This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights
Author's personal copy Sensors and Actuators B 199 (2014) 183–189
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Study of cyanoethyl pullulan as insulator for electrowetting Jianfeng Chen a , Yuhua Yu a , Kaidi Zhang a , Chuanyong Wu b , Ai Qun Liu c , Jia Zhou a,∗ a
ASIC and System State Key Lab, School of Microelectronics, Fudan University, 220 Handan Road, Shanghai 200433, China Digital BioSystems, 325 Sharon Park Dr #222, Menlo Park, CA 94025, United States c School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b
a r t i c l e
i n f o
Article history: Received 31 January 2014 Received in revised form 21 March 2014 Accepted 29 March 2014 Available online 12 April 2014 Keywords: Cyanoethyl pullulan (CEP) Insulator Dielectric Electrowetting EWOD
a b s t r a c t In this paper, the dielectric and electrowetting properties of cyanoethyl pullulan (CEP) are studied as the insulator in electrowetting on dielectric (EWOD) device. The CEP film is formed by spin-coating and low-temperature annealing in atmosphere. The characterization results show that CEP can serve as the dielectric of EWOD with a high dielectric constant of 18 and a great electrowetting performance when annealed at 100 ◦ C in atmosphere. The reversibility, stability, polarity-dependence, and frequencydependence of electrowetting on CEP are fully investigated. The spin-coated CEP exhibits little polarityrelated electrowetting while having better driving performance under negative-potential. Frequencyrelated electrowetting is more sensitive at low-frequency alternating current (AC) potentials than at high-frequency ones. It also shows reversible and stable electrowetting with a small irreversibility of 2◦ after a 700-cycle test. Based on the above results, an EWOD device with 1 m thick CEP dielectric layer is fabricated. The EWOD device successfully manipulates water droplet at a low driving voltage of 20 V and a large velocity under low voltage of DC or AC signals with different frequencies. The easy fabrication and excellent performance of CEP qualifies it as a superior dielectric material in the future EWOD devices and lab-on-a-chip system. © 2014 Elsevier B.V. All rights reserved.
1 CV 2 2
cos − cos 0 =
In recent years, the manipulation of liquid droplets, referred to as digital microfluidics (DMF), has been widely investigated as a platform for control of liquids on lab on a chip (LOC) in many applications [1–4]. As one of the advanced and promising DMF technologies, electrowetting-on-dielectric (EWOD) technology is feasible and efficient in controlling discrete droplets through electrowetting. During the electrowetting, the wetting property of a hydrophobic surface can be modified by an external electric field [5,6]. The EWOD device generally manipulates liquid by way of droplet creating, transporting, splitting, and merging [7]. A typical EWOD microfluidic device must contain four components: a conductive material serving as the drive electrodes to provide control signal and route, a dielectric material serving as the insulator to provide main capacitance for electrowetting, a hydrophobic material lowering the surface energy for successful liquid movement, and a droplet liquid working as a medium for typical applications [8]. The principle of EWOD device can be described by the Lippmann–Young (L–Y) equation [9]:
where 0 and are the contact angles (CAs) before and after the driving voltage, respectively. V is the applied voltage, C the capacitance of the dielectric in the device, and the surface tension, respectively. ε0 (8.85 × 10−12 F/m), ε, and t denote the permittivity of vacuum, the dielectric constant, and the thickness of the dielectric layer, respectively. As can be seen in Eq. (1), when the voltage is applied on the droplet, the CA of the droplet will be changed, leading to the irregular deformation of the droplet and further electrowetting forces to move the droplet. Furthermore, at a given voltage, the electrowetting ability of a droplet is highly dependent on the electrical properties of the dielectric. Therefore, an appropriate dielectric material layer with a high dielectric constant and a small thickness should be used to lower the actuation voltage for commercial point of care Microsystems [10]. Actually, there have been many researches focusing on fabrication and evaluation of high performance materials in EWOD devices to reduce the driving voltage, such as barium strontium titanate (BST) through metal organic chemical vapor deposition (MOCVD) process [11], Si3 N4 through plasma-enhanced chemical vapor deposition (PECVD) process [12], parylene through spin-coating [13] or vapor pyrolysis process [14], tetraethoxysilane (TEOS) deposited by PECVD [15], Al2 O3 deposited by atomic layer deposition (ALD) [16], and Ta2 O5
∗ Corresponding author. Tel.: +86 21 5566 4601. E-mail address:
[email protected] (J. Zhou). http://dx.doi.org/10.1016/j.snb.2014.03.112 0925-4005/© 2014 Elsevier B.V. All rights reserved.
