Digital microfluidics

Centrifugal microfluidic lab-on-a-chip system with automated sample lysis, DNA amplification and microarray hybridization for identification of enterohemorrhagic Escherichia coli culture isolates

Automated workflow that starts with a colony isolate and ends with a fluorescence signal on a DNA microarray.

Microbioreactors for Process Development and Cell-Based Screening Studies

Microbioreactors (MBRs) have emerged as potent cultivation devices enabling automated small-scale experiments in parallel while enhancing their cost efficiency. The widespread use of MBRs has contributed to recent advances in industrial and pharmaceutical biotechnology, and they have proved to be indispensable tools in the development of many modern bioprocesses. Being predominantly applied in early stage process development, they open up new fields of research and enhance the efficacy of biotechnological product development. Their reduced reaction volume is associated with numerous inherent advantages - particularly the possibility for enabling parallel screening operations that facilitate high-throughput cultivations with reduced sample consumption (or the use of rare and expensive educts). As a result, multiple variables can be examined in a shorter time and with a lower expense. This leads to a simultaneous acceleration of research and process development along with decreased costs.MBRs range from simple miniaturized cultivations vessels (i.e., in the milliliter scale with limited possibilities for process control) to highly complex and automated small-scale microreactors with integrated sensors that allow for comprehensive screenings in very short time or a precise reflection of large-scale cultivation conditions. Progressive developments and improvements in manufacturing and automation techniques are already helping researchers to make use of the advantages that MBRs offer. This overview of current MBR systems surveys the diverse application for microbial and mammalian cell cultivations that have been developed in recent years.

Molecular digital data storage using DNA

Molecular data storage is an attractive alternative for dense and durable information storage, which is sorely needed to deal with the growing gap between information production and the ability to store data. DNA is a clear example of effective archival data storage in molecular form. In this Review, we provide an overview of the process, the state of the art in this area and challenges for mainstream adoption. We also survey the field of in vivo molecular memory systems that record and store information within the DNA of living cells, which, together with in vitro DNA data storage, lie at the growing intersection of computer systems and biotechnology.

Density gradient calculation in a class of multiphase lattice Boltzmann models

The multiphase lattice Boltzmann (LB) models based on pairwise interactions show great potential for simulating multiphase flows due to the conceptual and computational simplicity. Although the dynamics of multiphase flows are reproduced by the pairwise interaction force, the gradient of density (or effective density, i.e., pseudopotential) is implicitly involved in these models via the specialized forcing scheme or the consistent scheme for ɛ^{3}-order term. This work focuses on the calculation of density gradient in this class of multiphase LB models. Theoretical analyses are first carried out to reveal the involvement and calculation of density gradient. On the basis of a low Mach number approximation, an improved scheme is then proposed to calculate the density gradient for the recent LB model with self-tuning equation of state. Analytical and numerical calculations show that the improved scheme is more accurate and can help to reduce the numerical error when the reduced temperature is relatively low.

Programmable hydraulic resistor for microfluidic chips using electrogate arrays

Flow rates play an important role in microfluidic devices because they affect the transport of chemicals and determine where and when (bio)chemical reactions occur in these devices. Flow rates can conveniently be determined using external peripherals in active microfluidics. However, setting specific flow rates in passive microfluidics is a significant challenge because they are encoded on a design and fabrication level, leaving little freedom to users for adjusting flow rates for specific applications. Here, we present a programmable hydraulic resistor where an array of "electrogates" routes an incoming liquid through a set of resistors to modulate flow rates in microfluidic chips post-fabrication. This approach combines a battery-powered peripheral device with passive capillary-driven microfluidic chips for advanced flow rate control and measurement. We specifically show a programmable hydraulic resistor composed of 7 parallel resistors and 14 electrogates. A peripheral and smartphone application allow a user to activate selected electrogates and resistors, providing 127 (27-1) flow resistance combinations with values spanning on a 500 fold range. The electrogates feature a capillary pinning site (i.e. trench across the flow path) to stop a solution and an electrode, which can be activated in a few ms using a 3 V bias to resume flow based on electrowetting. The hydraulic resistor and microfluidic chip shown here enable flow rates from ~0.09 nL.s-1 up to ~5.66 nL.s-1 with the resistor occupying a footprint of only 15.8 mm2 on a 1 × 2 cm2 microfluidic chip fabricated in silicon. We illustrate how a programmable hydraulic resistor can be used to set flow rate conditions for laminar co-flow of 2 liquids and the enzymatic conversion of a substrate by stationary enzymes (alkaline phosphatase) downstream of the programmable hydraulic resistor.

Simultaneous detection of two growth factors from human single-embryo culture medium by a bead-based digital microfluidic chip

The measurement of growth factors released in a culture medium is considered to be an attractive non-invasive approach, apart from the embryo morphology, to identify the condition of an embryo development after fertilization in vitro (IVF), but the available embryo culture medium in the current method is only a few microlitres. This small sample volume, also of small concentration, makes difficult the application of a conventional detection method, such as an enzyme-linked immunosorbent assay (ELISA). A reliable detection of the growth factor from each embryo culture medium of such a small concentration hence remains a challenge. Here for the first time we report the results of measurement of not just one, but two, growth factors, human IL-1β and human TNF-α, from an individual droplet of embryo culture medium with a bead-based digital microfluidic chip. The required sample volume for a single measurement is only 520 nL; the total duration of the on-chip process is less than 40 min. Using the culture media of human embryos with normal morphologic features, we found that the concentrations of TNF-α change little from day 3 to day 5-6, but the concentrations of IL-1β for some embryos might double from day 3 to day 5-6. For other embryos even with similar normal morphologic features, some growth factors, such as IL-1β, might exhibit different expressions during the culture period. Those growth factors could serve to distinguish the development conditions of each embryo, not merely from an observation of embryo morphology.

