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Active and passive microflow control has been demonstrated using gel formation by dilute aqueous solutions of triblock copolymers at elevated temperatures. Solutions of a poly(ethylene oxide)106-poly(propylene oxide)70-poly(ethylene oxide)106 polymer, which has the trade name Pluronic® F127, have been used as a sample system. Flow in a microchannel has been stopped in less than 33 ms by introducing heat with an integrated electric heater. Viscous heating under high shear rates also induces gel formation, which has been used for passive automated flow control in a microchannel.