Fully Screen Printed Thermocouple and Microheater Applied for Time-of-Flight Sensing in Microchannels

In order to control the flow inside microchannels, measuring the flow velocity of fluids is an important task. A possible way to determine flow velocity is by measuring the thermal time-of-flight. To this end, in this paper, a full screen printed combination of microheater and thermocouple is presented. Screen printing represents a technology that is attractive for fabricating low-cost sensor systems for microfluidic devices which can be directly integrated into the channel. The structure presented here has been manufactured using hightemperature stable screen printing inks. The thermocouple is calibrated and then the sensor setup is used to determine the flow velocity in a microchannel at various flow rates. The measurement is performed using a frequency domain approach by evaluating phase shifts of slow steady-state oscillations, and alternatively in the time-domain by estimating the heat transfer function from a step response measurement. The measurement results are compared to theoretically predicted values and show good agreement for a flow velocity range from 20 μl/min to 70 μl/min.