Recent advances in microfluidics for drug screening

With ever increasing drug resistance and emergence of new diseases, demand for new drug development is at an unprecedented urgency. This fact has led to extensive recent efforts to develop new drugs and novel techniques for efficient drug screening. However, new drug development is commonly hindered by cost and time span. Thus, developing more accessible, cost-effective methods for drug screening is necessary. Compared with conventional drug screening methods, a microfluidic-based system has superior advantages in sample consumption, reaction time, and cost of the operation. In this paper, the advantages of microfluidic technology in drug screening as well as the critical factors for device design are described. The strategies and applications of microfluidics for drug screening are reviewed. Moreover, current limitations and future prospects for a drug screening microdevice are also discussed.

Magnetic-Responsive Bendable Nozzles for Open Surface Droplet Manipulation

The handling of droplets in a controlled manner is essential to numerous technological and scientific applications. In this work, we present a new open-surface platform for droplet manipulation based on an array of bendable nozzles that are dynamically controlled by a magnetic field. The actuation of these nozzles is possible thanks to the magnetically responsive elastomeric composite which forms the tips of the nozzles; this is fabricated with Fe₃O₄ microparticles embedded in a polydimethylsiloxane matrix. The transport, mixing, and splitting of droplets can be controlled by bringing together and separating the tips of these nozzles under the action of a magnet. Additionally, the characteristic configuration for droplet mixing in this platform harnesses the kinetic energy from the feeding streams; this provided a remarkable reduction of 80% in the mixing time between drops of liquids about eight times more viscous than water, i.e., 6.5 mPa/s, when compared against the mixing between sessile drops of the same fluids.

Shaping and transporting diamagnetic sessile drops

Electromagnetic fields are commonly used to control small quantities of fluids in microfluidics and digital microfluidics. Magnetic control techniques are less well studied than their electric counterparts, with only a few investigations into liquid diamagnetism. The ratio of magnetic to surface energy (magnetic Bond number B m ) is an order of magnitude smaller for diamagnetic drops ( B m ≈ - 0.3 at 1.2 T applied field) than for paramagnetic drops ( B m ≈ 9.0 at 1.2 T applied field). This weaker interaction between the magnetic field and the diamagnetic drop has led to the phenomenon being overlooked in digital microfluidics. Here, we investigate shaping and transport of diamagnetic drops using magnetostatic fields. Our findings highlight how diamagnetic fluids can be used as a novel tool in the toolbox of microfluidics and digital microfluidics.

Droplet on Soft Shuttle: Electrowetting-on-Dielectric Actuation of Small Droplets

Here, we introduce the novel concept of a "soft shuttle" for transportation, manipulation, and diffusion studies of small liquid droplets using electrowetting on the dielectric mechanism. This method enables manipulation of droplets several times smaller than the electrode size and, importantly, minimizes evaporation, contamination, and exposure of the sample to high voltages. We demonstrate various modes of droplet loading, transporting, and unloading. Using advanced imaging processing techniques, we obtained detailed information about the shuttle and droplet centroids. Furthermore, varying water concentration on the soft shuttle allows for modulation of the diffusion kinetics of samples into the shuttle, which also can be controlled with soft shuttle actuation velocity. We believe that this novel approach for the manipulation of droplets will advance the field of droplet-based open microfluidics and can be potentially useful for applications in biotechnology, diagnostics, or analytical chemistry.

Electrocoalescence of liquid marbles driven by embedded electrodes for triggering bioreactions

Liquid marbles need to be controlled precisely to benefit applications, for instance, as microreactors on digital microfluidic platforms for chemical and biological assays. In this work, a strategy is introduced to coalesce liquid marbles via electrostatics, where two liquid marbles in contact can coalesce when a sufficiently high voltage is applied to embedded electrodes. With the understanding of the mechanism of coalescence through relating the electric stress and the restoring capillary pressure at the contact interface, this method coalesces liquid marbles efficiently. When compared with the existing electrocoalescence method, our approach does not require immersion of electrodes to trigger coalescence. We demonstrate this to exchange the medium for the culture of cell spheroids and to measure the cell metabolic activity through a CCK-8 assay. The manipulation of liquid marbles driven by electrostatics creates new opportunities to conduct chemical reactions and biomedical assays in these novel microreactors.

