We present a magnetically actuated microfluidic transistor (valve), an on-chip micro-valve that uses magnetorheological fluids micropatterned onto an elastomeric membrane to develop an ultra-compact solution for digital control of microfluidic circuits. We simulated the micro-valves theoretically and also characterized micro-fabricated devices experimentally. We simulated the effects of channel and valve geometry on the deformability of the valve. We experimentally characterized the effects of channel and valve geometry on the ability to reversibly close channels fully. We characterized the microfabricated valves in the flow rate range of 0.02-1 mu l/min. Among the various kinds of valves examined, for lower flow rates, circular valves with a 700 mu m diameter in 300 mu m width channel, and in higher flow rates, rectangular valves 700 mu m wide in the same channel show the best ability in terms of valve closure and response time. The fabrication and the integration of the proposed valve are compatible with a range of polymer microfabrication technologies having the advantage of simple fabrication, small size, and no external power requirement. These valve structures can be promising solutions for pumping and active flow control for portable analytical instrumentation, point-of-care diagnostic devices, and even wearable microfluidic devices. [2015-0296]