Integrated microfluidics system using surface acoustic wave and electrowetting on dielectrics technology

Abstract

This paper presents integrated microfluidic lab-on-a-chip technology combining surface acoustic wave (SAW) and electro-wetting on dielectric (EWOD). This combination has been designed to provide enhanced microfluidic functionality and the integrated devices have been fabricated using a single mask lithographic process. The integrated technology uses EWOD to guide and precisely position microdroplets which can then be actuated by SAW devices for particle concentration, acoustic streaming, mixing and ejection, as well as for sensing using a shear-horizontal wave SAW device. A SAW induced force has also been employed to enhance the EWOD droplet splitting function.

INTRODUCTION

Microfluidics involves the precise control and manipulation of fluids, typically in the sub-millimeter scale, and it normally includes a number of micro-components for handling fluids in micro-, nano-, or picolitre volumes.¹ Recently, digital microfluidics has been widely investigated as it offers a flexible and scalable system architecture and each droplet can be controlled precisely, independently and reconfigurably.^{2, 3, 4, 5, 6, 7, 8}

Surface acoustic wave (SAW) and electro-wetting-on-dielectric (EWOD) are two of the most widely reported digital microfluidic technologies.⁵ When liquid droplets are present in the path of a SAW, the acoustic force is able to create significant acoustic streaming in the droplet, which facilitates liquid movement, mixing, stirring, vibration, ejection, particle concentration and atomisation.^{7, 8, 9, 10} This ability to acoustically stream imparts energy into the droplet, which has been reported to be capable of increasing the droplet temperature as high as 140 °C (Refs. ^{9, 11}) and this can be undesirable in some applications.⁹

In contrast, EWOD devices use an electric field to modify the surface hydrophobicity for micro-droplet sample manipulation to move, merge and split droplets.^{4, 6} In particular, its ability to split microdroplets when capillary forces dominate is clearly of great interest.^{4, 6, 12, 13, 14, 15}

Comparing the two technologies, EWOD devices are better suited for droplet splitting and the precise position control of liquid micro-droplets, whereas the SAW technology is more suitable for in-droplet streaming or droplet ejection and also has a sensing capability. Clearly, there are possibilities to significantly enhance microfluidic functionality if these two technologies can be combined and preliminary work using test structures to demonstrate this has been briefly reported in conference papers.^{13, 16, 17} This paper explores the various combined microfluidic functions using SAW and EWOD technologies in a more comprehensive and systematic manner, which include SAW assisted EWOD splitting, EWOD assisted SAW particle concentration, and EWOD assisted shear-horizontal SAW sensing.

DEVICE DESIGN AND FABRICATION

Figure 1 shows a schematic of an integrated test structure and characterisation system which was designed to evaluate both the fabrication process parameters and materials, as well as to characterise basic EWOD and SAW devices for functions of enhanced droplet splitting, particle concentration/mixing and ejection, and SAW sensing. In Figure 1a, the EWOD electrode array is located in front of SAW interdigitated transducers (IDTs) so that droplets can be manipulated by either EWOD or SAW, or by both simultaneously. When the EWOD array is located between two symmetrically arranged SAW IDTs, the pair of the SAW IDTs is capable of performing SAW sensing of EWOD transported droplet samples.¹⁷ Important parameters associated with the performance of the SAW IDT are aperture width W, the IDT finger width d, which determines the SAW wavelength (λ = 4d when the design is a single electrode transducer with a metallisation ratio of 0.5), and the distance between the EWOD array and SAW IDTs D. In this work, the parameter values evaluated were d = 4, 8, 16, 32 μm, W = 200, 500, 1500, 2500, 5000, 7500 μm, and D = 656, 3216 μm. A cover plate was used for EWOD droplet splitting in order to minimize the required EWOD force.⁶ The key parameters for this splitting function are the ratio of channel gap g and electrode size L and in this work g and L were set to 160 μm and 1500 μm, respectively.

Figure 1.

Schematic of a SAW enhanced EWOD droplet splitting device. (a) Top view, (b) cross-section view (Ref. ¹³).

