A continuous droplet electroporation system for high throughput processing.

A continuous droplet electroporation (EP) system capable of handling a billion cells has been proposed and demonstrated using a proof-of-concept prototype design. Numerical simulations were conducted to design the new system and to compare the continuous droplet EP system with the previous single droplet EP system. Through parametric studies on the applied voltage and flow rate, a much higher cyan fluorescent protein transgene expression efficiency (38.8 ± 8.9%) was accomplished over that of the previous single droplet EP system. A parallel continuous droplet EP system is also demonstrated by introducing additional electrode pairs into the continuous droplet EP system to achieve ultrahigh throughput. Finally, the significance and meaning of the present work and future development direction have been discussed.

Colorimetric detection of pathogenic bacteria using platinum-coated magnetic nanoparticle clusters and magnetophoretic chromatography.

A colorimetric method that uses platinum-coated magnetic nanoparticle clusters (Pt/MNCs) and magnetophoretic chromatography is developed to detect pathogenic bacteria. Half-fragments of monoclonal Escherichia coli O157:H7 (EC) antibodies were functionalized to Pt/MNCs and used to capture E. coli bacteria in milk. After magnetic separation of free Pt/MNCs and Pt/MNC-EC complexes from the milk, a precision pipette was used to imbibe the E. coli-containing solution, then a viscous polyethylene glycol solution. Due to difference in viscosities, the solutions separate into two liquid layers inside the pipette tip. The Pt/MNC-EC complexes were separated from the free Pt/MNCs by applying an external magnetic field, then added to a tetramethylbenzidine (TMB) solution. Catalytic oxidation of TMB by Pt produced color changes of the solution, which enabled identification of the presence of 10 cfu mL(-1) E. coli bacteria with the naked eye. The total assay time including separation, binding and detection was 30 min.

Characterization of electrode alignment for optimal droplet charging and actuation in droplet-based microfluidic system.

The actuation method using electric force as a driving force is utilized widely in droplet-based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device.

Osmotic pressure of ionic liquids in an electric double layer: Prediction based on a continuum model.

An analysis has been performed for the osmotic pressure of ionic liquids in the electric double layer (EDL). By using the electromechanical approach, we first derive a differential equation that is valid for computing the osmotic pressure in the continuum limit of any incompressible fluid in EDL. Then a specific model for ionic liquids proposed by Bazant et al. [M. Z. Bazant, B. D. Storey, and A. A. Kornyshev, Phys. Rev. Lett. 106, 046102 (2011)] is adopted for more detailed computation of the osmotic pressure. Ionic liquids are characterized by the correlation and the steric effects of ions and their effects are analyzed. In the low voltage cases, the correlation effect is dominant and the problem becomes linear. For this low voltage limit, a closed form formula is derived for predicting the osmotic pressure in EDL with no overlapping. It is found that the osmotic pressure decreases as the correlation effect increases. The osmotic pressures at the nanoslit surface and nanoslit centerline are also obtained for the low voltage limit. For the cases of moderately high voltage with high correlation factor, approximate formulas are derived for estimating osmotic pressure values based on the concept of a condensed layer near the electrode. In order to corroborate the results predicted by analytical studies, the full nonlinear model has been solved numerically.

Extraction of Cations from an Ionic Liquid Droplet in a Dielectric Liquid under Electric Field.

Ionic liquids show great promise as excellent solvents or catalysts in energy and biological fields due to their unique chemical and physical properties. In this work, the characteristics of various ionic liquids are investigated with the electrophoresis of a charged droplet (ECD) method. Under normal situation, a charged droplet in a dielectric liquid shows back-and-forth bouncing motion between the positive and negative electrodes continuously. However, for some special ionic liquids, interesting retreating behavior of a charged droplet has been observed. This retreating behavior is due to the loss of positive charges of the droplet, and it suggests that only the positive ions are extracted from the droplet under the applied electric field. Based on this hypothesis of ion extraction, Fourier transform infrared (FTIR) spectroscopy analysis has been performed. The retreating behavior is also discussed from the intermolecular point of view according to the ion species.

Geometric characterization of optimal electrode designs for improved droplet charging and actuation.

Droplet charging characteristics depending on the geometry of charging electrodes have been investigated experimentally and numerically. In the experiments, two contrasting electrode systems are examined: pin-pin versus planar-planar types. To confirm the different charging behaviours on each electrode, an asymmetric system of a pin-planar type has also been examined. From the experimental and numerical results, it has been found that the droplet charge can be significantly increased (more than four times) with pin type electrodes compared with planar ones due to the increase in surface charge density by the intensification of the electric field around the charging electrode. Moreover, as the system scale becomes smaller, the superior charging effect becomes greater. Through comprehensive numerical studies on the effects of the cross-sectional area and length of a charging electrode, we have found the optimal geometric design of an electrode for droplet charging and actuation. The implications for basic understanding of the charging phenomenon and electrode design of microfluidic systems are discussed.

Digital electrophoresis of charged droplets.

A digital microfluidic system based on a direct electric charging and subsequent electrophoretic manipulation of droplets is made by simple fabrication at low cost. Digitally controlled two-dimensional droplet motions are realized by digital polarity control of an array of electrodes. By independent control of droplets and colorimetric detection, the coalescence and mixing of droplets is analyzed quantitatively. The gelation of sodium alginate and the crystallization of calcium carbonate by multiple droplet translations and coalescence and the actuation of glassy carbon beads are demonstrated to show the versatile manipulation capability of the proposed technology. Finally, we discuss the implications and potentials of the present technology.

Discrete electrostatic charge transfer by the electrophoresis of a charged droplet in a dielectric liquid.

We have experimentally investigated the electrostatic charging of a water droplet on an electrified electrode surface to explain the detailed inductive charging processes and use them for the detection of droplet position in a lab-on-a-chip system. The periodic bouncing motion of a droplet between two planar electrodes has been examined by using a high-resolution electrometer and an image analysis method. We have found that this charging process consists of three steps. The first step is inductive charge accumulation on the opposite electrode by the charge of a droplet. This induction process occurs while the droplet approaches the electrode, and it produces an induction current signal at the electrometer. The second step is the discharging of the droplet by the accumulated induced charge at the moment of contact. For this second step, there is no charge-transfer detection at the electrometer. The third step is the charging of the neutralized droplet to a certain charged state while the droplet is in contact with the electrode. The charge transfer of the third step is detected as the pulse-type signal of an electrometer. The second and third steps occur simultaneously and rapidly. We have found that the induction current by the movement of a charged droplet can be accurately used to measure the charge of the droplet and can also be used to monitor the position of a droplet under actuation. The implications of the current findings for understanding and measuring the charging process are discussed.