Search

The dynamic behavior of an electrowetting-on-dielectrics (EWOD) actuation process can be linked to certain fluid properties of the actuated polar liquid droplet. After actuation, inertia will lead to droplet oscillations, where the contact angle asymptotically approaches the newly created surface tension equilibrium value. The decay behavior, frequency, and amplitude of these oscillations can be related to material parameters, such as density, viscosity, and mass. In this paper, we study the characteristics of these oscillations, present simulation results, and develop a measurement setup for first observations of oscillations on deionized water droplets. We show that there is a big difference between two grounding schemes, in particular, regarding the dynamic movement of the liquid-gas interface. It turns out that only an electrical grounding from below leads to utilizable oscillations. Finally, we point towards applications for future Lab-on-a-Chip applications.

We present a method to design specific heater electrode distributions on substrates with high thermal conductivity such as sheet metal. The substrate is covered with a thin polymer insulation layer on both sides, which are, in turn, carrying screen printed, electrically conductive heater electrodes and another protective polymer layer. The temperature inside a predetermined optimization area is demanded to be as uniform as possible, which is required for various high-precision heating applications. A uniform temperature distribution is required when realizing a newly proposed "lab on metal" approach. For this purpose, an optimization algorithm yields the required distances between the electrodes. The resulting electrode distribution was numerically obtained for certain optimization areas. We show the electrode positions relative to the distance from the centermost electrode and the optimization error for each distribution. Finally we describe the fabrication of a test device and show infrared measurements of the FEM modeled uniform temperature distribution on the experimental realized device.

A highly parallelizable approach for the precise dosing of homogenous liquids is presented. Droplets of the dosing liquid rest on a superhydrophobic plate featuring a 3x3 array of holes, which evaporate with time and therefore reduce in size. If the size of the droplets is small enough, they are able to pass through the holes, which can be subsequently used for further applications. Furthermore, microheaters are screen-printed around the holes of the superhydrophobic plate, in order to accelerate the evaporation process. Experimental data regarding temperature distribution and the improvement due to the addition of the microheaters to the setup is provided.

We investigated the vibration characteristics of the torsional oscillation mode of an earlier proposed resonant viscosity sensor based on vibrating pipes. To do so, the sensor housing is opened, so that half of the resonator is freely accessible. The actuation and readout of the sensor is realized with electromagnetic coils on the remaining part of the housing and the oscillation velocity is measured on different positions on the uncovered sensor surface. We demonstrate the feasibility and suitability of the electromagnetic readout principle and identified an additional spurious oscillation. All measurements were performed with an air filled resonator to minimize the damping and maximize the oscillation amplitudes.

We present a setup for directed loading of standard microneedle arrays for transdermal drug delivery with the respective therapeutic agent. The necessity to dose medical drugs according to their particular utilization requires an exact volumetric measure of the particular drug, which is usually provided as a liquid. This is achieved by arranging a metallic plate above the array featuring a set of holes aligned with the microneedles underneath. The plate is coated with a superhydrophobic layer. To initiate the filling, droplets are deposited on said holes, where the volume needs to be above the desired load for an individual needle, but the exact dosage is not required. Evaporation of these sessile droplets, after some time, leads to the falling of the droplets through the microfluidic plate, delivering an exact amount of liquid drug to the needles underneath. The proposed setup is easy to implement and parallelize, assisting in the task of accurate and high throughput coating of microneedle-based transdermal drug delivery devices.

We study the fluid dynamics of polar high-k fluids immersed in non-polar insulating fluids, which are excited by Electrowetting-On-Dielectric (EWOD) forces inside a tube with pressure compensation channels (PCCs). The fluids are set in motion using the electrowetting effect resulting in a pressure difference inside the tube. A model of the pressure field for various viscosities of the surrounding fluid is obtained by finite element analysis of the Navier-Stokes equation. From the time-dependent pressure response and the associated viscous damping, a correlation with the fluid’s dynamic viscosity is obtained. In the next step, we study the pressure evolution after driving the polar fluid in one direction and stopping it rapidly at the edge of the driving electrode. The decay time of the pressure induced height difference inside the PCCs can also be related to the surrounding non-polar fluid’s viscosity. Subsequently, an alternative low-cost fabrication process of an EWOD stack is used to build the device simulated. The built device is tested by measuring silicon oils and a comparison between already established methods for viscosity sensing and our “tube” method is made.

This work demonstrates a low-cost fabrication method for Electrowetting-on-Dielectric (EWOD) structures for microsystem and Lab-on-a-chip applications. The electrowetting effect allows droplet movement by manipulating the surface tension equilibrium in an EWOD system. A voltage-induced imbalance of the contact angle (CA) at the triple contact line leads to droplet actuation into the direction of the smaller CA. The presented low-cost EWOD stack consists of a suitable substrate, actuation electrodes and dielectric/hydrophobic layers. Screen printed silver electrodes on glass substrates were investigated and characterized. Indium Tin Oxide (ITO) glass slides with a top coated Teflon fluoropolymer were used as electrical counter electrodes. As an application, a demonstrator of a simple optofluidic (OF) switch is simulated in terms of optical path switching time and fabricated using the silver screen printed setup. The incident beam of a laser light source becomes deflected depending on the position of an EWOD actuated droplet inside the device.