=
εε0 2 V 2t
1. Introduction
(1)
Author's personal copy 184
J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189
Fig. 1. Molecular structure of CEP.
formed by anodized or sputtered process [17,18]. Although the above materials can reduce the driving voltage greatly, the rigorous and complex fabrication process may limit mass production due to high cost, a great obstacle in the commercialization of EWOD-based LOC. Therefore, an ideal material for electrowetting should have a high dielectric constant and a great dielectric strength with easy fabrication process for LOC applications. In this paper, we report a novel material, cyanoethyl pullulan, as the insulator for EWOD devices. CEP is a cross-linked polymeric material with a molecular weight of 489,000. The molecular structure of CEP is shown in Fig. 1. It has many nitrile groups with polarization in the molecular side chains, accounting for a high dielectric constant of about 19 (1 kHz) [19,20]. With such attractive properties as easy solution-processability, low industrial cost, and nice flexibility, the CEP material has already been used as the high k gate insulator in organic field-effect transistors (OFET) [21–24]. However, due to the small fraction of the cross-linkable OH groups and the rigid ring structure of CEP, it is difficult to produce a very condensed film. As a result, it still suffers from the electrical leakage with a very thin layer as required in OFETs [20]. In EWOD field, no thickness limitation of the dielectric exists as in OFET and a thick CEP can also lower the driving voltage efficiently with its high dielectric constant. Moreover, the CEP layer can be easily formed through spin-coating and low temperature post-baking, which is very attractive to commercial applications of LOC. To our knowledge, the CEP material has rarely been used in EWOD devices [25,26] and its performances related to electrowetting have not been well studied. Herein, we study the properties of CEP material in detail and evaluate its performances for application in EWOD devices. The optimized fabrication process of CEP layer is obtained by electrical characterization and general CA measurements for the first time. Then asymmetry, reversibility, stability, and frequency-dependence of electrowetting on CEP are fully studied in the polarity-related, frequency-related, and long-term CA test. Finally, the CEP is used as the insulator in EWOD device, where the droplet manipulation is carried out. 2. Experimental 2.1. Materials and apparatus The glass substrate coated with 130 nm Indium tin oxide (ITO) layer with the sheet resistance of about 15 / was bought from Wesley technology Co., Ltd. The CEP powder was the commercial product form Shin-Etsu Chemical Co., Ltd. N,N-dimethylformamide (DMF) provided by Sigma–Aldrich was used as the solvent of CEP. The hydrophobic material Teflon® AF2400 solution (Grade 400S2-100-1, 1 wt%) was from DuPont. The positive photoresist (RZJ-304 from Suzhou Ruihong Electronic Chemical Co., Ltd.) was used in lithography. The glycerin and KCl standard solution (Sigma–Aldrich) were used to prepare the conductive liquid for CA measurements. The electrodes were patterned on the lithography machine (G33, Vacuum Machinery Plant in Chengdu, China). A reactive ion
etcher (RIE-10NR, Samco International, Japan) pretreated the substrate for subsequent coating. To deposit the CEP and hydrophobic layer, the spin-coater (WS-400BZ-6NPP, Laurell, US) and the hot plate (HP-303DU, SmartLab, US) were utilized for deposition and post-baking, respectively. Physical vapor deposition (PVD) machine (ASC-4000-C4 Type L, ULVAC, Japan) was applied to deposit Al metal layer. Film thicknesses were measured by the Profiler (Dektak XT, BRUKER, Germany). The impedance analyzer (4249A, Agilent, US) was used for capacitor–voltage (C–V) test. The CA measurements were carried out by the drop shape analyzer (DSA30, KRUSS, Germany). The driving signals for electrowetting were provided by the signal generator (FG503, MOTECH, Taiwan) and the amplifier (HA-405, MOTECH, Taiwan) with the aid of LabVIEW software and other circuit components (USB 6251, National Instruments (NI), US, etc.). The motion of droplets was recorded by the digital camera system (TK-C9201EC, JVC, Japan). 2.2. Preparation and measurements In our experiment, the glass substrate was first cleaned with acetone, ethanol, and deionized (DI) water in sequence, and then dried with pure N2 . If the bottom ITO electrodes need to be patterned, the lithography and etch process were carried out as follows. Firstly, the substrate was spin-coated with photoresist at 3000 rpm for 30 s and post-baked at 100 ◦ C for 10 min. The substrate was then exposed (5 s, 10 mW/cm2 ) through a mask, and developed in the developer for 40 s, followed by DI water rinsing and hard baking (120 ◦ C, 5 min). Finally, the wet etch process was carried out, followed by removing of the photoresist, washing, and drying. When the substrate with ITO electrodes was ready, the RIE treatment (30 sccm O2 , 100 W, 30 s) was employed to pretreat the surface for better adhesion between CEP and ITO. The CEP powder was dissolved in DMF to produce a 15% (wt/wt) solution, and then the solution was spincoated onto the substrate at 3000 rpm for 30 s to form a thin film of about 1 m. After that, the CEP samples were dried at room temperature (RT) or annealed at low temperatures (from 60 to 200 ◦ C) in atmosphere for an hour in case of different experiments. For CA measurements, a 60 nm thick Teflon layer was spincoated on CEP as the hydrophobic surface. 