Planar Microfluidic Devices Research Articles

Combining Hydrophilic and Hydrophobic Materials in 3D Printing for Fabricating Microfluidic Devices with Spatial Wettability

AbstractThe fabrication of microfluidic flow cells via projection micro‐stereolithography (PμSL) has excited researchers in recent years. However, due to the inherent process properties of most commercial PμSL, microfluidic devices are fabricated in a monolithic fashion with uniform material properties across a flow cell. Yet, the large surface‐to‐volume ratio in microfluidics demands to tailor microchannel surface properties—particularly in planar microchannel arrangements—with spatial control and micron‐scale resolution to form a desired flow profile, e.g., emulsion droplets. Here, the fabrication of planar microfluidic devices by PμSLbased 3D printing with spatial control over surface properties is presented. For that, homemade photopolymer formulations being either hydrophilic or hydrophobic are designed. Adding acrylic acid to a resin containing poly(ethylene glycol) diacrylate lowers the contact angle down to 0° against water creating a superhydrophilic surface. By utilizing 1H,1H,2H,2H‐perfluorodecyl acrylate, a photopolymer formulation allowing for 3D‐printing a hydrophobic microchannel surface with a contact angle >120° against water is obtained. Combining these two materials, microfluidic flow cells with spatially defined wettability are 3D‐printed for emulsion formation. Finally, the resin vat of the commercial PμSL printer is switched during the printing process for fabricating multimaterial geometries, as exemplarily applied for realizing a hydrophobic‐hydrophilic‐hydrophobic device for forming O/W/O double emulsions.

Effects of channel geometry and physicochemical properties of solutions on stable double emulsion production in planar microfluidic devices having triangular orifices

The planar microfluidic devices for producing double emulsions are beneficial in terms of accuracy and facility in fabrication. However, factors such as the flow rates, interfacial tensions, viscosities, channel geometry, and wettability of the devices affect the stability of the double emulsion production. In this study, we have focused on double emulsion production in a planar flow focusing device with triangle-shaped orifices. The local velocity in the channel can be controlled by modifying the channel design. Here, we have used two types of microfluidic devices with orifices and junctions of different shapes, denoted as mc-A and mc-B. By controlling the orifice angles and the width of the junctions, the stable flow regimes characterized in a capillary number space were expanded, and the production efficiency was increased. The effects of interfacial tensions of the sample solutions on the stability of double emulsion production were also examined. The double emulsions produced under stable conditions were highly uniform, and the diameter of the produced emulsions was well defined by the capillary numbers. However, the diameter of the double emulsions was mainly affected by the size of the orifice.

3D-printed planar microfluidic device on oxyfluorinated PET-substrate

The paper proposes the material extrusion additive manufacturing (via filament fabrication (FFF)) of planar microfluidics devices on the surface-modified PET-substrates production. The transformation of PET-substrates’ morphological structure under the action of a multi-gas-phase modifying mixture significantly improves the wetting and the adhesion of the PLA-filament to them. A significant change in the chemical composition and nanorelief of PET substrates' surfaces as a result of processing with a gas mixture based on helium, nitrogen, fluorine and oxygen is confirmed experimentally by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and fourie-transform infra-red spectroscopy (FTIR). The twelve-fold increase in the adhesion of the filament (measured) and the two-fold difference in the free surface energy values (calculated) for the original and the modified PET-substrates were observed. The functional characteristics' variances were accompanied with an approximately ten-fold increase in the degrees of heterogeneity for the carbon and oxygen concentration distributions and with the two-fold changes in the localization region and the average amplitude of the corresponding morphological spectra. The significant increase in the rate of mass transfer of an isotonic aqueous solution of sodium chloride through the surface microchannels of the constructed planar microfluidics device confirms the considerable changes in wettability of the modified PET substrates’ surfaces.

