SINGLE PLATE ELECTROWETTING ON DIELECTRIC BIOCHIP

Chankanok Chandee, Kessararat Ugsornrat, Patcharaporn Uyawapee

Department of Industrial Physics and Medical Instrumentation

King Mungkut’s University of Technology North Bangkok,Bangkok, Thailand

Email:Kessararatu@hotmail.com

Tawee Pogfai, Thitima Maturos, Disayut Pokarattakul and, Adisorn Tuantranont

Nanoelectronics and MEMS Laboratory National Electronics and Computer Technology Center

Pathumthani, Thailand Email:adisorn.tuantranont@nectec.or.th

Abstract— Electrowetting is well known method for microfluidic technology that plays a critical role in lab-on-a-chip systems by delivering chemical or biological samples. Once electrical potential is applied, the contact angels change and driving the droplet. In this report, a single-plate electrowetting on dielectric (EWOD) was fabricated and tested, containing both the control and ground square-shaped electrodes on one plate with dimension of 2 mm width and 2 mm long, fabricateded by thin film deposition methods and micromolding process. For droplet transportation, we designed electrical circuits using a microcontroller to control relays that switching voltage applied to control electrodes on and off. Interfacing RS232 with microcontroller makes the droplet transporting can be modified from personal computer by sending commands from a PC to a microcontroller.

Keywords-component; Electrowetting on Dielectric (EWOD); droplet; Microcontroller,Microfludics, Biochip

I. INTRODUCTION

Electrowetting on dielectrics (EWOD) is a phenomenon in which wettability of a droplet on an insulator-coated electrode surface is electrically changed to produce a motion. The electric potential causes to change contact angle of droplets, leading to droplet’s deformation and movement. Since the transport of an aqueous droplet based on EWOD was first demonstrated, successful developments of more complicated microfluidic operations such as creating, transporting, cutting, and merging of liquid droplets allowed EWOD to be adopted in many miniaturized biomedical applications. Today, electrowetting forms the backbone of digital microfluidics and many applications have been developed, including liquid lenses, switches, and reflective displays.

In addition, we designed droplet transport controlling using microcontroller. Microcontroller are dedicated to one task and run one specific program and often low-power device. The program is stored in ROM and generally does not change. Microcontrollers are widely used in today’s control systems for the many reasons, such as its design and simulation, small size, low-cost and easy to use.

In this work, we manufactured a single-plate EWOD microchip and designed the droplet transport controlling using

microcontroller. The next section shows fabrication of a single-plate EWOD microchip. Section III presents the design of microcontroller circuits and program. Section IVcontains experimental setup. Section V describes experimental result. Section VI concludes this paper.

II. FABRICATION OF A SINGLE-PLATE EWOD MICROCHIP In this section, we describe the fabrication of the single

plate EWOD device (Fig.1). Initially, the pattern of electrodes that is drawn by L-edit program is used in lithography process that creates a mask for sputtering, after those 100-nm thick Cr/Au electrodes are deposited by DC-sputtering process. They consist of twenty four control electrodes that are double in a linear array on the glass substrate. The first dielectric layer, namely, a 100-nm-thick silicon dioxide layer, is deposited by RF-sputtering. Then, a 20 nm hydrophobic Teflon® AF 1601 (Dupont, USA) layer is spin-coated on the silicon dioxide layer.

Mask: Lithography

Glass bottom plate

Cr/Au: DCi

SiO2: RF sputtering

Teflon® AF: Spin Coating

Single-plate EWOD

Droplet moving

978-1-4673-5696-1/12/$26.00 ©2012 IEEE

Figure 1. Fabrication of a single plate EWOD deviceFor single-plate EWOD device, the droplet can be moved

(transporting and merging a droplet) in fully controlled manner by applying voltage corresponding to a voltage control scheme as shown in Fig. 2.

Figure 2. A single plate EWOD device

III. THE DESIGN OF MICROCONTROLLER CIRCUITS AND PROGRAM.

For droplet transportation, we use a microcontroller to control relays that switching voltage applied to all 14 control electrodes on and off.

A. Circuit design For the design of the switch circuit, we use relay. A relay

circuit is typically a smaller switch or device which drives (opens/closes) an electric switch that is capable of carrying much larger current amounts. In the circuit we place diode across the coil of relay (Fig. 3) because when the relay is turned OFF, it produces a voltage in the opposite direction that can be much higher than the voltage of the supply. This voltage is called BACK EMF and only occurs when the relay is turned off suddenly when full current (or near full current) isfollowing

Figure 3. A relay circuit

Because when relay is off, voltage is not absolutely zero.We solve this problem we use two relays connected in series. To prevent unusual electrical disturbances from relays to microcontroller, we decided to use optocoupler or opto-isolator (Fig. 5). Optocoupler is an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output.

