Small- and wide-angle elastic light scattering study of fibrin structure

We show how small- and wide-angle elastic light scattering (q ~ 0.03-30 μm^-1) can be used to quantitatively characterize the structure of polymeric gels made of semi-flexible entangled fibers. We applied the technique to the study of fibrin gels grown from the polymerization of fibrinogen (FG) macromolecular monomers following activation by the enzyme thrombin (TH), at different concentrations and under different physical-chemical conditions of the gelling solution. Our findings show that the gel can be imagined as a random network of fibers of size d and density ρ, entangled together to form densely packed blobs of mass fractal dimension D_m and average size ξ, which may overlap by a factor η and exhibit a long-range order. Provided that d ≥ 50-100 nm, all of the above parameters can be recovered by the use of a global fitting function developed by us on the basis on the proposed gel model. When the fibers are thinner (d < ~50 nm), only the fiber mass/length ratio μ ~ ρ d ² can be retrieved instead of d and ρ. Our data confirm and quantify the major changes in the gel structure that can be obtained by varying either the salt concentration of the solution and/or the molar ratio TH/FG. Gels formed in Tris-HCl 50 mM/NaCl 150 mM, pH 7.4 and TH/FG = 0.01 are characterized by relatively small, fairly branched (D_m ~ 1.4-2.0) fibers with a mass/length ratio independent of concentration. On reducing the TH/FG ratio, the fibers become increasingly thicker, with d ~ 90 nm at TH/FG = 10^-5. When the salt concentration is reduced to NaCl 100 mM (TH/FG = 0.01) the fibers are less branched (D_m ~ 1.2-1.4), but much thicker, with μ increasing by an order of magnitude. These effects are quantitatively analyzed and discussed.