Fabrication and Characterization of a Microfluidic Flow Sensor Integrated into an Ultrathin PMMA Layer

This work presents the experimental results, characterization, and the devised fabrication procedure of a thermal-based flow sensor for microfluidic systems. In particular, a novel method of fabrication was developed to create the sensor’s electrodes, which are made of leaf gold and which are embedded within an ultrathin polymethylmethacrylate (PMMA) foil for mechanical support and insulation. By placing the foil in the center of a microfluidic channel, the thermal mass which is in direct contact with the sensor element and hence the overall heat capacity of the sensor system is minimized which allows for fast response times. To ensure low cost fabrication and material combability, PMMA is used as substrate material. The integration of the leaf gold electrodes into the sensor is achieved by using a specific bonding process combining solvent vapor and thermal bonding. Our four-wire sensor is characterized using well-established techniques for evaluating thermal-based flow sensors and anemometers in order to provide an accurate flow measurement. The sensor is designed to operate in the range from 10 μl per minute up to the low milliliter per minute range using channel dimensions of 200 μm height and 2 mm width. By adapting the channel geometries, it is easily possible to change the measurement range.