Growth kinetics and structure of fibrin gels

Light Scattering and Turbidimetry Techniques for the Characterization of Nanoparticles and Nanostructured Networks

Light scattering and turbidimetry techniques are classical tools for characterizing the dynamics and structure of single nanoparticles or nanostructured networks. They work by analyzing, as a function of time (Dynamic Light Scattering, DLS) or angles (Static Light Scattering, SLS), the light scattered by a sample, or measuring, as a function of the wavelength, the intensity scattered over the entire solid angle when the sample is illuminated with white light (Multi Wavelength Turbidimetry, MWT). Light scattering methods probe different length scales, in the ranges of ~5−500 nm (DLS), or ~0.1−5 μm (Wide Angle SLS), or ~1−100 μm (Low Angle SLS), and some of them can be operated in a time-resolved mode, with the possibility of characterizing not only stationary, but also aggregating, polymerizing, or self-assembling samples. Thus, the combined use of these techniques represents a powerful approach for studying systems characterized by very different length scales. In this work, we will review some typical applications of these methods, ranging from the field of colloidal fractal aggregation to the polymerization of biologic networks made of randomly entangled nanosized fibers. We will also discuss the opportunity of combining together different scattering techniques, emphasizing the advantages of a global analysis with respect to single-methods data processing.

Pinhole mirror-based ultra-small angle light scattering setup for simultaneous measurement of scattering and transmission

An ultra-small angle light scattering setup with the ability of simultaneous registration of scattered light by a charge-coupled device camera and the transmitted direct beam by a pin photodiode was developed. A pinhole mirror was used to reflect the scattered light; the transmitted direct beam was focused and passed through the central pinhole with a diameter of 500 μm. Time-resolved static light scattering measurement was carried out over the angular range 0.2° ≤θ≤ 8.9° with a time resolution of ∼33 ms. The measured scattering pattern in the q-range between 5 × 10^-5 and 1.5 × 10^-3 nm^-1 enables investigating structures of few micrometers to submillimeter, where q is the scattering vector. A LabVIEW-based graphical user interface was developed, which integrates the data acquisition of the scattering pattern and the transmitted intensity. The Peltier temperature-controlled sample cells of varying thicknesses allow for a rapid temperature equilibration and minimization of multiple scattering. The spinodal decomposition for coacervation (phase separation) kinetics of an aqueous mixture of oppositely charged polyelectrolytes was demonstrated.

Mesopore Controls the Responses of Blood Clot-Immune Complex via Modulating Fibrin Network

Formation of blood clots, particularly the fibrin network and fibrin network-mediated early inflammatory responses, plays a critical role in determining the eventual tissue repair or regeneration following an injury. Owing to the potential role of fibrin network in mediating clot-immune responses, it is of great importance to determine whether clot-immune responses can be regulated via modulating the parameters of fibrin network. Since the diameter of D-terminal of a fibrinogen molecule is 9 nm, four different pore sizes (2, 8, 14, and 20 nm) are rationally selected to design mesoporous silica to control the fibrinogen adsorption and modulate the subsequent fibrin formation process. The fiber becomes thinner and the contact area with macrophages decreases when the pore diameters of mesoporous silica are greater than 9 nm. Importantly, these thinner fibers grown in pores with diameters larger than 9 nm inhibit the M1-polorazation of macrophages and reduce the productions of pro-inflammatory cytokines and chemokines by macrophages. These thinner fibers reduce inflammation of macrophages through a potential signaling pathway of cell adhesion-cytoskeleton assembly-inflammatory responses. Thus, the successful regulation of the clot-immune responses via tuning of the mesoporous pore sizes indicates the feasibility of developing advanced clot-immune regulatory materials.

Effect of Fibrin Concentration on the In Vitro Production of Dermo-Epidermal Equivalents

Human plasma-derived bilayered skin substitutes were successfully used by our group to produce human-based in vitro skin models for toxicity, cosmetic, and pharmaceutical testing. However, mechanical weakness, which causes the plasma-derived fibrin matrices to contract significantly, led us to attempt to improve their stability. In this work, we studied whether an increase in fibrin concentration from 1.2 to 2.4 mg/mL (which is the useful fibrinogen concentration range that can be obtained from plasma) improves the matrix and, hence, the performance of the in vitro skin cultures. The results show that this increase in fibrin concentration indeed affected the mechanical properties by doubling the elastic moduli and the maximum load. A structural analysis indicated a decreased porosity for the 2.4 mg/mL hydrogels, which can help explain this mechanical behavior. The contraction was clearly reduced for the 2.4 mg/mL matrices, which also allowed for the growth and proliferation of primary fibroblasts and keratinocytes, although at a somewhat reduced rate compared to the 1.2 mg/mL gels. Finally, both concentrations of fibrin gave rise to organotypic skin cultures with a fully differentiated epidermis, although their lifespans were longer (25–35%) in cultures with more concentrated matrices, which improves their usefulness. These systems will allow the generation of much better in vitro skin models for the testing of drugs, cosmetics and chemicals, or even to “personalized” skin for the diagnosis or determination of the most effective treatment possible.