Lipid-Stabilized Double Emulsions Generated in Planar Microfluidic Devices

Microemulsions have found a wide range of applications exploiting their chemical and physical properties. Development of microfluidic-based approaches has allowed for the controlled production of highly monodispersed emulsions, including the formation of multiple and hierarchical emulsions. Conventional poly(dimethylsiloxane)-based microfluidic systems require tight spatial control over the surface chemistry when used for double emulsion generation, which can be challenging to achieve on the micrometer scale. Here, we present a two-dimensional device design, which can selectively be surface-treated in a straightforward manner and allows for the formation of uniform water/oil/water double emulsions by combining two distinct hydrophilic and hydrophobic surface properties. These surfaces are sufficiently separated in space to allow for imparting their functionalization without the requirement for lithographic approaches or complex flow control. We demonstrate that a mismatch between the wettability requirements of the continuous phase and the channel wall inherent in this approach can be tolerated over several hundreds of micrometers, opening up the possibility to use simple pressure-driven flows to achieve surface functionalization. The design architecture exhibits robust efficiency in emulsion generation while retaining simple device fabrication. We finally demonstrate the potential of this approach by generating water in oil in water emulsions with lipid molecules acting as surfactants.

Miniaturized Temperature-Controlled Planar Chromatography (Micro-TLC) as a Versatile Technique for Fast Screening of Micropollutants and Biomarkers Derived from Surface Water Ecosystems and During Technological Processes of Wastewater Treatment.

There is increasing interest in the development of simple analytical systems enabling the fast screening of target components in complex samples. A number of newly invented protocols are based on quasi separation techniques involving microfluidic paper-based analytical devices and/or micro total analysis systems. Under such conditions, the quantification of target components can be performed mainly due to selective detection. The main goal of this paper is to demonstrate that miniaturized planar chromatography has the capability to work as an efficient separation and quantification tool for the analysis of multiple targets within complex environmental samples isolated and concentrated using an optimized SPE method. In particular, we analyzed various samples collected from surface water ecosystems (lakes, rivers, and the Baltic Sea of Middle Pomerania in the northern part of Poland) in different seasons, as well as samples collected during key wastewater technological processes (originating from the "Jamno" wastewater treatment plant in Koszalin, Poland). We documented that the multiple detection of chromatographic spots on RP-18W microplates-under visible light, fluorescence, and fluorescence quenching conditions, and using the visualization reagent phosphomolybdic acid-enables fast and robust sample classification. The presented data reveal that the proposed micro-TLC system is useful, inexpensive, and can be considered as a complementary method for the fast control of treated sewage water discharged by a municipal wastewater treatment plant, particularly for the detection of low-molecular mass micropollutants with polarity ranging from estetrol to progesterone, as well as chlorophyll-related dyes. Due to the low consumption of mobile phases composed of water-alcohol binary mixtures (less than 1 mL/run for the simultaneous separation of up to nine samples), this method can be considered an environmentally friendly and green chemistry analytical tool. The described analytical protocol can be complementary to those involving classical column chromatography (HPLC) or various planar microfluidic devices.

High-throughput continuous production of liposomes using hydrodynamic flow-focusing microfluidic devices

The microfluidic hydrodynamic flow-focusing is a simple technique for nanoscale liposome formation that provides several advantages compared to the traditional manufacturing techniques. This work aimed to perform a systematic study of the liposome formation using planar microfluidic devices with different channel aspect-ratios, as an alternative to enhance the throughput of liposome synthesis. In general, liposomes with a low polydispersity and a precise control of the size were successfully produced from alteration of the flow rate ratio and channel aspect-ratio. The higher aspect-ratio enabled the most rapid generation of liposomes with similar diameter and significant lower polydispersity index than the obtained by other batch technique. Besides, β-carotene was successfully incorporated into liposomes with efficiency of approximately 60% and the incorporation ability was not specific to a choice of microfluidic device aspect-ratio. The results suggest that the use of microfluidic devices could be employed for liposome production with a possible advantage to minimize the degree of parallelization of processes. These results demonstrate the potential technical feasibility of microfluidic processes for future industrial applications.

