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The 16th International Conference on Machine Design and Production June 30 – July 03 2014, Izmir, Turkiye

FABRICATION OF MICROFLUIDIC DEVICES FOR DIELECTROPHORETIC AND

ACOUSTOPHORETIC APPLICATIONS USING HIGH-PRECISION MACHINING

Soheila Zeinali, [email protected] İhsan Doğramacı Bilkent Univesity, 06800,

Ankara,Turkey

Barbaros Çetin, [email protected] İhsan Doğramacı Bilkent Univesity, 06800,

Ankara, Turkey

M. Bülent Özer, [email protected] TOBB University of Economics and Technology, 06560,

Ankara, Turkey

Süleyman Büyükkoçak, [email protected] TOBB University of Economics and

Technology, 06560, Ankara, Turkey

ABSTRACT

In this study, the fabrication of microfluidic devices for dielectrophoretic and

acoustophoretic based applications with high-precision CNC machining has been presented.

For both devices, molds out of stainless steel have been fabricated, and polymer molding is

implemented. For dielectrophoretic device, the metal electrodes have been fabricated using

high-precision machining and embedded into the device during the molding process. For

acoustophoretic device, piezoelectric slides have been embedded into the device during the

molding process. This is a pioneer study for the fabrication of microfluidic devices with

embedded 3D electrodes and PZT slides.

Key words: Microfluidics, Dielectrophoresis, Acoustophoresis, High-precision machining

1. INTRODUCTION

There are basically two approaches for the fabrication of microfluidic devices, one is

substrate based fabrication (photolithography) and mold based fabrication techniques (soft

lithography) [Wang, 2007]. Photolithography has good ability to manufacture very small and

complicated microfluidic devices, but it generally needs multi-step processes which take

considerable time, and require expensive and high tech facilities in a clean-room

environment [Nguyen, 2006]. Mold-based techniques are very suitable for mass production

since once the mold is fabricated, the mold may be used for several times and after the

completion of the mold, following fabrication procedure is simple and very reproducible. One


common material used in the fabrication of the microchannels is the Polydimethylsiloxane

(PDMS) due to its low cost, low toxicity, ease of fabrication and transparency. One important

alternative to fabricate microfluidic devices is using high-precision mechanical machining for

direct substrate manufacturing and mold fabrication. Thus, a mold can be fabricated using

mechanical machining with no need for cleanroom facility and desired and controllable

accuracy for microfluidic devices. For mold based techniques, common materials are Silicon,

SU-8, polymeric materials and metallic materials. In terms of durability and robustness

polymeric and metallic materials are superior to other materials. For mechanical machining

microdevice fabrication, any of above materials may be selected, however some important

characteristics like machinability and cost need to be considered.

For electrokinetic-based microfluidic applications, embedded electrode structures are

required which are designed and located strategically within the microfluidic structure. One

important application is the DEP based particle manipulation. Dielectrophoresis (DEP) and

acoustophoresis (ACP) are two label-free techniques which is used to manipulate

(bio)particles within microfluidic devices. DEP is the movement of a particle in a non-uniform

electric field due to the interaction of the particle's dipole and spatial gradient of the electric

field. ACP is the movement of a particle in an ultrasonic field [Cetin, 2011]. Typically, DEP-

based applications require a microfluidic channel with a width of 50-100 µm, height of 20-50

µm and length of several millimeters sometimes integrated with metal electrodes. ACP-based

applications typically require a microfluidic channel with a width of 300-700 µm, height of

100-300 µm and a length of 1-2 centimeters. For DEP application, polymer-based devices

are common. For ACP applications, although silicon is the common material, recently

polymer-based devices have been also implemented [Islam, 2012; Adams, 2012]. For the

fabrication of the polymer based microfluidic devices, the use of a mold which is typically

fabricated using lithography based micro/nanofabrication techniques in a clean room

environment. However, with the introduction of the high-precision machining facilities, the

fabrication of the molds for microfluidic devices is possible, and introduces some

unique advantages like manufacturing of durable molds, no need for high tech cleanrooms,

fabrication of high aspect ratio microdevices and ease of parameter control.

