Nanoliter Dispensing Method by Degassed Poly(dimethylsiloxane) Microchannels and Its Application in Protein Crystallization

Valved Microwell Array Platforms for Stepwise Liquid Dispensing

The fabrication of microarray chips and the precise dispensing of nanoliter to microliter liquids are fundamental for high-throughput parallel biochemical testing. Conventional microwells, typically featuring a uniform cross section, fill completely in a single operation, complicating the introduction of multiple reagents for stepwise and combinatorial analyses. To overcome this limitation, we developed an innovative valved microwell array. Using ultraviolet (UV)-curing resin three-dimensional (3D) printing, these multilayer configurations can be rapidly fabricated through direct template printing and polydimethylsiloxane (PDMS) casting. Each microwell incorporates a microvalve structure, truncating fluids within the upper metering well and allowing transfer to the bottom reservoir well under centrifugal force. Sequential operations enable the introduction of multiple reagents, facilitating orthogonal combinations for complex assays. We explored four types of valving methods: DeepWell, Expansion, Bottleneck, and Membrane valve, each offering varying degrees of design complexity, operational efficiency, robustness, and precision. These methods constitute a versatile toolkit to accommodate a broad spectrum of analytical requirements. Our innovative approach redefines microwell architecture, direct manufacturing techniques, and stepwise fluid dispensation in microarrays.

Microarray Platforms Based on 3D Printing

This paper introduces an innovative method for the fabrication and infusion of microwell arrays based on digital light processing (DLP) 3D printing. A low-cost DLP 3D printer is employed to fabricate microstructures rapidly with a broad dynamic range while maintaining high precision and fidelity. We constructed microwell arrays with varying diameters, from 200 to 2000 μm and multiple aspect ratios, in addition to microchannels with widths ranging from 45 to 1000 μm, proving the potential and flexibility of this fabrication method. The superimposition of parallel microchannels onto the microwell array, facilitated by positive or negative pressure, enabled the transfer of liquid to the microwells. Upon removal of the microchannel chip, a dispensed microdroplet array was obtained. This array can be modulated by adjusting the volume of the microwells and the inflow fluid. The filled microwell array allows chip-to-chip dispensing to the microreactor array through binding and centrifugation, facilitating multistep and multireagent assays. The 3D printing approach also enables the fabrication of intricate cavity designs, such as micropyramid arrays, which can be integrated with parallel microchannels to generate spheroid flowcells. This device demonstrated the ability to generate spheroids and manipulate their environment. We have successfully utilized precise modulation of spheroids size and performed parallel drug dose-response assays to evaluate its effectiveness. Furthermore, we managed to execute dynamic drug combinations based on a compact spheroids array, utilizing two orthogonal parallel microchannels. Our findings suggest that both the combination and temporal sequence of drug administration have a significant impact on therapeutic outcomes.

Nanoliter-scale liquid metering and droplet generation based on a capillary array for high throughput screening

Herein, we developed a simple approach for quantitative metering of nanoliter-scale liquids in parallel based on a capillary array and applied it in high throughput screening protein crystallization conditions. The quantitative metering of liquids was achieved by using capillary force to spontaneously introduce the liquids into short capillaries with fixed length and inner diameter, and the nanoliter-scale droplets were generated by using a pneumatic pump to deliver liquids out from the capillary channels. We adopted measures of sharpening the capillary tips and performing a hydrophobic treatment on the tip surface to significantly reduce the capillary residues during the liquid aspirating and dispensing process, and thus improved the precision to 0.2%-3.5% relative standard deviations (RSD, n = 3) in metering droplets in the range of 280 pL-90 nL. We evaluated the performance of the system in metering liquids of different surface tensions and viscosity. On the basis of this approach, we built a capillary array system with 12 capillaries, by which parallel generation of 12 nL droplets of 12 samples could be achieved in 40 s with a relative standard deviation (RSD) of 1.2%. We applied the system in the screening of lysozyme crystallization conditions of 48 precipitants with 7.5 nL precipitant and 7.5 nL protein solutions in each crystallization droplet reactor, to demonstrate its potentials in large-scale high-throughput screening and analysis with different samples.

