Next generation digital microfluidic technology: Electrophoresis of charged droplets

3D Cell Culture Method in Channel-Free Water-in-Oil Droplets

A new channel-free water-in-oil (WO) droplet 3D cell culture method is proposed to address the challenges while maintaining the advantages of the conventional 3D cell culture methods. The proposed WO method can fundamentally solve the constraint of spheroids size, a common challenge in conventional 3D culture, by using droplet size controllability. The 3D cell culture performance of the WO method is verified by comparing it with the conventional 3D cell culture methods. A systematic investigation of the culture conditions of the WO method confirms the working range of cell concentration and droplet size, as well as the scalability of spheroid size. Adjusting droplet size and cell concentration enables rapid spheroid formation with large and high cell concentration droplets or fast spheroid growth with small and low cell concentration droplets, providing control over the spheroid size and growth rate according to the purpose. Furthermore, long-term culture is demonstrated for 1 month with the proposed method, showing the largest spheroid culture and demonstrating the possibility that this method can be used not only for spheroid formation but also for organoid studies. Finally, if a WO-based automated 3D cell culture system is developed, it will be a useful tool for organoid research.

Simultaneous Separating, Splitting, Collecting, and Dispensing by Droplet Pinch-Off for Droplet Cell Culture

Simultaneous separating, splitting, collecting, and dispensing a cell suspension droplet has been demonstrated by aspiration and subsequent droplet pinch-off for use in microfluidic droplet cell culture systems. This method is applied to cell manipulations including aliquots and concentrations of microalgal and mammalian cell suspensions. Especially, medium exchange of spheroid droplets is successfully demonstrated by collecting more than 99% of all culture medium without damaging the spheroids, demonstrating its potential for a 3D cell culture system. Through dimensional analysis and systematic parametric studies, it is found that initial mother droplet size together with aspiration flow rate determines three droplet pinch-off regimes. By observing contact angle changes during aspiration, the difference in the large and the small droplet pinch-off can be quantitatively explained using force balance. It is found that the capillary number plays a significant role in droplet pinch-off, but the Bond number and the Ohnesorge number have minor effects. Since the dispensed droplet size is mainly determined by the capillary number, the dispensed droplet size can be controlled simply by adjusting the aspiration flow rate. It is hoped that this method can contribute to various fields using droplets, such as droplet cell culture and digital microfluidics, beyond the generation of small droplets.

Safe and efficient RNA and DNA introduction into cells using digital electroporation system

We systematically compared the delivery and expression efficiencies according to cell types (plant and animal cells) and genetic materials (RNA and DNA) to deliver RNA using a digital electroporation system. Despite the significantly lower RNA delivery in Chlamydomoans reinhartii than DNA delivery due to RNA secondary structure and cell wall, the expression/delivery ratio of RNA was significantly higher than that of DNA (up to 90%), confirming the generally known fact that RNA is more favorable for expression than DNA. On the other hand, in K562 cells, the difference in RNA and DNA delivery efficiency was negligible. Therefore, structural differences between DNA and RNA affect delivery efficiency differently depending on the cell type. RNA delivery efficiency of K562 cells was high, but expression efficiency was much lower than that of microalgae. According to the proposed strategy, compatibility between K562 cells and the nucleic acids used in this study is presumed to be one of the reasons for this low expression efficiency. Gene regulation by delivering small interfering RNA (siRNA) was demonstrated in K562 cells, confirming the feasibility of the digital electroporation system for RNA interference (RNAi) research as a safe and efficient delivery system.

