Design and analysis of a low actuation voltage electrowetting-on-dielectric device

Electrowetting-on-dielectric (EWOD) technology has been considered as a promising candidate for digital microfluidic (DMF) applications due to its outstanding flexibility and integrability. The dielectric layer with a hydrophobic surface is the key element of an EWOD device, determining its driving voltage, reliability, and lifetime. Hereby, inspired by the ionic-liquid-filled structuring polymer with high capacitance independent on thickness, namely ion gel (IG), we develop a polymer (P)-ion gel-amorphous fluoropolymer, namely, PIGAF, composite film as a replaceable hydrophobic dielectric layer for fabrication of a high-efficiency and stable EWOD-DMF device at relatively low voltage. The results show that the proposed EWOD devices using the PIGAF-based dielectric layer can achieve a large contact angle (θ) change of ∼50° and excellent reversibility with a contact angle hysteresis of ≤5° at a relatively low voltage of 30 Vrms. More importantly, the EWOD actuation voltage did not change obviously with the PIGAF film thickness in the range of several to tens of microns, enabling the thickness of the film to be adjusted according to the demand within a certain range while keeping the actuation voltage low. An EWOD-DMF device can be prepared by simply stacking a PIGAF film onto a PCB board, demonstrating stable droplet actuation (motion) at 30 Vrms and 1 kHz as well as a maximum moving velocity of 69 mm/s at 140 Vrms and 1 kHz. The PIGAF film was highly stable and reliable, maintaining excellent EWOD performance after multiple droplet manipulations (≥50 cycles) or long-term storage of 1 year. The proposed EWOD-DMF device has been demonstrated for digital chemical reactions and biomedical sensing applications.

Previous studies have concentrated on harnessing the surface plasmonic effects of metallic nanoparticles (NPs) to improve polymer solar cell performance. Although ample examples have evidenced the viability of this methodology, the adverse effect of device photocurrent reduction via the intrinsic metal-mediated losses of plasmonic metal NPs, which hampers further enhancement of device efficiency, has rarely been recognized. To address this issue, we herein embedded Au NPs coated with a dielectric SiO2 layer into polymer solar cells, attempting to reduce the negative effects of these metallic nanostructures and thus increase photovoltaic photocurrent and efficiency. We constructed inverted polymer solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, and blended Au NPs coated with SiO2 layer, i.e. Au@SiO2 core–shell nanostructures into the active layer. Compared with plasmonic solar cells embedded with sole Au NPs, the device incorporating Au@SiO2 core–shell nanostructures indeed exhibited significantly augmented photocurrent density, though not a superior overall efficiency. The photocurrent density increase is attributed to the dielectric layer coating Au NPs, which mitigates metal-mediated losses such as exciton quenching, probably induced by the electron accumulation on the metallic surface, but which meanwhile is thin enough to maintain the plasmonic effects of the gold core upon photoexcitation. The study provides new insights into strategies harnessing plasmonic nanostructures to enhance photovoltaic performance, i.e. the balance between plasmonic effects and metal-mediated losses has to be comprehensively evaluated.

This paper presents a low temperature technological process ahle to integrate an all-glass microfluidic network with an ElectroWetting On Dielectric (EWOD) structure for the digital handling of liquids. The fluidic channels result from the wet-etching of the glass, while the electrodes necessary for the droplet movement are deposited on the bottom and top surfaces of the microfluidic structure. The bottom electrodes are produced by a selective and sequential photolithographic pattern of a stack of metals, insulation layer and hydrophobic film. The top common electrode is made by a continuous transparent conductive oxide, covered by a hydrophobic layer. Compatibility of the technological steps and mechanical robustness of the proposed device have been tested designing and fabricating a microfluidic network integrating a central chamber, with a volume of about 9 μΐ, two reservoirs, two microfluidic channels and 26 EWOD electrodes. The maximum temperature reached during the device fabrication was 330°C, which is two times lower than the one used for the anodic bonding of glass-based microfluidic network.

This paper presents the design, analysis, modeling and simulation of a novel RF MEMS series switches with low actuation voltage. A mechanical modeling is presented to describe the behavior of the series switch. The switch is designed with special mechanical structures. The novel mechanical and mathematical modeling of the switch leads to calculation of the accurate actuation voltage. The spring constant has been calculated in relation to the presence of the residual stress in the beam. The calculated spring constant for this beam is used to determine the accurate actuation voltage. The size of the switch is 60 ? 110 µm2. The designed RF MEMS series switch was simulated using Intellisuite MEMS tool. He calculated actuation voltage is 4.05 V and simulated one is 4.2 V for 0.6 µm beam thickness. The calculated result is also very close with simulated one. The proposed switch compared with other electrostatic switches has low actuation voltage and small size. The RF characteristics were simulated using HFSS software and the switch has good RF performance. The insertion loss of 0.067 dB, return loss of 26 dB and isolation of 16 dB were achieved at 40 GHz.