Electrochemical Switching of Plasmonic Colors Based on Polyaniline-Coated Plasmonic Nanocrystals

Plasmonic color generation has attracted much research interest because of the unique optical properties of plasmonic nanocrystals that are promising for chromatic applications, such as flat-panel displays, smart windows, and wearable devices. Low-cost, monodisperse plasmonic nanocrystals supporting strong localized surface plasmon resonances are favorable for the generation of plasmonic colors. However, many implementations so far have either a single static state or complexities in the particle alignment and switching mechanism for generating multiple displaying states. Herein, we report on a facile and robust approach for realizing the electrochemical switching of plasmonic colors out of colloidal plasmonic nanocrystals. The metal nanocrystals are coated with a layer of polyaniline, whose refractive index and optical absorption are reversibly switched through the variation of an applied electrochemical potential. The change in refractive index and optical absorption results in the modulation of the plasmonic scattering intensity with a depth of 11 dB. The electrochemical switching process is fast (∼5 ms) and stable (over 1000 switching cycles). A device configuration is further demonstrated for switching plasmonic color patterns in a transparent electrochemical device, which is made from indium tin oxide electrodes and a polyvinyl alcohol solid electrolyte. Our control of plasmonic colors provides a favorable platform for engineering low-cost and high-performance miniaturized optical devices.

On-Demand Droplet Collection for Capturing Single Cells

Droplet-based microfluidic techniques are extensively used in efficient manipulation and genome-wide analysis of individual cells, probing the heterogeneity among populations of individuals. However, the extraction and isolation of single cells from individual droplets remains difficult due to the inevitable sample loss during processing. Herein, an automated system for accurate collection of defined numbers of droplets containing single cells is presented. Based on alternate sorting and dispensing in three branch channels, the droplet number can be precisely controlled down to single-droplet resolution. While encapsulating single cells and reserving one branch as a waste channel, sorting can be seamlessly integrated to enable on-demand collection of single cells. Combined with a lossless recovery strategy, this technique achieves capture and culture of individual cells with a harvest rate of over 95%. The on-demand droplet collection technique has great potential to realize quantitative processing and analysis of single cells for elucidating the role of cell-to-cell variations.

Artificial Cells Capable of Long-Lived Protein Synthesis by Using Aptamer Grafted Polymer Hydrogel

Herein we report a new type of artificial cells capable of long-term protein expression and regulation. We constructed the artificial cells by grafting anti-His-tag aptamer into the polymer backbone of the hydrogel particles, and then immobilizing the His-tagged proteinaceous factors of the transcription and translation system into the hydrogel particles. Long-term protein expression for at least 16 days was achieved by continuously flowing feeding buffer through the artificial cells. The effect of various metal ions on the protein expression in the artificial cells was investigated. Utilizing the lac operator-repressor system, we could regulate the expression level of eGFP in the artificial cells by controlling the β-D-1-thiogalatopyranoside (IPTG) concentration in the feeding buffer. The artificial cells based on the aptamer grafted hydrogel provide a useful platform for gene circuit engineering, metabolic engineering, drug delivery, and biosensors.

A hinge-based aligner for fast, large-scale assembly of microfluidic chips

Microfluidics has shown its vitality in scientific research. But the lack of fast and straightforward approaches for aligning chip and easy-to-control on-chip valve still prevent microfluidic chips from becoming powerful commercial products. This work presents an aligner based on hinge structures, which we call a "hinge aligner", for aligning microfluidic chips. Two flat chip holders are connected by a connecting rod so that the chip holders can rotate relative to each other along the connecting rod, in the way a hinge works. The two chip holders contain pre-designed recesses for placing two chips which can align chips with 20 μm resolution. Meanwhile, with this hinge aligner, we can easily implement a fully sealed on-chip valve, which can prevent aqueous liquids from leaking even at 80 °C for 30 min. The real immunoassay result shows aligned microfluidic chips can detect protein with improved reproducibility in both high and low concentration of biomarkers.

Emerin induces nuclear breakage in Xenopus extract and early embryos

Emerin is an inner nuclear membrane protein often mutated in Emery–Dreifuss muscular dystrophy. Because emerin has diverse roles in nuclear mechanics, cytoskeletal organization, and gene expression, it has been difficult to elucidate its contribution to nuclear structure and disease pathology. In this study, we investigated emerin’s impact on nuclei assembled in Xenopus laevis egg extract, a simplified biochemical system that lacks potentially confounding cellular factors and activities. Notably, these extracts are transcriptionally inert and lack endogenous emerin and filamentous actin. Strikingly, emerin caused rupture of egg extract nuclei, dependent on the application of shear force. In egg extract, emerin localized to nonnuclear cytoplasmic membranes, and nuclear rupture was rescued by targeting emerin to the nucleus, disrupting its membrane association, or assembling nuclei with lamin A. Furthermore, emerin induced breakage of nuclei in early-stage X. laevis embryo extracts, and embryos microinjected with emerin were inviable, with ruptured nuclei. We propose that cytoplasmic membrane localization of emerin leads to rupture of nuclei that are more sensitive to mechanical perturbation, findings that may be relevant to early development and certain laminopathies.