Gellan microgels produced in planar microfluidic devices

Microgels were obtained from the droplets gelation of a water-in-oil emulsion containing gellan in the aqueous phase. Gellan gum (0.6% w/w) and calcium acetate (0.5 and 2.0% w/w) as gelling agent were used to produce the microgels. The effect of the continuous phase flow rate and salt concentration on the particle size and size polydispersity of the gellan microgels was evaluated, as well as microchannels configuration for encapsulation of hydrophilic compounds in these particles using planar microfluidics devices. The microgels exhibited uniform and spherical shape. A highly monodisperse particle size distribution was observed showing coefficient of variation below 5% and average size ranging from 63 to 113 μm. Increasing salt concentration caused an increase of the mean particle diameter and a decrease of the size polydispersity. However at high salt concentration the continuous flow rate did not exert significant effect on the microgel size since average size ranged from 106 to 113 μm. A potential use of gellan microgels as encapsulating matrix of active compounds was evaluated by adding a hydrophilic dye, Rhodamine B, in the aqueous phase. Results showed good retention capacity of the dye and stability over time of storage, indicating that microgels obtained by microfluidics technique may be used for the encapsulation of hydrophilic compounds, including those sensitive to temperature and pressure, as well as in drug delivery.

Flow environment and matrix structure interact to determine spatial competition in Pseudomonas aeruginosa biofilms.

Bacteria often live in biofilms, which are microbial communities surrounded by a secreted extracellular matrix. Here, we demonstrate that hydrodynamic flow and matrix organization interact to shape competitive dynamics in Pseudomonas aeruginosa biofilms. Irrespective of initial frequency, in competition with matrix mutants, wild-type cells always increase in relative abundance in planar microfluidic devices under simple flow regimes. By contrast, in microenvironments with complex, irregular flow profiles - which are common in natural environments - wild-type matrix-producing and isogenic non-producing strains can coexist. This result stems from local obstruction of flow by wild-type matrix producers, which generates regions of near-zero shear that allow matrix mutants to locally accumulate. Our findings connect the evolutionary stability of matrix production with the hydrodynamics and spatial structure of the surrounding environment, providing a potential explanation for the variation in biofilm matrix secretion observed among bacteria in natural environments.

Smart design of wettability-patterned gradients on substrate-independent coated surfaces to control unidirectional spreading of droplets.

Highly adherent wettability patterns on the substrate-independent superhydrophobic surfaces of trimethoxyoctadecylsilane modified titanium dioxide (TiO2)-based coatings were prepared by using commercial photolithography. Three custom unidirectional channels with gradient wettability patterns were obtained by spatially selective wettability conversion from superhydrophobic to superhydrophilic when the coatings were exposed to ultraviolet light (∼365 nm). The movement behavior of droplets on these unidirectional channels was studied and the displacement of droplet movement was effectively controlled. Integrating the idea of gradient wettability patterns into planar microfluidic devices (microreactors), a self-driven fluid transport was achieved to realize droplet metering, merging or reaction, and rapid transport. This self-driven fluid transport with gradient wettability patterns has great potential in fabricating a new category of pump-free microfluidic systems that can be used in various conditions.

Microfluidic step-emulsification in axisymmetric geometry.

Biphasic step-emulsification (Z. Li et al., Lab Chip, 2015, 15, 1023) is a promising microfluidic technique for high-throughput production of μm and sub-μm highly monodisperse droplets. The step-emulsifier consists of a shallow (Hele-Shaw) microchannel operating with two co-flowing immiscible liquids and an abrupt expansion (i.e., step) to a deep and wide reservoir. Under certain conditions the confined stream of the disperse phase, engulfed by the co-flowing continuous phase, breaks into small highly monodisperse droplets at the step. Theoretical investigation of the corresponding hydrodynamics is complicated due to the complex geometry of the planar device, calling for numerical approaches. However, direct numerical simulations of the three dimensional surface-tension-dominated biphasic flows in confined geometries are computationally expensive. In the present paper we study a model problem of axisymmetric step-emulsification. This setup consists of a stable core-annular biphasic flow in a cylindrical capillary tube connected co-axially to a reservoir tube of a larger diameter through a sudden expansion mimicking the edge of the planar step-emulsifier. We demonstrate that the axisymmetric setup exhibits similar regimes of droplet generation to the planar device. A detailed parametric study of the underlying hydrodynamics is feasible via inexpensive (two dimensional) simulations owing to the axial symmetry. The phase diagram quantifying the different regimes of droplet generation in terms of governing dimensionless parameters is presented. We show that in qualitative agreement with experiments in planar devices, the size of the droplets generated in the step-emulsification regime is independent of the capillary number and almost insensitive to the viscosity ratio. These findings confirm that the step-emulsification regime is solely controlled by surface tension. The numerical predictions are in excellent agreement with in-house experiments with the axisymmetric step-emulsifier.