Miniaturized sample preparation on a digital microfluidics device for sensitive bottom-up microproteomics of mammalian cells using magnetic beads and mass spectrometry-compatible surfactants

While LC-MS-based proteomics with high nanograms to micrograms of total protein has become routine, the analysis of samples derived from low cell numbers is challenged by factors such as sample losses, or difficulties encountered with the manual manipulation of small liquid volumes. Digital microfluidics (DMF) is an emerging technique for miniaturized and automated droplet manipulation, which has been proposed as a promising tool for proteomic sample preparation. However, proteome analysis of samples prepared on-chip by DMF has previously been unfeasible, due to incompatibility with down-stream LC-MS instrumentation. To overcome these limitations, we here developed protocols for bottom-up LC-MS based proteomics sample preparation of as little as 100 mammalian cells on a commercially available digital microfluidics device. To this end, we developed effective cell lysis conditions optimized for DMF, as well as detergent-buffer systems compatible with downstream proteolytic digestion on DMF chips and subsequent LC-MS analysis. A major step was the introduction of the single-pot, solid-phase-enhanced sample preparation (SP3) approach on-chip, which allowed the removal of salts and anti-fouling polymeric detergents, thus rendering sample preparation by DMF compatible with LC-MS-based proteome analysis. Application of DMF-SP3 to the proteome analysis of Jurkat T cells led to the identification of up to 2500 proteins from approximately 500 cells, and up to 1200 proteins from approximately 100 cells on an Orbitrap mass spectrometer, emphasizing the high compatibility of DMF-SP3 with low protein input and minute volumes handled by DMF. Taken together, we demonstrate the first sample preparation workflow for proteomics on a DMF chip device reported so far, allowing the sensitive analysis of limited biological material.

Droplet Incubation and Splitting in Open Microfluidic Channels

Droplet-based microfluidics enables compartmentalization and controlled manipulation of small volumes. Open microfluidics provides increased accessibility, adaptability, and ease of manufacturing compared to closed microfluidic platforms. Here, we begin to build a toolbox for the emerging field of open channel droplet-based microfluidics, combining the ease of use associated with open microfluidic platforms with the benefits of compartmentalization afforded by droplet-based microfluidics. We develop fundamental microfluidic features to control droplets flowing in an immiscible carrier fluid within open microfluidic systems. Our systems use capillary flow to move droplets and carrier fluid through open channels and are easily fabricated through 3D printing, micromilling, or injection molding; further, droplet generation can be accomplished by simply pipetting an aqueous droplet into an empty open channel. We demonstrate on-chip incubation of multiple droplets within an open channel and subsequent transport (using an immiscible carrier phase) for downstream experimentation. We also present a method for tunable droplet splitting in open channels driven by capillary flow. Additional future applications of our toolbox for droplet manipulation in open channels include cell culture and analysis, on-chip microscale reactions, and reagent delivery.

On-Chip Impedance for Quantifying Parasitic Voltages During AC Electrokinetic Trapping

OBJECTIVE

Assessing the effectiveness of microfluidic device structures for enabling electrokinetic or acoustic trapping requires imaging of model particles within each device under the requisite force fields. To avoid the need for extensive microscopy, the use of valuable biological samples, and reliance on a trained operator to assess efficacy of trapping, we explore electrical means to identify device geometry variations that are responsible for the poor trapping.

RESULTS

Using the example of AC electrokinetic trapping over an insulated channel in a contact-less dielectrophoresis mode, we present an on-chip method to acquire impedance spectra on the microfluidic device for quantifying the parasitic voltage drops.

CONCLUSION

Based on the parasitic voltage drops, device geometries can be designed to maximize fraction of the applied voltage that is available for dielectrophoretic manipulation and the measured on-chip impedance can rapidly inform downstream decisions on particle manipulation.

On-chip organic synthesis enabled using an engine-and-cargo system in an electrowetting-on-dielectric digital microfluidic device

This paper presents a microfluidic chemical reaction using an electrowetting-on-dielectric (EWOD) digital microfluidic device. Despite a number of chemical/biological applications using EWOD digital microfluidic devices, their applications to organic reactions have been seriously limited because most of the common solvents used in synthetic organic chemistry are not compatible with EWOD devices. To address this unsolved issue, we first introduce a novel technique using an "engine-and-cargo" system that enables the use of non-movable fluids (e.g., organic solvents) on an EWOD device. With esterification as the model reaction, on-chip chemical reactions were successfully demonstrated. Conversion data obtained from on-chip reactions were used to characterize and optimize the reaction with regard to reaction kinetics, solvent screening, and catalyst loading. As the first step toward on-chip combinatorial synthesis, parallel esterification of three different alcohols was demonstrated. Results from this study clearly show that an EWOD digital microfluidic platform is a promising candidate for microscale chemical reactions.

Guided droplet transport on synthetic slippery surfaces inspired by a pitcher plant

We show how anisotropic, grooved features facilitate the trapping and directed transport of droplets on lubricated, liquid-shedding surfaces. Capillary action pins droplets to topographic surface features, enabling transport along the feature while inhibiting motion across (or detachment from) the feature. We demonstrate the robustness of this capillary-based mechanism for directed droplet transport on slippery surfaces by combining experiments on synthetic, lubricant-infused surfaces with observations on the natural trapping surface of a carnivorous pitcher plant. Controlling liquid navigation on synthetic surfaces promises to unlock significant potential in droplet-based technologies. Our observations also offer novel insight into the evolution of the Nepenthes pitcher plant, indicating that the 'pitfall' trapping mechanism is enhanced by the lubricant-infused, macroscopic grooves on the slippery peristome surface, which guide prey into the trap in a way that is more tightly controlled than previously considered.