Patterning Hydrophobic Surfaces by Negative Microcontact Printing and Its Applications

Here, a negative microcontact printing method is developed to form hydrophilic polydopamine (PDA) patterns with micrometer resolution on hydrophobic including perfluorinated surfaces. In the process of the negative microcontact printing, a uniform PDA thin film is first formed on the hydrophobic surface. An activated polydimethylsiloxane (PDMS) stamp is then placed in contact with the PDA-coated hydrophobic surface. Taking advantage of the difference in the surface energy between the hydrophobic surface and the stamp, PDA is removed from the contact area after the stamp release. As a result, a PDA pattern complementary to the stamp is obtained on the hydrophobic surface. By using the negative microcontact printing, arrays of liquid droplets and single cells are reliably formed on perfluorinated surfaces. Microlens array with tunable focal length for imaging studies is further created based on the droplet array. The negative microcontact printing method is expected to be widely applicable in high-throughput chemical and biological screening and analysis.

Novel Substrates for Microarrays

In the microarray platform, the surface substrate is critical to the result quality in terms of signal consistency and detection sensitivity. Traditional substrates such as glass and nitrocellulose often entail complicated preparation processes such as the activation and functionalization of the reaction spots and surface blocking to prevent nonspecific molecule adsorption. In addition, coffee-ring morphology of the spots is a common issue in the traditional substrates. To address these issues, we introduced a novel substrate based on fluorinated ethylene propylene (FEP) membrane for microarrays.

Stacking chip for quantitative bioanalysis

This paper describes a microwell-based microdevice for performing quantitative bioanalysis. This microdevice combined the passive pumping by degassed polydimethylsiloxane (PDMS) with serial operations including solution dispensing, plates splitting and plates stacking. We name this microdevice "stacking chip". To use the stacking chip in quantitative bioanalysis, nanoliter solutions were first dispensed into the microwells through the degassed PDMS microchannels. Next, we split the microwell and microchannel plates assisted by the application of one drop of silicone oil, which resulted in a microwell array containing the reagent solutions. Microreactor arrays were formed by stacking the two microwell arrays containing the reagent solutions. With this microdevice, the enzymatic kinetics of alkaline phosphatase during the dissociation of the fluorescein diphosphate was measured and analyzed by the Michaelis-Menten model. The stacking chip is simple to fabricate and operate, and amenable to automation for high throughput analysis.

Hydrophilic Sponges for Leaf-Inspired Continuous Pumping of Liquids

A bio-inspired, leaf-like pumping strategy by mimicking the transpiration process through leaves is developed for autonomous and continuous liquid transport enabled by durable hydrophilic sponges. Without any external power sources, flows are continuously generated ascribed to the combination of capillary wicking and evaporation of water. To validate this method, durable hydrophilic polydimethylsiloxane sponges modified with polyvinyl alcohol via a "dip-coat-dry" method have been fabricated, which maintains hydrophilicity more than 2 months. The as-made sponges are further applied to achieve stable laminar flow patterns, chemical gradients, and "stop-flow" manipulation of the flow in microfluidic devices. More importantly, the ease-of-operation and excellent pumping capacity have also been verified with over 24 h's pumping and quasi-stable high flow rates up to 15 µL min-1. The present strategy can be easily integrated to other miniaturized systems requiring pressure-driven flow and should have potential applications, such as cell culture, micromixing, and continuous flow reaction.