Molecular packing structure of fibrin fibers resolved by X-ray scattering and molecular modeling

Fibrin is the major extracellular component of blood clots and a proteinaceous hydrogel used as a versatile biomaterial. Fibrin forms branched networks built of laterally associated double-stranded protofibrils. This multiscale hierarchical structure is crucial for the extraordinary mechanical resilience of blood clots, yet the structural basis of clot mechanical properties remains largely unclear due, in part, to the unresolved molecular packing of fibrin fibers. Here the packing structure of fibrin fibers is quantitatively assessed by combining Small Angle X-ray Scattering (SAXS) measurements of fibrin reconstituted under a wide range of conditions with computational molecular modeling of fibrin protofibrils. The number, positions, and intensities of the Bragg peaks observed in the SAXS experiments were reproduced computationally based on the all-atom molecular structure of reconstructed fibrin protofibrils. Specifically, the model correctly predicts the intensities of the reflections of the 22.5 nm axial repeat, corresponding to the half-staggered longitudinal arrangement of fibrin molecules. In addition, the SAXS measurements showed that protofibrils within fibrin fibers have a partially ordered lateral arrangement with a characteristic transverse repeat distance of 13 nm, irrespective of the fiber thickness. These findings provide fundamental insights into the molecular structure of fibrin clots that underlies their biological and physical properties.

Filamentous and step-like behavior of gelling coarse fibrin networks revealed by high-frequency microrheology

By a micro-experimental methodology, we study the ongoing molecular process inside coarse fibrin networks by means of microrheology. We made these networks gelate around a probe microbead, allowing us to observe a temporal evolution compatible with the well-known molecular formation of fibrin networks in four steps: monomer, protofibril, fiber and network. Thanks to the access that optical-trapping interferometry provides to the short-time scale on the bead's Brownian motion, we observe a Kelvin-Voigt mechanical behavior from low to high frequencies, range not available in conventional rheometry. We exploit that mechanical model for obtaining the characteristic lengths of the filamentous structures composing these fibrin networks, whose obtained values are compatible with a non-affine behavior characterized by bending modes. At very long gelation times, a ω^7/8 power-law is observed in the loss modulus, theoretically related with the longitudinal response of the molecular structures.

Revealing the molecular origins of fibrin's elastomeric properties by in situ X-ray scattering

Fibrin is an elastomeric protein forming highly extensible fiber networks that provide the scaffold of blood clots. Here we reveal the molecular mechanisms that explain the large extensibility of fibrin networks by performing in situ small angle X-ray scattering measurements while applying a shear deformation. We simultaneously measure shear-induced alignment of the fibers and changes in their axially ordered molecular packing structure. We show that fibrin networks exhibit distinct structural responses that set in consecutively as the shear strain is increased. They exhibit an entropic response at small strains (<5%), followed by progressive fiber alignment (>25% strain) and finally changes in the fiber packing structure at high strain (>100%). Stretching reduces the fiber packing order and slightly increases the axial periodicity, indicative of molecular unfolding. However, the axial periodicity changes only by 0.7%, much less than the 80% length increase of the fibers, suggesting that fiber elongation mainly stems from uncoiling of the natively disordered αC-peptide linkers that laterally bond the molecules. Upon removal of the load, the network structure returns to the original isotropic state, but the fiber structure becomes more ordered and adopts a smaller packing periodicity compared to the original state. We conclude that the hierarchical packing structure of fibrin fibers, with built-in disorder, makes the fibers extensible and allows for mechanical annealing. Our results provide a basis for interpreting the molecular basis of haemostatic and thrombotic disorders associated with clotting and provide inspiration to design resilient bio-mimicking materials. STATEMENT OF SIGNIFICANCE: Fibrin provides structural integrity to blood clots and is also widely used as a scaffold for tissue engineering. To fulfill their biological functions, fibrin networks have to be simultaneously compliant like skin and resilient against rupture. Here, we unravel the structural origin underlying this remarkable mechanical behaviour. To this end, we performed in situ measurements of fibrin structure across multiple length scales by combining X-ray scattering with shear rheology. Our findings show that fibrin sustains large strains by undergoing a sequence of structural changes on different scales with increasing strain levels. This demonstrates new mechanistic aspects of an important biomaterial's structure and its mechanical function, and serves as an example in the design of biomimicking materials.

Fast and reliable advanced two-step pore-size analysis of biomimetic 3D extracellular matrix scaffolds

The tissue microenvironment is a major contributor to cellular functions, such as cell adhesion, migration and invasion. A critical physical parameter for determining the effect of the microenvironment on cellular functions is the average pore-size of filamentous scaffolds, such as 3D collagen fiber matrices, which are assembled by the polymerization of biopolymers. The scaffolds of these matrices can be analyzed easily by using state-of-the-art laser scanning confocal imaging. However, the generation of a quantitative estimate of the pore-size in a 3D microenvironment is not trivial. In this study, we present a reliable and fast analytical method, which relies on a two-step 3D pore-size analysis utilizing several state-of-the-art image analysis methods, such as total variation (TV) denoising and adaptive local thresholds, and another crucial parameter, such as pore-coverage. We propose an iterative approach of pore-size analysis to determine even the smallest and obscure pores in a collagen scaffold. Additionally, we propose a novel parameter, the pseudo-pore-size, which describes a virtual scaffold porosity. In order to validate the advanced two-step pore-size analysis different types of artificial collagens, such as a rat and bovine mixture with two different collagen concentrations have been utilized. Additionally, we compare a traditional approach with our method using an artificially generated network with predefined pore-size distributions. Indeed, our analytical method provides a precise, fast and parameter-free, user-independent and automatic analysis of 3D pore topology, such as pore-sizes and pore-coverage. Additionally, we are able to determine non-physiological network topologies by taking the pore-coverage as a goodness-of-fit parameter.