5 L DI water, or conductive liquid, consisting of 50% (v/v) glycerin and 50% (v/v) KCl solution with an electrical conductivity of about 0.013 S/m, was dripped on the surface of Teflon. A 0.1 mm platinum (Pt) wire was inserted into the droplet as the top electrode. During measurements, voltages were applied to the droplet through Pt electrode while grounding the bottom ITO electrode (see Fig. 2(a)). The DC or AC voltage was applied on and off the droplet with the shift time of 2 s. If a positive (negative) voltage is applied on Pt electrodes, it is denoted as +DC (−DC). The shape of the droplet, changing with the on and off of the applied voltage, was captured and analyzed to obtain the CAs. For electrical characterization, 150 nm Al layer was deposited and patterned on top of the CEP surface to form the 500 m circle electrodes through a hard mask by PVD process, as shown in Fig. 2(b). The ITO and the Al electrodes were used as the bottom and top electrodes, respectively, for CV tests to find out the dielectric constant of CEP. A parallel-plate EWOD device was fabricated with the CEP as the dielectric layer and the ITO drive electrodes of 1.5 mm × 1.5 mm (Fig. 2(c)). The bottom plate consists of ITO drive electrodes, 1 m thickness CEP dielectric layer, and 60 nm Teflon hydrophobic layer. The top plate was an unpatterned ITO glass as the transparent common electrode and a layer of 60 nm Teflon as the hydrophobic layer. The gap between the plates was 150 m. To actuate the droplets, the common electrode on the top plate was connected to the ground, while the driving electrodes on the bottom plate
Author's personal copy J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189
185
Dielectric Constant
24 RT O 100 C
22 20 18 16 14 12 10 1
10
100
1000
Frequency (kHz) Fig. 4. Dielectric constant of CEP film at various frequencies.
Fig. 2. Schematics of sample preparation and structure setup for (a) CA measurement; (b) dielectric characterization; and (c) EWOD manipulation.
were connected to the common terminals of SPDT (single pole double throw) relays. The potential on the bottom electrodes was switched between a high potential and zero potential through the relays, which were programmed and controlled by NI devices and computer. The motion of the droplets was recorded by the digital camera system. 3. Results and discussion 3.1. Dielectric properties The CEP is a polymer with the glass transition temperature (Tg ) of about 250 ◦ C [27]. It is believed that appropriate annealing process can result in stress relief and local structural rearrangement of the polymer chains with improved dielectric properties [28].
18.5
Dielectric Constant
18.0 17.5 17.0 16.5 16.0 15.5 15.0
0
50 100 150 Annealing Temperature (º C)
200
Fig. 3. Dielectric constant of CEP film at 100 kHz as a function of annealing temperature.
Fig. 3 shows the dielectric constant of CEP film dried at RT or annealed at various temperatures of 60 ◦ C, 120 ◦ C, 150 ◦ C, or 200 ◦ C for an hour in atmosphere, obtained from C–V measurements at the frequency of 100 kHz. Films annealed at below 100 ◦ C shows an increase in the dielectric constant whereas films anneals at above 100 ◦ C shows a decrease with the increase of the annealing temperature. When the CEP films are dried at RT or annealed at 100 ◦ C in atmosphere for an hour, it reaches the lowest and highest dielectric constants of 15.5 and 19, respectively. Fig. 4 shows the variation of dielectric constant with frequency (100 Hz–1 MHz) for CEP films dried at RT or annealed at 100 ◦ C, which also demonstrates that CEP annealed at appropriate temperature achieves better dielectric properties than that dried at RT. The obvious reduction of the dielectric constant with increasing frequency may be attributed to the tendency of induced dipoles to orient themselves in the direction of the applied field. Different from previous reports [21,23,25], in which the spin-coated CEP must be dried in vacuum for a long time and then annealed in vacuum or argon ambient, the CEP film in our work is formed through hot-baking in atmosphere. Such an easier process causes no change in its main dielectric properties. It is clear that, with the similar spin-coating and annealing process [29–31], CEP shows a much larger dielectric constant than that of SU8 (∼4), the traditional and popular EWOD dielectric material. 3.2. Electrowetting properties 3.2.1. Static electrowetting To further characterize the performance of CEP as the insulator for electrowetting, CA measurements are carried out as shown in Fig. 2(a). +DC voltages are applied on DI water droplet. Fig. 5 shows the results. CAs under different annealing temperature demonstrates similar trend with the increase of the applied voltage. The CAs presents hysteresis when voltage is below 8 V while approaching saturation when voltage is over 30 V. In the voltage range from 10 V to 30 V, the change of CAs obeys the electrowetting theory [32,33] consistently, also shown by the inset of Fig. 5. The inset is the cosine value of CAs versus the square value of the applied voltage, where data is selected from the obvious electrowetting section (from 10 V to 30 V). It can be seen that the electrowetting of CEP shows a high linearity with R2 = 0.995. In addition, the CEP film with different annealing process behaves in a changing trend approximate to what is discussed in the dielectric constant test above, presenting the largest and smallest CA change when annealing at 100 ◦ C and dried at RT, respectively. With annealing at 100 ◦ C, 40◦ change of CA (from 125◦ to 85◦ ) can be reached under 25 V,