3D-Printed High-Density Droplet Array Chip for Miniaturized Protein Crystallization Screening under Vapor Diffusion Mode

Here we describe the combination of three-dimensional (3D) printed chip and automated microfluidic droplet-based screening techniques for achieving massively parallel, nanoliter-scale protein crystallization screening under vapor diffusion mode. We fabricated high-density microwell array chips for sitting-drop vapor diffusion crystallization utilizing the advantage of the 3D-printing technique in producing high-aspect-ratio chips. To overcome the obstacle of 3D-printed microchips in performing long-term reactions caused by their porousness and gas permeability properties in chip body, we developed a two-step postprocessing method, including paraffin filling and parylene coating, to achieve high sealability and stability. We also developed a simple method especially suitable for controlling the vapor diffusion speed of nanoliter-scale droplets by changing the layer thickness of covering oil. With the above methods, 84 tests of nanoliter-scale protein crystallization under vapor diffusion mode were successfully achieved in the 7 × 12 droplet array chip with a protein consumption of 10 nL for each test, which is 20-100 times lower than that in the conventional large-volume screening system. Such a nanoliter-scale vapor diffusion system was applied to two model proteins with commercial precipitants and displayed advantages over that under microbatch mode. It identified more crystallization conditions, especially for the protein samples with lower concentrations.

Accelerated Biofluid Filling in Complex Microfluidic Networks by Vacuum-Pressure Accelerated Movement (V-PAM)

Rapid fluid transport and exchange are critical operations involved in many microfluidic applications. However, conventional mechanisms used for driving fluid transport in microfluidics, such as micropumping and high pressure, can be inaccurate and difficult for implementation for integrated microfluidics containing control components and closed compartments. Here, a technology has been developed termed Vacuum-Pressure Accelerated Movement (V-PAM) capable of significantly enhancing biofluid transport in complex microfluidic environments containing dead-end channels and closed chambers. Operation of the V-PAM entails a pressurized fluid loading into microfluidic channels where gas confined inside can rapidly be dissipated through permeation through a thin, gas-permeable membrane sandwiched between microfluidic channels and a network of vacuum channels. Effects of different structural and operational parameters of the V-PAM for promoting fluid filling in microfluidic environments have been studied systematically. This work further demonstrates the applicability of V-PAM for rapid filling of temperature-sensitive hydrogels and unprocessed whole blood into complex irregular microfluidic networks such as microfluidic leaf venation patterns and blood circulatory systems. Together, the V-PAM technology provides a promising generic microfluidic tool for advanced fluid control and transport in integrated microfluidics for different microfluidic diagnosis, organs-on-chips, and biomimetic studies.

Digital PCR using micropatterned superporous absorbent array chips

Digital PCR (dPCR) is an emerging technology for genetic analysis and clinical diagnostics. To facilitate the widespread application of dPCR, here we developed a new micropatterned superporous absorbent array chip (μSAAC) which consists of an array of microwells packed with highly porous agarose microbeads. The packed beads construct a hierarchically porous microgel which confers superior water adsorption capacity to enable spontaneous filling of PDMS microwells for fluid compartmentalization without the need of sophisticated microfluidic equipment and operation expertise. Using large λ-DNA as the model template, we validated the μSAAC for stochastic partitioning and quantitative digital detection of DNA molecules. Furthermore, as a proof-of-concept, we conducted dPCR detection and single-molecule sequencing of a mutation prevalent in blood cancer, the chromosomal translocation t(14;18), demonstrating the feasibility of the μSAAC for analysis of disease-associated mutations. These experiments were carried out using the standard molecular biology techniques and instruments. Because of its low cost, ease of fabrication, and equipment-free liquid partitioning, the μSAAC is readily adaptable to general lab settings, which could significantly facilitate the widespread application of dPCR technology in basic research and clinical practice.

Vacuum-driven power-free microfluidics utilizing the gas solubility or permeability of polydimethylsiloxane (PDMS)

Suitable pumping methods for flow control remain a major technical hurdle in the path of biomedical microfluidic systems for point-of-care (POC) diagnostics. A vacuum-driven power-free micropumping method provides a promising solution to such a challenge. In this review, we focus on vacuum-driven power-free microfluidics based on the gas solubility or permeability of polydimethylsiloxane (PDMS); degassed PDMS can restore air inside itself due to its high gas solubility or gas permeable nature. PDMS allows the transfer of air into a vacuum through it due to its high gas permeability. Therefore, it is possible to store or transfer air into or through the gas soluble or permeable PDMS in order to withdraw liquids into the embedded dead-end microfluidic channels. This article provides a comprehensive look at the physics of the gas solubility and permeability of PDMS, a systematic review of different types of vacuum-driven power-free microfluidics, and guidelines for designing solubility-based or permeability-based PDMS devices, alongside existing applications. Advanced topics and the outlook in using micropumping that utilizes the gas solubility or permeability of PDMS will be also discussed. We strongly recommend that microfluidics and lab-on-chip (LOC) communities harness vacuum energy to develop smart vacuum-driven microfluidic systems.