Effect of iron oxide nanoparticles on fibrin gel formation and its fractal dimension

In this paper, we studied the influence of nonmagnetic iron oxide nanoparticles on fibrin gel formation and its structure using dynamic light scattering. The surface of nanoparticles produced by a new method in acoustoplasma discharge with cavitation has specific morphology and accelerates the rate of fibrin gel formation, i.e., activates the enzyme thrombin. We studied changes in the form of autocorrelation functions of the scattered light intensity for fibrinogen-thrombin samples with different thrombin concentrations as well as the nanoparticles addition. Appearance of the power-law term in the function was an indicator of gel formation in the sample. Application of Martin's theory allows estimating the exponent φ of power-law function and the contribution of the diffusive mode of protofibrils. We found that an increase in thrombin concentration or its activation with iron oxide nanoparticles leads to decreasing contribution of the diffusive mode, and increasing contribution of the exponent of power-law function. The values of fractal dimension D_f calculated using Muthukumar's theory are 1.61 ± 0.13 and 1.69 ± 1.11 for samples with low and high concentrations of thrombin respectively and 1.77 ± 0.08 for the sample with thrombin activated by nanoparticles. Such an increase in fractal dimension shows an increase in the complexity of the fibrin gel structure (or density).

Fibrinography: A Multiwavelength Light-Scattering Assay of Fibrin Structure

We have previously developed a fibrin structural assay dedicated to purified fibrinogen-thrombin system. Here, we extend the pertinence of this test, called Fibrinography, to tissue factor-triggered plasma coagulation. We show that Fibrinography determines quantitatively the structure of fibrin fibers in plasma with an excellent reproducibility. We compare this assay with the commonly used single wavelength turbidity method, showing that the latter is not a proper structural assay, but determines essentially the fibrinogen content in plasma. In addition, we also show, in model plasmas, that Fibrinography is able to discriminate normal and hypocoagulant plasmas, and even between hypercoagulant plasmas. Therefore, Fibrinography, by measuring the final step of the coagulation cascade, may be used to evaluate patients’ plasma in hypo- or hypercoagulant diseases.

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

Fibrin hydrogels are promising carrier materials in tissue engineering. They are biocompatible and easy to prepare, they can bind growth factors and they can be prepared from a patient's own blood. While fibrin structure and mechanics have been extensively studied, not much is known about the relation between structure and diffusivity of solutes within the network. This is particularly relevant for solutes with a size similar to that of growth factors. A novel methodological approach has been used in this study to retrieve quantitative structural characteristics of fibrin hydrogels, by combining two complementary techniques, namely confocal fluorescence microscopy with a fiber extraction algorithm and turbidity measurements. Bulk rheological measurements were conducted to determine the impact of fibrin hydrogel structure on mechanical properties. From these measurements it can be concluded that variations in the fibrin hydrogel structure have a large impact on the rheological response of the hydrogels (up to two orders of magnitude difference in storage modulus) but only a moderate influence on the diffusivity of dextran solutes (up to 25% difference). By analyzing the diffusivity measurements by means of the Ogston diffusion model we further provide evidence that individual fibrin fibers can be semi-permeable to solute transport, depending on the average distance between individual protofibrils. This can be important for reducing mass transport limitations, for modulating fibrinolysis and for growth factor binding, which are all relevant for tissue engineering.

STATEMENT OF SIGNIFICANCE

Fibrin is a natural biopolymer that has drawn much interest as a biomimetic carrier in tissue engineering applications. We hereby use a novel combined approach for the structural characterization of fibrin networks based on optical microscopy and light scattering methods that can also be applied to other fibrillar hydrogels, like collagen. Furthermore, our findings on the relation between solute transport and fibrin structural properties can lead to the optimized design of fibrin hydrogel constructs for controlled release applications. Finally, we provide new evidence for the fact that fibrin fibers may be permeable for solutes with a molecular weight comparable to that of growth factors. This finding may open new avenues for tailoring mass transport properties of fibrin carriers.

A constitutive model for a maturing fibrin network

Blood clot formation is crucial to maintain normal physiological conditions but at the same time involved in many diseases. The mechanical properties of the blood clot are important for its functioning but complicated due to the many processes involved. The main structural component of the blood clot is fibrin, a fibrous network that forms within the blood clot, thereby increasing its mechanical rigidity. A constitutive model for the maturing fibrin network is developed that captures the evolving mechanical properties. The model describes the fibrin network as a network of fibers that become thicker in time. Model parameters are related to the structural properties of the network, being the fiber length, bending stiffness, and mass-length ratio. Results are compared with rheometry experiments in which the network maturation is followed in time for various loading frequencies and fibrinogen concentrations. Three parameters are used to capture the mechanical behavior including the mass-length ratio. This parameter agrees with values determined using turbidimetry experiments and is subsequently used to derive the number of protofibrils and fiber radius. The strength of the model is that it describes the mechanical properties of the maturing fibrin network based on it structural quantities. At the same time the model is relatively simple, which makes it suitable for advanced numerical simulations of blood clot formation during flow in blood vessels.

Molecular mechanisms of the effect of ultrasound on the fibrinolysis of clots

BACKGROUND

Ultrasound accelerates tissue-type plasminogen activator (t-PA)-induced fibrinolysis of clots in vitro and in vivo.

