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Droplet based microfluidic systems, in recent years, have demonstrated numerous advantages and exciting potential for Lab-On-a Chip applications. In order to fully realize the potential benefits of this technology, one requires precision dispensing and manipulation of droplets of known volume and sample concentration, in a rapid and controlled manner. In this article, we demonstrate the rapid and controlled microactuation of aqueous samples and subsequent dispensing of variable volume droplets in nanolitre to picolitre regime by using a coplanar tapered electrode structure that leverages the phenomena of liquid dielectrophoresis (L-DEP). The transient behavior of the tapered liquid jet departs significantly from that of a uniform liquid jet case and is not adequately explained in terms of a simplified lumped capacitance model as in the case of the uniform jet, during the L-DEP actuation. A more generalized numerical model is developed for the tapered actuation scheme to account for the experimental observations. We furthermore demonstrate that the density of the dispensed droplets can be proactively controlled by the judicious placement of electrode bumps and pinches in the electrode structure thus overcoming the limitations imposed by Rayleigh's instability criterion. The proposed droplet dispensing schemes are superior to existing L-DEP based dispensing schemes which are restricted in size and spacing of the dispensed droplets by Rayleigh's instability criteria and furthermore mostly restricted to equi-volume droplets.

Liquid Dielectrophoresis (L-DEP) has been successfully leveraged at microscopic scales and shown to provide a controllable means of on-chip precision dispensing and manipulation of sub-nanoliter single emulsion droplets. In this paper, we report on the dynamics of a DEP actuated emulsion jet prior to break-up and compare its characteristic behavior based on the lumped parameter model of Jones et al. (R. Ahmed and T. B. Jones, J. Micromech. Microeng., 2007, 17, 1052). Furthermore, features and aspects of these emulsion jets, their break-up and formation of sub-nanoliter emulsion droplets is studied in further detail. Applications of the proposed scheme in dispensing encapsulated sub-nanoliter droplets is envisioned in various fields including microTAS, on-chip handling and storage of cells and other biological samples for longer duration in controlled environments as well as solving the more general encapsulation issues in surface microfluidic devices. Scalability of the proposed scheme is shown by producing controlled sample-oil single emulsion droplets (aqueous samples in oil) in the range of 50-400 picoliters.

A miniaturized microfluidic device was developed to facilitate electromanipulation of bacterial respiratory pathogens. The device comprises a microchip with circular aluminum electrodes patterned on glass, which is housed in a microfluidic system fabricated utilizing polydimethylsiloxane. The device provides sample preparation capability by exploiting positive dielectrophoresis (DEP) in conjunction with pulsed voltage for manipulation and disruption of Bordetella pertussis bacterial cells. Positive DEP capture of B. pertussis was successfully demonstrated utilizing 10 Vrms and 1 MHz ac fields. Application of dc pulses (300 V amplitude and 50 micros pulsewidth applied 1 s apart) across the aluminum electrodes resulted in electrodisruption and lysis of B. pertussis bacterial cells. Real-time polymerase chain reaction, a 2(3) factorial experimental design and transmission electron microscopy were used to evaluate bacterial cell manipulation and factors affecting bacterial cell disruption. The main factors affecting bacterial cell disruption were electric field strength, the electrical conductivity of the cell suspension sample, and the combined effect of number of pulses and sample conductivity. The bacterial deoxyribonucleic acid target remained undamaged as a result of DEP and cell lysis experimentation. Our findings suggest that a simple miniaturized microfluidic device can achieve important steps in sample preparation on-chip involving respiratory bacterial pathogens.

Miniaturized fluidic systems have been developed in recent years and offer new and novel means of leveraging the domain of microfluidics for the development of micro-total analysis systems (microTAS). Initially, such systems employed closed microchannels in order to facilitate chip-based biochemical assays, requiring very small quantities of sample and/or reagents and furthermore providing rapid and low-cost analysis on a compact footprint. More recently, advancements in the domain of surface microfluidics have suggested that similar low volume sample handling and manipulation capabilities for bioassays can be attained by leveraging the phenomena of liquid dielectrophoresis and droplet dielectrophoresis (DEP), without the need for separate pumps or valves. Some of the key aspects of this surface microfluidic technology and its capabilities are discussed and highlighted in this paper. We, furthermore, examine the integration and utility of liquid DEP and droplet DEP in providing rapid and automated sample handling and manipulation capabilities on a compact chip-based platform.

We present a prototype microfluidic device developed for the continuous dielectrophoretic (DEP) fractionation and purification of sample suspensions of biological cells. The device integrates three fully functional and distinct units consisting of an injector, a fractionation region, and two outlets. In the sheath and sample injection ports, the cell sample are hydrodynamically focused into a stream of controlled width; in the DEP fractionation region, a specially shaped nonuniform (isomotive) electric field is synthesized and employed to facilitate the separation, and the sorted cells are then delivered to two sample collection ports. The microfluidic behavior of the injector region was simulated and then experimentally verified. The operation and performance of the device was evaluated using yeast cells as model biological particles. Issues relating to the fabrication and operation of the device are discussed in detail. Such a device takes a significant step towards an integrated lab-on-a-chip device, which could interface/integrate to a number of other on-chip components for the device to undertake the whole laboratory procedure.

An integrated microfluidic system for combined manipulation, pre-concentration, and lysis of samples containing Bordetella pertussis by dielectrophoresis and electroporation has been developed and implemented. The microfluidic device was able to pre-concentrate the amount of B. pertussis cells present in 200 microl of a B. pertussis suspension stock into a 20 microl volume. The device exhibited optimal sample pre-concentration of 6.7x at a stock value of 10(3) cfu/ml and at a flow rate of 250 microl/h. Electro-disruption experiments showed that on-chip-based electroporation is an effective solution for lysis of B. pertussis cells that is easily integrated with dielectrophoresis assisted pre-concentration procedures. Pulsed voltage applied, number of pulses, and presence of potassium chloride in a B. pertussis suspension showed a reduction in B. pertussis cell viability by electroporation; and transmission electron microscopy confirmed B. pertussis cell disruption by electroporation. Genetic amplification and detection of the pre-concentrated sample employing an integrated chip-based system demonstrated a complete chip approach for pathogen detection.