For the test structures reported in this paper, EWOD electrodes (1500 × 1500 and 1000 × 1000 μm) were available with an electrode separation of 10 μm. These structures were also used for EWOD assisted shear-horizontal SAW sensing devices, using the second identical SAW IDT that was positioned symmetrically on the other side of the EWOD array.¹⁷

The devices were fabricated using a single-mask process. Figure 2 shows the fabrication flow. The process started on piezoelectric substrates commonly used for SAW devices, such as 128° Y-cut LiNbO₃ for Rayleigh SAW waves, and 36° Y-cut LiTaO₃ for shear SAW waves. A lift-off process was chosen to pattern the SAW and EWOD electrodes on the integrated device substrates. For this lift-off process, 1.5 to 1.8 μm of image reversal (negative) photoresist AZ5214E was spun onto the substrate and patterned using contact lithography, followed by metal electrode deposition using sputtering. The two different electrode metals used were tantalum (Figure 2a) and aluminum (Figure 2b). Sputtered aluminum has low resistance, which is one of the important candidates when making RF devices such at the SAW IDTs. It can also be employed in EWOD fabrication, coated with Parylene-C dielectric and CYTOP (Asahi Glass Co.) hydrophobic layers.¹⁸ Tantalum has the advantages that its anodic oxide high dielectric constant can help to lower the EWOD operation voltage.⁴ However, due to its high sheet resistivity (∼8 Ω/□ at 400 nm thickness), it is mainly used in integrating with low frequency SAW devices in this study. After lift-off, the tantalum EWOD electrodes were anodized to form the EWOD Ta₂O₅ dielectric layer. This was followed by a layer of hydrophobic CYTOP, deposited on the electrodes⁴ using spin-coating.

Figure 2.

Process flows for a saw and EWOD integrated device using (a) tantalum electrodes; (b) aluminum electrodes.

EXPERIMENTS AND RESULTS

SAW assisted EWOD microfluidic

The capability to split droplets is an important function that can be used to precisely separate and sort individual droplets.^{4, 6} Using EWOD to implement this process requires a voltage to be applied to the two side electrodes to turn the surface from hydrophobic into hydrophilic while the central electrode surface is kept hydrophobic.^{4, 6} This results in the droplet to be separated being first pulled in opposite directions onto the two side electrodes. In this configuration, the droplet forms the shape where pinching in the middle of the droplet creates a “necking” zone.^{4, 6} When the necking is broken as a result of elongating the droplet, it successfully splits into two.

However, there are design limitations using such a configuration to split microdroplets. A successful droplet separation process relies on sufficient EWOD splitting force to combat the cohesive force within the droplet and a cover plate is generally required to implement EWOD based splitting. The available EWOD force is related to the liquid contact angle (CA) change that can be achieved. However, the CA saturates after a given voltage is applied. If the droplet has not been separated at this point, then separation does not occur no matter how high the applied driving voltage. The critical parameters for successful droplet splitting include the separation gap (g) between the top cover plate and bottom chip surface, and the electrode geometry (L).⁶ It has been reported that in order to implement this function using a hydrophobically coated droplet-in-air EWOD system, the chip-cover plate separation to electrode size ratio (g:L) should be less than 0.05.⁶ Clearly, these parameters need to be fully characterised in order to determine the design rules before integrating EWOD technology into lab-on-chip applications.⁶ Figure 3 (frame 3) shows how, when such design rules are not obeyed, the “necking” between the resulting two droplets reaches saturation status (increasing the EWOD driving voltage has no effect), which results in failure to split the droplet. In this case, the integration of SAW actuation can be employed to help augment the EWOD splitting process. In addition to the standard arrangements of EWOD electrodes, a SAW IDT is placed in such a manner as to direct a large acoustic force or power towards the centre of the “necking” region and help facilitate breaking the droplet boundaries. This combination is designed to deliver droplet-splitting with more relaxed EWOD design constraints.

Figure 3.

Download video file ^{(1.8MB, mpg)}

Droplet splitting—(1) Initial location of droplet, (2&3) EWOD (L = 1500 mm electrodes) splitting force applied, which cannot fully separate the droplets, (4) application of SAW (IDT parameters: W = 500 μm, d = 16 μm, 30 pairs) energy to split the liquid into two droplets (Ref. ¹³) (enhanced online) .