OBJECTIVE

To identify mechanisms for the enhancement of t-PA-induced fibrinolysis of clots.

METHODS

Turbidity is an accurate and convenient method, not previously used, to follow the effects of ultrasound. Deconvolution microscopy was used to determine changes in structure, while fluorescence recovery after photobleaching was used to characterize the kinetics of binding/unbinding and transport.

RESULTS

The ultrasound pulse repetition frequency affected clot lysis times, but there were no thermal effects. Ultrasound in the absence of t-PA produced a slight but consistent decrease in turbidity, suggesting a decrease in fibrin diameter due solely to the action of the ultrasound, likely caused by an increase in protofibril tension because of vibration from ultrasound. Changes in fibrin network structure during lysis with ultrasound were visualized in real time by deconvolution microscopy, revealing that the network becomes unstable when 30-40% of the protein in the network was digested, whereas without ultrasound, the fibrin network was digested gradually and retained structural integrity. Fluorescence recovery after photobleaching during lysis revealed that the off-rate of oligomers from digesting fibers was little affected, but the number of binding/unbinding sites was increased.

CONCLUSIONS

Ultrasound causes a decrease in the diameter of the fibers due to tension as a result of vibration, leading to increased binding sites for plasmin(ogen)/t-PA. The positive feedback of this structural change together with increased mixing/transport of t-PA/plasmin(ogen) is likely to account for the observed enhancement of fibrinolysis by ultrasound.

Collagen I self-assembly: revealing the developing structures that generate turbidity

Type I collagen gels are routinely used in biophysical studies and bioengineering applications. The structural and mechanical properties of these fibrillar matrices depend on the conditions under which collagen fibrillogenesis proceeds, and developing a fuller understanding of this process will enhance control over gel properties. Turbidity measurements have long been the method of choice for monitoring developing gels, whereas imaging methods are regularly used to visualize fully developed gels. In this study, turbidity and confocal reflectance microscopy (CRM) were simultaneously employed to track collagen fibrillogenesis and reconcile the information reported by the two techniques, with confocal fluorescence microscopy (CFM) used to supplement information about early events in fibrillogenesis. Time-lapse images of 0.5 mg/ml, 1.0 mg/ml, and 2.0 mg/ml acid-solubilized collagen I gels forming at 27°C, 32°C, and 37°C were collected. It was found that in situ turbidity measured in a scanning transmittance configuration was interchangeable with traditional turbidity measurements using a spectrophotometer. CRM and CFM were employed to reveal the structures responsible for the turbidity that develops during collagen self-assembly. Information from CRM and transmittance images was collapsed into straightforward single variables; total intensity in CRM images tracked turbidity development closely for all collagen gels investigated, and the two techniques were similarly sensitive to fibril number and dimension. Complementary CRM, CFM, and in situ turbidity measurements revealed that fibril and network formation occurred before substantial turbidity was present, and the majority of increasing turbidity during collagen self-assembly was due to increasing fibril thickness.

Spatiotemporal characterization of a fibrin clot using quantitative phase imaging

Studying the dynamics of fibrin clot formation and its morphology is an important problem in biology and has significant impact for several scientific and clinical applications. We present a label-free technique based on quantitative phase imaging to address this problem. Using quantitative phase information, we characterized fibrin polymerization in real-time and present a mathematical model describing the transition from liquid to gel state. By exploiting the inherent optical sectioning capability of our instrument, we measured the three-dimensional structure of the fibrin clot. From this data, we evaluated the fractal nature of the fibrin network and extracted the fractal dimension. Our non-invasive and speckle-free approach analyzes the clotting process without the need for external contrast agents.

Biomimetic fibrin–hyaluronan hydrogels for nucleus pulposus regeneration

Aim: To develop a biomimetic polymeric injectable hydrogel that can support nucleus pulposus (NP) regeneration. Materials & methods: Natural polymer-based hydrogels were synthesized using fibrinogen (FBG) and hyaluronic acid (HA), conjugated by a novel two-step procedure. Bovine NP cells were cultured in FBG–HA conjugate-based 3D beads in vitro and in a nucleotomized organ culture model. Results: FBG–HA conjugate-based hydrogels prepared with 235 KDa HA at a FBG/HA w/w ratio of 17:1 showed superior gel stability and mechanical properties and markedly increased glycosaminoglycan synthesis compared with a FBG/HA mixture-based hydrogels or fibrin gels. Gene-expression levels of NP markers were maintained in vitro. In organ culture, NP cells seeded in FBG–HA conjugate-based hydrogels showed better integration with native NP tissue compared with fibrin gels. Moreover, FBG–HA conjugate-based hydrogels restored compressive stiffness and disc height after nucleotomy under dynamic load. Conclusion: Specific FBG–HA conjugate-based hydrogels may be suitable as injectable materials for minimally invasive, biological NP regeneration.