In this study, fabrication of microfluidic devices for dielectrophoretic and acoustophoretic

based applications with high-precision CNC machining have been presented. Both devices,

molds out of stainless steel have been fabricated, and polymer molding has been

implemented. For DEP device, metal electrodes have been fabricated using high-precision


machining, and embedded into the device during the molding process. For ACP device,

piezoelectric slides have been embedded into the device during the molding process. DEP

and ACP devices are tested to check any leakage and fabrication lack. Both devices have

been implemented for particle manipulation.

2. FABRICATION OF THE DEP DEVICE

The fabrication process of the DEP device mainly has three steps: fabrication of the mold,

fabrication of the electrodes, fabrication of the device. CNC-based high-precision machining

has been implemented to fabricate the mold and the electrodes out of stainless steel. High-

precision machining has been performed by a 3-axis CNC milling machine (DECKEL MAHO

HSC55, Germany).

Fabrication of the Mold

Mold fabrication starts with design of a CAD model for the device. Figure 1-(a) shows CAD

model of the mold for the DEP device. Screws at the side of the mold are considered to apply

force and prevent the leakage of the PDMS between the tips of the electrode and channel.

For each electrode two guide pins are considered within the mold for easy placement of

electrodes.

(a) CAD model (b) Manufactured

Figure 3. CAD model and photograph of the DEP device

Machining steps of the fabrication process are as follows:

• Face milling operation to ensure smoothness of stock surface

• Counter milling operation foe final dimensions

• Drilling operation for M2 screws in the side of the mold

• Contour milling of the microchannel and the guide pins


Table 1. Machining parameters of the mold of the DEP device

Spindle

Rotational Speed (RPM)

Tool Diameter (mm)

Feed Rate (mm/min)

Depth of Cut (mm)

Rough 12000 2 350 0.1

Semi-Finish 15000 1 150 1

Finish 15000 1 150 1

• Tap milling for M2 screws in the side of the mold

• Pocket milling to open mold cavity

The machining parameters of the mold are tabulated in Table 1. Figure 1-(b) shows

fabricated stainless steel mold. Microchannel has the height of 200µm, width of 100µm and

length of 60 mm. In order to have robust pipe connection in the reservoirs, the depth of the

microchannel cavity has chosen as 3mm. The total machining took approximately 2 hours.

Fabrication of the Electrodes

CNC-based machining has been used to fabricate the electrodes from stainless steel. The

CAD model and manufactured electrodes are shown in Figure 2-(a). Large electrode is 16

mm in length, 1 mm in width and 10 mm in height. The small electrode has the height of 11.5

mm and three tips with width of 500 µm. The space between each tap is 1.5 mm. Two elliptic

guide slots are considered to have easy placement of electrodes in the mold and robust

contact with channel wall. Bottom sides of the electrodes have cavities for the ease of

removal process of the electrodes from the mold during the peel-off, and for a better sealing

at bottom side of the electrodes when the microchannel is bonded to the glass.

(a) CAD model (b) Manufactured

Figure 2. CAD model and photograph of manufactured electrodes


Table 2. Machining parameters of the electrodes of the DEP device

Spindle


Tool Diameter (mm)

Feed Rate (mm/min)

Depth of Cut (mm)

Rough 15000 2 850 0.25

Semi-Finish 16000 0.8 150 0.1

Finish 18000 0.6 80 0.1

The steps of the machining process of the electrodes can be summarized as follows:

• Face milling operation to have smooth stock surface

• Pocket milling for cavity and guide slots

• Counter milling for outside of the stock

The machining parameters of the mold are tabulated in Table 2. The fabricated electrodes

are presented in Figure 2-(b). The total machining time for the small and large electrodes

took approximately 30 minutes and 1 hour, respectively.