The main purpose of an opto-isolator is to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other side.

Input current come from output pin of programmed microcontroller and output of this circuit is to be connected to a control electrode. This circuit is for one control electrode. There are 14 control electrodes so we have to use 14 relay circuits and 14 microcontroller output pin. Altium Designer 09 (Fig. 4) is used for PCB designing.The complete circuit board with 14 output ports shown in Fig. 5.

Figure 4. Altium Designer 09

Figure 5. A complete circuit board

B. Microcontroller programming In this work, we use a PIC16f877 microcontroller to control

the droplet transportation on a EWOD microchip.Microcontroller is programmed in C programming language.The program contains RS232 interface to send and receive data between microcontroller and computer by using HyperTerminal program (Fig. 6) and has commands for droplet control as follows:

1) On HyperTerminal screen, has an initial command “Enter Key:” waiting for key press to be entered. We set ‘w’ to be a key.

2) After key press, “Input Delay:” message has shown waiting for delay times (ms) to be entered.

3) After we enter delay time, the message “press start” appears. This command waiting for four types of key press, ‘s’ key for droplet moving forward, ‘b’ for droplet moving backward, ‘m’ for droplet merging and ‘p’ is the pause key.

With these commands, we can control the moving style of droplet simply by press button on a personal computer’s keyboard.

Figure 6. Hyperterminal Program

IV. EXPERIMENTAL SETUP

In this experiment, we use the single-plate EWOD microchip to study droplet transport. The single-plate EWOD microchip is equipped with an acrylic block that has wires connected to circuit board. The droplet of deionized water are initially placed on the first and the last control electrode. For droplet actuation, the single-plate EWOD microchip is connected to the control system consists of a circuit board, a function generator, an amplifier, a transformer, and an oscilloscope. The experimental setup is shown in Fig.7.

The control system uses a function generator to provide AC signals. The signal is then amplified by an amplifier to generate control signal for droplet motion.

To control the droplet, we connect circuit board to a laptop via serial port (RS232) and USB-to-Serial adapter toreceive commands from keyboard.

Figure 7. Control system and experimental setup

The applied voltage is varied from 100-800 volt with 18 kHz frequency. The volume of droplets is varied from 20-40 µl.

Droplets are controlled by pressing the keyboard as mentioned above. First, we press ‘w’ key to enter the programand enter delay times in milliseconds. After that, when HyperTerminal screen appears “press start” message, we press ‘m’ key to start droplet merging. When we need to stop the droplets, we can press ‘p’ key. The droplet moving can be recorded by a personal digital camera.

V. EXPERIMENTAL RESULT For a droplet transporting, a droplets of deionized water are

initially placed on the first and the last control electrode (Fig. 8 (a)). For droplet actuation, single-plate EWOD microchip is connected to the control system. A droplet can be transported by applied voltage at the control electrode (Fig.8).

(a) (d)

(b) (e)

(c)

Figure 8. (a)-(f). A droplets are merging by microcontroller

In this experiment, a droplet of deionized water began moving at voltage of 600V and 30 ul (microliters). The 30 droplet 30 ul is the most appropriate volume for droplet actuation. From the adjustment of droplet motion’s speed by changing delay times, we found that the most appropriate value is 1500 ms.

VI.CONCLUSION

A single-plate electrowetting on dielectric microchip is fabricated and experimentally studied. For droplet actuation, the tested result show that a deionized water droplets are merged successfully across all electrodes of single-plate EWOD microchip by using applied voltage about 600-800 Vat frequency of 18 kHz.

From the design, fabrication and testing of integrated circuits with microcontroller to control the motion of deionized water droplets on control electrodes, as you can see the design tools can be utilized easily for droplets actuation. To be applied in the fields, such as the analysis of chemistry, biology and medical properties of interested samples without the use of complex tools. It can be easily controlled by personal computer or laptop. Moreover, we can control speed of motion of droplets by setting the delay times through the HyperTerminal program. This program can be applied for the new application by create new program for microcontroller.

ACKNOWLEDGMENT

This work is supported by Nanoelectronics and MEMSLaboratory, National Electronics and Computer Technology Center (NECTEC), National Science and Technology Development Agency, (NSTDA).

REFERENCES

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[3] J. Lienemann, A. Greiner, and J. G. Korvink, “ Modelling, Simulation and Optimization of Electrowetting,” IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, vol. 25(2), pp. 234-247, 2006.

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