Fabrication of a 3 dimensional dielectrophoresis electrode by a metal inkjet printing method
© Lee et al.; licensee Springer. 2013
Received: 23 September 2013
Accepted: 5 December 2013
Published: 18 December 2013
We proposed a micro electrode fabrication method by a metal inkjet printing technology for the bio-applications of dielectrophoresis (DEP). The electrodes are composed of bottom planar gold (Au) electrodes and three dimensional (3D) silver (Ag) electrodes fabricated locally on the Au electrode through metal inkjet printing. We observed the negative DEP characteristics of the 4 μm polystyrene beads on the both electrodes at the 500 kHz, AC 20 Vpp point. The number of beads trapped on the printed Ag electrode is 79 and 25 on the planar Au electrode because of spatially larger electric field in a 3D electrode system.
Three dimensional (3D) electrodes are effectively used in a wide range of applications such as electronic circuit components, micro sensors, micro actuators, and microfluidic system. In particular, the 3D electrode is useful for the dielectrophoresis (DEP) of biological samples including cells, proteins, and particles. Compared to the planar electrode, the 3D electrode structure allows it to transmit a large DEP force to the biological samples due to the high gradient electrical field generated between the 3D electrodes . Therefore, the high gradient electrical field in 3D DEP chips could enhance separation and trapping performance. There are many investigations into the trapping and separation of a biological sample using 3D DEP chips.
Conventional microfabrication technologies have several drawbacks in 3D electrode fabrication. It is difficult to form 3D electrodes that have a micrometer scale thickness using thermal evaporation and sputtering. Metal electroplating can be used to fabricate 3D electrodes with high aspect ratios, but complicated procedures are required. The inkjet printing technology has been studied for the patterning of the 3D electrode. Inkjet printing can be directly utilized in fabricating 3D structures with a simple process at short run-time and low cost. The inkjet printing method does not require complicated additional fabrication process steps such as photoresist mold and metal seed layer formation for electroplating, in order to pattern a 3D metal structure. For example, various metals can be directly patterned using inkjet printing on the substrate by consuming a small metal source. Therefore, inkjet printing is a very suitable technology to fabricate a 3D metal structure. However, the inkjet printing technology has not yet been applied to the microfluidic systems due to its low fabrication resolution [2–4].
We propose a metal inkjet printing method for the fabrication of the 3D DEP electrode for micro particles manipulation in a microfluidic chamber. For a high resolution inkjet printing, fluorocarbon (FC) film patterning was utilized to make the ink-phobic boundaries which prevent droplets from spreading and enable selective patterning. A microfluidic chamber was assembled with the electrode chip after printing silver (Ag) 3D electrodes for protecting particles and adjusting for fluid control. The DEP performance was verified using the 4 μm polystyrene beads and AC drive voltage control [5–7].
Inkjet-printed 3D DEP chip design
DEP characteristics of the beads
The DEP force is positive when the Clausius-Mossotti (CM) factor (ƒCM) has a positive value while the negative DEP force is generated when ƒCM has a negative value. Moreover, the electric field gradient and the radius of the particles are directly affected by the DEP force . According to the electric field distribution, we can predict where the beads are trapped on the chip. In the positive DEP, the particles are gathered toward the higher electric field region while the particles move toward the lower electric field region in the negative DEP. In the case of the polystyrene bead, it has negative DEP characteristics at almost every frequency range.
Results and discussion
In this work, we successfully demonstrated the new fabrication method for fabricating 3D electrodes by a metal inkjet printing and selective FC film patterning. FC film patterning was very effective at making ink-phobic boundaries to prevent droplets from spreading after the Ag inkjet printing. Ag ink successfully spread and was trapped only on the Au surface without FC film, while the FC films acted as a barrier to prevent the over-spreading after the ink-jetting process. The overall morphology resulted in a dented rectangle with rounded corners due to the coffee ring effect and the thickness of the Ag electrode is non-uniform. However, we could control the thickness of an electrode by regulating the overlap-rate of the inkjet jetting and the number of metal inkjet droplets. The DEP performance of the inkjet-printed Ag electrodes was examined using the 4 μm polystyrene beads and AC drive voltage control. We could confirm that more beads were trapped on the 3D electrodes than on the planar electrodes because a large amount of the beads could be spatially exposed to the high electric field in a 3D electrode system. For future research, we need to improve the morphology of the printed structure by controlling printing conditions such as substrate temperature, surface wettability, and ink vapor pressure.
This work was supported by the Korea Institute of Industrial Technology.
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