Recent advancements in Electrowetting on Dielectric (EWOD) systems, such as simplified fabrication, low-voltage actuation, and the development of more reliable materials, are expanding the potential applications of electrowetting actuators. One application of EWOD actuators is in RF devices to enable dynamic reconfiguration and allow real-time adjustments to frequency and bandwidth. In this paper, a method is introduced to actuate a panel using EWOD forces. In the EWOD system, the velocity of the plate increases by maximizing the actuation force, minimizing the moving mass (droplets and metalized plate), and reducing resistance (contact line drag, fluid drag). However, some of these are competing factors. For instance, the actuation force can be increased by increasing the number of droplets, but this also increases the inertia and the drag force. An analytical model of EWOD actuation is presented to understand system performance tradeoffs. The model is validated with an EWOD experiment, and the data demonstrate less than a 7.8% error between the measured and predicted maximum plate velocities for different voltage inputs. In addition, this study presents a 3D numerical FEM model to analyze the velocity profile and viscous force in the thin droplets, focusing on variations along the droplet's height, which cannot be captured experimentally. The main advantage of the proposed system over previous works is the simple 2D manufacturing process, which allows embedding metalized plates and RF circuit boards, in addition to being compact, portable, and low-cost. In addition, the proposed method does not have any mechanical components, which can increase the system's reliability in a harsh environment.

Optimum thickness of hydrophobic layer for operating voltage reduction in EWOD systems

In this study, it is demonstrated that an organic memory structure using pentacene and citrate-stabilized silver nanoparticles (Ag NPs) as charge storage elements on dielectric SiO2 layer and silicon substrate. The Ag NPs were synthesized by thermal reduction method of silver trifluoroacetate with oleic acid. The synthesized Ag NPs were analyzed with high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) for their crystalline structure. The capacitance versus voltage (C-V) curves obtained for the Ag NPs embedded capacitor exhibited flat-band voltage shifts, which demonstrated the presence of charge storages. The citrate-capping of the Ag NPs was confirmed by ultraviolet-visible (UV-VIS) and Fourier transformed infrared (FTIR) spectroscopy. With voltage sweeping of +/-7 V, a hysteresis loop having flatband voltage shift of 7.1 V was obtained. The hysteresis loop showed a counter-clockwise direction. In addition, electrical performance test for charge storage showed more than 10,000 second charge retention time. The device with Ag NPs can be applied to an organic memory device for flexible electronics.

This paper reviews the principles, operations, and applications of bubble-based electrowetting-on-dielectric (EWOD). EWOD has proved to be an efficient tool in digital microfluidics that employs discrete droplets, and various applications that use the principles of EWOD have been developed from lab-on-a-chip to optical systems. Similar to its use with droplets, EWOD can also be applied to gaseous bubbles. This review begins with a discussion of the principles of EWOD for a bubble on an electrode covered with a hydrophobic dielectric layer. It then addresses EWOD actuation and the transportation of a bubble in an aqueous medium, along with a physical explanation of bubble motion. The operation of EWOD is then extended to the on-chip creation/elimination and splitting of bubbles. In particular, micro-mixers and pumps are discussed as potential applications of these operations. Unlike droplets, bubbles can be easily oscillated by external excitation, which provides additional functionalities. By integrating EWOD with external excitation, a number of new advanced applications are introduced, including the capture/separation of particles and the propulsion of objects. In these advanced operations, cavitational microstreaming flows and acoustic radiation forces are mainly responsible for the physical mechanisms. This paper also discusses these advanced operations along with their underlying physics. It is expected that in addition to bubble oscillation, other bubble actuation modes will create new functionalities and new potential applications.

In order to get rid of the dependence on expensive photolithography technology and related facilities, an economic and simple design and fabrication technology for digital microfluidics (DMF) is proposed. The electrodes pattern was generated by inkjet printing nanosilver conductive ink on the flexible Polyethylene terephthalate (PET) substrate with a 3D circuit board printer, food wrap film was attached to the electrode array to act as the dielectric layer and Teflon® AF was sprayed to form a hydrophobic layer. The PET substrate and food wrap film are low cost and accessible to general users. The proposed flexible DMF chips can be reused for a long time by replacing the dielectric film coated with hydrophobic layer. The resolution and conductivity of silver traces and the contact angle and velocity of the droplets were evaluated to demonstrate that the proposed technology is comparable to the traditional DMF fabrication process. As far as the rapid prototyping of DMF is concerned, this technology has shown very attractive advantages in many aspects, such as fabrication cost, fabrication time, material selection and mass production capacity, without sacrificing the performance of DMF. The flexible DMF chips have successfully implemented basic droplet operations on a square and hexagon electrode array.

