SEM of aged plasma clots. Plasma clots (50 μL) were formed by addition of thrombin (to 0.5 NIH units/mL) and CaCl₂ (to 20 mmol/L) to citrated human plasma and then transferred to cylindrical wells milled in carbon planchettes. After 3 hours of incubation, clots were overlaid with HBS, pH 7.4. After a 30-minute permeation period, excess liquid was removed and each sample was dehydrated, critical point-dried, and sputter-coated as described in Materials and Methods. Samples were examined in a Philips Model 515 scanning electron microscope, and micrographs were taken at magnifications of 1,850× (A; bar = 10 μm) and 18,300× (B; bar = 1 μm).