Author's personal copy 186
J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189 0.8
110
cosθ - cosθ0
130
0.4
Contact angle (º )
120
0.2 0.0
110
0
200
400
RT 60°C-1h 100°C-1h 120°C-1h 150°C-1h 200°C-1h
100 90 80 0
600
800
1000
V2
RT-Positive 100°C-1h-Positive 120°C-1h-Positive RT-Negative 100°C-1h-Negative 120°C-1h-Negative
105 100 95
+DC and -DC Voff +DC Von
90 -DC Von
85 -100
10
20
30
40
which is far lower than that on traditional materials such as silicon dioxide, parylene [11], or SU8 with the same dielectric thickness. The voltage can be further lowered by reducing the thickness of CEP and/or using alternative hydrophobic film [34]. 3.2.2. Polarity-related electrowetting As to L–Y equation, the change of CA is proportional to the square of the applied voltage, which means that the polarity of voltage has no influence on the CA, and the curve of contact angle versus the applied voltage should be a symmetric parabola with the V = 0 axis prior to the angle saturation. However, polarity-related electrowetting exists and has been reported in some dielectric system [11,35–37]. In most cases, alternating signals are used for driving EWOD chip. Such polarity-related electrowetting has rarely been considered and has not been well explained yet. Fig. 6 shows the CAs measured from −40 V to +40 V by using CEP film in different annealing conditions. In overall, electrowetting on CEP demonstrates better symmetry than amorphous fluoropolymers [35] or tantalum pentoxide [37]. However, in details shown in the inset of Fig. 6, we can find out that the acceptable symmetry only maintains when the absolute value of the applied voltage is lower than 20 V. Otherwise, the asymmetry occurs, and the
130 120 110 100 90 80 0
10
20
100
200
300 400 Cycles
500
600
700
Fig. 7. Long-term contact angle measurement by applying +DC/−DC voltage on and off different CEP film built at RT, 100 ◦ C and 120 ◦ C.
Fig. 5. Contact angle change with applied voltage on CEP film under different annealing temperatures (inset is the cosine value of CA versus the square value of applied voltage).
-40 -30 -20 -10
0
50
Voltage (V)
Contact Angle (º )
Contact Angle (º )
0.6
30
40
50
Voltage (V) Fig. 6. Contact angle measured from −40 V to +40 V by using CEP film under different annealing condition (inset: replot the data of CEP under RT drying and 100 ◦ C annealing).
difference of CAs between +DC and −DC voltages increases as voltage rises. The CAs under −DC voltage decreases faster than those under +DC voltage, which indicates better electrowetting performances under −DC. Besides, the CA reaches lower saturation values at negative voltages. The average saturated CAs under −DC of CEP by different annealing process is 5◦ smaller than those at +DC. Moreover, as shown in the inset of Fig. 6, the dielectric layer annealed at 100 ◦ C shows polarity-related electrowetting at a higher voltage than that dried at RT. In fact, compared with the film dried at RT, the difference of CAs between +DC and −DC is smaller when CEP undergoes annealing at 100 ◦ C, showing smaller asymmetry electrowetting. In order to verify the polarity effect and study the reversibility and stability of electrowetting on CEP material, the long-term electrowetting test is carried out. 5 L conductive liquid droplet containing glycerin and KCl is used for the long-term CA test so that the applied voltage drops predominantly across the dielectric layers and the droplet is not easy to evaporate. Three CEP samples with RT drying and annealing at 100 ◦ C, and 120 ◦ C are tested under +DC and −DC voltages (26 V) for about 700 cycles (1500 s), respectively. Their CAs with DC on/off are recorded. Fig. 7 gives the results. Firstly, the CAs at −DC are smaller than those under +DC voltage, showing better electrowetting performance consistent with the results in Fig. 6, even under long-term repeatable test. Secondly, the CAs under −DC shows less fluctuation than those under +DC voltage, indicating better reversibility of electrowetting on CEP by −DC than that under +DC voltage. In macroscopic view of longterm test, there are tiny decrease of initial CAs and increase of active CAs under voltages for several hundred cycles (less than 4◦ , and the smallest is 2◦ ). It is mainly due to the evaporation of the droplet under the long time test and can be prevented in practical EWOD devices. Meanwhile, in detailed view of the short-term test (dozens of test cycles), we can find out that CA recovery occurs with a minimum hysteresis of less than 1◦ under −DC voltage. Such outstanding reversibility of CEP makes it an excellent dielectric candidate to the EWOD device [12,38]. Thirdly, the good stability of CEP is found even after a long-time electric testing. CA changing under driving voltage and recovering with no breakdown in the long-term test indicates the better performance of the spin-coating CEP than that of Ta2 O5 [39]. 3.2.3. Frequency-related electrowetting In most microfluidic electrowetting devices, AC voltage is generally used even its driving force is a little smaller than that under DC voltage. The benefits of using AC signal are described by Chen [40]
Author's personal copy J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189
187
20
CA Reduction (ºº )
18 16 100 Hz 500 Hz 1 KHz 5 KHz 10 KHz
14 12 10 8 -100
0
100 200 300 400 500 600 700 800 Cycles
Fig. 8. Reduction of contact angle by applying AC voltage with different frequencies on and off for long cycles test.