A comprehensive mechanism of fibrin network formation involving early branching and delayed single- to double-strand transition from coupled time-resolved X-ray/light-scattering detection

The formation of a fibrin network following fibrinogen enzymatic activation is the central event in blood coagulation and has important biomedical and biotechnological implications. A non-covalent polymerization reaction between macromolecular monomers, it consists basically of two complementary processes: elongation/branching generates an interconnected 3D scaffold of relatively thin fibrils, and cooperative lateral aggregation thickens them more than 10-fold. We have studied the early stages up to the gel point by fast fibrinogen:enzyme mixing experiments using simultaneous small-angle X-ray scattering and wide-angle, multi-angle light scattering detection. The coupled evolutions of the average molecular weight, size, and cross section of the solutes during the fibrils growth phase were thus recovered. They reveal that extended structures, thinner than those predicted by the classic half-staggered, double-stranded mechanism, must quickly form. Following extensive modeling, an initial phase is proposed in which single-bonded "Y-ladder" polymers rapidly elongate before undergoing a delayed transition to the double-stranded fibrils. Consistent with the data, this alternative mechanism can intrinsically generate frequent, random branching points in each growing fibril. The model predicts that, as a consequence, some branches in these expanding "lumps" eventually interconnect, forming the pervasive 3D network. While still growing, other branches will then undergo a Ca(2+)/length-dependent cooperative collapse on the resulting network scaffolding filaments, explaining their sudden thickening, low final density, and basic mechanical properties.

Modeling of fibrin gels based on confocal microscopy and light-scattering data

Fibrin gels are biological networks that play a fundamental role in blood coagulation and other patho/physiological processes, such as thrombosis and cancer. Electron and confocal microscopies show a collection of fibers that are relatively monodisperse in diameter, not uniformly distributed, and connected at nodal points with a branching order of ∼3-4. Although in the confocal images the hydrated fibers appear to be quite straight (mass fractal dimension D(m) = 1), for the overall system 1<D(m)<2. Based on the confocal images, we developed a method to generate three-dimensional (3D) in silico gels made of cylindrical sticks of diameter d, density ρ, and average length <L>, joined at randomly distributed nodal points. The resulting 3D network strikingly resembles real fibrin gels and can be sketched as an assembly of densely packed fractal blobs, i.e., regions of size ξ, where the fiber concentration is higher than average. The blobs are placed at a distance ξ0 between their centers of mass so that they are overlapped by a factor η =ξ/ξ0 and have D(m) ∼1.2-1.6. The in silico gels' structure is quantitatively analyzed by its 3D spatial correlation function g(3D)(r) and corresponding power spectrum I(q) = FFT(3D[g3D(r)]), from which ρ, d, D(m), η, and ξ0 can be extracted. In particular, ξ0 provides an excellent estimate of the gel mesh size. The in silico gels' I(q) compares quite well with real gels' elastic light-scattering measurements. We then derived an analytical form factor for accurately fitting the scattering data, which allowed us to directly recover the gels' structural parameters.

Fast two-dimensional bubble analysis of biopolymer filamentous networks pore size from confocal microscopy thin data stacks

The average pore size ξ0 of filamentous networks assembled from biological macromolecules is one of the most important physical parameters affecting their biological functions. Modern optical methods, such as confocal microscopy, can noninvasively image such networks, but extracting a quantitative estimate of ξ0 is a nontrivial task. We present here a fast and simple method based on a two-dimensional bubble approach, which works by analyzing one by one the (thresholded) images of a series of three-dimensional thin data stacks. No skeletonization or reconstruction of the full geometry of the entire network is required. The method was validated by using many isotropic in silico generated networks of different structures, morphologies, and concentrations. For each type of network, the method provides accurate estimates (a few percent) of the average and the standard deviation of the three-dimensional distribution of the pore sizes, defined as the diameters of the largest spheres that can be fit into the pore zones of the entire gel volume. When applied to the analysis of real confocal microscopy images taken on fibrin gels, the method provides an estimate of ξ0 consistent with results from elastic light scattering data.

Multiangle static and dynamic light scattering in the intermediate scattering angle range

We describe a light scattering apparatus based on a novel optical scheme covering the scattering angle range 0.5° ≤ θ ≤ 25°, an intermediate regime at the frontier between wide angle and small angle setups that is difficult to access by existing instruments. Our apparatus uses standard, readily available optomechanical components. Thanks to the use of a charge-coupled device detector, both static and dynamic light scattering can be performed simultaneously at several scattering angles. We demonstrate the capabilities of our apparatus by measuring the scattering profile of a variety of samples and the Brownian dynamics of a dilute colloidal suspension.

Magnetic rotational spectroscopy with nanorods to probe time-dependent rheology of microdroplets

In situ characterization of minute amounts of fluids that rapidly change their rheological properties is a challenge. In this paper, the rheological properties of fluids were evaluated by examining the behavior of magnetic nanorods in a rotating magnetic field. We proposed a theory describing the rotation of a magnetic nanorod in a fluid when its viscosity increases with time exponentially fast. To confirm the theory, we studied the time-dependent rheology of microdroplets of 2-hydroxyethyl-methacrylate (HEMA)/diethylene glycol dimethacylate (DEGDMA)-based hydrogel during photopolymerization synthesis. We demonstrated that magnetic rotational spectroscopy provides rich physicochemical information about the gelation process. The method allows one to completely specify the time-dependent viscosity by directly measuring characteristic viscosity and characteristic time. Remarkably, one can analyze not only the polymer solution, but also the suspension enriched with the gel domains being formed. Since the probing nanorods are measured in nanometers, this method can be used for the in vivo mapping of the rheological properties of biofluids and polymers on a microscopic level at short time intervals when other methods fall short.

