Integration of electrowetting technology inside an all-glass microfluidic network

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.

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.

This paper presents an Electrowetting-on-Dielectric (EWOD) device with optimized insulating layers operated by low actuation voltage. The device consists of an electrode array on a silicon substrate, covered by a dielectric layer and a hydrophobic layer. To characterize the performance of the device, simulations are performed for the dielectric layer of Sio2 and the hydrophobic layer of Sio2, Su-8 and Parylene C at different voltages. The volume finite difference approach of the Coventorware software was used to carry out the simulations. Two different molar of di-ionized water droplet were considered in the simulations. It was observed that the device having the Sio2 dielectric layer and the Parylene C hydrophobic layer moved the 1M KCL (potassium chloride) droplet at the actuation voltage of 25V.

We demonstrated a novel electrowetting-on-dielectric (EWOD) device for digital microfluidic using a flexible, transparent, and conducting graphene electrode. We showed that a graphene electrode had a lower leakage current and a higher breakdown voltage compared to a gold electrode when they were under the same dielectric layer such as Teflon. The droplet movement was manipulated on graphene-based EWOD using a sequential switching voltage controlled by a programmable logic controller (PLC) relay array.

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.

The present work investigates the effect of variation of length of electrode for a EWOD (electrowetting on dielectric) based microfluidic flow. The discussed model takes into account the effect of energy gradient on droplet movement. The voltage and pitch length are kept constant at 60 V and 200μm respectively. The parameter measured is the velocity of the droplet in the direction along channel length. Various configurations were simulated and the best configuration was attributed to the one with the highest and stable energy gradient. Simulations of droplet movement are done on Coventor Ware. The increase in length of electrode has been found to increase the velocity of droplet movement.

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

Flow patterns surrounding particles suspended near electrodes within an electrolyte solution can be induced with an electric field due to an electrohydrodynamic (EHD) force. Depending on electrolyte, particle, and field properties, a variety of particle packing and stability states have been observed in EHD flow. In this work, we report evidence of EHD flow-induced aggregation of 2-μm sulfonated latex beads in NaCl and NaOH electrolyte solutions, in inverted particle-electrode orientations. Experimental conditions were chosen to match previous work where aggregation was observed at a bottom electrode. Particles remain stable at the top electrode for times greater than 1 h, and aggregation behavior is quantified in terms of growth rate and particle packing density and order. Similar aggregation behavior is seen at both top and bottom electrodes, with aggregate growth occurring more quickly within NaCl solutions than NaOH solutions at both the bottom and top electrode. In addition, an observed secondary location of stability for the vertical position of particles in NaOH electrolyte at the bottom electrode is not seen at the top electrode. Comparing these metrics to predictions made by a scaling model for EHD flow, particle aggregation behavior is successfully predicted at both top and bottom electrodes, but some of the observed differences in aggregate packing are not. Thus we suggest that modifications to existing models or their interpretation may be needed to improve predictions of the behavior of such systems.

Electrowetting is a technique which allows to modify the wettability of a surface through the action of an electric field. At the time of its discovery the phenomenon was believed to have no practical use due to the electrochemical reactions occurring on the surface, which in fact limit the overall effect obtainable. In the last few decades the interest toward this technique arose and the first applications appeared. Fundamental to allow such renewed interest was the use of a dielectric hydrophobic layer interposed between the metallic surface and the electrolyte: it is known in this case as Electrowetting on Dielectric (EWOD). Possible applications of this technique include variable focal lenses, microfluidic systems and displays. The latter in particular can find possible wide industrial diffusion due to the development of the Liquavista technology by Samsung. The challenge of the next future is the transfer of the electrowetting technology from rigid and planar displays to flexible and curved ones. Flexible electronics are expected to be the technology of the future and applications include wearable devices, flexible screens and smart labels for packaging. The present work investigates the possibility of creating compact layers of copper oxide via copper anodization for the EWOD technology. Copper was chosen instead of metals that can be more efficiently anodized due to its ease of deposition by mean of wet techniques. In fact copper can be easily electroless deposited on flexible substrates, while metals like aluminium or titanium need PVD deposition or thermal coupling techniques. Other advantages of copper include high conductivity and good mechanical properties. The first part of the study involves the fundamental study of electrowetting on copper oxide coated with a fluoropolymer. Hysteresis and saturation of contact angle are analysed, as well as the adherence of the behaviour to the Young-Lippman equation. In the second part a flexible substrate is tested for electrowetting on a curved surface. Such specimen is manufactured by electroless plating of copper on PET, anodizing the resulting layer and coating the oxide layer thus obtained with a fluoropolymer. Finally the result is compared with an equivalent sample obtained coupling an aluminium layer with a polymeric substrate and thermally bonding them. The result is then anodized to obtain a compact oxide layer and coated with the fluoropolymer.