Fabrication of the Device

Assembly and fabrication process of the DEP device can be summarized as:

• Placement of electrodes in the cavity by aid of guide pins

• Tightening screws in the side wall to prevent the leakage of the PDMS in the interface

of microchannel and electrodes

• Mixing PDMS and curing agent with the ratio of 10:1, and pouring into the mold

• Placing the setup in an oven for PDMS curing (75°C for 45 min)

• Peeling cured PDMS with embedded electrodes from the mold

• Punching out the inlet and outlet reservoirs

• Bonding PDMS to glass slide using plasma bonding


Figure 3 presents the mold and assembled device used for DEP chip fabrication.

(a) Mold (b) Assembled device

Figure 3. Mold and the assembled DEP device with 3D sidewall electrodes

3. FABRICATION OF THE ACP DEVICE

The fabrication process of the ACP device mainly has two steps: fabrication of the mold and

fabrication of the device. CNC-based high-precision machining has been implemented to

fabricate the mold out of stainless steel. High-precision machining has been performed by a

3-axis CNC milling machine (DECKEL MAHO HSC55, Germany).

Fabrication of the Mold

For ACP device, mold fabrication also starts with designing a CAD model and CNC-based

high precision mechanical machining has been implemented to fabricate the mold out of

stainless steel. Machining steps of the fabrication process are as follows:

• Face milling operation to ensure smoothness of stock surface

• Counter milling operation for final dimensions

• Contour milling of the microchannel and the guide pins

Figure 4 shows fabricated stainless steel mold for ACP device fabrication. In this mold,

microchannel is the height of 130 µm, width of 375 µm and length of 30 mm. The depth of

microchannel cavity as 3mm for the ease and robustness of the fluidic connections on the

reservoirs. The total machining took approximately 2 hours.

Fabrication of the Device

ACP device consists of polymeric channel with piezoelectric (PZT) slides placed in mold

sidewalls. Assembly and fabrication steps for ACP device are summarized as:

• Placement of PZT slides in the cavity

• Mixing PDMS and curing agent with the ratio of 10:1, and pouring into the mold


Figure 4. Manufactured mold for ACP device

• Placing the setup in an oven for PDMS curing (75°C for 45 min)

• Peeling cured PDMS with embedded PZT slides from the mold

• Punching out the inlet and outlet reservoirs

• Bonding PDMS to glass slide using plasma bonding

Figure 5 shows the mold and assembled device used for ACP chip fabrication.

(a) Mold (b) Assembled device

Figure 5. Mold and the assembled ACP device with embedded PZT slides

Table 3. Machining parameters of the ACT device

Spindle


Tool Diameter (mm)

Feed Rate (mm/min)

Depth of Cut (mm)

Rough 15000 2 850 0.25

Semi-Finish 16000 0.8 150 0.2

Finish 18000 0.6 80 0.2


4. RESULTS AND DISCUSSIONS

The fluid is loaded into the fabricated DEP device, and the device is tested for any fluid

leakage (especially around the electrodes), and no leakage has been observed in the device.

Moreover, the device has been examined in details using optical microscopy to find any

PDMS leakage between the channel and electrode interface. It has been observed that there

was direct contact between the electrode and the fluid inside microchannel which is

necessary condition for DEP applications. DEP device has been implemented for n-DEP

particle separation. For the experiment, buffer solution of DI water and 5 µm in diameter

Latex particles has been loaded from inlet reservoir. AC voltage of 5 V and 500 KHz has

been applied to electrodes by using a function generator (Agilent 33250A, Arbitrary

Waveform Generator). Latex particles have been repelled from small electrode due to

negative DEP force (negative DEP is typical for latex particles at this frequency [Cetin,

2011]), and the particles are collected in the outlet reservoir B as expected.

One more advantage of the proposed technique is that the electrodes embedded within the

device are 3D which offers some unique advantages [Cetin and Zeinali, 2014]. Fabrication of

3D sidewall electrodes is not straightforward using lithography-based techniques. Although

some methods have been proposed in the literature [Li, 2014], a fabrication process with

high repeatability, and which enables application of small voltages is still a challenge.