Internal structures of agar-gelatin co-hydrogels by light scattering, small-angle neutron scattering and rheology

Internal structures of agar-gelatin co-hydrogels were investigated as a function of their volumetric mixing ratio, [Formula: see text] , 1.0 and 2.0 using dynamic light scattering (DLS), small-angle neutron scattering (SANS) and rheology. The degree of non-ergodicity ( X = 0.2 ± 0.02) , which was extracted as a heterodyne contribution from the measured dynamic structure factor data remained less than that of homogeneous solutions where ergodicity is expected (X = 10. The static structure factor, I(q) , results obtained from SANS were interpreted in the Guinier regime (low-q , which implied the existence of ≈ 250 nm long rod-like structures (double-helix bundles), and the power law (intermediate-q regions) yielded I (q) ~ q(−α) with α = 2.3 , 1.8 and 1.6 for r = 0.5 , 1.0 and 2.0. This is indicative of the presence of Gaussian chains at low r , while at r = 2 there was a propensity of rod-shaped structures. The gel strength and transition temperatures measured from frequency sweep and temperature ramp studies were suggestive of the presence of a stronger association between the two biopolymer networks at higher r . The results indicate that the internal structures of agar-gelatin co-hydrogels were highly dependent on the volumetric mixing ratio.

Nanostructure of the fibrin clot

The nanostructure of the fibrin fibers in fibrin clots is investigated by using spectrometry and small angle x-ray scattering measurements. First, an autocoherent analysis of the visible light spectra transmitted through formed clots is demonstrated to provide robust measurements of both the radius and density of the fibrin fibers. This method is validated via comparison with existing small-angle and dynamic light-scattering data. The complementary use of small angle x-ray scattering spectra and light spectrometry unambiguously shows the disjointed nature of the fibrin fibers. Indeed, under quasiphysiological conditions, the fibers are approximately one-half as dense as their crystalline fiber counterparts. Further, although the fibers are locally crystalline, they appear to possess a lateral fractal structure.

In situ formation, manipulation, and imaging of droplet-encapsulated fibrin networks

The protein fibrin plays a principal role in blood clotting and forms robust three dimensional networks. Here, microfluidic devices have been tailored to strategically generate and study these bionetworks by confinement in nanoliter volumes. The required protein components are initially encapsulated in separate droplets, which are subsequently merged by electrocoalescence. Next, distinct droplet microenvironments are created as the merged droplets experience one of two conditions: either they traverse a microfluidic pathway continuously, or they "park" to fully evolve an isotropic network before experiencing controlled deformations. High resolution fluorescence microscopy is used to image the fibrin networks in the microchannels. Aggregation (i.e."clotting") is significantly affected by the complicated flow fields in moving droplets. In stopped-flow conditions, an isotropic droplet-spanning network forms after a suitable ripening time. Subsequent network deformation, induced by the geometric structure of the microfluidic channel, is found to be elastic at low rates of deformation. A shape transition is identified for droplets experiencing rates of deformation higher than an identified threshold value. In this condition, significant densification of protein within the droplet due to hydrodynamic forces is observed. These results demonstrate that flow fields considerably affect fibrin in different circumstances exquisitely controlled using microfluidic tools.

Cl- and F- anions regulate the architecture of protofibrils in fibrin gel

Ischemic heart disease is the leading cause of serious morbidity and mortality in Western society. One of the therapeutic approaches is based on the use of thrombolitic drugs that promote clot lysis. Even if the mechanisms leading to clot lysis are not completely understood, it is widely accepted that they depend on the complex biochemical reactions that occur among fibrin fibers and fibrinolitic agents, and by their ready diffusion into the fibers. Here we investigate the effects of specific anions on the architecture of protofibrils within fibrin fibers in fibrin gels prepared in a para-physiological solution. The results obtained through small-angle X-ray scattering (SAXS) demonstrate that the characteristic axial and longitudinal repeat distances among protofibrils are strongly affected by the action of Cl(-) and F(-) anions.

Modifications in solvent clusters embedded along the fibers of a cellulose polymer network cause paper degradation

Plants, algae, and their derivatives (paper, textiles, etc.) are complex systems that are chiefly composed of a web of cellulose fibers. The arrangement of solvents within the polymeric structure is of great importance since cellulose degradation is strongly influenced by water accessibility and external agents. In this paper we develop a model that is able to deconvolve the scattering contributions of both polymeric structures and solvent clusters trapped along the polymeric fibers. The surface morphology of cellulose fibers and the spatial distribution of water-filled pores and their dimensions have been recovered from small angle neutron scattering and atomic force microscopy data in function with paper degradation. In addition to providing a boost to the effort to preserve cellulose-supported material (included cultural heritage), the relevance of our model resides in the exploitation of a large number of biopolymer networks that are known to share structures similar to that of cellulose.