Compact Three-Dimensional Digital Microfluidic Platforms with Programmable Contact Charge Electrophoresis Actuation

Digital microfluidics (DMF) has garnered considerable interest as a straightforward, rapid, and programmable technique for controlling microdroplets in various biological, chemical, and medicinal research disciplines. This study details the construction of compact and low-cost 3D DMF platforms with programmable contact charge electrophoresis (CCEP) actuations by employing electrode arrays composed of a small commercial pin socket and a 3D-printed housing. We demonstrate basic 3D droplet manipulation on the platform, including horizontal and vertical transport via lifting and climbing techniques, and droplet merging. Furthermore, phenolphthalein reaction and precipitation process are evaluated using the proposed 3D DMF manipulations as a proof of concept for chemical reaction-based analysis and synthesis. The threshold voltage (or electrical field) and maximum vertical transport velocity are quantified as a function of applied voltage and electrode distance to determine the CCEP actuation conditions for 3D droplet manipulations. The ease of manufacturing and flexibility of the proposed 3D DMF platform may provide an effective technique for programmable 3D manipulation of droplets in biochemical and medical applications, such as biochemical analysis and medical diagnostics.

Digital Microfluidic Mixing via Reciprocating Motions of Droplets Driven by Contact Charge Electrophoresis

Contact charge electrophoresis (CCEP) is an electrically controllable manipulation technique of conductive droplets and particles by charging and discharging when in contact with the electrode. Given its straightforward operation mechanism, low cost, and ease of system construction, it has gained traction as a versatile and potential strategy for the realistic establishment of lab-on-a-chip (LOC) in various engineering applications. We present a CCEP-based digital microfluidics (DMF) platform with two parallel electrode modules comprising assembled conventional pin header sockets, allowing for efficient mixing through horizontal and vertical shaking via droplet reciprocating motions. The temporal chromic change caused by the chemical reaction between the pH indicator and base solutions within the shaking droplets is quantitatively analyzed under various CCEP actuation conditions to evaluate the mixing performance in shaking droplets by vertical and horizontal reciprocating motions on the DMF platform. Furthermore, mixing flow patterns within shaking droplets are successfully visualized by a high-speed camera system. The suggested techniques can mix samples and reagents rapidly and efficiently in droplet-based microreactors for DMF applications, such as biochemical analysis and medical diagnostics.

Effect of Deformation on Droplet Contact Charge Electrophoresis

The effect of deformation on the droplet contact charge electrophoresis (CCEP) was investigated for consistent droplet movement control. Through systematic experiments and numerical simulations, it has been found that overcharging by deformation is up to about 130% of the sphere and is mainly driven by the concentration of the electric field near the tip of the droplet rather than an increase in the surface area. Dimensional analysis revealed a consistent droplet CCEP motion with the electric capillary number range of 0.01-0.09. We also found that the dimensionless droplet charge follows a universal curve proportional to the electric capillary number, regardless of the droplet size, and the weak dependence on the droplet size shown in the experimental results is due to hydrodynamic effects, not electrostatic ones. Changes in droplet velocity distribution with droplet size and the electric capillary number were also investigated. Using the perfect conductor theory and Stokes law, we derived an analytical relationship between the droplet center velocity and the electric capillary number and analyzed the experimental results based on this relationship. This study implies that if proper hydrodynamic correction is applied, the droplet CCEP and its deformation effect can be explained by a perfect conductor theory.

Wall Effects on Hydrodynamic Drag and the Corresponding Accuracy of Charge Measurement in Droplet Contact Charge Electrophoresis

Droplet size dependent wall effects on hydrodynamic drag and the corresponding droplet contact charge estimations were experimentally and theoretically investigated. The consistent reduction in the dimensionless droplet contact charges proportional to droplet size was reported and explained by the parallel and approaching wall effects on the drag coefficient. Extrapolation of the size dependent droplet charge data showed that the droplet charge follows the perfect conductor theory when the droplet radius approaches zero. The proposed model was applied to the drag calculation to estimate and compare dimensionless charges before and after consideration of the wall effects. The droplet free fall test concluded that the droplets in the current experimental setup follow Stokes' law. The theoretical velocity profile of the droplet approaching the wall perpendicularly is proposed considering the approaching wall effect on hydrodynamic drag and verified by comparison with the experiment. The droplet size dependent velocity profile shape change was also explained by this approaching wall effect. The shape of the asymmetric velocity profile along the direction of droplet movement was explained by the effect of the image charge through direct numerical calculation of the electric force. The direct calculation of the electric force also showed that the electric correction at the center of the cuvette is negligible; thus, it is sufficient to consider only hydrodynamic correction for accurate charge measurement in this experimental system. The present study will contribute to the accurate measurement of the droplet charges under contact charge electrophoresis. It also provides the basis for precise control of droplet movement in lab-on-a-chip devices.