A flow-through holed PDMS membrane as a reusable microarray spotter for biomedical assays

We propose the exploitation of a holed-designed poly(dimethyl)siloxane (PDMS) membrane as an innovative microarray spotter. The membrane is fabricated by a simple technological approach and can be reused several times. A good level of reproducibility is found upon spotting fluorescent proteins at different concentrations over large areas.

An ultralow background substrate for protein microarray technology

A polydopamine modified fluoro-polymer provides a new microarray substrate with ultralow background and uniform spot morphology.

A robust microfluidic device for the synthesis and crystal growth of organometallic polymers with highly organized structures

A simple and robust microfluidic device was developed to synthesize organometallic polymers with highly organized structures. The device is compatible with organic solvents. Reactants are loaded into pairs of reservoirs connected by a 15 cm long microchannel prefilled with solvents, thus allowing long-term counter diffusion for self-assembly of organometallic polymers. The process can be monitored, and the resulting crystalline polymers are harvested without damage. The device was used to synthesize three insoluble silver acetylides as single crystals of X-ray diffraction quality. Importantly, for the first time, the single-crystal structure of silver phenylacetylide was determined. The reported approach may have wide applications, such as crystallization of membrane proteins, synthesis and crystal growth of organic, inorganic, and polymeric coordination compounds, whose single crystals cannot be obtained using traditional methods.

Recent developments in microfluidics for cell studies

As a technique for precisely manipulating fluid at the micrometer scale, the field of microfluidics has experienced an explosive growth over the past two decades, particularly owing to the advances in device design and fabrication. With the inherent advantages associated with its scale of operation, and its flexibility in being incorporated with other microscale techniques for manipulation and detection, microfluidics has become a major enabling technology, which has introduced new paradigms in various fields involving biological cells. A microfluidic device is able to realize functions that are not easily imaginable in conventional biological analysis, such as highly parallel, sophisticated high-throughput analysis, single-cell analysis in a well-defined manner, and tissue engineering with the capability of manipulation at the single-cell level. Major advancements in microfluidic device fabrication and the growing trend of implementing microfluidics in cell studies are presented, with a focus on biological research and clinical diagnostics.

Individually addressable arrays of replica microbial cultures enabled by splitting SlipChips

Isolating microbes carrying genes of interest from environmental samples is important for applications in biology and medicine. However, this involves the use of genetic assays that often require lysis of microbial cells, which is not compatible with the goal of obtaining live cells for isolation and culture. This paper describes the design, fabrication, biological validation, and underlying physics of a microfluidic SlipChip device that addresses this challenge. The device is composed of two conjoined plates containing 1000 microcompartments, each comprising two juxtaposed wells, one on each opposing plate. Single microbial cells are stochastically confined and subsequently cultured within the microcompartments. Then, we split each microcompartment into two replica droplets, both containing microbial culture, and then controllably separate the two plates while retaining each droplet within each well. We experimentally describe the droplet retention as a function of capillary pressure, viscous pressure, and viscosity of the aqueous phase. Within each pair of replicas, one can be used for genetic analysis, and the other preserves live cells for growth. This microfluidic approach provides a facile way to cultivate anaerobes from complex communities. We validate this method by targeting, isolating, and culturing Bacteroides vulgatus, a core gut anaerobe, from a clinical sample. To date, this methodology has enabled isolation of a novel microbial taxon, representing a new genus. This approach could also be extended to the study of other microorganisms and even mammalian systems, and may enable targeted retrieval of solutions in applications including digital PCR, sequencing, single cell analysis, and protein crystallization.

Toward integrated molecular diagnostic system (i MDx): principles and applications

Integrated molecular diagnostic systems ( iMDx), which are automated, sensitive, specific, user-friendly, robust, rapid, easy-to-use, and portable, can revolutionize future medicine. This review will first focus on the components of sample extraction, preservation, and filtration necessary for all point-of-care devices to include for practical use. Subsequently, we will look for low-powered and precise methods for both sample amplification and signal transduction, going in-depth to the details behind their principles. The final field of total device integration and its application to the clinical field will also be addressed to discuss the practicality for future patient care. We envision that microfluidic systems hold the potential to breakthrough the number of problems brought into the field of medical diagnosis today.