Modes and break periods of electrowetting liquid bridge

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

Separation of Floating Oil Drops Based on Drop-Liquid Substrate Interfacial Tension

Though various strategies exist for the transport of oil drops suspended on a liquid substrate, selective manipulation of different kinds of drops based on their respective characteristics remains a challenge. In practice, it is possible to have multiple drops having different wetting states with the liquid substrate, whose separation is desired. In this work, we exploit curvature-induced capillary forces for the selective manipulation (transport as well as separation) of oil droplets based on their interfacial tension (IFT) with the underlying liquid substrate. To demonstrate this, we have selected two oils having different IFTs with the aqueous liquid substrate and tuned their curvature-induced capillary interaction (inward or outward from the source) by controlled addition of the surfactant. We experimentally realize three droplet manipulation regimes: repulsion, attraction, and separation regime. In the repulsion and attraction regimes, both the drops behave in a similar manner. Strikingly, in the separation regime, drops can be effectively separated based on their IFT; low IFT droplets are attracted toward the source, while high IFT droplets do the reverse.

Electrowetting Behavior and Digital Microfluidic Applications of Fluorescent, Polymer-Encapsulated Quantum Dot Nanofluids

Digital microfluidics is a liquid-handling technology capable of rapidly and autonomously controlling multiple discrete droplets across an array of electrodes and has seen continual growth in the fields of chemistry, biology, and optics. This technology is enabled by rapidly switching the wettability of a surface through the application of an electric field: a phenomenon known as electrowetting-on-dielectric. The results reported here elucidate the wetting behavior of fluorescent quantum dot nanofluids by varying the aqueous-solubilizing polymers, changing the size of the nanocrystals, and the addition of surfactants. Nanofluid droplets were demonstrated to have very large changes in contact angle (>100°) by employing alternating current voltage to aqueous droplets within a dodecane medium. The stability of quantum dot nanofluids is also evaluated within a digital microfluidics platform, and the optical properties are not perturbed even under high voltages (250 V). Multiple fluorescent droplets with varying emission can be simultaneously actuated and rapidly mixed (<10 s) to generate a new nanofluid with optical properties different from the parent solutions.

Facile Actuation of Organic and Aqueous Droplets on Slippery Liquid-Infused Porous Surfaces for the Application of On-Chip Polymer Synthesis and Liquid-Liquid Extraction

Digital microfluidics employs water-repellant surfaces to exquisitely manipulate droplets of water for chemical analysis. However, the actuation and manipulation of organic droplets is still relatively unexplored as it is significantly more difficult to synthesize organic-repellent surfaces compared to water-repellent surfaces. Here, we present the fabrication of slippery liquid-infused porous surfaces (SLIPS) based on a porous polymer monolithic approach. The synthesized SLIPS were able to repel organic liquids such as hexane and methanol with a contact angle of 42.1 ± 0.4° and 69.0 ± 1.8°, respectively, as well as water with a contact angle of 115.8 ± 0.8°. More importantly for digital microfluidic applications, the sliding angle of liquids tested was between 4° and 6°. As a result, droplets containing magnetically susceptible material could be facilely manipulated on the SLIPS surface. A systematic actuation study was carried out to explore how actuation parameters including speed, paramagnetic particle (PMP) concentrations, and droplet volume impacted the outcomes (droplet actuation, disengagement, and PMP extraction). Two different applications were used to demonstrate the utility of actuating organic droplets on SLIPS surfaces including on-chip liquid-liquid extractions of natural products (NPs) from marine bacteria and droplet-based polymer synthesis with different polymerization conditions. Both applications employ an aqueous droplet and organic droplet interface at which either phase transfer or a chemical reaction is carried out. Two NPs (prodigiosin from Pseudoalteromonas rubra and violacein from Pseudoalteromonas luteoviolacea) were extracted, from aqueous droplets containing the bacteria, into butanol droplets and characterized with matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS). Nylon 6,6 was synthesized on-chip via magnetic actuation of organic droplets containing adipoyl chloride and hexamethylamine. Relative intensities of the characteristic polymer masses suggest that droplet-based microfluidic synthesis on slips can be used to probe reaction conditions. The compatibility of SLIPS with both aqueous and organic solutions opens up a wider number of droplet-based sample preparation protocols and chemical transformations.

Open Microfluidic Capillary Systems

Open microfluidic capillary systems are a rapidly evolving branch of microfluidics where fluids are manipulated by capillary forces in channels lacking physical walls on all sides. Typical channel geometries include grooves, rails, or beams and complex systems with multiple air-liquid interfaces. Removing channel walls allows access for retrieval (fluid sampling) and addition (pipetting reagents or adding objects like tissue scaffolds) at any point in the channel; the entire channel becomes a "device-to-world" interface, whereas such interfaces are limited to device inlets and outlets in traditional closed-channel microfluidics. Open microfluidic capillary systems are simple to fabricate and reliable to operate. Prototyping methods (e.g., 3D printing) and manufacturing methods (e.g., injection molding) can be used seamlessly, accelerating development. This Perspective highlights fundamentals of open microfluidic capillary systems including unique advantages, design considerations, fabrication methods, and analytical considerations for flow; device features that can be combined to create a "toolbox" for fluid manipulation; and applications in biology, diagnostics, chemistry, sensing, and biphasic applications.

