Richard B. Fair

Electrowetting-based actuation of liquid droplets for microfluidic applications

A microactuator for rapid manipulation of discrete microdroplets is presented. Microactuation is accomplished by direct electrical control of the surface tension through two sets of opposing planar electrodes fabricated on glass. A prototype device consisting of a linear array of seven electrodes at 1.5 mm pitch was fabricated and tested. Droplets (0.7–1.0 μl) of 100 mM KCl solution were successfully transferred between adjacent electrodes at voltages of 40–80 V. Repeatable transport of droplets at electrode switching rates of up to 20 Hz and average velocities of 30 mm/s have been demonstrated. This speed represents a nearly 100-fold increase over previously demonstrated electrical methods for the transport of droplets on solid surfaces.

Chemical and Biological Applications of Digital-Microfluidic Devices

Digital-microfluidic lab-on-a chip (LoC) technology offers a platform for developing diagnostic applications with the advantages of portability, sample and reagent volume reduction, faster analysis, increased automation, low power consumption, compatibility with mass manufacturing, and high throughput. In addition to diagnostics, digital microfluidics is finding use in airborne chemical detection, DNA sequencing by synthesis, and tissue engineering. In this article, we review efforts to develop various LoC applications using electrowetting-based digital microfluidics. We describe these applications, their implementation, and associated design issues.

Dynamics of electro-wetting droplet transport

A model is formulated to describe the dynamics of electro-wetting-induced transport of liquid droplets. The velocity of droplet transport as a function of actuation voltage is derived. The operating parameters include the viscosity of the droplet and the medium through which it actuates, contact-line friction, system geometry, and surface tension. Numerical coefficients are extracted from experimental data to represent the effect of operating parameters on electro-wetting dynamics. The power dissipation of droplet transport is analyzed which reveals the key limiting factors for device operation as well the effect of scaling on device power requirements. # 2002 Published by Elsevier Science B.V.

Electrowetting-based on-chip sample processing for integrated microfluidics

In this work, results and data are reported on key aspects of sample processing protocols performed on-chip in a digital microfluidic lab-on-a-chip. We report the results of experiments on aspects of sample processing, including on-chip preconcentration and dilution, on-chip sample injection or dispensing, and sample mixing. It is shown that high speed transport and mixing of analytes and reagents can be performed using biological solutions without system contamination.

Electrowetting-based droplet mixers for microfluidic systemsElectronic supplementary information (ESI) available: six mpeg videos showing some mixing schemes used in Fig. 7. See http://www.rsc.org/suppdata/lc/b2/b210825a/

Mixing of analytes and reagents is a critical step in realizing a lab-on-a-chip. However, mixing of liquids is very difficult in continuous flow microfluidics due to laminar flow conditions. An alternative mixing strategy is presented based on the discretization of liquids into droplets and further manipulation of those droplets by electrowetting. The interfacial tensions of the droplets are controlled with the application of voltage. The droplets act as virtual mixing chambers, and mixing occurs by transporting the droplet across an electrode array. We also present an improved method for visualization of mixing where the top and side views of mixing are simultaneously observed. Microliters of liquid droplets are mixed in less than five seconds, which is an order of magnitude improvement in reported mixing times of droplets. Flow reversibility hinders the process of mixing during linear droplet motion. This mixing process is not physically confined and can be dynamically reconfigured to any location on the chip to improve the throughput of the lab-on-a-chip.

Microfluidics-Based Biochips: Technology Issues, Implementation Platforms, and Design-Automation Challenges

Microfluidics-based biochips are soon expected to revolutionize clinical diagnosis, deoxyribonucleic acid (DNA) sequencing, and other laboratory procedures involving molecular biology. In contrast to continuous-flow systems that rely on permanently etched microchannels, micropumps, and microvalves, digital microfluidics offers a scalable system architecture and dynamic reconfigurability; groups of unit cells in a microfluidics array can be reconfigured to change their functionality during the concurrent execution of a set of bioassays. As more bioassays are executed concurrently on a biochip, system integration and design complexity are expected to increase dramatically. This paper presents an overview of an integrated system-level design methodology that attempts to address key issues in the synthesis, testing and reconfiguration of digital microfluidics-based biochips. Different actuation mechanisms for microfluidics-based biochips, and associated design-automation trends and challenges are also discussed. The proposed top-down design-automation approach is expected to relieve biochip users from the burden of manual optimization of bioassays, time-consuming hardware design, and costly testing and maintenance procedures, and it will facilitate the integration of fluidic components with a microelectronic component in next-generation systems-on-chips (SOCs).

