Thrombin effects on fibrin fiber size by atomic force microscopy. (A) Three representative atomic force microscopy images of fibrin fibers formed with human plasminogen-depleted IF-1 purified fibrinogen (1 mg/mL), 2.5 mM CaCl₂, and 0.1, 1.0, and 10 U/mL thrombin. (B) Fibrin fiber radius obtained by atomic force microscopy in liquid at 0.1, 1.0, and 10 U/mL thrombin. Each image is 4 × 4 μm, the scale bar indicates 500 nm, and 5 cross sections along the fibrin fiber are shown that were used in fiber diameter calculations. The results represent the mean values ± SD; n = 25 fibers. Statistical significance, using a 1-way ANOVA, is denoted with ***P < .005 and ****P < .001 for comparison between 0.1 U/mL and the remaining thrombin concentrations.