Fabrication of lateral porous silicon membranes for planar microfluidics by means of ion implantation

We introduce a new fabrication method based on ion implantation to create lateral porous silicon membranes and integrate them into planar microfluidic devices. Our proposed method relies on the fact that the formation of porous silicon by anodization highly depends on the dopant type and concentration, which can be manipulated by ion implantation. In order to confine the porosification at desired locations within silicon steps bridging microchannels, we use boron and phosphorus implantation to respectively create a p++ layer buried in an n-type silicon substrate, and a protective n-type surficial layer. The use of a metal electrode patterned onto the silicon step for current injection during anodization enables pores to propagate laterally during the membrane formation. The optimal implantation doses and energies leading to the required boron and phosphorus profiles are determined by means of process simulation and further confirmed by SIMS analysis. We demonstrate that the proposed fabrication process leads to the creation of lateral porous silicon membranes with open-ended pores adequately bridging microchannels and that we are able to manipulate the pore size (∼3–30nm) and membrane porosity (∼15–65%) by adjusting the current density during anodization. The adequate dead-end filtration capability of the fabricated membranes was tested and demonstrates the interest of the presented fabrication process for microfluidic applications.

Using Adhesive Patterning to Construct 3D Paper Microfluidic Devices.

We demonstrate the use of patterned aerosol adhesives to construct both planar and nonplanar 3D paper microfluidic devices. By spraying an aerosol adhesive through a metal stencil, the overall amount of adhesive used in assembling paper microfluidic devices can be significantly reduced. We show on a simple 4-layer planar paper microfluidic device that the optimal adhesive application technique and device construction style depends heavily on desired performance characteristics. By moderately increasing the overall area of a device, it is possible to dramatically decrease the wicking time and increase device success rates while also reducing the amount of adhesive required to keep the device together. Such adhesive application also causes the adhesive to form semi-permanent bonds instead of permanent bonds between paper layers, enabling single-use devices to be non-destructively disassembled after use. Nonplanar 3D origami devices also benefit from the semi-permanent bonds during folding, as it reduces the likelihood that unrelated faces may accidently stick together. Like planar devices, nonplanar structures see reduced wicking times with patterned adhesive application vs uniformly applied adhesive.

An optimised detector for in-situ high-resolution NMR in microfluidic devices

Integration of high-resolution nuclear magnetic resonance (NMR) spectroscopy with microfluidic lab-on-a-chip devices is challenging due to limited sensitivity and line broadening caused by magnetic susceptibility inhomogeneities. We present a novel double-stripline NMR probe head that accommodates planar microfluidic devices, and obtains the NMR spectrum from a rectangular sample chamber on the chip with a volume of 2μl. Finite element analysis was used to jointly optimise the detector and sample volume geometry for sensitivity and RF homogeneity. A prototype of the optimised design has been built, and its properties have been characterised experimentally. The performance in terms of sensitivity and RF homogeneity closely agrees with the numerical predictions. The system reaches a mass limit of detection of 1.57nmols, comparing very favourably with other micro-NMR systems. The spectral resolution of this chip/probe system is better than 1.75Hz at a magnetic field of 7T, with excellent line shape.

Digital Microfluidic System with Vertical Functionality

Digital (droplet) microfluidics (DµF) is a powerful platform for automated lab-on-a-chip procedures, ranging from quantitative bioassays such as RT-qPCR to complete mammalian cell culturing. The simple MEMS processing protocols typically employed to fabricate DµF devices limit their functionality to two dimensions, and hence constrain the applications for which these devices can be used. This paper describes the integration of vertical functionality into a DµF platform by stacking two planar digital microfluidic devices, altering the electrode fabrication process, and incorporating channels for reversibly translating droplets between layers. Vertical droplet movement was modeled to advance the device design, and three applications that were previously unachievable using a conventional format are demonstrated: (1) solutions of calcium dichloride and sodium alginate were vertically mixed to produce a hydrogel with a radially symmetric gradient in crosslink density; (2) a calcium alginate hydrogel was formed within the through-well to create a particle sieve for filtering suspensions passed from one layer to the next; and (3) a cell spheroid formed using an on-chip hanging-drop was retrieved for use in downstream processing. The general capability of vertically delivering droplets between multiple stacked levels represents a processing innovation that increases DµF functionality and has many potential applications.