This work presents a study on how threshold voltage (VT) varies with the change in shapes (rectangular and interdigitated finger) and size of the coplanar electrode in electrowetting-on-dielectric (EWOD) device. The EWOD device is fabricated on glass substrate with aluminimum (Al) electrode. Next, synthesized nanocomposite thin film of BST/Teflon® AF is coated on the coplanar electrodes to act as both dielectric and hydrophobic layer. Experiments show that change in electrode dimension (or shape), such as electrode length (L), gap (G) and no. of teeth (T) result in different contact line (CL) between the droplet and the actuated electrode. This leads to change in VT for droplet transport. It is found that higher contact line results in lower threshold voltage, while lower contact line results in higher threshold voltage. This finding matches with the reported theory in literature.

High-speed droplet actuation on single-plate electrode arrays

RF-MEMS switches may be divided into capacitive and metal-to-metal types in terms of type of connection. In capacitive switches in the OFF-state the beam does not attach to the transmission line due to the presence of a thin dielectric layer and it gets released easily. Capacitive switches are better compared with the metal-to-metal types due to their ability to transmit signals of higher frequency and power. In this paper, a novel RF MEMS shunt capacitive switch with a unique spring design is proposed. The proposed design benefits from high isolation, low loss, low actuation voltage, small size and weight compared to similar switches and is designed for application in the Ka-band. Step structure is used in the mechanical design of the switch to reduce the air gap between the bridge and the transmission line that has resulted in reduction of actuation voltage to 2.2 V. The proposed switch has been placed upon a coplanar waveguide (CPW) line with an impedance of 50 Ohms. $${\text{SiO}}_{2}$$ with a thickness of 1000 A has been used as dielectric to increase isolation and down-state capacitance. The unique design of the spring results in the reduction of stress to a minimum at all points of the switch especially the springs at the time of applying the actuation voltage. This advantage would reduce switch failures as time passes and results in increased life of the switch. Up-state capacitance is 93.3 fF and down-state capacitance is 2.64 pF. Therefore, a capacitive ratio of 28 is obtained. The dynamic behavior of the switch is studied by using the COMSOL Multiphysics software package and the RF characteristics have been obtained by using the HFSS software and show good RF performance. The results show that in the up-state, the designed switch including S11 is less than − 11.5 dB and S21 is more than − 0.85 dB at 1–40 GHz. In the down-state, the switch has excellent isolation in the Ka-band. Maximum isolation is − 71 dB that occurs at the resonance frequency of 30.5 GHz.

By means of a computer simulation the self-consistent charge transport with the current densities j(x,t), the respective charges ρ(x,t), field strengths F(x,t), and potential distributions V(x,t) in SiO2 layers are obtained as a function of the insulator depth x and the injection time t. The SiO2 layers are considered as open layers on silicon substrate or they are embedded in metal-oxide-semiconductor (MOS) structures. The given currents of primary electrons, the field-dependent ballistic currents of secondary electrons and holes as well as the Fowler–Nordheim injection of electrons from the substrate into the dielectric layer are taken into account. This method allows a defined charge storage and the explanation of complicated emission, charging-up, and breakdown processes within insulating layers during electron bombardment and/or high-field charge injection from adjacent electrodes, e.g., in MOS structures.

A through-silicon via (TSV) with a polymeric insulation layer is promising in three-dimensional packaging technology because of its outstanding electrical properties. In this study, we present an electroless grafting method to realize the fabrication of a thin insulation layer in high aspect ratio TSVs. The proposed method is based on the polymerization of a vinyl monomer and an aryl diazonium salt on a silicon surface. Both poly(acrylic acid) and poly(methyl methacrylate) insulation layers were successfully prepared in TSVs to demonstrate the feasibility of the proposed method to fabricate hydrophilic and hydrophobic layers. The resulting polymer insulation layer exhibited high step coverage and superior dielectric properties. The electroless grafting method is a novel synthesis route to prepare polymeric insulation layers with outstanding dielectric properties in high aspect ratio TSVs.

This paper describes effect of characteristics of dielectric layer on performances of droplet transport in EWOD (Electro wetting on dielectric) microfluidic devices. EWOD is a technology for manipulating droplets discretely by controlling the surface wettability electrically. Films of Parylene C, PDMS, and SU-8 were prepared for the dielectric layer and effects of deposition method, thickness and permittivity of these films were investigated experimentally. The results show that water droplets can be easily transported at low voltage when the dielectric film with high permittivity is deposited thinly and uniformly.