Proposed technique is a promising candidate especially for the microchannels with high

aspect ratio. With the current CNC machining, aspect ratio up to 5 (W: 100 µm, H: 500 µm) is

realizable if needed.

The fabricated ACP device has also been tested and examined to check for any fluid leakage

and fabrication defects within the device, especially in the regions around PZT slides. No

leakage and defects are observed. ACP device has been used successfully for the

manipulation of the 5 µm latex particles towards the channel centerline. It should be also

noted that with the proposed technique, the coupling between the microfluidic device and the

PZT slide seems to be adequate, which is important for the acoustic coupling of the PZT

slide with the microfluidic device. The PZT slides actuated by using a power amplifier

(Electronics & Innovation 1040L Power Amplifier) at 30V and 2MHz. The frequency of the

signal is directly determined by the size of the channel, however the applied voltage is higher

than that of a typical silicon-based ACP device. This is due to the use of PDMS as the device

material. Due to the high voltage and the acoustic heating, some bubble formation is

observed within the microchannels (this issue is also mentioned in the literature [Islam,


2012]). To overcome this adverse effect some Peltier units based cooling system will be

designed in near future.

Although high precision mechanical machining is implemented for the fabrication of DEP and

ACT based devices, it is not preferable for the hydrodynamic based microfluidic devices [Wu

and Hjort, 2009] in which sharp corners are required for the successful manipulation of

particles within microchannel networks. Some further efforts need to be dedicated to

implement high precision mechanical machining for the fabrication of microchannels with

sharp corners.

5. CONCLUSION

In this study, fabrication procedures for a DEP device and an ACP device for particle

manipulation have been proposed using high precision mechanical machining. Fabrication

process for each device has been described in details, and some preliminary experiments

regarding the particle manipulation have been performed. Although utilizing mechanical

(CNC-based) machining provides limitations for high resolution and complex structures, has

some unique advantages over the lithography-based fabrication such as: manufacturing long

lasting and durable molds, no need for high facility cleanrooms, ease of controllability due to

fewer control parameters and ease of fabrication of high aspect ration channels for high

throughput microfluidic device. This is a pioneer study for the fabrication of microfluidic

devices with embedded 3D electrodes and PZT slides. Following these preliminary

experiments, systematic experiments with latex particles both for DEP and ACP devices

which will be followed by experiments with (bio)particles will be our future research

directions.

ACKNOWLEDGMENT

Financial support from the Turkish Scientific and Technical Research Council, Grant No.

112M102, is greatly appreciated.

REFERENCES

1. Wang, L., Flanagan, LA., Lee, AP., (2007), “Sidewall vertical electrodes for lateral field

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2. Nguyen, NT., Wereley, ST., (2006), Fundamentals and Applications of Microfluidics.

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3. Cetin, B., Li, D., (2011), “Dielectrophoresis in microfluidics technology”, Electrophoresis,

Vol. 32, pp 2410-2427.

4. Islam, AT., Siddique, AH., Ramulu, TS., Reddy, V., Eu, Y-J., Cho, SH., Kim, C.,

(2012), “Ultrasonic alignment of bio-functionalized magnetic beads and live cells in

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“High-throughput, temperature-controlled microchannel acoustophoresis device made

with rapid prototyping”, J. Micromech. Microeng., Vol. 22 (7), 075017

6. Cetin, B., Zeinali, S. (2014), “Modeling of on-chip (bio)particle separation and counting

using 3D electrode structures”, Proc. 4th Micro and Nano Flows Conference, UCL,

London, UK, 7-10 September 2014 (accepted)

7. Li, M., Li, W. H., Zhang, J., Alici, G., Wen, W., (2014), “A review of microfabrication

techniques and dielectrophoretic microdevices for particle manipulation and separation”,

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FABRICATION OF MICROFLUIDIC DEVICES FOR DIELECTROPHORETIC ...me.bilkent.edu.tr/wp-content/uploads/2014/08/UMTIK_Manuscript... · FABRICATION OF MICROFLUIDIC DEVICES FOR DIELECTROPHORETIC