EWOD (electrowetting on dielectric) digital microfluidics powered by finger actuation
Summary (2 min read)
- Microfluidic devices are attractive for portable applications, including point-of-care (POC) diagnosis, bio-surveillance, environment sampling, and forensic science.
- Previous studies proposed different approaches to circumventing these challenges.
- Battery-powered microfluidic devices with integrated microvalves and micropumps, for example, have been reported for carrying out biological assays.
Device design and fabrication
- To successfully manipulate micro-droplets using EWOD, one must generate voltage pulses of sufficient amplitude to overcome capillary, inertial, and viscous forces.
- Because the capillary resistance originated from the contact angle hysteresis of an aqueous droplet surrounded in air (as opposed to the popular oil environment) is larger than the inertial or viscous resistance in most cases, performance of EWOD devices is often measured in the air environment without resorting to the filler oil or oil impregnation.
- The bottom glass plate contains an array of 1 × 1 mm2 gold electrodes, which was fabricated using standard micro-fabrication processes.
- A shadow mask was used to define electrical contact pads.
- The 1118 | Lab Chip, 2014, 14, 1117–1122 dielectric layer thickness is further limited by the capacitance allowed per EWOD electrode, which must be kept below that of the piezoelectric element to minimize the voltage dividing effect.
EWOD actuation voltage
- To characterize the threshold actuation voltage required for EWOD actuation as a function of the dielectric layer thickness, an external programmable power source was used to apply precisely defined voltage pulses.
- A water droplet of ~0.3 μL in volume was spotted onto the EWOD device and subsequently split into two nominally identical daughter droplets.
- After one of these droplets was positioned on one of the electrodes, the amplitude of the voltage pulse applied to the adjacent electrode was gradually increased until the droplet was successfully transported.
- The threshold actuation voltage was recorded for each dielectric layer thickness and the results are plotted in Fig. 2 .
- Also shown as a solid line in Fig. 2 are the voltages required theoretically (eqn (1)) for the contact angle to change from 120° to 70°, which is an empirically determined range for droplet actuation in the given EWOD device.
Mechanical energy conversion
- The prediction (straight line) agrees reasonably well with the experimental results over the entire range.
- The results are shown for three different bending angles.
- At relatively small tip bending angles (~15°), the total voltage output increases linearly with each additional piezoelectric element.
Basic droplet operations
- The authors next demonstrated successful finger-powered actuation of water droplets on EWOD devices with 0.8 μm-thick PECVD SiNx dielectric layers.
- Referring to Fig. 6, element 2 was not entirely released when element 3 was deflected, such that the front contact line of the droplet would stay across the gap between electrodes 2 and 3.
- The authors also demonstrated droplet splitting by simultaneously deflecting two non-adjacent piezoelectric elements while keeping the middle one non-deflected.
- Elongated in the longitudinal direction by a wetting force exerted at the two ends while keeping the middle non-wetting, as shown in Fig.
- The actuation voltage on either side is approximately 40–50 V, produced by one single piezoelectric element with a bending angle <90°.
Applications to biological assays
- In these experiments, where higher EWOD voltages are necessary to actuate droplets, the authors used silicon nitride layers of thickness 2.5 μm because they help prevent electrolysis of water under their low-frequency (~1 Hz) finger-driven actuation scheme.
- Since the reagent solution may chemically attack the hydrophobic coatings, the authors pre-treated both the upper plate and bottom substrate of their EWOD devices with silicone oil.
- The reagent droplet was then transported towards the glucose sample droplet and merged, as shown in Fig. 9(b–d).
- In their experiment, the enzyme substrate was used to detect an ALP conjugated antibody.
- In summary, the authors demonstrated finger-actuated digital microfluidics based on the electrowetting-on-dielectric (EWOD) The Royal Society of Chemistry 2014 phenomenon by using piezoelectric energy conversion of human power.
- The authors demonstrated generation of voltage pulses of amplitudes >100 V using laminated polymer piezoelectric elements connected in series.
- Using the scheme, the authors confirmed basic EWOD droplet operations such as droplet transport, splitting and merging and further demonstrated an implementation of basic assay steps in glucose detection and immunoassay.
- Due to the low-frequency nature of finger actuation, a relatively thick dielectric layer was used to help prevent possible electrolysis.
- The authors work offers a promising solution for expanded applications of EWOD-based digital microfluidics in portable systems.
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Q1. What are the contributions in "Ewod (electrowetting on dielectric) digital microfluidics powered by finger actuation" ?
In this paper, a finger-actuated digital microfluidics based on electrowetting-on-dielectric ( EWOD ) was demonstrated by using piezoelectric energy conversion of human power.