Slip-driven microfluidic devices for nucleic acid analysis

Slip-driven microfluidic devices can manipulate fluid by the relative movement of microfluidic plates that are in close contact. Since the demonstration of the first SlipChip device, many slip-driven microfluidic devices with different form factors have been developed, including SlipPAD, SlipDisc, sliding stripe, and volumetric bar chart chip. Slip-driven microfluidic devices can be fabricated from glass, quartz, polydimethylsiloxane, paper, and plastic with various fabrication methods: etching, casting, wax printing, laser cutting, micromilling, injection molding, etc. The slipping operation of the devices can be performed manually, by a micrometer with a base station, or autonomously, by a clockwork mechanism. A variety of readout methods other than fluorescence microscopy have been demonstrated, including both fluorescence detection and colorimetric detection by mobile phones, direct visual detection, and real-time fluorescence imaging. This review will focus on slip-driven microfluidic devices for nucleic acid analysis, including multiplex nucleic acid detection, digital nucleic acid quantification, real-time nucleic acid amplification, and sample-in-answer-out nucleic acid analysis. Slip-driven microfluidic devices present promising approaches for both life science research and clinical molecular diagnostics.

"Learning on a chip:" Microfluidics for formal and informal science education

Microfluidics is a technique for the handling of small volumes of liquids on the order of picoliters to nanoliters and has impact for miniaturized biomedical science and fundamental research. Because of its multi- and interdisciplinary nature (i.e., combining the fields of biology, chemistry, physics, and engineering), microfluidics offers much potential for educational applications, both at the university level as well as primary and secondary education. Microfluidics is also an ideal "tool" to enthuse and educate members of the general public about the interdisciplinary aspects of modern sciences, including concepts of science, technology, engineering, and mathematics subjects such as (bio)engineering, chemistry, and biomedical sciences. Here, we provide an overview of approaches that have been taken to make microfluidics accessible for formal and informal learning. We also point out future avenues and desired developments. At the extreme ends, we can distinguish between projects that teach how to build microfluidic devices vs projects that make various microscopic phenomena (e.g., low Reynolds number hydrodynamics, microbiology) accessible to learners and the general public. Microfluidics also enables educators to make experiments low-cost and scalable, and thereby widely accessible. Our goal for this review is to assist academic researchers working in the field of microfluidics and lab-on-a-chip technologies as well as educators with translating research from the laboratory into the lecture hall, teaching laboratory, or public sphere.

Extraction of nucleic acids from blood: unveiling the potential of active pneumatic pumping in centrifugal microfluidics for integration and automation of sample preparation processes

This paper describes the development of an on-chip nucleic acid (NA) extraction assay from whole blood using a centrifugal microfluidic platform that allows for pneumatic actuation of liquids during rotation. The combination of pneumatic and centrifugal forces makes it possible to perform fluidic operations without the need for integrating active control elements on the microfluidic cartridge. The cartridge is fabricated from thermoplastic polymers (e.g., Zeonor 1060R) and features a simple design that is compatible with injection molding. In addition, the cartridge is interfaced with two external vials for off-chip storage of the blood sample and retrieval of the eluted NA solution, respectively. On-chip capture of NAs is performed using an embedded solid-phase extraction matrix composed of commercial glass microfiber filters (Whatman GF/D and GF/F). The yield of the automated, on-chip extraction protocol, determined by measuring absorbance at 260 nm, is comparable to some of the best manually operated kits (e.g., Qiagen QIAamp DNA Mini Kit) while providing low assay-to-assay variability due to the high level of control provided by the platform for each processing step. The A260/A280 and A260/A230 ratios of the absorbance spectra also reveal that protein contamination of the sample is negligible. The capability of the pneumatic platform to circulate air flux through the microfluidic conduit was used to dry leftover ethanol residues retained in the capture matrix during washing. This method, applied in combination with localized heating, proved effective for reducing ethanol contamination in eluted samples from ∼12% to 1% (v/v).

High-Precision Stereolithography of Biomicrofluidic Devices

Stereolithography (SL) is emerging as an attractive alternative to soft lithography for fabricating microfluidic devices due to its low cost and high design efficiency. Low molecular weight poly(ethylene glycol)diacrylate (MW = 258) (PEG-DA-258) has been used for SL 3D-printing of biocompatible microdevices at submillimeter resolution. However, 3D-printing resins that simultaneously feature high transparency, high biocompatibility, and high resolution are still lacking. It is found that photosensitizer isopropyl thioxanthone can, in a concentration-dependent manner, increase the absorbance of the resin (containing PEG-DA-258 and photoinitator Irgacure-819) by over an order of magnitude. This increase in absorbance allows for SL printing of microdevices at sub pixel resolution with commercially available desktop printers and without compromising transparency or biocompatibility. The assembly-free, rapid (<15 h) 3D-printing of a variety of complex 3D microfluidic devices such as a 3D-fluid router, a passive chaotic micro-mixer, an active micro-mixer with pneumatic microvalves, and high-aspect ratio (37:1) microchannels of single pixel width is demonstrated. These manufacturing capabilities are unavailable in conventional microfluidic rapid prototyping techniques. The low absorption of small hydrophobic molecules and microfluidic labeling of cultured mammalian cells in 3D-printed PEG-DA-258 microdevices is demonstrated, indicating the potential of PEG-DA-based fabrication of cell-based assays, drug discovery, and organ-on-chip platforms.