Magnetically-Guided Self-Assembly of Fibrin Matrices with Ordered Nano-Scale Structure for Tissue Engineering

The development of effective biological scaffold materials for tissue engineering and regenerative medicine applications hinges on the ability to present precise environmental cues to specific cell populations to guide their position and function. Natural extracellular matrices have an ordered nano-scale structure that can modulate cell behaviors critical for developmental control, including directional cell motility. Here we describe a method for fabricating fibrin gels with defined architecture on the nanometer scale in which magnetic forces are used to position thrombin-coated magnetic micro-beads in a defined 2-dimensional array and thereby guide the self-assembly of fibrin fibrils through catalytic cleavage of soluble fibrinogen substrate. Time-lapse and confocal microscopy confirmed that fibrin fibrils nucleate near the surface of the thrombin-coated beads and extend out in a radial direction to form these gels. When controlled magnetic fields were used to position the beads in hexagonal arrays, the fibrin nano-fibrils that polymerized from the beads oriented preferentially along the bead--bead axes in a geodesic (minimal path) pattern. These biocompatible scaffolds supported adhesion and spreading of human microvascular endothelial cells, which exhibited co-alignment of internal actin stress fibers with underlying fibrin nano-fibrils within some membrane extensions at the cell periphery. This magnetically-guided, biologically-inspired microfabrication system is unique in that large scaffolds may be formed with little starting material, and thus it may be useful for in vivo tissue engineering applications in the future.

Measurement of the ultrasonic attenuation coefficient of human blood plasma during clotting in the frequency range of 8 to 22 MHz

The blood coagulation mechanism consists of a series of concatenated chemical reactions, governed by the coagulation factors present in the blood plasma, after the activation of the clot mechanism. The last reaction corresponds to the fibrinogen conversion into fibrin, followed by the fibrin polymerisation and production of a stable fibrin network. During the clotting process, there is a sol-gel transformation of the medium. The subject of the present paper is the measurement of the ultrasonic attenuation coefficient for human blood plasma during the coagulation process, in the frequency range of 8 to 22 MHz. The clot was obtained after the procedure to measure the prothrombin time (approximately 12 s): mixing 150 microL of reconstituted lyophilised normal plasma with 300 microL of reconstituted lyophilised thromboplastin immersed in a water bath with the temperature controlled at 36.5 degrees C. The attenuation coefficient for pure plasma remained constant within the measurement period of 10 s and at frequencies of 8, 9, 10, 15, 20, 21 and 22 MHz. On the other hand, there is a detectable time-decay of the attenuation coefficient for samples of plasma going through the coagulation process and at frequencies of 8, 9, 10 and 15 MHz. The time-decay becomes less and less detectable as the frequency increases and it becomes completely undetectable at 20, 21 and 22 MHz.

Gelation dynamics and gel structure of fibrinogen

Gelation dynamics and gel structure of fibrinogen induced by serine protease, thrombin, was investigated by light scattering, real space observation using confocal laser scanning microscopy (CLSM), and turbidity. Effects of additives, such as (linear) saccharides, glucose to dextran, and cyclodextrin, were studied focusing on the interaction with fibrin(ogen) and thrombin. Light scattering measurement was ascertained to be able to characterize the gelation process and growth kinetics. Stepwise (two-step) gelation process, formation of fibrin monomers and protofibrils followed by the lateral aggregation to form fibrin fibers and gel network, was clearly ascertained. Gelation point could be characterized quantitatively. At the gelation point, dynamic light scattering exhibited a self-similar nature of the fibrin gel network, and the fractal dimension was evaluated in good accordance with the reconstructed 3D image of gel network by CLSM. The interaction between the additives and fibrin(ogen) and thrombin were studied by the inhibition test using synthesized substrate. Temporal variation of microstructure of fibrin gel network (lateral fiber growth) was investigated by turbidity in detail. Addition of saccharides affects significantly the network formation as revealed by turbidity. The interaction of dextran with fibrin fibers was examined by fluorescence microscopy, too, and the characteristic spatial distribution was observed.

Fibrinogen and fibrin

Fibrinogen is a large, complex, fibrous glycoprotein with three pairs of polypeptide chains linked together by 29 disulfide bonds. It is 45 nm in length, with globular domains at each end and in the middle connected by alpha-helical coiled-coil rods. Both strongly and weakly bound calcium ions are important for maintenance of fibrinogen's structure and functions. The fibrinopeptides, which are in the central region, are cleaved by thrombin to convert soluble fibrinogen to insoluble fibrin polymer, via intermolecular interactions of the "knobs" exposed by fibrinopeptide removal with "holes" always exposed at the ends of the molecules. Fibrin monomers polymerize via these specific and tightly controlled binding interactions to make half-staggered oligomers that lengthen into protofibrils. The protofibrils aggregate laterally to make fibers, which then branch to yield a three-dimensional network-the fibrin clot-essential for hemostasis. X-ray crystallographic structures of portions of fibrinogen have provided some details on how these interactions occur. Finally, the transglutaminase, Factor XIIIa, covalently binds specific glutamine residues in one fibrin molecule to lysine residues in another via isopeptide bonds, stabilizing the clot against mechanical, chemical, and proteolytic insults. The gene regulation of fibrinogen synthesis and its assembly into multichain complexes proceed via a series of well-defined steps. Alternate splicing of two of the chains yields common variant molecular isoforms. The mechanical properties of clots, which can be quite variable, are essential to fibrin's functions in hemostasis and wound healing. The fibrinolytic system, with the zymogen plasminogen binding to fibrin together with tissue-type plasminogen activator to promote activation to the active enzyme plasmin, results in digestion of fibrin at specific lysine residues. Fibrin(ogen) also specifically binds a variety of other proteins, including fibronectin, albumin, thrombospondin, von Willebrand factor, fibulin, fibroblast growth factor-2, vascular endothelial growth factor, and interleukin-1. Studies of naturally occurring dysfibrinogenemias and variant molecules have increased our understanding of fibrinogen's functions. Fibrinogen binds to activated alphaIIbbeta3 integrin on the platelet surface, forming bridges responsible for platelet aggregation in hemostasis, and also has important adhesive and inflammatory functions through specific interactions with other cells. Fibrinogen-like domains originated early in evolution, and it is likely that their specific and tightly controlled intermolecular interactions are involved in other aspects of cellular function and developmental biology.

