The effect of shear on in vitro platelet and leukocyte material-induced activation

The failure to understand the mechanisms of biomaterial-associated thrombosis prevents us from improving the blood compatibility of stents and mechanical heart valves. Blood-material interactions trigger a complex series of events and anticoagulant and anti-platelet therapies are needed to reduce the risks of thrombotic complications with most cardiovascular materials. While material interaction with platelets has been widely studied, little is currently known on material-induced leukocyte activation in the presence of shear. In vitro experiments were performed to assess the effect of flow on blood cell activation induced by medical grade metals, ST316L and TiAl6V4. Blood was circulated in flow chambers preloaded with or without metal wires at shear rates of 100, 500, and 1500 s⁻¹. Platelet and leukocyte activation, leukocyte-platelet aggregation, and tissue factor expression on monocytes were measured by flow cytometry. Metal surfaces were characterized by scanning electron microscopy. Under physiological shear rates, no significant platelet microparticle formation was observed. However, significant CD11b up-regulation, leukocyte-platelet aggregates, and tissue factor expression were observed at 100 s⁻¹. As shear rate increased to 1500 s⁻¹, leukocyte activation reduced to control values. TiAl6V4-induced leukocyte activation was generally lower than that of ST316L. Adhesion significantly decreased with increasing shear rate to 1500 s⁻¹. In blood, increase within physiological shear rates led to a significant reduction in in vitro material-induced leukocyte activation, suggesting that difference between material biocompatibility may be better identified at low shear rates or under pathological shear conditions.