Dynamic microscale flow patterning using electrical modulation of zeta potential

The ability to move fluids at the microscale is at the core of many scientific and technological advancements. Despite its importance, microscale flow control remains highly limited by the use of discrete channels and mechanical valves, and relies on fixed geometries. Here we present an alternative mechanism that leverages localized field-effect electroosmosis to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an ac voltage in sync with an external electric field, creating nonuniform electroosmotic flow distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. Furthermore, we provide a characterization of the system in terms of time response and dielectric breakdown, as well as engineering guidelines for its robust design and operation. We believe that the ability to create tailored microscale flow using solid-state actuation will open the door to entirely new on-chip functionalities.

Droplet Sorting and Manipulation on Patterned Two-Phase Slippery Lubricant-Infused Surface

Slippery lubricant-infused surfaces are composite materials consisting of a solid matrix permanently infused by a lubricant. Such surfaces have proved to be highly repellent to various liquids immiscible with the lubricant. Depending on the underlying surface chemistry, different lubricants can be used, including perfluorinated or alkylated oils. Here, we construct patterned slippery surfaces that consist of virtual channels permanently impregnated with an organic oil and surrounded by areas permanently impregnated with a perfluorinated oil. We demonstrate that water droplets preferentially occupy the organic-oil-lubricated virtual channels. Based on a simple model, we evaluate the forces acting on droplets crossing over to the regions impregnated with perfluorinated oil and show that the cloaking of the droplets plays an important role. We study the actuation of droplets in virtual oil-in-oil channels based on gravity and magnetic fields. Finally, we construct a variety of organic-oil-lubricated channel architectures permitting droplet sorting according to size. We believe that this novel approach for the formation of virtual all-liquid surface-tension-confined channels based on lubricant-infused surfaces, channel networks, or patterns will advance the field of droplet-based microfluidics. The approach presented can be potentially useful for applications in biotechnology, diagnostics, or analytical chemistry.

Velocity Saturation in Digital Microfluidics

In digital microfluidics, discrete droplets of fluid are made to move on an open surface with no microchannels. These systems are commonly operated by application of electrical driving forces to an array of electrodes. While these driving forces are well characterized, the dissipative forces opposing droplet movement have not been as thoroughly examined. In recognition of this deficit, we used force-velocity plots to characterize droplet movement in digital microfluidics, which was found to be consistent with a simple theoretical framework for understanding dissipation effects for droplets in two-plate, air-filled devices. Interestingly, in some conditions, a previously unreported ″velocity saturation″ effect was observed. When examined across a range of different liquids, the forces at which this saturation occurs seem to be lower for liquids with smaller surface tensions. Furthermore, when driven at forces that cause saturation, physical phenomena are observed that are akin to what has been reported for stationary droplets in the electrowetting literature. These phenomena are detrimental to device performance, leading to a new "force window" approach that delineates the optimum operation conditions for different liquids. We propose that these findings may be useful for a wide range of applications for experts and new users alike in this growing field.

High density DNA data storage library via dehydration with digital microfluidic retrieval

DNA promises to be a high density data storage medium, but physical storage poses a challenge. To store large amounts of data, pools must be physically isolated so they can share the same addressing scheme. We propose the storage of dehydrated DNA spots on glass as an approach for scalable DNA data storage. The dried spots can then be retrieved by a water droplet using a digital microfluidic device. Here we show that this storage schema works with varying spot organization, spotted masses of DNA, and droplet retrieval dwell times. In all cases, the majority of the DNA was retrieved and successfully sequenced. We demonstrate that the spots can be densely arranged on a microfluidic device without significant contamination of the retrieval. We also demonstrate that 1 TB of data could be stored in a single spot of DNA and successfully retrieved using this method.

Micro-Electrode-Dot-Array Digital Microfluidic Biochips: Technology, Design Automation, and Test Techniques

Digital microfluidic biochips (DMFBs) are being increasingly used for DNA sequencing, point-of-care clinical diagnostics, and immunoassays. DMFBs based on a micro-electrode-dot-array (MEDA) architecture have recently been proposed, and fundamental droplet manipulations, e.g., droplet mixing and splitting, have also been experimentally demonstrated on MEDA biochips. There can be thousands of microelectrodes on a single MEDA biochip, and the fine-grained control of nanoliter volumes of biochemical samples and reagents is also enabled by this technology. MEDA biochips offer the benefits of real-time sensitivity, lower cost, easy system integration with CMOS modules, and full automation. This review paper first describes recent design tools for high-level synthesis and optimization of map bioassay protocols on a MEDA biochip. It then presents recent advances in scheduling of fluidic operations, placement of fluidic modules, droplet-size-aware routing, adaptive error recovery, sample preparation, and various testing techniques. With the help of these tools, biochip users can concentrate on the development of nanoscale bioassays, leaving details of chip optimization and implementation to software tools.

