or cardiologists do not know much about the mechanical properties of fibrin. Perhaps this topic is the least understood aspect of fibrin. Why is this? Maybe it is because this subject comprises very biophysical aspects, such that the language and techniques are foreign to most scientists outside of polymer chemistry. Structural biology may be almost as alien, with the difficulty of understanding reciprocal space in X-ray crystallography or the methods of specimen preservation for electron microscopy, but in this case the end result, i.e. the images, are more immediately comprehensible.

I think that some of the results described here will be surprising, even to experts on other aspects of fibrin. Probably the extent of what we do not yet know about this topic will also be unexpected. We do not know the answers to some of the most fundamental questions in this field. On the basic science side, we do not even know the origin of the viscoelastic properties of fibrin. On the clinical side, we know almost nothing about the mechanical properties of in vivo clots or thrombi, even though these properties are critical for the physiological function of fibrin and for the treatment of pathological conditions, such as bleeding and thrombotic disorders, including angioplasty and thrombolytic therapy. I will begin by describing briefly reasons why I think both scientists and clinicians should have a rudimentary understanding of the mechanical properties of fibrin.

Section snippets

Why are the mechanical properties of fibrin important?

The mechanical properties of fibrin are essential for its functions. In hemostasis, the clot must form a plug to stop bleeding and this structure must be strong enough to withstand the pressure of arterial blood flow. In the case of thrombus formation, the mechanical properties are also important. If a vessel is partially occluded, the viscoelastic properties of the thrombus will determine whether the flowing blood will cause it to deform reversibly or irreversibly, rupture, or embolize.

What kind of polymeric structure is a fibrin clot?

I assume that readers know major aspects of fibrinogen structure and fibrin polymerization. If not, there are good reviews available. Here I only summarize some basics. Fibrinogen is an elongated protein, 45 nm in length, and is made up of globular domains at each end connected by α-helical coiled-coils to a globular region in the middle. During clotting, thrombin converts fibrinogen to fibrin by cleaving fibrinopeptides from the central domain, exposing knobs that can then interact with holes

Branching of fibers makes the clot a network

Some proteins polymerize to make fibers, but the fibers do not form a network. For example, type I collagen and actin assemble into fibrous structures but only form a network through the intervention of other proteins. On the other hand, other proteins, such as gelatin and elastin, yield networks through intermolecular linkages but without making fibers. Ferry proposed that an interspersed array of individual fibers form the basis of the fine fibrin gel [5]. However, subsequently he and other

Fibrin is a viscoelastic polymer

Elastic solids are characterized by Hooke's law, which states that the strain, or deformation, is proportional to the stress, or force applied per area, but stress is independent of the rate of strain. On the other hand, in the classical theory of hydrodynamics, viscous materials are characterized by Newton's law, which says that the stress is proportional to the rate of strain but independent of the strain itself. Viscoelastic materials, such as fibrin, like rubber, plastic, and a great many

What are the elastic properties of fibrin?

A finite elastic modulus or stiffness appears at about the gel point or clotting time, when the network is first established [14], [15], [16] (Fig. 6). The gel point comes early in clotting, perhaps when only about 10% of the protein has been incorporated, depending on the environmental conditions. The network that is formed at the gel point is a scaffold that sets the plan for the clot structure formed by addition of further material [17]. Then, the stiffness increases as a function of time,

The origin of clot stiffness is unknown

Superficially, the measurements of fibrin elastic and inelastic components resemble those of some types of rubber, except for the magnitudes and frequency range [13]. However, clot elasticity cannot be rubber-like. Electron microcopy shows thick branching fibers (Fig. 4), instead of a random-coil network with highly flexible strands required for rubber-like elasticity. Furthermore, the average mass between branch points can be calculated, using the formula for a rubber-like polymer, Mc=cRT/G,

What are the inelastic properties of fibrin?