Fig. 9. Droplet created (a–c) and transported from left to right (c and d) under −DC driving voltage in a CEP-based EWOD device.
3.2.4. Electrowetting actuation A parallel-plate EWOD device is fabricated with 1 m CEP deposited by optimized process (annealed at 100 ◦ C in atmosphere for an hour) as the dielectric layer for further study. Fig. 9 is the pictures captured from a piece of video, where the droplet is successfully created and transported under −DC voltage actuation. The minimum actuation voltage of the EWOD device is 22 V and 26 V for −DC and +DC control signals, respectively, which is comparable with Ta2 O5 [17,39] when the dielectric thickness is scaled to the same value. To further characterize the EWOD device using CEP and study the influence of the driving signal, the velocity test is carried out. The driving velocities under different signals are obtained by the “voltage-dependent maximum velocity” [48]. The velocity is represented by using L × fmax , where L is the width of driving electrodes and fmax is the maximum switching frequency on adjacent electrodes, respectively. By using DC and AC voltage (square wave with the same value of the DC signals) with different frequencies, the voltage-depended velocity is shown in Fig. 10. It can be seen that under different types of driving signals, the droplet velocities are all approximately proportional to the square of the applied voltage, which agrees with
DC AC-100 Hz AC-1 KHz AC-5 KHz AC-10 KHz AC-20 KHz
100 Velocity (mm/s)
as follows. First is the reduction of contact angle hysteresis. Using AC voltage electrowetting continuously perturbs the force balance at the contact line and essentially prevents the pinning effects of the contact line from the surface, which may lead to CA hysteresis [41]. Second is the delay of CA saturation. Many experimental results have shown that the CA saturation occurs at a smaller value and at a higher effective voltage for AC rather than that for DC electrowetting [42,43], although the underlying physics has still not been fully understood. Third is the reduction of ion adsorption, which is another possible reason leading to irreversibility or large CA hysteresis. As for AC voltage, different frequencies have been used by different material systems [44,45] because the frequency is important for efficient drive. On one hand, the L–Y equation is only valid for AC electrowetting when the liquid can be treated as a perfect conductor. This means the AC frequency should be larger than the resonance frequency of sessile drops (typically of few hundred hertz) and far smaller than a critical frequency fc [32]. Some other undesirable effects such as satellite droplet generation may occur when the frequency is higher than fc . On the other hand, the electrowetting efficiency diminishes with frequency increasing beyond the fc . This is verified by Jones [46,47] who proposed the “height-of-rise measurements” to test the relationship between electrowetting forces and driving frequency. In our experiment, the frequency-related electrowetting was carried out by long-term electrowetting test on CEP sample annealed at 100 ◦ C. The square wave is chosen as the driving AC signal applied on a 5 L conductive liquid droplet, because the rootmean-square (rms) voltage is equal to its peak value. Signals with a value of 24 V and frequencies of 100 Hz, 500 Hz, 1 kHz, 5 kHz, and 10 kHz are applied on the droplet for 2 s to record the changed CAs and off the droplet for 2 s to measure the recovered CAs. The test cycle is 750. Fig. 8 demonstrates the reduction of CAs (recovered CA minus changed CA). The largest reduction of CAs occurs at 100 Hz and 500 Hz with an average value around 17◦ , which is a little smaller than that under −DC voltage. As the frequency increases, the reduction of CAs decreases, indicating the reduced electrowetting ability of CEP. That means a strong effect of frequency on CEP, i.e. a higher value of voltage should be used to achieve efficient electrowetting when using high frequency signals. Besides, electrowetting performances under AC signals do not deteriorate much even in a long time test, showing that the reversibility and stability of CEP is acceptable and AC driving signals can also be used for CEP. In summary, a low frequency signal (<1 kHz) or DC signal is more preferable in lowering the driving voltage of EWOD device when using CEP as the insulator.
