Resonance induced impedance sensing of human blood cells

A challenging problem in AC impedance sensing of particles (e.g., blood cells in plasma) with micro electrodes is that with the shrinking of electrode surface area the electrode double layer capacitance decreases. Combined with the parallel stray capacitance, the system impedance is dominated by these capacitive components. Hence the sensitivity for particle sensing decreases. In this paper, we propose a new approach to solve the problem. The idea is to use resonant sensing by connecting an external parallel inductor to the system. At the resonant frequency, the capacitive components in the system were nullified by the inductor, leaving the electrolyte and particle impedance to be a major component in the system impedance. We then successfully demonstrated this idea by sensing 5μm polystyrene beads. More important, this technique was extended to sensing blood cells in diluted human whole blood and leukocyte rich plasma. The measured signal pulse height histogram matched well with known volume distribution of erythrocytes and leukocytes.