Grow with the Flow: When Morphogenesis Meets Microfluidics

Developmental biology has advanced the understanding of the intricate and dynamic processes involved in the formation of an organism from a single cell. However, many gaps remain in the knowledge of embryonic development, especially regarding tissue morphogenesis. A possible approach to mimic such phenomena uses pluripotent stem cells in in vitro morphogenetic models. Herein, these systems are summarized with emphasis on the ability to better manipulate and control cellular interfaces with either liquid or solid materials using microengineered tools, which is critical for attaining deeper insights into pattern formation and stem cell differentiation during organogenesis. The role of conventional and customized cell-culture systems in supporting important advances in the field of morphogenesis is discussed, and the fascinating role that material sciences and microengineering currently play and are expected to play in the future is highlighted. In conclusion, it is proffered that continued microfluidics innovations when applied to morphogenesis promise to provide important insights to advance many multidisciplinary fields, including regenerative medicine.

Magnetic digital microfluidics on a bioinspired surface for point-of-care diagnostics of infectious disease

Magnetic digital microfluidics uses magnetic force to manipulate droplets on a Teflon-coated substrate through the added magnetic particles. To achieve a wide range of droplet manipulation, hydrophilic patterns, known as surface energy traps, are introduced onto the Teflon-coated hydrophobic substrate. However, the Teflon-coated substrate is difficult to modify because it is nonwettable, and existing techniques for patterning surface energy traps have many limitations. Inspired by the mussel adhesion mechanism, we use polydopamine, a bioinspired substance that adheres strongly to almost any materials, to pattern surface energy traps on the Teflon-coated substrate with a great ease. We have optimized the polydopamine coating protocol and characterized the surface properties of the polydopamine surface energy traps. Droplet operations including particle extraction, liquid dispensing, liquid shaping, and cross-platform transfer have been demonstrated on the polydopamine surface energy trap-enabled magnetic digital microfluidic platform in both single-plate and two-plate configurations. Furthermore, the detection of hepatitis B surface antigen using ELISA has been demonstrated on the new magnetic dgitial microfluidic platform. This new bioinspired magnetic digital microfluidic platform is easy to fabricate and operate, showing a great potential for point-of-care applications.

Organic-free, versatile sessile droplet microfluidic device for chemical separation using an aqueous two-phase system

This work presents a novel portable, versatile sessile droplet microfluidic (SDMF) device to perform liquid manipulation operations such as confining, splitting and colorimetric detection. Furthermore, chemical isolations based on an aqueous two-phase system (ATPS) for separating an analyte of choice from a complicated sample matrix can be carried out. ATPS extractions can replace conventional liquid-liquid extractions and take away the need for harmful organic solvents. Superhydrophobic (SH) surfaces were fabricated from a commercially available material, Ultra-Ever Dry® (UED®). On these SH surfaces, surface energy traps (SETs) were produced either by air plasma treatment (simultaneously) or laser micromachining (sequentially) to dock/pin an ATPS containing droplet onto the surface. Splitting of droplets or removing a precise volume of the top phase from a pinned extraction system was achieved with a sandwich-chip approach. For this, an additional SET patterned substrate was placed on top of the droplet and subsequently lifted. This multipurpose platform was used to isolate Cd from a mixture of several other metal ions (i.e. Mn, Ni, Cu, Pb, Fe) for its subsequent interference-free detection. An ATPS consisting of sodium sulfate and polyethylene glycol (PEG) as phase forming components and potassium iodine as extractant allowed separation of cadmium with an extraction efficiency of q(Cd2+) = 98.5%. Using a portable, cost-effective, smartphone-based UV/vis spectrometer, Cd was detected with a LoD of 3.4 ppm. Alternatively, the multipurpose platform can also be used as sampling platform for a benchtop UV/vis spectrometer, where a LoD of 0.53 ppm was obtained. Potential applications of the presented platform include sample preparation and separation that can be achieved by aqueous two-phase extractions, such as proteins, antibodies, DNA, cells, organic molecules and metal ions.

Heterogeneous Immunoassay Using Channels and Droplets in a Digital Microfluidic Platform

This work presents a heterogeneous immunoassay using the integrated functionalities of a channel and droplets in a digital microfluidic (DMF) platform. Droplet functionality in DMF allows for the programmable manipulation of discrete sample and reagent droplets in the range of nanoliters. Pressure-driven channels become advantageous over droplets when sample must be washed, as the supernatant can be thoroughly removed in a convenient and rapid manner while the sample is immobilized. Herein, we demonstrate a magnetic bead-based, enzyme-linked immunosorbent assay (ELISA) using ~60 nL of human interleukin-6 (IL-6) sample. The wash buffer was introduced in the form of a wall-less virtual electrowetting channel by a syringe pump at the flow rate of 10 μL/min with ~100% bead retention rate. Critical parameters such as sample wash flow rate and bead retention rate were optimized for reliable assay results. A colorimetric readout was analyzed in the International Commission on Illumination (CIE) color space without the need for costly equipment. The concepts presented in this work are potentially applicable in rapid neonatal disease screening using a finger prick blood sample in a DMF platform.