80 60 40 20 0
20
30
40
50
60
70
80
Voltage (V) Fig. 10. The voltage dependent velocity affected by type of driving signal in EWOD device using CEP as insulator.
Author's personal copy 188
J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189
the theory [33]. From DC signal to AC signal with increased frequency, the threshold value of driving voltage increases from about 20 V to 40 V. At the same value of voltage, the velocity acquired under DC signal is larger than that under AC signals, and the velocity decreases as the frequency of the AC signal increases. In other words, the EWOD device with CEP layer expresses the largest driving force under DC driving signal, and the driving performance deteriorates as the frequency increases under AC driving signals. This behavior of EWOD device using CEP insulator conforms to the electrowetting properties of CEP discussed above. 4. Conclusions In this paper, the use of CEP in EWOD devices is fully studied. This material can be easily fabricated by spin-coating method, and performs the best dielectric properties when it is annealed at 100 ◦ C in atmosphere with a dielectric constant of about 18 (100 kHz). The polarity-related electrowetting on CEP maintains good symmetry under low voltages. It presents better driving performance at the negative voltage than the positive one when the applied voltage is close to the saturation value. The long-term electrowetting test verifies the polarity effect, and shows great reversibility and stability of electrowetting on CEP insulator. The frequency-related electrowetting of CEP indicates its good electrowetting performances even under AC signal. As the frequency increases, the electrowetting force declines. Therefore, a low-frequency signal (<1 kHz) or a DC signal is proposed to lower the driving voltage of EWOD device when using CEP as the insulator. Based on the fundamental experiments, a parallel-plate EWOD device is fabricated by using CEP, demonstrating successful actuation of water droplet at the low DC voltage of 20 V. The voltage-dependent velocity shows that the DC signal presented larger electrowetting driving force than the AC signals do. When using AC driving signals, the driving force decreases as the frequency increases. In summary, the CEP with appropriate anneal can perform good dielectric and electrowetting properties as the insulator of EWOD device. Easy fabrication, high dielectric constant, and great reversibility and stability on electrowetting qualify CEP as a superior dielectric material for EWOD devices. It shows a great promise of applications in the commercial lab on chip. Acknowledgments This work is supported by the National Science Foundation of China with grant no. 61176110. References [1] R.B. Fair, Digital microfluidics: is a true lab-on-a-chip possible? Microfluidics Nanofluidics 3 (2007) 245–281. [2] R. Sista, Z.S. Hua, P. Thwar, A. Sudarsan, V. Srinivasan, A. Eckhardt, et al., Development of a digital microfluidic platform for point of care testing, Lab Chip 8 (2008) 2091–2104. [3] L. Malic, D. Brassard, T. Veres, M. Tabrizian, Integration and detection of biochemical assays in digital microfluidic LOC devices, Lab Chip 10 (2010) 418–431. [4] M.J. Jebrail, M.S. Bartsch, K.D. Patel, Digital microfluidics: a versatile tool for applications in chemistry, biology and medicine, Lab Chip 12 (2012) 2452–2463. [5] M.G. Pollack, R.B. Fair, A.D. Shenderov, Electrowetting-based actuation of liquid droplets for microfluidic applications, Appl. Phys. Lett. 77 (2000) 1725–1726. [6] J. Lee, H. Moon, J. Fowler, T. Schoellhammer, C.J. Kim, Electrowetting and electrowetting-on-dielectric for microscale liquid handling, Sens. Actuators A: Phys. 95 (2002) 259–268. [7] S.K. Cho, H.J. Moon, C.J. Kim, Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits, J. Microelectromech. Syst. 12 (2003) 70–80. [8] H. Liu, S. Dharmatilleke, D.K. Maurya, A.A.O. Tay, Dielectric materials for electrowetting-on-dielectric actuation, Microsyst. Technol. Micro Nanosyst. Inf. Storage Process. Syst. 16 (2010) 449–460. [9] M. Vallet, B. Berge, L. Vovelle, Electrowetting of water and aqueous solutions on poly(ethylene terephthalate) insulating films, Polymer 37 (1996) 2465–2470.