Piston-cylinder based micro liquid–liquid extraction with GC–qMS for trace analysis of targeted chlorinated organic compounds in water

A fast and reliable approach for the measurement of sub parts-per-billion levels of targeted chlorinated compounds like tetrachloroethane, hexachloroethane, hexachlorobutadiene, pentachlorobenzene, and hexachlorobenzene in various water matrices such as waste water is described. The method employed a novel piston-cylinder-based micro liquid–liquid extraction technique using hexane as an extraction solvent. The device, known commercially as the MIXXOR, substantially accelerates extraction time by a factor of more than 100 times and reduces solvent consumption by a factor of 25 times when compared with the solvent extraction technique using wrist-action mechanical agitation. A recently introduced 6% cyanopropylphenyl –94% dimethylpolysiloxane capillary column offering a high degree of inertness was used for the separation of the analytes. A quadrupole mass spectrometer equipped with a triple-axis detector was also employed to enhance the instrument detection limit. With this technique, a complete separation for the analytes in water can be conducted in less than 10 min using a three-port SilFlow planar microfluidic device for back-flushing. Repeatability of retention times for all compounds were found to be less than 0.04% (n = 10). The compounds cited can be analyzed from 1 ng/mL to 10 μg/mL, with a detection limit and correlation coefficient of at least 0.5 ng/mL and 0.999, respectively. A relative precision of less than 1.2% relative standard deviation (RSD) (n = 20) at the 50 ng/mL level, with analyte recovery of greater than 99% (n = 3) from 10 ng/mL to 10 μg/mL, was obtained.

Back-flushing and heart cut capillary gas chromatography using planar microfluidic Deans’ switching for the separation of benzene and alkylbenzenes in industrial samples

Planar microfluidic devices coupled with modern electronic pressure control have allowed gas chromatography (GC) practitioners to easily manipulate chromatographic systems to achieve heart cut and back-flushing configurations. These planar microfluidic devices have enhanced the connectivity between different components of GC instrumentation and have improved the inertness and minimised system dead volumes compared to classical chromatographic unions and valves. In the present contribution the setup and configuration of two multidimensional GC (MDGC) platforms is described for achieving the separation and quantification of trace level target C6–C8 alkylbenzenes in styrene monomer and Isoparaffin™ solvents, using flame ionisation detection (FID). The performance of these MDGC platforms indicated excellent retention time (0.2% relative standard deviation, RSD) and peak area repeatability (1% RSD) for all analytes of interest. The limit of detection (LOD) was 0.8mgkg−1 for benzene in styrene monomer, and 2.4–2.8mgkg−1 for C6–C8 alkylbenzenes such as benzene, toluene, ethylbenzene and xylene in Isoparaffin™ solvent.

Determination of furfurals in Manuka honey using piston-cylinder liquid–liquid extraction and gas chromatography

A rapid analytical approach for the direct measurement of furfurals such as 2-furfural and 5-methyl-2-furfural at parts-per-billion level in Manuka honey is described. The approach employs a piston-cylinder based liquid–liquid extraction device using chloroform extraction solvent. This device substantially reduces extraction time by a factor of 120 times compared to solid phase micro-extraction and reduces solvent consumption by a factor of 25 times compared to liquid–liquid extraction with mechanical agitation. A recently commercialised capillary column offering a high degree of inertness permits separation and detection of the analytes at ultra-trace level without derivatisation. A three-port planar microfluidic device with a mid-point pressure is also incorporated to back-flush heavier compounds in the matrix to improve column longevity and overall system cleanliness. With this approach, analysis is conducted in less than 7min. Repeatability of retention times for all compounds is less than 0.1% (n=20). The compounds cited can be analysed over a range from 1ng/g to 10μg/g in honey with a 5ng/g limit of quantification (LOQ) and correlation coefficients of at least 0.999. Relative precision is less than 2.8% RSD (n=20) at 50ng/g level with analyte extraction efficiency of greater than 99% (n=3) over a range from 5ng/g to 10μg/g in the matrix described. The analytical system requires only minimal maintenance and is suitable for remote site deployment. Under the analytical conditions established and with a practical LOQ of 5ng/g, 100 samples can be analysed before septum/liner/o-ring replacements are needed. As a preventive measure, the pre-column can be replaced once every six months to maintain chromatographic fidelity.