Design and testing of an interpolating mixing architecture for electrowetting-based droplet-on-chip chemical dilution

An on-chip dilution scheme for an electrowetting based microfluidic system is presented in this paper. The dilution scheme uses the mixing and splitting of two droplets of different concentrations to result in an intermediate concentration, as the fundamental operation. A range of dilution factors can be obtained repeating the simple two-droplet mixing and splitting cycles, in various combinations. The accuracy of the dilution scheme primarily depends on two factors-variation in droplet volume during dispensing and splitting, and homogenous mixing. The proposed on-chip dilution architecture was tested for the dilution factors of 2, 4 and 8, using a red food dye as the sample solution and 0.1M KCl solution as the buffer. The concentrations were measured using an optical absorbance measurement system, consisting of an LED and photodiode. The error in the dilution factor was found to be /spl sim/15% for a dilution factor of 4 and /spl sim/25% for a dilution factor of 8. Droplet volume variations in dispensing and splitting contribute to /spl sim/80% of this error.

Rapid droplet mixers for digital microfluidic systemsElectronic supplementary information (ESI) available: video clips of 2 2, 2 3, 2 4 electrode array and split-and-merge mixing. See http://www.rsc.org/suppdata/lc/b3/b307628h/

The mixing of analytes and reagents for a biological or chemical lab-on-a-chip is an important, yet difficult, microfluidic operation. As volumes approach the sub-nanoliter regime, the mixing of liquids is hindered by laminar flow conditions. An electrowetting-based linear-array droplet mixer has previously been reported. However, fixed geometric parameters and the presence of flow reversibility have prevented even faster droplet mixing times. In this paper, we study the effects of varying droplet aspect ratios (height:diameter) on linear-array droplet mixers, and propose mixing strategies applicable for both high and low aspect ratio systems. An optimal aspect ratio for four electrode linear-array mixing was found to be 0.4, with a mixing time of 4.6 seconds. Mixing times were further reduced at this ratio to less than three seconds using a two-dimensional array mixer, which eliminates the effects of flow reversibility. For lower aspect ratio (</=0.2) systems, we present a split-and-merge mixer that takes advantage of the ability to perform droplet splitting at these ratios, resulting in a mixing time of less than two seconds.

Integrated chemical/biochemical sample collection, pre-concentration, and analysis on a digital microfluidic lab-on-a-chip platform

An ideal on-site chemical/biochemical analysis system must be inexpensive, sensitive, fully automated and integrated, reliable, and compatible with a broad range of samples. The advent of digital microfluidic lab-on-a-chip (LoC) technology offers such a detection system due to the advantages in portability, reduction of the volumes of the sample and reagents, faster analysis times, increased automation, low power consumption, compatibility with mass manufacturing, and high throughput. We describe progress towards integrating sample collection onto a digital microfluidic LoC that is a component of a cascade impactor device. The sample collection is performed by impacting airborne particles directly onto the surface of the chip. After the collection phase, the surface of the chip is washed with a micro-droplet of solvent. The droplet will be digitally directed across the impaction surface, dissolving sample constituents. Because of the very small droplet volume used for extraction of the sample from a wide colection area, the resulting solution is realatively concentrated and the analytes can be detected after a very short sampling time (1 min) due to such pre-concentration. After the washing phase, the droplet is mixed with specific reagents that produce colored reaction products. The concentration of the analyte is quantitatively determined by measuring absorption at target wavelengths using a simple light emitting diode and photodiode setup. Specific applications include automatic measurements of major inorganic ions in aerosols, such as sulfate, nitrate and ammonium, with a time resolution of 1 min and a detection limit of 30 nm/m3. We have already demonstrated the detection and quantification of nitroaromatic explosives without integrating the sample collection. Other applications being developed include airborne bioagent detection.

Effect of complex formation on diffusion of arsenic in silicon

When As diffuses into Si, only a fraction of the As remains electrically active. Because of the importance of As as an emitter dopant, it is necessary to understand the nature of the inactive As and how it affects the solubility and diffusion of As+ ions. A model is proposed in which As+ diffuses via a simple vacancy mechanism while in quasiequilibrium with [VSiAs2] complexes. The flux of mobile monatomic As+ is modified according to the extent of [VSiAs2] complex formation. The structure of this defect and its formation energy (≈ 1.8 eV) are discussed. An effective diffusion coefficient is derived using this model: DAs=2DiCA /(1+8 K2′ CA3) where CA is the As+ concentration and K2′ is a collective parameter that depends upon As+ surface concentration and the diffusion temperature. Experimental verification of the correctness of this equation is given. The important results of this quantitative analysis show that DAs reaches a maximum value with increasing As concentration, and then decreases monotonically...