Kinetics of Fibrinopeptide Release by Thrombin as a Function of CaCl2Concentration: Different Susceptibility of FPA and FPB and Evidence for a Fibrinogen Isoform-Specific Effect at Physiological Ca2+Concentration†

The kinetics of release of fibrinopeptide A (FPA) and B (FPB) by thrombin were investigated on unfractionated fibrinogen samples as a function of CaCl(2) concentration. A 50 mM Tris, 104 mM NaCl, pH 7.4 (TBS) buffer, to which 1 mM EDTA-Na(2) (TBE) or 2.5 (TBC2.5), 14 (TBC14), and 30 mM CaCl(2) (TBC30) was alternatively added, was employed. The % FPA versus time curves were fitted with single stretched-exponential growth functions, where the stretch parameter beta likely reflects substrate polydispersity (beta = 1, monodisperse). For TBE, TBS, TBC14, and TBC30, we found beta approximately 1, with corresponding normalized rate constants (K(a)) of 3.8, 4.2, 2.7, and 1.9 x 10(-5) [(NIHu/L)s](-1). Surprisingly, in TBC2.5 we found beta = 0.69, with an "average" K(a) of 3.5 x 10(-5) [(NIHu/L)s](-1). This effect disappeared [beta = 0.97, K(a) = 2.7 x 10(-5) [(NIHu/L)s](-1)] with an increase in the ionic strength I to that of TBC30 with 186 mM NaCl (TBCaNa buffer). FPB releases were instead consistent with a nonstretched consecutive exponential growth function, except in TBC30 where some FPB appeared to be cleaved independently. Log-log plots of K(a) versus Ca(2+) concentration, Cl(-) concentration, or I showed a strong linear correlation with only the latter two except in TBCaNa, again suggesting specific effects of the physiological Ca(2+) concentration and I on FPA release. The corresponding K(b) plots showed instead that both total depletion and high Ca(2+) hampered FPB release. To further investigate the TBC2.5 beta = 0.69 effect, FG polydispersity was assessed by Western blot analyses. The thrombin-binding gamma'-chain isoform was approximately 4%, resulting in a bound:free thrombin ratio of approximately 25:75. With regard to the C-terminal ends of the Aalpha-chains, approximately 45% were either intact or lightly degraded, while the remaining approximately 55% were more degraded. Fitting the % FPA release data in TBC2.5 with a sum of two exponentials resulted in a faster component and a slower component (K(a1)/K(a2) approximately 6), with a ratio of approximately 48:52. While a role for the gamma'-chain isoform cannot be excluded, this good correlation with the C-terminal degradation of the Aalpha-chains suggests their calcium-dependent involvement in FPA release.

Formation of fibrin gel in fibrinogen-thrombin system: static and dynamic light scattering study

The dynamics of thrombin-induced fibrin gel formation was investigated by means of static and dynamic light scattering. The decay time distribution function, obtained by the dynamic light scattering, clearly revealed a stepwise gelation process: the formation of fibrin and protofibril from fibrinogen followed by the lateral aggregation of protofibrils to form fibrin fibers and the formation of a three-dimensional network consisting of fibers. This conversion process was correlated with the angular dependence of the scattered light intensity (static light scattering). The correlation function of dynamic light scattering was analyzed in terms of sol-gel transition and gel structure. The correlation function showed a stretched exponential type behavior before the sol to gel transition point, and it showed a power law behavior at the gelation point.

Structure of fibrin gels studied by elastic light scattering techniques: dependence of fractal dimension, gel crossover length, fiber diameter, and fiber density on monomer concentration

The concentration dependence of the structure of fibrin gels, formed following fibrinogen activation by thrombin at a constant molar ratio, was investigated by means of elastic light scattering techniques. The scattered intensity distributions were measured in absolute units over a wave-vector range q of about three decades ( approximately 3x10(2)-3x10(5) cm(-1)). A set of gel-characterizing parameters were recovered by accurately fitting the data with a single function recently developed by us [F. Ferri et al., Phys. Rev. E 63, 031401 (2001)], based on a simple structural model. Accordingly, the gels can be described as random networks of fibers of average diameter d and density rho, entangled together to form densely packed and spatially correlated blobs of mass fractal dimension D(m) and average size (or crossover length) xi. As previously done for d, we show here that the recovered xi is also a good approximation of a weight average, namely, d approximately sqrt[<d2>(w)] and xi approximately <xi>(w). By varying the fibrinogen concentration c(F) between 0.034-0.81 mg/ml, gels with 100> or =xi> or =10 microm, 100< or =d< or =200 nm, 1.2< or =D(m)< or =1.4, and constant rho approximately 0.4 mg/ml were obtained. The power-law c(F) dependencies that we found for both xi and d are consistent with the model, provided that the blobs are allowed to partially overlap by a factor eta likewise scaling with c(F) (2> or =eta> or =1). Recasting the whole dataset on a single master curve provided further evidence of the similarity between the structures of all the gels, and confirmed the self-consistency of the model.