Increased viscosity of hemoglobin-based oxygen carriers retards NO-binding when perfused through narrow gas-permeable tubes

► Using a gas-permeable narrow tube (25 µm), NO-binding of HBOCs was observed. ► PEGylation, encapsulation, and HES-addition of Hb increase the viscosity. ► They retard the NO-binding as compared with an Hb solution. ► Addition of HES to Hb-vesicles further retards the reaction to the level of RBCs. ► Increased particle size and viscosity might mitigate NO-related vasoconstriction.