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Visualization and Mechanical Manipulations of Individual Fibrin Fibers Suggest that Fiber Cross Section Has Fractal Dimension 1.3

M. Guthold ^*, W. Liu ^*, B. Stephens ^*, S. T. Lord , R. R. Hantgan , D. A. Erie ^¶, R. M. Taylor, Jr.  and R. Superfine ^||

^* Department of Physics, Wake Forest University, Winston-Salem, North Carolina; Department of Pathology and Lab Medicine, University of North Carolina, Chapel Hill, North Carolina; Department of Biochemistry, Wake Forest University School of Medicine, Winston-Salem, North Carolina; and ^¶ Departments of Chemistry, Computer Science, and ^|| Physics, University of North Carolina, Chapel Hill, North Carolina

Correspondence: Address reprint requests to Asst. Prof. Martin Guthold, Dept. of Physics, Wake Forest University, Reynolda Station 7507, Winston-Salem, NC 27109. Tel.: 336-758-4977; E-mail: gutholdm{at}wfu.edu.

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 D^1.30±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 D^1.25±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 (D) 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.

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