[10] D. Mark, S. Haeberle, G. Roth, F. von Stetten, R. Zengerle, Microfluidic lab-on-achip platforms: requirements, characteristics and applications, Chem. Soc. Rev. 39 (2010) 1153–1182. [11] H. Moon, S.K. Cho, R.L. Garrell, C.-J. Kim, Low voltage electrowetting-ondielectric, J. Appl. Phys. 92 (2002) 4080–4087. [12] J.K. Lee, K.W. Park, H.R. Kim, S.H. Kong, Dielectrically stabilized electrowetting on AF1600/Si3 N4 /TiO2 dielectric composite film, Sens. Actuators B: Chem. 160 (2011) 1593–1598. [13] Y.Y. Lin, R.D. Evans, E. Welch, B.N. Hsu, A.C. Madison, R.B. Fair, Low voltage electrowetting-on-dielectric platform using multi-layer insulators, Sens. Actuators B: Chem. 150 (2010) 465–470. [14] M. Dhindsa, S. Kuiper, J. Heikenfeld, Reliable and low-voltage electrowetting on thin parylene films, Thin Solid Films 519 (2011) 3346–3351. [15] D.P. Papageorgiou, A. Tserepi, A.G. Boudouvis, A.G. Papathanasiou, Superior performance of multilayered fluoropolymer films in low voltage electrowetting, J. Colloid Interface Sci. 368 (2012) 592–598. [16] J.-H. Chang, D.Y. Choi, S. Han, J.J. Pak, Driving characteristics of the electrowetting-on-dielectric device using atomic-layer-deposited aluminum oxide as the dielectric, Microfluidics Nanofluidics 8 (2009) 269–273. [17] Y. Li, W. Parkes, L.I. Haworth, A.A. Stokes, K.R. Muir, P. Li, et al., Anodic Ta2 O5 for CMOS compatible low voltage electrowetting-on-dielectric device fabrication, Solid-State Electron. 52 (2008) 1382–1387. [18] Y.Y. Lin, E.R.F. Welch, R.B. Fair, Low voltage picoliter droplet manipulation utilizing electrowetting-on-dielectric platforms, Sens. Actuators B: Chem. 173 (2012) 338–345. [19] M. Taniguchi, T. Kawai, Vertical electrochemical transistor based on poly(3-hexylthiophene) and cyanoethylpullulan, Appl. Phys. Lett. 85 (2004) 3298–3300. [20] W.T. Xu, S.W. Rhee, Compromise of electrical leakage and capacitance density effects: a facile route for high mobility and sharp subthreshold slope in low-voltage operable organic field-effect transistors, J. Mater. Chem. 21 (2011) 998–1004. [21] M. Na, S.W. Rhee, Electronic characterization of Al/PMMA poly(methyl methacrylate)/p-Si and Al/CEP(cyanoethyl pullulan)/p-Si structures, Org. Electron. 7 (2006) 205–212. [22] R. Parashkov, E. Becker, G. Ginev, T. Riedl, H.H. Johannes, W. Kowalsky, Allorganic thin-film transistors made of poly(3-butylthiophene) semiconducting and various polymeric insulating layers, J. Appl. Phys. 95 (2004) 1594–1596. [23] W. Xu, S.W. Rhee, Organic field-effect transistors with cross-linked high-k cyanoethylated pullulan polymer as a gate insulator, Org. Electron. 11 (2010) 996–1004. [24] W.T. Xu, C. Guo, S.W. Rhee, High performance organic field-effect transistors using cyanoethyl pullulan (CEP) high-k polymer cross-linked with trimethylolpropane triglycidyl ether (TTE) at low temperatures, J. Mater. Chem. C 1 (2013) 3955–3960. [25] B. Bhattacharjee, H. Najjaran, Droplet sensing by measuring the capacitance between coplanar electrodes in a digital microfluidic system, Lab Chip 12 (2012) 4416–4423. [26] B. Bhattacharjee, Study of Droplet Splitting in an Electrowetting Based Digital Microfluidic System, The University of British Columbia, Okanagan, 2012. [27] S.P.S. Yen, C.R. Lewis, P.J. Cygan, T.R. Jow, High dielectric constant material development, in: IEEE 35th International Symposium on Power Sources, 1992, pp. 381–386. [28] W.T. Xu, F. Wang, S.W. Rhee, Quasi-ordering in spontaneously associated surface dipoles: an intrinsic interfacial factor for high-k polymer insulated organic field-effect transistors, J. Mater. Chem. 22 (2012) 1482–1488. [29] S. Keller, G. Blagoi, M. Lillemose, D. Haefliger, A. Boisen, Processing of thin SU-8 films, J. Micromech. Microeng. 