An integrated droplet-digital microfluidic system for on-demand droplet creation, mixing, incubation, and sorting

Droplet microfluidics is a technique that has the ability to compartmentalize reactions in sub nano- (or pico-) liter volumes that can potentially enable millions of distinct biological assays to be performed on individual cells. In a typical droplet microfluidic system, droplets are manipulated by pressure-based flows. This has limited the fluidic operations that can be performed in these devices. Digital microfluidics is an alternative microfluidic paradigm with precise control and manipulation over individual droplets. Here, we implement an integrated droplet-digital microfluidic (which we call 'ID2M') system in which common fluidic operations (i.e. droplet generation, cell encapsulation, droplet merging and mixing, droplet trapping and incubation, and droplet sorting) can be performed. With the addition of electrodes, we have been able to create droplets on-demand, tune their volumes on-demand, and merge and mix several droplets to produce a dilution series. Moreover, this device can trap and incubate droplets for 24 h that can consequently be sorted and analyzed in multiple n-ary channels (as opposed to typical binary channels). The ID2M platform has been validated as a robust on-demand screening system by sorting fluorescein droplets of different concentration with an efficiency of ∼96%. The utility of the new system is further demonstrated by culturing and sorting tolerant yeast mutants and wild-type yeast cells in ionic liquid based on their growth profiles. This new platform for both droplet and digital microfluidics has the potential to be used for screening different conditions on-chip and for applications like directed evolution.

Determination of Aqueous Two-Phase System Binodals and Tie-Lines by Electrowetting-on-Dielectric Droplet Manipulation

Handling the aqueous two-phase systems (ATPSs) formed by liquid-liquid phase separation (LLPS) relies on the accurate construction of binodal curves and tie-lines, which delineate the polymer concentrations required for phase separation and depict the properties of the resulting phases, respectively. Various techniques to determine the binodal curves and tie-lines of ATPSs exist, but most rely on manually pipetting relatively large volumes of fluids in a slow and tedious manner. We describe a method to determine ATPS binodals and tie-lines that overcomes these disadvantages: microscale droplet manipulation by electrowetting-on-dielectric (EWOD). EWOD enables automated handling of droplets in an optically transparent platform that allows for in situ droplet observation. Separated phases are clearly visible, and the volumes of each phase are readily determined. Additionally, in considering the molecular crowding present in living cells, this work examines the role of a macromolecule in prompting LLPS. These results show that EWOD-driven droplet manipulation effectively interrogates the phase dynamics of ATPSs and macromolecular crowding in LLPS.

Mobile Microfluidics

Microfluidics platforms can program small amounts of fluids to execute a bio-protocol, and thus, can automate the work of a technician and also integrate a large part of laboratory equipment. Although most microfluidic systems have considerably reduced the size of a laboratory, they are still benchtop units, of a size comparable to a desktop computer. In this paper, we argue that achieving true mobility in microfluidics would revolutionize the domain by making laboratory services accessible during traveling or even in daily situations, such as sport and outdoor activities. We review the existing efforts to achieve mobility in microfluidics, and we discuss the conditions mobile biochips need to satisfy. In particular, we show how we adapted an existing biochip for mobile use, and we present the results when using it during a train ride. Based on these results and our systematic discussion, we identify the challenges that need to be overcome at technical, usability and social levels. In analogy to the history of computing, we make some predictions on the future of mobile biochips. In our vision, mobile biochips will disrupt how people interact with a wide range of healthcare processes, including medical testing and synthesis of on-demand medicine.

Rapid generation of chemical combinations on a magnetic digital microfluidic array

Combinatorial screening is frequently used to identify chemicals with synergistic effects by measuring the response of biological entities exposed to various chemical-dose combinations. Conventional microwell-based combinatorial screening is resource-demanding, and the closed microfluidics-based screening requires sophisticated fluidic control systems. In this work, we present a novel combinatorial screening platform based on the surface energy trap (SET)-assisted magnetic digital microfluidics. This platform, known as FlipDrop, rapidly generates chemical combinations by coupling two droplet arrays with orthogonal chemical concentration gradients with a simple flip. We have illustrated the working principle of FlipDrop by generating combinations of quantum dots. We have also successfully demonstrated the screening of quantum dot fluorescence resonance energy transfer (QD-FRET) on the FlipDrop platform by measuring the FRET response. This report demonstrates that FlipDrop is capable of rapidly generating chemical combinations with unprecedented ease for combinatorial screening.

FlipDrop is a combinatorial screening platform. It enables rapid generation of chemical combinations by flipping and coupling two droplet arrays generated by surface energy traps on the magnetic digital microfluidic platform.

A review of sorting, separation and isolation of cells and microbeads for biomedical applications: microfluidic approaches

We have reviewed the microfluidic approaches for cell/particle isolation and sorting, and extensively explained the mechanism behind each method.

High throughput screening of complex biological samples with mass spectrometry – from bulk measurements to single cell analysis

We review the state of the art in HTS using mass spectrometry with minimal sample preparation from complex biological matrices. We focus on industrial and biotechnological applications.

The Many Roads to an Ideal Paper-based Device

The recent Zika and Ebola virus outbreaks highlight the need for low-cost diagnostics that can be rapidly deployed and used outside of established clinical infrastructure. This demand for robust point-of-care (POC) diagnostics is further driven by the increasing burden of drug-resistant diseases, concern for food and water safety, and bioterrorism. As has been discussed in previous chapters, paper-based tests provide a simple and compelling solution to such needs.