Growth kinetics and structure of fibrin gels

The structure and kinetics of fibrin gels grown from fibrinogen solutions under quasiphysiological conditions, but in absence of Ca⁺⁺, were investigated by means of elastic light scattering. By combining classical light scattering and low-angle elastic light scattering, an overall wave-vector range of about three decades was spanned, from q~3×10² to q~3×10⁵ cm^-1. The scattered intensity distribution of the gels was measured in absolute units and fitted to a single function, which was able to reproduce accurately the data over the entire wave-vector range. From the fitting, it was possible to estimate the average diameter d of the fibrin fibers, the average crossover length ξ of the gel, and establish the fractal nature of the gel structure, with a measure of its fractal dimension D_m. The measure of the intensity in absolute units also allowed the estimate of the density ρ of the fibrin fibers and provided an independent measure of their size. The kinetics of formation of the gel was described in terms of a simple growth model: the scaffold of the network is formed very early in the course of the gelation process, at a ``networking time,'' t_n, which is much smaller than the time required to form the final gel. At times t>t_n, the gel structure remains substantially unchanged and the successive growth consists only in a thickening of the gel fibers. Gels prepared under the same physical-chemical conditions, but at different fibrinogen concentrations, exhibited rather similar structures and kinetics, showing that the modalities of the gelation process are mainly governed by the solution conditions, and only secondarily by the fibrinogen concentration. For gels at fibrinogen concentration of ~0.24 mg/ml, the gel parameters were d~130 nm, ξ~27 μm, D_m~1.3, and ρ~0.4 g/cm³. Our d and ρ values are in very good agreement with electron microscopy- and turbidity-derived literature data, respectively, while ξ seems to be related to the mesh size of the initial scaffold formed at t_n, rather than to the mesh size of the final aged gel.