Point-of-Care Detection of Hematocrit in a Microfluidic System Integrating Microfabricated Carbon Electrodes

This work describes an electrochemical microfluidic system for point-of-care detection of hematocrit. Hematocrit, the volume percentage of red blood cells in whole blood, is an important medical indicator. Its deviations from normal ranges (40–54% for men and 36–48% for women) are symptoms of various medical conditions such as anemia and leukemia.

We fabricated carbon microelectrodes via a microfabrication strategy. Carbon precursors, negative photoresist SU-8, were photolithographically patterned on quartz substrates and pyrolyzed at 900 ^OC. Specific pyrolysis conditions were optimized to obtain carbon electrodes from large SU-8 features (greater than 10 mm in length) containing more internal stress and residual gases. The SU-8 features were hard-baked at 200 ^OC under vacuum pumping to completely evaporate residual gases and to enhance the adhesion. Then, the hard-baked SU-8 were pyrolyzed via a conventional two-step pyrolysis: 1) hard-baking at 200 ^OC for 30 mins and 2) pyrolysis at 900 ^OC for 1 hour. SU-8 pyrolyzed carbon electrodes were combined with a single PDMS microchannel to complete a device.

To test the feasibility for detection of hematocrit, current responses of red blood cells suspended in phosphate buffered saline were measured at 100 V for 30 secs. A linear trend line of current versus red blood cell concentration was obtained. The results indicate that our device can determine hematocrit within 3.67 % precision and 3.97 % accuracy. Excellent performance of carbon electrodes was attributed to their surface properties. Compared to metal electrodes, carbon electrodes have less bio-fouling tendencies. Large electrode surface areas with porous structures also increase the system voltage efficiency by decreasing the charge transfer overpotential at electrode/electrolyte interfaces.