Bidirectional field-flow particle separation methodin a dielectrophoretic chip

with 3D electrodes

Date 　 ： 2012/12/24

Name 　： Po Yuna Cheng( 鄭博元 )

Teacher ： Professor Hsu

OutlineIntroduction

Fabrication of the DEP device

Separation methods

Experiment

Conclusions

Introduction

This paper proposes a bidirectional field-flow separation method in a dielectrophoretic chip with 3D electrodes.

The 3D electrodes structure that is used in this device is not only used for generating an uniform DEP force across the microfluidic channel but also for achieving a gradient of the velocity (and in this way a variable hydrodynamic force) in the microfluidic device.

Separation methodsBidirectional separation in a DEP chip with 3D electrode array

The separation method using DEP chip with 3D electrode array can be described in four steps (Fig. 2).

Fig. 2. Main steps of the separation technique.

First, the solution with the mixture of two particle populations is injected into the microfluidic chamber. Secondly, by applying an electric field the two populations are separated into different locations according to their electrical properties.

In the third step, a fresh buffer solution flows through in one direction, and one of the population is collected at one of the outlets. Finally, the second population is collected at the second outlet by flowing the fresh buffer in the perpendicular direction.

Fig. 3. The electric field distribution in the DEP structure.

Fig. 4 shows an ANSYS simulation of the flow through the electrode-pillars. As a result, these geometries lead to a huge gradient in the flow velocity.

As a result, these geometries lead to a huge gradient in the flow velocity. The zone with higher fluid velocity experiences larger hydrodynamic forces (Zone A) increases and the particles that are trapped using positive DEP force can be removed.

While, in the low velocity regions (Zone B) the hydrodynamic force is too weak to remove the particles that experience negative DEP.

Fabrication of the DEP device

Fig. 5. Fabrication processes of DEP chip.

Four holes were first etched through in a glass wafer with a thickness of 500 um for the inlet–outlet access of fluid using a low-stress amorphous si/si carbide/photoresist mask (Fig.5a).

After that a 4 in. silicon wafer with a thickness of 100m was wafer-to-wafer anodically bonded to a 4 in.Pyrex glass wafer with a thickness of 500 um at 305◦C with an applied voltage of 800V for 20 min (Fig. 5b).

Next, the microchannel walls and electrode array were defined in the silicon wafer using deep RIE technology through a silicon oxide mask (Fig. 5c).

Next, the top glass wafer with two inlet/outlets was bonded to the patterned silicon wafer using second anodic bonding at 400◦C with an applied voltage of 1200V and an applied force of 1500N (Fig. 5d).Subsequently, the bottom glass wafer was thinned from 500um to around 100m using an optimized wet etching process in a HF 49%/HCl 37% solution (10/1) (Fig. 5e) .

The via holes were wet etched through the thinned bottom glass wafer in the same solution using a low-stress amorphous silicon mask, to provide a path to connect the outside metal electrodes and silicon electrodes (Fig. 5f).

Experiment

The feasibility of the separation mechanism in the DEP chip with 3D electrode array was proved using populations of viable and non-viable yeast cells were used.

100 mg of yeast, 100 mg of sugar and 2ml DI water were incubated in an Eppendorf tube at 37 ◦C for 2 h. Next, the cell culture was divided into two with one population being boiled for several minutes in 5ml boiling DI water (dead cells).

Both viable and non-viable populations were mixed and resuspended in the separation buffer, which was a mixture of phosphate buffered saline (PBS) and DI water. The conductivity of the separation buffer was around 20 uS cm.-1.

The final concentration of the cells was 107 cells/ml. A function generator and a linear amplifier were used for the drive signal generation of the dielectrophoretic chip.

The drive signal was increased from 0 to 25V peak to peak gradually with the signalfrequency being in the range of 20–100 kHz.

Fig. 7. Optical image with the main steps of the separation technique: (a) insertionof dead and live cell, (b) trapping of the cells by positive and negative DEPand (c) collection of one cell population by flowing a fresh buffer solution.

Fig. 7 showed the bidirectional separation processes for viable and non-viable yeast cells in a DEP chip with triangular electrode array.

The mixture of viable and non-viable yeast cells was injected into the microchannel (Fig. 7a);

non-viable yeast cells experiencing positive DEP move to the highest electric field regions and viable yeast cells experiencing negative DEP move to the lowest electric field regions (Fig. 7b);

next, non-viable yeast cells were collected from the other one outlet by flushing a fresh buffer solution (Fig. 7c).

Finally the other population is collected at the opposite outlet using same method.

Conclusions

This paper proposed a bidirectional separation method in a DEP chip with 3D electrode array, which also functions as microfluidic channels.

These electrodes serve a double function. The first function is to generate positive and negative dielectrophoretic force, trapping two populations of cells in different locations. The second function is to produce a gradient of fluid velocity.

As a consequence, the resulted hydrodynamic force will drag out the population trapped by positive dielectrophoresis. After the removal of the electric field, the remaining population can then be collected at the outlet.

Thank you for your attention