Figure 3 shows a sequence of photos from experiment when splitting a droplet by applying SAW actuation from the left side of the EWOD electrode array consists of three electrodes (L = 1500 μm). The SAW IDTs used here has an aperture size W = 500 μm, width d = 16 μm with 30 pairs of metal fingers operating using a 60 MHz input signal on a LiNbO₃ substrate. This has been characterised using a network analyzer in Ref. 17. The SAW is directed at the “necking” position after EWOD has initiated the separation and reached saturation. This approach can be successfully employed to split droplets, when a maximum degree of necking forms in the middle of the splitting droplet.⁶ Figure 3 (frame 3) shows the droplet deformation when the voltage has been increased to 65 V at which point the EWOD contact angle change starts to saturate, and the maximum degree of necking is observed. Further voltage increments do not help split the droplet, due to EWOD contact angle saturation.⁶ The extra force to divide the droplet is provided by a SAW, with a power of 15 dBm being sufficient to successfully separate the droplet, as depicted in Figure 3 (frame 4). This result clearly shows that SAW power can be used to promote droplet splitting with EWOD when saturation occurs before successful separation has been achieved.

During SAW assisted EWOD splitting, it is important to ensure that the SAW power level is not too strong or damage can occur on the hydrophobic coatings of both the top plate and the substrate. Figure 4a shows an example of such damage that destroys the normal function of the device and the cause is believed to be the SAW power reflecting between the cover plate and the bottom chip surface (Figure 4b). The hydrophobic layer appears cracked with parts of it delaminated from the substrate. Such surface damage routinely happens when the cover plate is present and the SAW excitation power exceeds 24 dBm. By comparison without the cover plate, this type of damage rarely occurs even with much higher SAW power settings.

Figure 4.

(a) Damage caused by powerful SAW wave (Power >24 dBm, IDT parameters: W = 500 μm, d = 16 μm, 30 pairs) to the hydrophobic coating on both the top cover plate and substrate, causing liquid to be trapped at the damaged zones. (b) Schematic of powerful SAW streaming forces reflecting between the cover plate and bottom chip surface which are believed to be the cause of such damage.

EWOD assisted SAW particle concentration, mixing and droplet ejection

EWOD electrodes in arrays, such as a 5 × 8 array,^{12, 13} can be individually addressed under software control to guide and hold multiple droplets at predetermined locations. The droplet positions are simply determined by the electrode positions, which can be placed anywhere on the device with much higher precision. For processes that use a single mask for patterning EWOD electrodes and SAW IDTs the positional accuracy is defined by the mask. If they are patterned by separate masks the registration error between layers is typically better than 2 μm if contact printing lithography is employed during fabrication. This provides opportunities for improving on the microfluidic manipulation capability of SAW, for which precise movement and position control is relatively more complex to implement than with EWOD.^{16, 17, 19, 20, 21, 22} The best reported SAW based positional placement is 100 μm using feedback detection control,^{19, 20, 21, 22} but this does require considerable external equipment to implement. As mentioned previously, SAW can be employed to mix the content of droplets. However, when actuating the droplet in this mode they need to be accurately positioned and well anchored^{16, 17} to efficiently mix, stream, and perform well controlled concentration of particulates in the droplet.¹⁰ EWOD is ideally suited to deliver accurate positional control. The location of the SAW IDTs with respect to the droplet position is important as this defines the different flow patterns that result inside the droplets under SAW actuation.¹⁰

Figure 5 illustrates how EWOD technology can be used to precisely move the droplets into the SAW actuation zone (SAW IDTs: W = 1500 μm, d = 16 μm), so that either streaming/mixing or particle concentration can be realized.^{10, 23} Polystyrene beads with an average diameter of 6 μm in DI (deionized) water solution have been used for this demonstration with the SAW power level being set at 18 dBm. Figure 5a shows that when the droplet is moved to the centre of the SAW wave by EWOD, SAW induced liquid streaming can be observed, which causes either mixing or pumping depending on the SAW power level. Figures 5b, 5c show that when the droplet is positioned towards the edge of the SAW actuation area on an EWOD electrode, the particles are concentrated in the centre of the droplet. The ability to precisely control the droplet’s position using an EWOD array simplifies the design and process required to implement particle concentration in droplets. By moving the droplet up and down along the EWOD electrode array, a dynamic cycle of concentration- mixing-concentration can be easily realized, which is potentially useful in applications such as cell concentration from blood samples and concentration of pathogens in miniaturized biosensors.²³ For finer control of the droplet position (higher resolution), electrodes with a smaller length can be employed with multiple electrodes being switched to move and locate the droplet.¹⁸

Figure 5.

(a) Mixing of polystyrene beads inside a droplet when located by EWOD in the middle of the SAW (Power = 18 dBm, IDT parameters: W = 1500 μm, d = 16 μm, 30 pairs). (b) Polystyrene beads (average diameter = 6 μm) in DI (deionized) water starting to concentrate in the centre of a droplet that has used EWOD to move the droplet towards the SAW wave edge. (c) Polystyrene beads concentrated in the centre of the droplet.

