Reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (/spl mu/TAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion.

Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits

The suitability of electrowetting-on-dielectric (EWD) microfluidics for true lab-on-a-chip applications is discussed. The wide diversity in biomedical applications can be parsed into manageable components and assembled into architecture that requires the advantages of being programmable, reconfigurable, and reusable. This capability opens the possibility of handling all of the protocols that a given laboratory application or a class of applications would require. And, it provides a path toward realizing the true lab-on-a-chip. However, this capability can only be realized with a complete set of elemental fluidic components that support all of the required fluidic operations. Architectural choices are described along with the realization of various biomedical fluidic functions implemented in on-chip electrowetting operations. The current status of this EWD toolkit is discussed. However, the question remains: which applications can be performed on a digital microfluidic platform? And, are there other advantages offered by electrowetting technology, such as the programming of different fluidic functions on a common platform (reconfigurability)? To understand the opportunities and limitations of EWD microfluidics, this paper looks at the development of lab-on-chip applications in a hierarchical approach. Diverse applications in biotechnology, for example, will serve as the basis for the requirements for electrowetting devices. These applications drive a set of biomedical fluidic functions required to perform an application, such as cell lysing, molecular separation, or analysis. In turn, each fluidic function encompasses a set of elemental operations, such as transport, mixing, or dispensing. These elemental operations are performed on an elemental set of components, such as electrode arrays, separation columns, or reservoirs. Examples of the incorporation of these principles in complex biomedical applications are described.

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Digital microfluidics: is a true lab-on-a-chip possible?

In this work, a method is presented for controlling on-chip droplet dispensing by electro-wetting actuation in conjunction with capacitance feedback. The method exploits the built-in capacitance of an electro-wetting device to meter the droplet volume and control the dispensing process. A self-contained system is built to provide continuous-flow loading, capacitance measurement, and electro-wetting chip control. Automated droplet generation at rates up to 120 droplets/min is demonstrated for droplets of 0.1 M KCl aqueous solution dispensed onto 1 mm pitch buried electrodes and a 500 μm channel gap. The reproducibility of the droplet volumes is tested against dispensing parameters including production rate, fluid viscosity, channel aspect ratio, dispensing needle position and dispensing volume. The overall reproducibility is ±5% for production rates varying from 8 to 45 droplets/min and ±6% for fluidic viscosity ranging from 1 to 58 Cst, which implies the application of the method to wide ranges of solutions and pressures. Feedback metering compares well with droplet dispensing without capacitance feedback, which gives a reproducibility of ±13% for on-chip dispensing from a reservoir and ±12% for constant pressure-assisted dispensing.

Automated on-chip droplet dispensing with volume control by electro-wetting actuation and capacitance metering

The present invention relates to droplet-based surface modification and washing. According to one embodiment, a method of splitting a droplet is provided, the method including providing a droplet microactuator including a droplet including one or more beads and immobilizing at least one of the one or more beads. The method further includes conducting one or more droplet operations to divide the droplet to yield a set of droplets including a droplet including the one or more immobilized beads and a droplet substantially lacking the one or more immobilized beads.

Droplet-based surface modification and washing

A hydrodynamic scaling model of droplet actuation in an electrowetting-on-dielectric (EWD) actuator is presented that takes into account the effects of contact angle hysteresis, drag from the filler fluid, drag from the solid walls, and change in the actuation force while a droplet traverses a neighboring electrode. Based on this model, the threshold voltage, VT, for droplet actuation is estimated as a function of the filler medium of a scaled device. It is shown that scaling models of droplet splitting and liquid dispensing all show a similar scaling dependence on [t/er(d/L)]1/2, where t is insulator thickness and d/L is the aspect ratio of the device. It is also determined that reliable operation of a EWD actuator is possible as long as the device is operated within the limits of the Lippmann–Young equation. The upper limit on applied voltage, Vsat, corresponds to contact-angle saturation. The minimum 3-electrode splitting voltages as a function of aspect ratio d/L < 1 for an oil medium are less than Vsat. However, for an air medium the minimum voltage for 3-electrode droplet splitting exceeds Vsat for d/L ≥ 0.4. EWD actuators were fabricated to operate with droplets down to 35pl. Reasonable scaling results were achieved.

A scaling model for electrowetting-on-dielectric microfluidic actuators

In this work new data, models, and applications are presented of an ultra-low power, microfluidic device for use in integrated bio-microelectrofluidic systems (Bio-MEFS). The metal-insulator-solution transport (MIST) device is based on the high-speed manipulation of discrete droplets of analytes and reagents under voltage control, and is the MOSFET equivalent for MEFS.

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T. Zhang

Papers

A micro-watt metal-insulator-solution-transport (MIST) device for scalable digital bio-microfluidic systems