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In this Letter, a microdevice driven by capillary action is developed for the high-throughput on-chip separation of plasma from a drop of blood. The microfluidic device consists of an array of polymethyl methacrylate channels 2 m deep and 2 m wide. It is fabricated by hot embossing and thermal bonding from a Si master mold. Then, it is modified using oxygen plasma and poly-l-lysine to obtain a hydrophilic surface. After the injection of a blood sample, the plasma flows rapidly and arrives at the outlet within 3 min, demonstrating a red blood cell separation ratio greater than 99 . Moreover, no haemolysis was observed. Since the proposed device is driven solely by capillary action, its passive mode of operation requires no microfluidic pumps or any other additional power supply. It can therefore be miniaturised for incorporation in a diagnostic system.