We study the fluid dynamics of polar high-k fluids immersed in non-polar insulating fluids, excited by electrowetting-on-dielectric (EWOD) forces inside a tube with pressure compensation channels. The fluids are set in motion using the electrowetting effect resulting in a pressure difference inside the tube. A model of the pressure field for various viscosities of the surrounding fluid is obtained by finite element analysis of the Navier-Stokes equation. From the time dependent pressure response and the associated viscous damping, a correlation with the fluid's dynamic viscosity is obtained. In the next step, we study the pressure evolution after driving the polar fluid in one direction and stopping it rapidly at the edge of the driving electrode. The decay time of the pressure induced height difference inside the compensation channels can also be related to the surrounding non-polar fluid's viscosity. Subsequently, a cheap alternative fabrication process of an EWOD stack will be shown and used to build a tube oscillator simulated in the steps before.

Quantification of protein interactions in living cells is of key relevance for understanding cellular signaling. With current techniques, however, it is difficult to determine binding affinities and stoichiometries of protein complexes in the plasma membrane. We introduce here protein micropatterning as a convenient and versatile method for such investigations. Cells are grown on surfaces containing micropatterns of capture antibody to a bait protein, so that the bait gets rearranged in the live cell plasma membrane. Upon interaction with the bait, the fluorescent prey follows the micropatterns, which can be readout with fluorescence microscopy. In this study, we addressed the interaction between Lck and CD4, two central proteins in early T-cell signaling. Binding curves were recorded using the natural fluctuations in the Lck expression levels. Surprisingly, the binding was not saturable up to the highest Lck expression levels: on average, a single CD4 molecule recruited more than nine Lck molecules. We discuss the data in view of protein- and lipid-mediated interactions.

We present a method to study the pressure compensation behavior of polar fluids immersed in non-polar insulating fluids, which are actuated using the "electrowetting on dielectric (EWOD)" effect inside a closed tube with a pressure compensation channel. The electrowetting effect yields to a pressure difference inside the tube, the so called Laplace pressure. The pressure field for various viscosities of the surrounding fluid is modeled using finite element analysis (FEA) for the Navier-Stokes equation. The time dependent pressure response and the viscous damping of the fluid oscillations help to establish a relation to the fluid's dynamic viscosity. In a second step we compare the pressure response for a given pressure compensation channel diameter after actuating the polar fluid. When the polar droplet reaches the edge of the electrode, the driving force vanishes and the pressures before and behind the polar droplet begin to settle to its initial value. The generated pressure difference can be linked to the surrounding non-polar fluid's viscosity and density. Finally, we introduce the design and fabrication of an EWOD device capable of performing the actuation schemes considered in the simulations.

Quantification of protein interactions in living cells is of key relevance for understanding cellular signaling. With current techniques, however, it is difficult to determine binding affinities and stoichiometries of protein complexes in the plasma membrane. We introduce here protein micropatterning as a convenient and versatile method for such investigations. Cells are grown on surfaces containing micropatterns of capture antibody to a bait protein, so that the bait gets rearranged in the live cell plasma membrane. Upon interaction with the bait, the fluorescent prey follows the micropatterns, which can be readout with fluorescence microscopy. In this study, we addressed the interaction between Lck and CD4, two central proteins in early T-cell signaling. Binding curves were recorded using the natural fluctuations in the Lck expression levels. Surprisingly, the binding was not saturable up to the highest Lck expression levels: on average, a single CD4 molecule recruited more than nine Lck molecules. We discuss the data in view of protein- and lipid-mediated interactions.

Dielectric elastomers are used for electromechanical energy conversion in actuators and in harvesting mechanical energy from renewable sources. The electrical breakdown strength determines the limit of a dielectric elastomer for its use in actuators and energy harvesters. We report two experimental configurations for the measurement of the stretch dependence of the electrical breakdown strength of dielectric elastomers, and compare the electrical breakdown fields for compliant and rigid electrodes on the elastomer. We show that the electrode configuration strongly influences the electrical breakdown field strength. Further, we compare the stretch dependent dielectric function and breakdown of the acrylic elastomer VHB 4910 TM from 3M TM , and of the natural rubber ZruElast TM A1040 TM from Zrunek rubber technology. While the dielectric permittivity of VHB decreases with increasing stretch ratio, the dielectric constant of rubber is insensitive to stretch. Our results suggest natural rubber as a versatile material for dielectric elastomer energy harvesting. (Some figures may appear in colour only in the online journal)