The progress of microfluidic technology has produced successful devices in various fields. Like in many other applications, the capability for the evaluation of evaluating cellular functions with a minimum amount of cells and reagents will also be beneficial in fields including stem cell research, drug screening, and cell biology. To this end, an issue is on-chip cultivation, sampling, and monitoring of changes of physical and/or chemical parameters. To realize these functions, we used electrochemical principles. In this study, we fabricated a microsystem that can carry out necessary procedures for monitoring of ammonia metabolism of hepatocytes.

　The microsystem consisted of a glass substrate with valve electrodes and sensing electrodes, and a polydimethylsiloxane (PDMS) substrate with microflow channels and a culture chamber (Fig.1). Electrowetting-based valves controlled transport of extracted culture medium and an electrolyte solution of an air-gap Severinghaus-type ammonia sensor. The extracted culture was transported to the sensing area one by one with a predetermined interval and the ammonia concentration was measured using the sensor. The decrease in the ammonia concentration as a result of metabolism could clearly be followed on the chip (Fig.2). The amount of cells required in the system was less than one-hundredth of that in conventional analyses.

Fig. 1 Ammonia monitoring chip

Fig. 2 Monitoring of ammonia metabolism of hepatocytes. Hepatocytes were seeded at densities of 11 cells/mm2 (○), 115 cells/mm2 (□) in the microsystem, and 11 cells/mm2 using a commercial kit (●).