Liquid-liquid diffusion crystallization improves the X-ray diffraction of EndoS, an endo-β-N-acetylglucosaminidase from Streptococcus pyogenes with activity on human IgG

Endoglycosidase S (EndoS) is an enzyme secreted by Streptococcus pyogenes that specifically hydrolyzes the β-1,4-di-N-acetylchitobiose core glycan on immunoglobulin G (IgG) antibodies. One of the most common human pathogens and the cause of group A streptococcal infections, S. pyogenes secretes EndoS in order to evade the host immune system by rendering IgG effector mechanisms dysfunctional. On account of its specificity for IgG, EndoS has also been used extensively for chemoenzymatic synthesis of homogeneous IgG glycoprotein preparations and is being developed as a novel therapeutic for a wide range of autoimmune diseases. The structural basis of its enzymatic activity and substrate specificity, however, remains unknown. Here, the purification and crystallization of EndoS are reported. Using traditional hanging-drop and sitting-drop vapor-diffusion crystallization, crystals of EndoS were grown that diffracted to a maximum of 3.5 Å resolution but suffered from severe anisotropy, the data from which could only be reasonably processed to 7.5 Å resolution. When EndoS was crystallized by liquid-liquid diffusion, it was possible to grow crystals with a different space group to those obtained by vapor diffusion. Crystals of wild-type endoglycosidase and glycosynthase constructs of EndoS grown by liquid-liquid diffusion diffracted to 2.6 and 1.9 Å resolution, respectively, with a greatly diminished anisotropy. Despite extensive efforts, the failure to reproduce these liquid-liquid diffusion-grown crystals by vapor diffusion suggests that these crystallization methods each sample a distinct crystallization space.

A pneumatic valve controlled microdevice for bioanalysis

This paper describes a pneumatic valve controlled microdevice for performing mixing and reaction. This microdevice combined the degassed polydimethylsiloxane (PDMS) pumping method with a syringe-actuated valve system to control the dispensing and mixing of nanoliter solutions. The syringe was used to manually generate vacuum and to open the valves. Upon the opening of the valve, the microchamber was filled with the solution, which was driven by the external atmosphere through the degassed PDMS microchannel. With this microdevice, the enzymatic kinetics of alkaline phosphatase converting the fluorescein diphosphate was studied, and the Michaelis-Menten kinetics was analyzed. The microdevice has the advantages of simplicity and low cost in fabrication and operation.

High-throughput counter-diffusion capillary crystallization andin situdiffraction using high-pressure freezing in protein crystallography

Post-crystallization treatments such as manual fishing of crystals and soaking in cryoprotectant solutions, especially of large macromolecular complexes and membrane proteins, are cumbersome and often lead to crystal damage and reduced diffraction data quality. Here, a capillary crystallization plate is presented that simultaneously allows counter-diffusion crystallization at the nanolitre scale in a high-throughput screening mode, low-temperaturein situdiffraction data collection from crystals after cryoprotection and low-temperaturein situdata collection of crystals without the addition of any cryoprotectant after high-pressure (HP) freezing. The development of this plate and plunge cooling of crystals in the capillaries is a major step towards implementing automatedin situhigh-throughput crystal diffraction data collection at a synchrotron beamline. In combination with HP freezing this offers a new opportunity to obtain structural information from fragile crystals of supramolecular complexes that might otherwise not be feasible.

A "place n play" modular pump for portable microfluidic applications

This paper presents an easy-to-use, power-free, and modular pump for portable microfluidic applications. The pump module is a degassed particle desorption polydimethylsiloxane (PDMS) slab with an integrated mesh-shaped chamber, which can be attached on the outlet port of microfluidic device to absorb the air in the microfluidic system and then to create a negative pressure for driving fluid. Different from the existing monolithic degassed PDMS pumps that are generally restricted to limited pumping capacity and are only compatible with PDMS-based microfluidic devices, this pump can offer various possible configures of pumping power by varying the geometries of the pump or by combining different pump modules and can also be employed in any material microfluidic devices. The key advantage of this pump is that its operation only requires the user to place the degassed PDMS slab on the outlet ports of microfluidic devices. To help design pumps with a suitable pumping performance, the effect of pump module geometry on its pumping capacity is also investigated. The results indicate that the performance of the degassed PDMS pump is strongly dependent on the surface area of the pump chamber, the exposure area and the volume of the PDMS pump slab. In addition, the initial volume of air in the closed microfluidic system and the cross-linking degree of PDMS also affect the performance of the degassed PDMS pump. Finally, we demonstrated the utility of this modular pumping method by applying it to a glass-based microfluidic device and a PDMS-based protein crystallization microfluidic device.

