EWOD devices

FIGURE 3.50 Side view of an EWOD device (not to scale). The bottom plate served as the base for a pattern of polysilicon EWOD electrodes (1 mm2, 4-pm gap) buried under the thermal oxide. A top plate was formed from ITO on glass both plates were coated with Teflon-AF. The plates were joined with double-sided adhesive tape as a spacer [518], Reprinted with permission from the American Chemical Society. 

EWOD device. Shaded circles represent droplets of liquid. Squares are electrodes where the dotted hatching indicates the electrode is on. Directed lines specify the direction of motion. The multi-shaded droplet shows the diffusion and mixing of two chemicals here mixing is enhanced by the fluid dynamics created inside the droplet due to its imposed motion... 

Finally, given that pressure on the boundary is directly related to the local contact angle [20], we again use experimental data for the contact angle versus voltage characteristics of the EWOD device [22] to compute the electrode voltages needed to achieve the boundary pressures a. In general, there will be some uncertainty about the device parameters. 

Simulations were run assuming the device characteristics described in [22]. A 3 x 3 electrode grid was used to actuate and control the droplet where each square electrode is 1.4 mm on a side. All voltage actuations in these simulations are within the limits of the UCLA EWOD device. 

Our simulations demonstrate the potential for performing particle placement and separation in the current UCLA EWOD devices with a reasonable number of electrodes. It is interesting to note that existing EWOD systems have enough control authority to steer a single particle along complex trajectories and to steer two particles along simple paths. 

UCLA EWOD device [22], we linearly map each component of the solution vector a so that these constraints are satisfied. As far as the steering algorithm is concerned, this mapping effectively changes the magnitude of h, which only affects the speed, and not the direction, of the particle... 

EWOD-based digital microfluidics have already been combined with MALDI-TOF-MS by Wheeler et al.13 However, the Wheeler et al. device would have several drawbacks when used in pre-steady state kinetic studies, including lower throughput - for reasons explained in the Section 12.3 - and the absence of a mixing element. Further, while Paik et al.5 have demonstrated droplet mixers for EWOD-based systems, their system is simply not fast enough to be of use in pre-steady state kinetics. 

The coupling of microfabricated on-chip SPE devices to MALDI-MS has previously been presented by our group. Another approach to on-chip SPE—the microfluidic compact disk Gyro Lab MALDI SPl (Gyros AB, Sweden)— has been presented and commercialized by Gyros AB, where centrifugal force is used to transport liquids for the purpose of SPE before MALDI-MS. EWOD and electrocapture are techniques that can be used instead of SPE for purification and concentration before MALDI-MS. 

Recently, EWOD actuation chips were developed into a multiplexed device that was used to simultaneously cleanup four samples. A sequence of seven actuation steps were performed for each sample (1) generation of sample droplets (0.02 p.L), (2) transport and drying of sample droplets, (3) generation of rinsing droplets, (4) transport of rinsing droplets to the sample sites for selective dissolution of urea, (5) transport and disposal of the rinsing droplets, (6) generation of MALDI matrix solution droplets, and (7) delivery of matrix droplets to the dried peptide spots. 

Control of Micro-fluidics, Fig. 5 The EWOD system manipulates fluids by charging a dielectric layer underneath the liquid that effectively changes the local surface tension properties of the liquid/gas interface creating liquid motion. Existing (move, split, join, and mix) capabilities of electrowetting devices are shown schematically above alongside the new particle steering capability discussed in this entry. The view is from the top of the... 

Disadvantages of conventional microchips include resistance to hydrodynamic flow (backpressure), adsorption of some biomolecules on walls, and limited robustness (e.g., clogging narrow channels). Digital microfluidic systems overcome the problems with mixing reagents, which are normally associated with conventional microfluidic devices that use laminar hydrodynamic flow. Such microscale platforms are normally based on the electrowetting-on-dielectric (EWOD) principle [66]. In these digital microchips. 

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