18 (2008). [30] S.K. Fan, Y.W. Hsu, C.H. Chen, Encapsulated droplets with metered and removable oil shells by electrowetting and dielectrophoresis, Lab Chip 11 (2011) 2500–2508. [31] J. Chen, Y. Yu, J. Li, Y. Lai, J. Zhou, Size-variable droplet actuation by interdigitated electrowetting electrode, Appl. Phys. Lett. 101 (2012). [32] F. Mugele, J.C. Baret, Electrowetting, From basics to applications, J. Phys.: Condens. Matter 17 (2005) R74–R705. [33] J. Berthier, Micro-Drops and Digital Microfluidics, Elsevier, 2007. [34] I.F. Guha, J. Kedzierski, B. Abedian, Low-voltage electrowetting on a lipid bilayer formed on hafnium oxide, Appl. Phys. Lett. 99 (2011). [35] E. Seyrat, R.A. Hayes, Amorphous fluoropolymers as insulators for reversible low-voltage electrowetting, J. Appl. Phys. 90 (2001) 1383–1386. [36] S.K. Fan, H.P. Yang, T.T. Wang, W. Hsu, Asymmetric electrowetting – moving droplets by a square wave, Lab Chip 7 (2007) 1330–1335. [37] H. Lian-Xin, K. Bonhye, C.C.J. Kim, Evaluation of anodic Ta2 O5 as the dielectric layer for EWOD devices, in: IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012, pp. 428–431. [38] M. Khodayari, J. Carballo, N.B. Crane, A material system for reliable low voltage anodic electrowetting, Mater. Lett. 69 (2012) 96–99. [39] L.X. Huang, B. Koo, C.J. Kim, Sputtered–anodized Ta2 O5 as the dielectric layer for electrowetting-on-dielectric, J. Microelectromech. Syst. 22 (2013) 253–255. [40] L. Chen, E. Bonaccurso, Electrowetting — From statics to dynamics, Adv. Colloid Interface Sci. (2013). [41] F. Li, F. Mugele, How to make sticky surfaces slippery: contact angle hysteresis in electrowetting with alternating voltage, Appl. Phys. Lett. 92 (2008). [42] S. Chevalliot, S. Kuiper, J. Heikenfeld, Experimental validation of the invariance of electrowetting contact angle saturation, J. Adhes. Sci. Technol. 26 (2012) 1909–1930.
Author's personal copy J. Chen et al. / Sensors and Actuators B 199 (2014) 183–189 [43] Y.S. Nanayakkara, S. Perera, S. Bindiganavale, E. Wanigasekara, H. Moon, D.W. Armstrong, The effect of AC frequency on the electrowetting behavior of ionic liquids, Anal. Chem. 82 (2010) 3146–3154. [44] S.K. Fan, P.W. Huang, T.T. Wang, Y.H. Peng, Cross-scale electric manipulations of cells and droplets by frequency-modulated dielectrophoresis and electrowetting, Lab Chip 8 (2008) 1325–1331. [45] I.A. Eydelnant, U. Uddayasankar, B.Y. Li, M.W. Liao, A.R. Wheeler, Virtual microwells for digital microfluidic reagent dispensing and cell culture, Lab Chip 12 (2012) 750–757. [46] T.B. Jones, J.D. Fowler, Y.S. Chang, C.J. Kim, Frequency-based relationship of electrowetting and dielectrophoretic liquid microactuation, Langmuir 19 (2003) 7646–7651. [47] T.B. Jones, K.L. Wang, D.J. Yao, Frequency-dependent electromechanics of aqueous liquids: electrowetting and dielectrophoresis, Langmuir 20 (2004) 2813–2818. [48] M.G. Pollack, A.D. Shenderov, R.B. Fair, Electrowetting-based actuation of droplets for integrated microfluidics, Lab Chip 2 (2002) 96–101.
Biographies Jianfeng Chen is a graduate student of ASIC and System State Key Lab, School of Microelectronics, Fudan University. His research interest focuses on novel EWOD devices and integration with electrochemical sensing technology.
189
Yuhua Yu is a graduate student of ASIC and System State Key Lab, School of Microelectronics, Fudan University. Her research interest focuses on MEMS/NEMS-based biochemical sensors and their modification with nanomaterial. Kaidi Zhang is a graduate student of ASIC and System State Key Lab, School of Microelectronics, Fudan University. His research interest focuses on digital microfluidic devices and their applications in biology. Chuanyong Wu is an adjunct professor of the School of Microelectronics, Fudan University. Dr. Chuanyong Wu is also the founder and CEO of Digital BioSystems Co., Ltd. He received his Ph.D. in Physics from Louisiana State University. His research interests are in the applications of microfluidic devices and the commercialization of microfluidic system. Ai Qun Liu is currently a Professor at the School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore. He received his PhD degree from National University of Singapore (NUS) in 1994. His research interests include Microelectromechanical Systems (MEMS) device physics and fabrication technology, optical and photonic MEMS, photonic bandgap crystal and structure, micro-optofluidics and biophotonic-on-a-chip. Jia Zhou is a professor of ASIC and System State Key Lab, School of Microelectronics, Fudan University. Prof. Jia Zhou received her PhD degree from Fudan University in 2004. Her research interests are in nano/microfluidics, MEMS/NEMS-based chemical, biochemical and biomedical sensors and their applications.
