A Review of Electrowetting on Dielectric Applications of Carbon Nanotubes

Introduction

Electrowetting on dielectric (EWOD) is a popular technology used in digital microfluidic devices to move microscale quantity of liquid on a substrate with electrodes [1]. A typical EWOD device consists of a substrate made of glass, silicon, paper, polyimide and more possible choices of material. The substrate is patterned with several electrodes and is coated with a thin uniform layer of a dielectric material. The entire device is then coated with a hydrophobic material to obtain a high contact angle for liquid droplets as well as to avoid adsorption of biomolecules. On application of high voltage to the electrodes an electric field is established over the corresponding electrode which reduces the contact angle of the liquid with the hydrophobic surface. The reduction in contact angle causes the liquid to “wet” the electrode and thus sticking to the surface for the period during which electric field is prevalent. The change in contact angle is given by the Young-Lippman equation (Equation 1) [2].

$$\cos \theta_v = \cos \theta_0 + \frac{1}{\gamma_{LG}} \frac{E_r E_0}{2t} V^2$$

Upon removal of the high voltage and therefore electric field the droplet “de-wets” the surface. This wetting and dewetting behavior can be used to move droplet form one electrode to the other thereby achieving transport of liquid on the microfluidic chip. Figure 1 shows the working of a parallel plate EWOD device. EWOD microfluidic technology has been used in many fields such as biofuels [3], blood-based assays [4] and many more. A growing demand of this technology is observed in applications involving flexible displays, electronic skin, etc [5, 6]. Such applications require the device to be transparent and have low thickness of the various layers on the chip. Further, an interest in using novel materials for the conducting electrodes and dielectrics is also observed due to the high demand in conductivity and physical properties.

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Utility of CNTs in EWOD

A number of groups have utilized the transparency of ITO as a conductive material, gold as a stretchable material. However, some attempts at using graphene or carbon nanotubes have shown promise to exploit the electrical as well as physical properties for making EWOD devices [5, 6]. Xuebin Tan, et al. studied the potential of thin layered graphene as the electrode layer in EWOD devices. They established that graphene is a strong candidate to be used in electrowetting on dielectric devices compared to other common conductive materials such as gold, for properties like higher breakdown voltage, electrical conductivity optical transparency and flexibility, as well as lower cost [6]. Gruner, et al. explored the use of SWCNTs as the conductive electrode layer in a EWOD microfluidic device. The group fabricated EWOD devices with a spray coated CNT films as the conductive layer, contact angle and droplet translation was tested to evaluate the performance of their nanotube EWOD devices. They demonstrated carbon nanotube electrodes in EWOD devices having indistinguishable performance when compared to ITO based electrode EWOD devices. Chen et al. grew CNT structures on the ground plate in their EWOD device in order to exploit the thermal conductivity of CNTs for the purpose of thermal management on microfluidic chip [8]. Lipomi, et al. also used the technique of spray coating for obtaining a conductive layer of CNTs [13] to make stretchable wearable pressure sensors indicating the strong potential of CNTs for easy fabrication of wearable electronics. Facilitating one more step towards development of low cost microfluidic devices, Ko, et al. have thoroughly discussed the opportunity of printing CNT electrodes on paper for digital microfluidic application using an inkjet printer [14]. CNTs properties are also used as a research tool to study the performance of electrowetting in EWOD systems. For example, Wang et al. used the ability to make nanostructures using CNTs on for parylene dielectric layer in order to compare the effect of roughness in contact angle saturation [15]. Shahini, et al. used the CNTs as electroporation electrodes for cell lysis [16]. Previously Shih, et al. demonstrated a versatile EWOD device for synthetic biology that included a region on the EWOD chip with bare gold-chromium electrodes for electroporation of cells [3]. Further, more complex applications of CNTs include detection of biomolecules like enzymes, proteins and DNA. The incorporation of carbon nanotubes has the potential to address a variety of long-standing issues with respect to bio-sensing [17]. Similarly, EWOD technology has been attractive due to its promising potential in biological, biochemical and biomedical applications [18].

Discussion

A number of reports that used the properties of CNTs for different applications have been reviewed and the overlap of functionality and application has been identified in this short review.

The tremendous progress in the fabrication techniques and research of CNTs has proved the strong potential for utility in electrowetting based nanoscale and microscale devices. EWOD is a technology that is used in many fields including microelectronics, bio applications, chemical sensing and many more. The incorporation of CNTs to EWOD has been successful and rewarding and holds promise to spread the reach of EWOD in even more areas of science and technology.