New electrode configuration to Impedance wide-band frequency measurements for micro flow cytometry

María Cabello Fusarés a, Óscar Castillo-Fernández a, Mauricio Moreno b, Francesc Xavier Muñoz a

a CNM-IMB CSIC, Campus UAB, 08193 Bellaterra, Spain

bElectronics Department of University of Barcelona (UB),08028 Barcelona, Spain

e-mail: oscar.castillo@imb-cnm.csic.es

Keywords: Microfluidics, Impedance Spectroscopy, Micro flow cytometry

We present a new electrode configuration design for the development of micro flow impedance cytometry. The new design combines a high metal-liquid interface area with a suitable microfluidic design that allows a significant reduction of the sensing volume. Moreover, their big interface area performs a real impedance response for a wide band of frequencies. An improvement of the electrodes sensitivity is obtained while their frequency performance allows efficient measurements of biological samples. Furthermore, the device is easy and cheap to fabricate and an equivalent 3D electrode response has been achieved. The utilization of microelectrodes is essential to scale the sensing volumes to the particle size, nevertheless, because of the electrode-electrolyte interface, they present complex frequency responses. In Impedance micro-cytometry, there are several ways to integrate the microelectrodes into de walls of the microchannel. Co-planar electrodes are feasible and easy to fabricate [1], however, their disposition distorts the electric field generating problems with the sensitivity. There are some design options to keep the electric field uniformity in the sensing region: electrodes faced at the lateral walls [2], electrodes faced at the roof and floor walls of the channel [3] or the use of eternal elements to deform the electric field [4]. The electrodes design developed in this work (see Fig 1), consist of 3 big circular electrodes of 1mm diameter. The contact impedance is reduced due to the increase of the interface area electrode-electrolyte. In order to integrate the electrodes into the microchannel, the cavity through which the electric current flows narrows to a dimension of 10 µm, becoming in a very small effective electrodes (and sensing volume). Moreover, the fluidic device incorporates two inlet junctions to produce hydrodynamic focalization to the particles, and some microbubble trap zones.

In order to validate the three-electrode design, it was modelled by COMSOL Multiphysics 3D simulations. The electrode-electrolyte interface is modelled with a contact impedance with ρ=1·108, Cs=16 F/m2, while cell membrane is modelled with ρ=1·106, Cs=0.08 F/m2. Particles are detected by a differential measure obtained from the two sensing volumes; if there is no particle, the current measurement in both sensing volumes is identical whereas when a particle is situated between two opposite electrodes, it will work as a resistance and the detected current will change (see Fig 2). The electrical impedance of the electrodes in simulations shows a resistive behavior for a wide band of frequencies (from 102Hz to 107Hz) represented by bode plots (see Fig 4,A). A real device was developed by standard microfabrication processes. The channels were replicated on PDMS, the electrodes were printed on glass slides and both parts were bounded by Oxygen plasma, closing the channels, see Fig 3. The channel and electrodes were filled with electrolyte solution with Phosphate Buffered Saline (PBS), connecting the electrodes. The impedance response of the device was obtained (see Fig 4, B), being comparable to the results obtained by simulation.

References: [1] O. Castillo-Fernandez, R. Rodriguez-Trujillo, G. Gomila, & J. Samitier, , 16(1-2) (2013) 91–99. [2]L. Wang, et al. Lab on a Chip, 2007. 7(9): p. 1114-1120. [3] N. Haandbæk, O. With, S. Bürgel, F. Heer, & A. Hierlemann, Lab on a Chip, (2014)3313–3324 Microfluidics and Nanofluidics, 2013: p. 1-9. [4].C. Clausen, et al. Micromachines, 2014. 6(1): p. 110.

Figure 1. Channel and electrodes design with a detail of the sensing volume.

Figure 2. Simulation that verify the uniformity of the electric field in all channel high in the two sensing areas.

Figure 3: Image of the final device. PDMS channels replicated form SU8 master, bounded to glass substrate with gold electrodes.

Figure 4: Bode representation of the electrode Impedance response. In A is represented the calculated impedance by COMSOL. B represents the real measurements of the electrodes submerged in PBS, measured with a impedance analyser.