Microfluidic Chip for Continuous Monitoring of Hormone Secretion from Live Cells Using an Electrophoresis-Based Immunoassay

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Abstract
A microfluidic device has been developed for the determination of insulin secreted from islets of Langerhans by a capillary electrophoresis competitive immunoassay. Online assays were performed by electrophoretically sampling anti-insulin antibody (Ab), fluorescein isothiocyanate-labeled insulin (FITC-insulin), and insulin from separate reservoirs and allowing them to mix as they traveled through a 4-cm reaction channel heated to 38 °C. From the reaction channel, samples were injected onto a 1.5-cm-long electrophoresis channel where the FITC-insulin and FITC-insulin−Ab complex were separated in 5 s using an electric field of 500 V/cm. Detection limits for insulin were 3 nM in this mode of operation. Assays could be collected at 15-s intervals with continuous sampling and online mixing for up to 30 min with no intervention. Relative standard deviation was 2−6% depending on the insulin concentration. Response time to a step change in insulin concentration was 30 s. For live cell monitoring, single islets were placed into a reservoir on the chip and fluid in the immediate vicinity was continuously sampled to detect insulin secretion from the islet. Monitoring of insulin secretion with electropherograms taken at 15-s intervals resolved secretory profiles characteristic of first- and second-phase insulin secretion. The method should be amenable to other cell or tissue types for measurements of release with high temporal resolution.