Microfluidics using spatially defined arrays of droplets in one, two, and three dimensions

Spatially defined arrays of droplets differ from bulk emulsions in that droplets in arrays can be indexed on the basis of one or more spatial variables to enable identification, monitoring, and addressability of individual droplets. Spatial indexing is critical in experiments with hundreds to millions of unique compartmentalized microscale processes--for example, in applications such as digital measurements of rare events in a large sample, high-throughput time-lapse studies of the contents of individual droplets, and controlled droplet-droplet interactions. This review describes approaches for spatially organizing and manipulating droplets in one-, two-, and three-dimensional structured arrays, including aspiration, laminar flow, droplet traps, the SlipChip, self-assembly, and optical or electrical fields. This review also presents techniques to analyze droplets in arrays and applications of spatially defined arrays, including time-lapse studies of chemical, enzymatic, and cellular processes, as well as further opportunities in chemical, biological, and engineering sciences, including perturbation/response experiments and personal and point-of-care diagnostics.

Oil microsealing: a robust micro-compartmentalization method for on-chip chemical and biological assays

A simple and robust method to compartmentalize aqueous solutions into an array of independent microchambers is presented. The array of microchambers fabricated in poly(dimethylsiloxane) are filled with the sample solution through a microfluidic channel and then sealed with oil to isolate the microchambers from each other. A water reservoir close to the microchambers allows the maintainance and incubation of sub-nanoliter solutions (e.g., at 37 °C) within the chambers for hours without any problem of evaporation. Once assembled, the device is self-sustainable and can be used for different application purposes. As a demonstration, the device configuration is shown to be suitable for spatiotemporal control of the inner solution conditions by light stimulation through a photomask. This method was applied for the generation of regular EmGFP (emerald green fluorescent protein) expression arrays, selective photobleaching, photopatterning of calcium concentration, and cell culture in independent microchambers.

Protein crystallization using microfluidic technologies based on valves, droplets, and SlipChip

To obtain protein crystals, researchers must search for conditions in multidimensional chemical space. Empirically, thousands of crystallization experiments are carried out to screen various precipitants at multiple concentrations. Microfluidics can manipulate fluids on a nanoliter scale, and it affects crystallization twofold. First, it miniaturizes the experiments that can currently be done on a larger scale and enables crystallization of proteins that are available only in small amounts. Second, it offers unique experimental approaches that are difficult or impossible to implement on a larger scale. Ongoing development of microfluidic techniques and their integration with protein production, characterization, and in situ diffraction promises to accelerate the progress of structural biology.

Degassing-assisted patterning of cell culture surfaces

We developed an alternative patterning technique which is capable of producing both topographic and biochemical features for cell culture studies. This technique is based on microaspiration induced with a degassed poly (dimethylsiloxane) (PDMS) mold. After degassing in a rough vacuum chamber and placed on a sample surface, liquid solution can be aspired through channels and cavities created in the PDMS mold. Depending on the composition of the solution and the associated drying or incubation processes, a variety of surface patterns can be produced without applying external pressure. For demonstration, we fabricated agarose gel microwells and biomolecule patterns either on a glass plate or in a cell culture Petri dish, both applicable for cell culture studies.

Multiparameter screening on SlipChip used for nanoliter protein crystallization combining free interface diffusion and microbatch methods

This paper describes two SlipChip-based approaches to protein crystallization: a SlipChip-based free interface diffusion (FID) method and a SlipChip-based composite method that simultaneously performs microbatch and FID crystallization methods in a single device. The FID SlipChip was designed to screen multiple reagents, each at multiple diffusion equilibration times, and was validated by screening conditions for crystallization of two proteins, enoyl-CoA hydratase from Mycobacterium tuberculosis and dihydrofolate reductase/thymidylate synthase from Babesia bovis, against 48 different reagents at five different equilibration times each, consuming 12 microL of each protein for a total of 480 experiments using three SlipChips. The composite SlipChip was designed to screen multiple reagents, each at multiple mixing ratios and multiple equilibration times, and was validated by screening conditions for crystallization of two proteins, enoyl-CoA hydratase from Mycobacterium tuberculosis and dihydrofolate reductase/thymidylate synthase from Babesia bovis. To prevent cross-contamination while keeping the solution in the neck channels for FID stable, the plates of the SlipChip were etched with a pattern of nanowells. This nanopattern was used to increase the contact angle of aqueous solutions on the surface of the silanized glass. The composite SlipChip increased the number of successful crystallization conditions and identified more conditions for crystallization than separate FID and microbatch screenings. Crystallization experiments were scaled up in well plates using conditions identified during the SlipChip screenings, and X-ray diffraction data were obtained to yield the protein structure of dihydrofolate reductase/thymidylate synthase at 1.95 A resolution. This free-interface diffusion approach provides a convenient and high-throughput method of setting up gradients in microfluidic devices and may find additional applications in cell-based assays.

