Impedance-based cell density measurement with inkjet printed flexible sensor

Cell density is an important parameter in the monitoring of biotechnological processes. An inline measurement principle is described based on impedance spectroscopy. The sensor is fabricated by inkjet printing of silver nanoparticle ink on a polyethylene terephthalate substrate. Yeast concentrations ranging from 0.5 g/l - 10 g/l have been measured. The cells were suspended in deionized water and in 0.1 M phosphate buffered saline to observe the effect on the impedance spectra. The impedance measurements were performed between the frequency range of 20 Hz - 1 MHz with an excitation voltage of 10 mV. Measurements in deionized water showed an increase in admittance of 94.6 mu Omega(-1) per 1 g/l of increase in cell concentration at 10 kHz. The effect of the cell membrane polarization resulting in a change of the capacitance is observable in phosphate buffered saline. The increase in capacitance observed is 360 pF per 1 g/l of increase in cell concentration. Thus, an inline, label-free method for cell-density monitoring is possible. It can be seen that impedance spectroscopy with a flexible sensor is a useful tool to monitor cell density and its relationship with the surrounding medium.