An elastic material with no inelasticity will deform quickly with applied stress, maintain constant deformation for long periods of stress and immediately regain its initial shape when the stress is removed. In contrast, an inelastic material with viscous energy loss will show delays in deforming with applied stress and regaining its shape afterward and will undergo continued deformation, or creep, during sustained application of constant stress.

At low frequency of oscillation (or slow

What is the mechanism of inelastic or irreversible deformation?

As described above, in fibrin there is irreversible deformation but no structural damage. The mechanisms of irreversible deformation, or the inelastic component, are unknown, but two possibilities have been suggested in the previous section for different time scales. For fine clots over long times, the knobs-into-holes bonds that hold fibrin together may occasionally break and then be re-formed in a different location [21], [24], [25]. This may be plausible for a fine clot where only a few such

Effects of factor XIIIa on viscoelastic properties

The plasma transglutaminase, Factor XIIIa, introduces a pair of ɛ-amino-(γ-glutamyl)lysine isopeptide bridges between the C-terminal γ chains of two fibrin molecules and numerous other bridges between specific donor and acceptor sites in the C-terminal two-thirds of the α chains [26]. The introduction of these cross-links has dramatic effects on the viscoelastic properties of fibrin [14], [27]. The stiffness of the clot is increased substantially and the creep or irreversible deformation is

Clot stiffness increases at high deformation

At low strains or deformations, stress is directly proportional to strain and the slope of the curve, or the elastic modulus, is constant [5]. At large strains, the slope increases dramatically, so that the modulus or stiffness of the clot increases up to a factor of 20 fold [32]. This phenomenon, which is called strain hardening or stiffening in materials science, is opposite to what happens for rubber. Strain hardening may be important biologically because it allows fibrin clots to be

Viscoelastic properties are sensitive to small changes in polymerization and clot structure

From studies of dysfibrinogenemias, in which a single base change results in an amino acid substitution or truncation, it may be concluded that viscoelastic properties are one of the most sensitive measures of the effects of such modifications on fibrin polymerization and clot structure. An extreme case is the clot formed from fibrinogen Dusart, with a substitution at Aα554 and accompanying disulfide attachment of albumin [33]. In this case, lateral aggregation is severely impaired because of

Effects of deformation on clot structure

Uniaxial stretching of fibrin films caused extensive orientation of the fibers in the direction of pull [38]. The initial random orientation returned on return of the fibrin film to its original dimensions. The notion that clot elasticity arises from bending of fibers was reinforced by observations of bent fibers in the stretched fibrin films.

A small-angle X-ray study of fibrin films showed a prominent peak that appeared to be in a location consistent with a repeat of 23 nm, based on a

Dissociation of network strands by competing reagents

Peptides with the sequence of the amino terminal end of the α chain of fibrin, glycine-proline-arginine, the knobs that are exposed by removal of the A fibrinopeptides and then fit into the holes in the γC domains at the ends of the molecule, bind strongly to fibrin(ogen) and inhibit fibrin polymerization [40]. Peptides that mimic the amino terminal end of the β chain, glycine-histidine-arginine, also inhibit polymerization but to a lesser extent. When glycine-proline-arginine-proline was

Plasma clots, platelet-rich plasma clots and whole blood clots

Clots made from purified proteins, used for nearly all of the studies in the literature, are very different than in vivo clots or thrombi. Other proteins present in plasma have large effects on the structure and mechanical properties of plasma clots [42], but little is known about the mechanisms involved. Platelets have even more dramatic effects on clot structure, organizing the fibrin around the platelet aggregates (Fig. 2B) [43], [44]. However, little is known about the mechanical properties

Acknowledgements

I thank Rustem Litvinov, Jean-Philippe Collet and Bernard Lim for suggestions on this manuscript. Research described here originating from my laboratory was funded by NIH grant HL30954.

Dedication

John Ferry was a gentleman, humble in spite of being a pioneer and having founded a whole area of polymer research. He was one of the first scientists to study purified fibrinogen, having been part of Edwin Cohn's group, which isolated the plasma proteins. His papers from the 1940's are remarkable for

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