Visualization and mechanical manipulations of individual fibrin fibers suggest that fiber cross section has fractal dimension 1.3

We report protocols and techniques to image and mechanically manipulate individual fibrin fibers, which are key structural components of blood clots. Using atomic force microscopy-based lateral force manipulations we determined the rupture force, F-R, of fibrin fibers as a function of their diameter, D, in ambient conditions. As expected, the rupture force increases with increasing diameter; however, somewhat unexpectedly, it increases as F-R similar to D1.30 +/- 0.06. Moreover, using a combined atomic force microscopy-fluorescence microscopy instrument, we determined the light intensity, I, of single fibers, that were formed with fluorescently labeled fibrinogen, as a function of their diameter, D. Similar to the force data, we found that the light intensity, and thus the number of molecules per cross section, increases as I similar to D1.25 +/- 0.11. Based on these findings we propose that fibrin fibers are fractals for which the number of molecules per cross section increases as about D-1.3. This implies that the molecule density varies as rho(D) similar to D-0.7, i.e., thinner fibers are denser than thicker fibers. Such a model would be consistent with the observation that fibrin fibers consist of 70 - 80% water and only 20 - 30% protein, which also suggests that fibrin fibers are very porous.