Medical devices such as coronary catheters and mechanical heart valves (MHVs) and extracorporeal cardiopulmonary bypass (CPB) and extracorporeal membrane oxygenation (ECMO) circuits activate factor XII (FXII) and trigger clotting via the intrinsic pathway of coagulation (1). Heparin prevents catheter thrombosis during percutaneous coronary intervention and clotting of CPB and ECMO circuits, whereas warfarin suppresses clotting with MHVs. Although effective, heparin and warfarin require coagulation monitoring and dose adjustments, which is inconvenient, and protamine sulfate used to reverse heparin after CPB surgery can cause potentially fatal allergic reactions. Furthermore, the high doses of heparin used to prevent clotting of extracorporeal circuits can cause bleeding, as can warfarin. Therefore, safer and more convenient anticoagulant agents are needed to prevent clotting on medical devices.