Nonuniform Internal Structure of Fibrin Fibers: Protein Density and Bond Density Strongly Decrease with Increasing Diameter

The Applicability of Current Turbidimetric Approaches for Analyzing Fibrin Fibers and Other Filamentous Networks

Turbidimetry is an experimental technique often used to study the structure of filamentous networks. To extract structural properties such as filament diameter from turbidimetric data, simplifications to light scattering theory must be employed. In this work, we evaluate the applicability of three commonly utilized turbidimetric analysis approaches, each using slightly different simplifications. We make a specific application towards analyzing fibrin fibers, which form the structural scaffold of blood clots, but the results are generalizable. Numerical simulations were utilized to assess the applicability of each approach across a range of fiber lengths and diameters. Simulation results indicated that all three turbidimetric approaches commonly underestimate fiber diameter, and that the “Carr-Hermans” approach, utilizing wavelengths in the range of 500−800 nm, provided <10% error for the largest number of diameter/length combinations. These theoretical results were confirmed, under select conditions, via the comparison of fiber diameters extracted from experimental turbidimetric data, with diameters obtained using super-resolution microscopy.

Strength, deformability and toughness of uncrosslinked fibrin fibers from theoretical reconstruction of stress-strain curves

Structural mechanisms underlying the mechanical properties of fibrin fibers are elusive. We combined tensile testing of uncrosslinked fibrin polymers in vitro and in silico to explore their material properties. The experimental stress (σ) - strain (ε) curves for fibrin fibers are characterized by elastic deformations with a weaker elastic response for ε<160% due to unraveling of αC tethers and straightening of fibrin protofibrils, and a stronger response for ε>160% owing to unfolding of the coiled coils and γ nodules in fibrin monomers. Fiber rupture for strains ε>212% is due to dissociation of the knob-hole bonds and rupture of D:D interfaces. We developed the Fluctuating Bilinear Spring model to interpret the σ-ε profiles in terms of the free energy for protofibril alignment ΔG0 = 10.1-11.5 kBT, Young's moduli for protofibril alignment Yu = 1.9-3.2 MPa and stretching Ya = 5.7-9.7 MPa, strain scale ε˜≈ 12-40% for fiber rupture, and protofibril cooperativity m= 3.6-8. We applied the model to characterize the fiber strength σcr≈ 12-13 MPa, deformability εcr≈ 222%, and rupture toughness U≈ 9 MJ/m3, and to resolve thermodynamic state functions, 96.9 GJ/mol entropy change for protofibril alignment (at room temperature) and 113.6 GJ/mol enthalpy change for protofibril stretching, which add up to 210.5 GJ/mol free-energy change. Fiber elongation is associated with protofibril dehydration and sliding mechanism to create an ordered protofibril array. Fibrin fibers behave like a hydrogel; protofibril dehydration and water expulsion account for ∼94-98% of the total free-energy changes for fiber elongation and rupture. STATEMENT OF SIGNIFICANCE: Structural mechanisms underlying the mechanical properties of fibrin fibers, major components of blood clots and obstructive thrombi, are elusive. We performed tensile testing of uncrosslinked fibrin polymers in vitro and in silico to explore their material properties. Fluctuating Bilinear Spring theory was developed to interpret the stress-strain profiles in terms of the energy for protofibril alignment, elastic moduli for protofibril alignment and stretching, and strain scale for fiber rupture, and to probe the limits of fiber strength, extensibility and toughness. Fibrin fibers behave like a hydrogel. Fiber elongation is defined by the protofibril dehydration and sliding. Structural rearrangements in water matrix control fiber elasticity. These results contribute to fundamental understanding of blood clot breakage that underlies thrombotic embolization.

Composite Inks for Extrusion Printing of Biological and Biomedical Constructs

Extrusion-based three-dimensional (3D) printing is an emerging technology for the fabrication of complex structures with various biological and biomedical applications. The method is based on the layer-by-layer construction of the product using a printable ink. The material used as the ink should possess proper rheological properties and desirable performances. Composite materials, which are extensively used in 3D printing applications, can improve the printability and offer superior performances for the printed constructs. Herein, we review composite inks with a focus on composite hydrogels. The properties of different additives including fibers and nanoparticles are discussed. The performances of various composite inks in biological and biomedical systems are delineated through analyzing the synergistic effects between the composite ink components. Different applications, including tissue engineering, tissue model engineering, soft robotics, and four-dimensional printing, are selected to demonstrate how 3D-printable composite inks are exploited to achieve various desired functionality. This review finally presents an outlook of future perspectives on the design of composite inks.