This paper demonstrates several enhanced microfluidic functions achieved by integrating two different micro droplet actuation systems (Surface Acoustic Waves (SAW) and ElectroWetting-On-Dielectric (EWOD)). The paper reports how SAW can be employed to help EWOD create and split droplets by augmenting the EWOD force to complete the operation. It also demonstrates how EWOD can be used to guide and precisely position microdroplets for further efficient SAW actuated movement (pumping), concentration and ejection of droplets. The paper also highlights how this combination can also be employed to position droplets for shear wave SAW sensing.

We report electrowetting on dielectric (EWOD) experiments using graphene; a transparent, flexible and stretchable nanomaterial. Graphene sheets were synthesized by chemical vapor deposition, and transferred to various substrates (including glass slides and PET films). Reversible contact angle changes were observed on the Teflon-coated graphene electrode with both AC and DC voltages. Nyquist plots of the EWOD reveal that the graphene electrode has higher capacitive impedance than gold electrodes under otherwise identical conditions, suggesting a lower density of pin-holes and defects in the Teflon/graphene electrode than in the Teflon/gold electrode. Furthermore, we have observed reduced electrolysis of the electrolyte and smaller leakage current in the dielectric layer (Teflon) on graphene electrodes than on Au electrodes at the same Teflon thickness and applied voltage. We expect that the improved EWOD properties using graphene as an electrode material will open the door to various applications, including flexible displays and droplet manipulation in three-dimensional microfluidics.

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.

With the development of micro chemical and medical analysis in MEMS, micro-fluid-control chip becomes more and more popular. Based on discrete droplet control system, chemical and medical analysis is applied in the micro chip lab. The change of voltage between the dielectric layer of the electrode controls the wettability of a droplet on a dielectric solid surface and then makes it possible to create,transport,merge and cut the micro droplets. This paper presents a numerical method to simulate the droplet motion by electrowetting on dielectric (EWOD). The second-order projection method is carried out to solve N-S equations and level set function. The interface is captured as zero level set. Dynamic contact angle for fluid contacting a solid surface by the electric potential is used to evaluate the wettability of fluid by EWOD. The numerical scheme is based on staggered MAC grid. Numerical simulations are used to capture the interface between two kinds of fluids in micro channels, the shapes of micro droplets resting on the horizontal solid surface under different electric potential and the transportation of a micro droplet in a parallel-plate channel when electrodes are activated.

Wearable sweat sensors can analyze the abundant composition of solutes and metabolites in sweat to reflect the health state of the wearers in real time. The realization of active motion control for sweat droplets is significant for a multifunctional sweat monitoring device with several analysis chambers. Here, a wearable droplet‐based human sweat monitoring platform (WSMP), by combining an electrowetting on dielectrics (EWOD) device and a triboelectric nanogenerator (TENG), is demonstrated. It allows to collect and transport sweat droplets in different chambers by dielectric wetting effect and eventually merge and react with a pH indicator. The mechanical and electrical model of WSMP is introduced to describe the relationship between the open‐circuit voltage of the TENG and the voltage applied on the EWOD device. The high‐voltage electrical field generated by the TENG can change the wettability of solid–liquid interfaces and realize the controlling of droplet motion. The contact angle of electrolyte droplets changes over 30% with the triboelectric voltage of 5 kV. The driving, merging, and color reaction can be realized by actively controlling the motion of droplets. Finally, a wearable WSMP worn on the shank successfully demonstrates the preliminary detection of the pH level of human sweat.

Towards the fabrication of fully printed flexible P(VDF-TrFE)-based piezoelectric sensors