Titration of surface tissue factor required to trigger fibrin formation. Immunostaining of platelet GPIbα (top) and fibrin(ogen) (bottom) after 5-minute whole blood perfusion over collagen/TF arrays at wall shear rates of 100, 500, and 1000 s⁻¹ (A). Surface tissue factor concentration in each feature increased from left to right from 0 to 25 molecules-TF/μm² (scale bar = 1 mm). Increased magnification of the first row of each array for each shear rate for surface TF concentration ranging from 1.9 to 25 molecules-TF/μm² (scale bar = 300 μm; yellow indicates colocalized GPIbα and fibrin(ogen)) (B). Column averages of fluorescent intensity (FI) of the fibrin(ogen) staining are plotted against the TF surface concentration for 100 s⁻¹ (C), 500 s⁻¹ (D), and 1000 s⁻¹ (E) for 3 independent experiments (3 × 30 spots per plotted data point). Data were fit to a 4-parameter logistic equation to determine the TF EC50 values for each shear rate.