Influence of Surface Wettability on Discharges from Water Drops in Electric Fields

It is known that electrified droplets deform and may become unstable when the electric field they are exposed to reaches a certain critical value. These instabilities are accompanied by electric discharges due to the local enhancement of the electric field caused by the deformed droplets. Here we report and highlight an interesting aspect of the behavior of unstable water droplets and discharge generation: by implementing wettability engineering, we can manipulate these discharges. We demonstrate that wettability strongly influences the shape of a droplet that is exposed to an electric field. The difference in shape is directly related to differences in the critical value of the applied electric field at which inception of discharge occurs. Using theoretical models, we can predict and sufficiently support our observations. Thus, by tailoring the wettability of the surface, we can control droplet's behavior from expediting the discharge inception to completely restricting it.

Statistical Analysis of Droplet Charge Acquired during Contact with Electrodes in Strong Electric Fields

Aqueous droplets acquire charge when they contact electrodes in high-voltage electric fields. Although many researchers have investigated droplet charging under various conditions, the droplet charges are typically reported simply in terms of a mean and standard deviation. Here, we show that droplets often acquire significantly less charge for a single contact compared to the previous and subsequent contacts. These "low-charge events," which are not observed with charging of metal balls, yield up to a 60% decrease in charge acquired by the droplet and occur regardless of the applied field strength, droplet conductivity, or droplet volume. In all cases examined here, the occurrence of low-charge events to good approximation follows a negative binomial distribution (i.e., a Pascal distribution) with a mean probability of 13%. We further demonstrate that approximately 16% of charging events are characterized by "irregular" Taylor cone dynamics, suggesting that instabilities in the electrically driven deformation of the approaching liquid interface may be responsible for the low-charge events. The results indicate that workers using systems involving droplet charging should take into account the high likelihood of droplets randomly acquiring less charge than expected.

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.

Optimization of the droplet electroporation method for wild type Chlamydomonas reinhardtii transformation

We performed the transformation of a wild type Chlamydomonas reinhardtii by optimizing previously developed droplet EP method. For more effective and faster optimization, we used DNA dying fluorescent molecule (Yo-Pro-1) for finding optimal EP conditions instead of using protein expression based evaluation method. By examining wider range of electrical parameter space together with the analysis of total current flow of EP process, we found optimal EP conditions. The obtained optimal EP conditions were verified by CFP transgene expression experiments. By applying the optimal EP conditions to the transformation of C. reinhardtii, we obtained transformants and analyzed them using PCR. Finally, implications and future work are discussed.

Droplet Conductivity Strongly Influences Bump and Crater Formation on Electrodes during Charge Transfer

Aqueous droplets acquire charge when they contact electrodes in high voltage electric fields, but the exact mechanism of charge transfer is not understood. Recent work by Elton et al. revealed that electrodes are physically pitted during charge transfer with aqueous droplets. The pits are believed to result when a dielectric breakdown arc occurs as a droplet approaches the electrode and the associated high current density transiently locally melts the electrode, leaving distinct crater-like deformations on the electrode surface. Here we show that the droplet conductivity strongly modulates the pitting morphology but has little effect on the amount of charge transferred. Electron and atomic force microscopy shows that deionized water droplets yield no observable deformations, but as the salt concentration in the droplet increases above 10^-3 M, the deformations become increasingly large. The observed intensity of the flash of light released during the dielectric breakdown arc also increases with droplet conductivity. Surprisingly, despite the large difference in pitting morphology and corresponding arc intensity, droplets of any conductivity acquire similar amounts of charge. These results suggest that the energy transferred during dielectric breakdown is primarily responsible for electrode pitting rather than the total amount of energy released during charge transfer.

