High resolution capacitance sensor array for real-time monitoring of cell viability

Capacitance based cell sensing has shown promising results for monitoring cell viability. Studies have shown direct correlation between cell health and measured capacitance. Prior sensors have used charge based capacitance measurement, which has limitations in offset correction, range, and resolution. An alternative approach to monitoring small changes in electrode capacitance is to measure frequency change of a current controlled ring oscillator containing a sensing electrode that acts as a variable capacitor. This sensor design has no need for on-chip offset correction, because the range of the digital output of the chip depends on the counter size rather than input voltage. In addition, the resolution for this sensor is limited only by the amount of read time given to each pixel. For an oscillation frequency of 40 MHz the sensors are expected to achieve a resolution of 2.6 aF. The sensor has been incorporated into a 4×4 array prototype which will be packaged for operation in cell culture and used in viability studies of mammalian cells.