Control experiments for controlling PF4 density coated on the gold surface and on platelets. (A-B) AFM images of gold surface (A, left), polyethylene glycol (PEG)–coated gold surface (A, middle), and PF4-coated PEG/gold surface (A, right) show no significant differences in roughness between gold surface before (B, black) and after coating with PEG (B, blue) but reduced roughness after coating with PF4 (B, red). This illustrates that PF4 molecules covered the gold surface in high density and excludes that the reduced binding force of KKO in the solid-phase system is due to suboptimal antigen density. (C) The binding strength of KKO to 25 µg/mL PF4-coated platelets shows a force profile similar to that obtained on platelets coated with 10 µg/mL PF4 (Figure 1B, middle). Images in panels A and B were modified from Nguyen et al¹⁸  with permission.