Contact Charge Electrophoresis: Fundamentals and Microfluidic Applications

Contact charge electrophoresis (CCEP) uses steady electric fields to drive the oscillatory motion of conductive particles and droplets between two or more electrodes. In contrast to traditional forms of electrophoresis and dielectrophoresis, CCEP allows for rapid and sustained particle motions driven by low-power dc voltages. These attributes make CCEP a promising mechanism for powering active components for mobile microfluidic technologies. This Feature Article describes our current understanding of CCEP as well as recent strategies to harness it for applications in microfluidics and beyond.

Electrostatic Origins of the Positive and Negative Charging Difference in the Contact Charge Electrophoresis of a Water Droplet

The positive and negative charging difference in the contact charge electrophoresis of a water droplet suspended in oil was investigated to find out the origin of this charging difference. Through numerous experiments and numerical analysis, the charging difference has been found to be mainly originated from electrostatic sources. Two electrostatic sources were found in the present experimental setup, and by excluding those two sources the charging difference was successfully diminished. The present findings well explain previous experimental results and also provide design guidelines for consistent droplet movement control in contact charge electrophoresis-based digital microfluidic systems. Finally, further discussions on the obtained results, its implications, and future work are discussed.

Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids

The dispensing of tiny droplets is a basic and crucial process in a myriad of applications, such as DNA/protein microarray, cell cultures, chemical synthesis of microparticles, and digital microfluidics. This work systematically demonstrates droplet dispensing into immiscible fluids through electric charge concentration (ECC) method. It exhibits three main modes (i.e., attaching, uniform, and bursting modes) as a function of flow rates, applied voltages, and gap distances between the nozzle and the oil surface. Through a conventional nozzle with diameter of a few millimeters, charged droplets with volumes ranging from a few μL to a few tens of nL can be uniformly dispensed into the oil chamber without reduction in nozzle size. Based on the features of the proposed method (e.g., formation of droplets with controllable polarity and amount of electric charge in water and oil system), a simple and straightforward method is developed for microparticle synthesis, including preparation of colloidosomes and fabrication of Janus microparticles with anisotropic internal structures. Finally, a combined system consisting of ECC-induced droplet dispensing and electrophoresis of charged droplet (ECD)-driven manipulation systems is constructed. This integrated platform will provide increased utility and flexibility in microfluidic applications because a charged droplet can be delivered toward the intended position by programmable electric control.

On-chip analysis, indexing and screening for chemical producing bacteria in a microfluidic static droplet array

Economic production of chemicals from microbes necessitates development of high-producing strains and an efficient screening technology is crucial to maximize the effect of the most popular strain improvement method, the combinatorial approach. However, high-throughput screening has been limited for assessment of diverse intracellular metabolites at the single-cell level. Herein, we established a screening platform that couples a microfluidic static droplet array (SDA) and an artificial riboswitch to analyse intracellular metabolite concentration from single microbial cells. Using this system, we entrapped single Escherichia coli cells in SDA to measure intracellular l-tryptophan concentrations. It was validated that intracellular l-tryptophan concentration can be evaluated by the fluorescence from the riboswitch. Moreover, high-producing strains were successfully screened from a mutagenized library, exhibiting up to 145% productivity compared to its parental strain. This platform will be widely applicable to strain improvement for diverse metabolites by developing new artificial riboswitches.

Coalescence dynamics of viscous conical drops

When two oppositely charged drops come into light contact, a liquid meniscus bridge with double-cone geometry forms between the drops. Recent experiments have demonstrated the existence of a critical cone angle above which the meniscus bridge pinches off and the drops do not coalesce. This striking behavior-which has implications for processes ranging from the coarsening of emulsions to electrospray ionization in mass spectrometry-has been studied theoretically and experimentally for inertial liquid drops. Little is known, however, about the influence of the liquid viscosity on the critical cone angle. Here, we use high-fidelity numerical simulations to gain insight into the coalescence dynamics of conical drops at intermediate Reynolds numbers. The simulations, which account for viscous, inertial, and surface tension effects, predict that the critical cone angle increases as the viscosity of the drops decreases. When approaching the inertial regime, however, the predicted critical angle quickly stabilizes at approximately 27°, as observed in experiments.