Flow velocity measurement in microchannels using magnetoresistive chips

On-chip spin valve sensors were used to measure the flow velocity within fabricated poly(dimethylsiloxane) (PDMS) microchannels. Superparamagnetic particles with dimensions of 250 nm in diameter were introduced into a 100 /spl mu/m wide microchannel, bonded to the chip surface. The flow velocity was determined by measuring the changes in resistance of a pair of spin-valve sensors, 1.65 mm apart from each other, caused by the passage of ensembles of superparamagnetic particles. Flow velocities ranging from 50 to 300 /spl mu/m/s were estimated, depending on the pressure used to drive the flow. The application of this system provides a means to measure and control flow velocities on microfluidic devices.