Cell-Free Microfluidic Determination of P-glycoprotein Interactions with Substrates and Inhibitors

The membrane protein P-glycoprotein (P-gp) plays key roles in the oral bioavailability of drugs, their blood brain barrier passage as well as in multidrug resistance. For new drug candidates it is mandatory to study their interaction with P-gp, according to FDA and EMA regulations. The vast majority of these tests are performed using confluent cell layers of P-gp overexpressing cell lines that render these tests laborious. In this study, we introduce a cell-free microfluidic assay for the rapid testing of drug- P-gp interactions. Cell-derived vesicles are prepared from MDCKII-MDR1 overexpressing cells and immobilized on the surface of a planar microfluidic device. The drug is delivered continuously to the vesicles and calcein accumulation is monitored by means of a fluorescence assay and total internal reflection fluorescence (TIRF) microscopy. Only small amounts of compounds (~10μl) are required in concentrations of 5, 25 and 50μM for a test that provides within 5min information on the apparent dissociation constant of the drug and P-gp. We tested 10 drugs on-chip, 9 of which are inhibitors or substrates of P-glycoprotein and one negative control. We benchmarked the measured apparent dissociation constants against an alternative assay on a plate reader and reference data from FDA. These comparisons revealed good correlations between the logarithmic apparent dissociation constants (R(2) = 0.95 with ATPase assay, R(2) = 0.93 with FDA data) and show the reliability of the rapid on-chip test. The herein presented assay has an excellent screening window factor (Z'-factor) of 0.8, and is suitable for high-throughput testing.

Three-Dimensional Hydrodynamic Focusing Method for Polyplex Synthesis

Successful intracellular delivery of nucleic acid therapeutics relies on multiaspect optimization, one of which is formulation. While there has been ample innovation on chemical design of polymeric gene carriers, the same cannot be said for physical processing of polymer-DNA nanocomplexes (polyplexes). Conventional synthesis of polyplexes by bulk mixing depends on the operators' experience. The poorly controlled bulk mixing process may also lead to batch-to-batch variation and consequent irreproducibility. Here, we synthesize polyplexes by using a three-dimensional hydrodynamic focusing (3D-HF) technique in a single-layered, planar microfluidic device. Without any additional chemical treatment or postprocessing, the polyplexes prepared by the 3D-HF method show smaller size, slower aggregation rate, and higher transfection efficiency, while exhibiting reduced cytotoxicity compared to the ones synthesized by conventional bulk mixing. In addition, by introducing external acoustic perturbation, mixing can be further enhanced, leading to even smaller nanocomplexes. The 3D-HF method provides a simple and reproducible process for synthesizing high-quality polyplexes, addressing a critical barrier in the eventual translation of nucleic acid therapeutics.

Multidimensional GC using planar microfluidic devices for the characterization of phenolic antioxidants in fuels

A multidimensional gas chromatographic approach using planar microfluidic devices for Deans switching has been developed and implemented for the characterization of sterically hindered phenolic compounds used as antioxidants in fuels. Detection and quantitation was conducted with MS in selected ion monitoring mode. A complete analysis is conducted in less than 15 min with precision greater than 5.5% at 1 and 25 ppm w/w (ppm(w)). LODs of 50 ppb w/w (ppb(w)) or better in selected ion monitoring mode and a linear range of 100 ppb(w) to 100 ppm(w) with a correlation coefficient greater than 0.998 were attained for all analytes. Unique to this analytical configuration is the use of a mass spectrometer capable of monitoring the column effluent from either dimension by incorporating a high-temperature rotary valve and a three-port planar microfluidic device. High-molecular-weight (C25-C40) fuel contaminants eluting from the first column can be selectively sent to the mass spectrometer for profile characterization in scan mode. These compounds would otherwise be retained substantially by the low-phase-ratio analytical column employed in the second dimension.