For droplet ejection using SAW actuation on a hydrophobic surface, the high power of the SAW is more likely to move (or pump) the droplet away, rather than hold it in a fixed position before ejection. In this case EWOD electrodes can be employed to help anchor a droplet at a precise position before ejection or used to position multiple droplets in different locations for ejection using SAW actuation. Figure 6 gives an example of how EWOD electrodes can be employed to anchor a droplet on hydrophobic surfaces, before a SAW (SAW IDT parameters: W = 2500 μm, d = 16 μm, 30 pairs) is applied to eject the droplet. Without a voltage being applied to an EWOD electrode, the droplet simply moves (pumped) away from the actuated IDT as the SAW power is gradually increased. Figure 6b shows the droplet shape deforming under the application of SAW power while its position is anchored on the initially hydrophobic surface of the EWOD electrode. The capability of being able to hold a droplet’s position potentially increases the accuracy of the angle of droplet ejection or enables more vigorous SAW actuated mixing to be undertaken on hydrophobic surfaces. It is expected that a higher EWOD voltage would change the initial ejection angle of the droplet. Multiple droplet ejection with directional control can be achieved when more EWOD electrodes are present in front of the SAW IDTs.

Figure 6.

EWOD controlled SAW mixing and ejection. (Top view (left), angled view (centre) and side (right). (a) No EWOD or SAW forces; (b) EWOD (DC voltage 60 V) anchoring droplet on the hydrophobic surface when SAW induces powerful mixing; (c) increasing SAW (IDT parameters: W = 2500 μm, d = 16 μm, 30 pairs) power (or removing EWOD anchoring) (26 dBm) force results in droplet ejection (Ref. ¹³).

Droplet manipulation on shear wave SAW sensor

Rayleigh-type modes are rapidly attenuated by the liquid media^{9, 10, 24} and this makes them not suitable for use as bio-sensing elements. In contrast, shear horizontal surface acoustic wave (sSAW) devices have been employed as an attractive platform for biosensors. However, actuating droplets using sSAW devices presents a challenge due to the low dissipation energy that is coupled into liquid.⁹ Some designated sSAW substrates such as 36° Y-cut LiTaO₃ do have a Rayleigh wave mode which could be used to manipulate the droplets on the same substrate.²⁵ However, the piezoelectric coupling coefficient is weak hence a much higher RF power input is required, reducing the system efficiency while increasing Joule heating. Fortunately, EWOD electrodes can be employed to efficiently pre-position droplets on the sSAW sensing device, which overcomes the problem that there might not be sufficient force available from the sSAW to move the droplet in a stable and precise manner.

The sSAW sensing function of an integrated EWOD-SAW device has been demonstrated by using the sSAW sensor to detect the droplet’s position while it is being moved using EWOD. Figure 7a shows a block diagram of the circuitry of a typical SAW sensing system. The sSAW device was used as a resonator in an oscillator circuit, connected to an RF amplifier. Similar circuitry designed for SAW frequency measurement has been reported²⁶ and this has since been widely applied. A frequency counter was used to monitor the oscillation frequency. The change of the resonant frequency has been used as the sensing signal, which is sensitive to the mass loading, elasticity, viscosity, permittivity, and conductivity change on the sSAW surface. Figure 7b shows the resonance frequency change resulting from the sSAW sensing circuitry when a 5 μl DI water droplet was moved across a SAW wave transmitted between two IDTs (W = 2500 μm, d = 16 μm, 30 pairs). It is clear that the resonant frequency signal is closely related to the droplet position.

Figure 7.

(a) SAW sensing circuitry (b) Measured frequency when the droplet is located at different positions (1 & 5 droplet out of SAW propagation path, 2 & 4 at the side of the SAW wave, 3 at the centre of the SAW wave) (Ref. ¹³).

CONCLUSIONS

This paper demonstrated the practical integration of SAW and EWOD technologies to realise multi-functional digital microfluidics for lab-on-chip applications. SAW IDTs have been used to help droplets to be split when the EWOD driving force saturates before sufficient force is available for separation. On the other hand, EWOD can be employed to control the SAW particle concentration, mixing and ejection. In addition shear-horizontal SAW sensing has been demonstrated using EWOD technology to position droplets for the test.