Fibrinogen and Fibrin

Fibrinogen is a large glycoprotein, synthesized primarily in the liver. With a normal plasma concentration of 1.5-3.5 g/L, fibrinogen is the most abundant blood coagulation factor. The final stage of blood clot formation is the conversion of soluble fibrinogen to insoluble fibrin, the polymeric scaffold for blood clots that stop bleeding (a protective reaction called hemostasis) or obstruct blood vessels (pathological thrombosis). Fibrin is a viscoelastic polymer and the structural and mechanical properties of the fibrin scaffold determine its effectiveness in hemostasis and the development and outcome of thrombotic complications. Fibrin polymerization comprises a number of consecutive reactions, each affecting the ultimate 3D porous network structure. The physical properties of fibrin clots are determined by structural features at the individual fibrin molecule, fibrin fiber, network, and whole clot levels and are among the most important functional characteristics, enabling the blood clot to withstand arterial blood flow, platelet-driven clot contraction, and other dynamic forces. This chapter describes the molecular structure of fibrinogen, the conversion of fibrinogen to fibrin, the mechanical properties of fibrin as well as its structural origins and lastly provides evidence for the role of altered fibrin clot properties in both thrombosis and bleeding.

Aggregates Dramatically Alter Fibrin Ultrastructure

Among the many factors influencing fibrin formation and structure (concentration, temperature, composition, pH, etc.), it has been suggested that the polydispersity of fibrinogen may play an important role. We propose here a detailed investigation of the influence of this parameter on fibrin multiscale structure. Two commercial fibrinogen preparations were used, a monodisperse and a polydisperse one. First, the respective compositions of both fibrinogen preparations were thoroughly determined by measuring the fibrin-stabilizing factor; fibronectin; α, β, and γ intact chain contents; the γ/γ' chains ratio; the N-glycosylation; and the post-translational modifications. Slight variations between the composition of the two fibrinogen preparations were found that are much smaller than the compositional variations necessary to alter significantly fibrin multiscale structure as observed in the literature. Conversely, multiangle laser light scattering-coupled size exclusion chromatography and dynamic light scattering measurements showed that the polydisperse preparation contains significant amounts of aggregates, whereas the other preparation is essentially monodisperse. The multiscale structure of the fibrins produced from those two fibrinogen preparations was determined by using x-ray scattering, spectrophotometry, and confocal microscopy. Results show that fibers made from the aggregate-free fibrinogen present a crystalline longitudinal and lateral structure and form a mikado-like network. The network produced from the aggregates containing fibrinogen looks to be partly built around bright spots that are attributed to the aggregate. The multiscale structure of mixtures between the two preparations shows a smooth evolution, demonstrating that the quantity of aggregates is a major determining factor for fibrin multiscale structure. Indeed, the effect of a few percent in the mass of aggregates is larger than any other effect because of compositional differences under the same reaction conditions. Finally, we propose a mechanistic interpretation of our results, which points at a direct role of the aggregates during polymerization, which disrupts the ideal ordering of monomers inside fibrin protofibrils and fibers.

Fibrinogen and fibrin: An illustrated review

Since its discovery over 350 years ago, studies of fibrinogen have revealed remarkable characteristics. Its complex structure as a large (340 kDa) hexameric homodimer supports complex roles in hemostasis and homeostasis. Fibrinogen synthesis is regulated at the transcriptional and translational levels, undergoing both constitutive (basal) secretion from liver, and inducible upregulation in response to inflammatory events. In addition, alternative splicing yields fibrinogen variants with unique properties and contributions to coagulation biochemistry. During coagulation, fibrinogen conversion to fibrin occurs via thrombin‐mediated proteolytic cleavage that produces intermediate protofibrils and then mature fibers that provide remarkable biochemical and mechanical stability to clots. Fibrin formation, structure, and stability are regulated by various genetic, biochemical, and environmental factors, allowing for dynamic kinetics of fibrin formation and structure. Interactions between fibrinogen and/or fibrin and plasma proteins and receptors on platelets, leukocytes, endothelial cells, and other cells enable complex functions in hemostasis, thrombosis, pregnancy, inflammation, infection, cancer, and other pathologies. Disorders in fibrinogen concentration and/or function increase risk of bleeding, thrombosis, and infection. This illustrated review covers fundamental aspects of fibrinogen and fibrin biology, biochemistry, biophysics, epidemiology, and clinical applications. Continued efforts to enhance our understanding of fibrinogen and fibrin in these processes are likely to advance treatment and prevention of many human diseases.