SlipChip

The SlipChip is a microfluidic device designed to perform multiplexed microfluidic reactions without pumps or valves. The device has two plates in close contact. The bottom plate contains wells preloaded with many reagents; in this paper plates with 48 reagents were used. These wells are covered by the top plate that acts as a lid for the wells with reagents. The device also has a fluidic path, composed of ducts in the bottom plate and wells in the top plate, which is connected only when the top and bottom plate are aligned in a specific configuration. Sample can be added into the fluidic path, filling both wells and ducts. Then, the top plate is "slipped", or moved, relative to the bottom plate so the complementary patterns of wells in both plates overlap, exposing the sample-containing wells of the top plate to the reagent-containing wells of the bottom plate, and enabling diffusion and reactions. Between the two plates, a lubricating layer of fluorocarbon was used to facilitate relative motion of the plates. This paper implements this approach on a nanoliter scale using devices fabricated in glass. Stability of preloaded solutions, control of loading, and lack of cross-contamination were tested using fluorescent dyes. Functionality of the device was illustrated via crystallization of a model membrane protein. Fabrication of this device is simple and does not require a bonding step. This device requires no pumps or valves and is applicable to resource-poor settings. Overall, this device should be valuable for multiplexed applications that require exposing one sample to many reagents in small volumes. One may think of the SlipChip as an easy-to-use analogue of a preloaded multi-well plate, or a preloaded liquid-phase microarray.

A chip-to-chip nanoliter microfluidic dispenser

A high-throughput microfluidic device is developed to handle liquid dispensation in nanoliter range. The dispenser system shows no cross-contamination between the microwells, indicating its great potential in large-scale screening experiments. An array of 115 nl PCR reactions, as well as the single channel addressable chip demonstrate the high flexibility and wide applications of this novel system.

Microfluidic crystallization

Microfluidics offers a wide range of new tools that permit one to revisit the formation of crystals in solution and yield insights into crystallization processes. We review such recent microfluidic devices and particularly emphasize lab-on-chips dedicated to the high-throughput screening of crystallization conditions of proteins with nanolitre consumption. We also thoroughly discuss the possibilities offered by the microfluidic tools to acquire thermodynamic and kinetic data that may improve industrial processes and shed a new light on nucleation and growth mechanisms.

A fast cell loading and high-throughput microfluidic system for long-term cell culture in zero-flow environments

We present a simple technique for cell loading, culturing, and phenotypic study in a multi-chamber microfluidic device made of polydimethylsiloxane (PDMS). This technique is based on the use of degassing induced aspiration of PDMS which allows loading cells into micro-cavities within 1 min. A large number of triangle cavities are patterned aside main flow channels with narrow connections so that cells can be loaded by aspirating into each cavity. In our device, high throughput and long-term monitoring can be done with minimum shear force of the flow. As a demonstration, we show a controlled loading at single cell level and the phenotypic variation of gene expression of the yeast strain w303 as a function of copper ion concentration of the medium.

Concentration control for protein crystallization via a continuously-fed crystallization chamber

A continuously-fed crystallization chamber that allows for kinetic path control through the crystallization phase diagram (from labile/nucleation to metastable/growth) was fabricated and used to crystallize lysozyme. A lumped kinetic model was developed, and parameters for heterogeneous nucleation kinetics were determined. Heterogeneous nucleation was found to have faster nucleation kinetics and slower growth kinetics than homogeneous nucleation, as expected. The major contributions of the new device are (1) to allow better control of the chemical environment for studies of crystal nucleation and growth, and (2) to allow lumped-model analysis of those studies to extract kinetic parameters.