Study Of 3D Electrodes Based On Conductive Polymer Composites In Microfluidic Systems

Electric field is widely used in microfluidic systems to perform various functions such as to mix fluids, to manipulate cells or particles and so on. Most of these electric operations are usually realized with thin metallic electrodes, generally obtained by evaporating or sputtering techniques in traditional IC process. In this case, electrode thickness is limited to few hundred nanometers.3D electrodes, equivalent to microfluidic chips on scale, are able to generate uniform electric fields over height of the channel. Moreover, 3D electrodes are usually presented as most robust, able to sustain higher current densities and to produce lesser joule heat than thin electrodes. All these features are especially suitable for biological microfluidic applications such as cell dielectrophoresis, electrofusion, lysis and trapping. However, 3D electrodes are technologically more challenging and expensive to produce than thin electrodes and they remain difficult to bond with substrate like glass because of sealing issues.In this paper, a new kind of conductive polymer composites is selected to fabricate 3D microelectrodes. The material, obtained by mixing fillers powder and polydimethylsiloxane(PDMS), preserves PDMS processing properties. These PDMS-based electrodes present two main advantages: 1.they can be fabricated in a large range of thicknesses and geometries, 2. they can be patterned by molding instead of traditional techniques which promotes efficiency and saves costs.The electric-field profile generated by parallel plate electrodes is analyzed by using finite element method and Comsol Multiphysics simulation software. In particular, the distribution of dielectrophoresis(DEP) force amplitude and orientation inside the chip is computed. Compared with bilateral symmetric structure, the electric field intensity produced by parallel plate electrodes is larger. What’s more, cells are more likely to gather in the middle uniform field so as to form long-train structure. Furthermore, in what ways fillers form conductive paths and conduction mechanism of composite are also discussed and analyzed preliminarily.Next, we sum up a complete set of preparation flow combined with common PDMS processing techniques. Two different kinds of fillers, silver powder and graphite powder, are chose to prepare Ag-PDMS and C-PDMS. The relationship between resistivity and graphite content conforms to percolation theory. The resistivity of conductive polymer composites is also relevant to certain preparation factors such as the addition of solvent, the dosage of curing agent and the size of fillers. With the aid of precision impedance analyzer, AC characteristics are studied. The impedance of composites is greatly influenced by signal frequency, especially for samples of smaller proportion. On the contrary, the signal amplitude has little impact on impedance. Through microscopic observation, particles mainly appear as two kinds of structure, aggregate and chain. At last, Shore hardness of C-PDMS is tested by a shore A durometer. As our results show, the physical performance of material is largely affected with the increasing addition of filler, which means poor processibility and plasticity. Therefore, our suggestion is to seek site of a balance between excellent conductivity and poor processibility according to practical situations.Finally, owing to a profound knowledge of conductive polymer composites, a cell-alignment chip integrated with plate electrodes is fabricated by soft lithography. Cell alignment experiment for blood red cells and breast cancer cells show that cell chains are obviously formed under the dielectrophoretic force and cell-alignment rate is rather high. Experiments verify the feasibility and effectiveness of 3D electrodes made of conductive polymer composite, which is also a good start for further research.