Simulation of an Electrical Impedance biosensor using COMSOL

Biosensors are being used for highly sensitive and selective recognition of biological macromolecules. The impedance based biosensor utilizes the electrical impedance measurements between interdigitated electrodes integrated into a microchanel containing the target cells in a suspension medium. This paper describes the simulation of electric impedance of an interdigitated microelectrode for detecting E.coli cells. The detection is based on the measuring the change in impedance caused by the presence of bacteria. In the experimental methods an impedance analyzer was used to measure the electrical impedance over a range of frequencies. The first impedance measurement was done with the microfluidic system containing the suspension medium alone. The subsequent impedance measurement was performed with a known concentration of E.coli cells added to the same suspension medium.

To verify the experimental results, a COMSOL time harmonic electric current analysis was performed using the electromagnetic module. The default element type was Lagrange – Quadratic and a parametric linear solver was used. The model consisted of cells suspended in a medium on top of a pair of electrodes. The width and spacing of the electrodes was 5 microns and the cells were confined within a distance of 10 microns from the surface of the electrodes. Simulations were performed over the frequency range from 100 Hz to 1MHz. An AC voltage of 150 millivolts was imposed on the electrodes. All others surfaces had an insulating boundary condition. Boundary integration was used to calculate the current flow through the measurement electrodes. The magnitude of the impedance was calculated by calculating the ratio of the applied voltage and the current. The experimental results were compared with the simulation results and were found to be in qualitative agreement. It was observed that the impedance values depend on the suspension medium used, the concentration of the E.coli cells and the dimensions of the interdigitated electrode.