Understanding wax screen-printing: A novel patterning process for microfluidic cloth-based analytical devices

In this work, we first introduce the fabrication of microfluidic cloth-based analytical devices (μCADs) using a wax screen-printing approach that is suitable for simple, inexpensive, rapid, low-energy-consumption and high-throughput preparation of cloth-based analytical devices. We have carried out a detailed study on the wax screen-printing of μCADs and have obtained some interesting results. Firstly, an analytical model is established for the spreading of molten wax in cloth. Secondly, a new wax screen-printing process has been proposed for fabricating μCADs, where the melting of wax into the cloth is much faster (∼5 s) and the heating temperature is much lower (75 °C). Thirdly, the experimental results show that the patterning effects of the proposed wax screen-printing method depend to a certain extent on types of screens, wax melting temperatures and melting time. Under optimized conditions, the minimum printing width of hydrophobic wax barrier and hydrophilic channel is 100 μm and 1.9 mm, respectively. Importantly, the developed analytical model is also well validated by these experiments. Fourthly, the μCADs fabricated by the presented wax screen-printing method are used to perform a proof-of-concept assay of glucose or protein in artificial urine with rapid high-throughput detection taking place on a 48-chamber cloth-based device and being performed by a visual readout. Overall, the developed cloth-based wax screen-printing and arrayed μCADs should provide a new research direction in the development of advanced sensor arrays for detection of a series of analytes relevant to many diverse applications.     

Development of paper-based microfluidic analytical device for iron assay using photomask printed with 3D printer for fabrication of hydrophilic and hydrophobic zones on paper by photolithography

This paper describes a paper-based microfluidic analytical device for iron assay using a photomask printed with a 3D printer for fabrication of hydrophilic and hydrophobic zones on the paper by photolithography. Several designed photomasks for patterning paper-based microfluidic analytical devices can be printed with a 3D printer easily, rapidly and inexpensively. A chromatography paper was impregnated with the octadecyltrichlorosilane n-hexane solution and hydrophobized. After the hydrophobic zone of the paper was exposed to the UV light through the photomask, the hydrophilic zone was generated. The smallest functional hydrophilic channel and hydrophobic barrier were ca. 500 μm and ca. 100 μm in width, respectively. The fabrication method has high stability, resolution and precision for hydrophilic channel and hydrophobic barrier. This test paper was applied to the analysis of iron in water samples using a colorimetry with phenanthroline.     

Flexible microfluidic cloth-based analytical devices using a low-cost wax patterning technique

This paper describes the fabrication of microfluidic cloth-based analytical devices (μCADs) using a simple wax patterning method on cotton cloth for performing colorimetric bioassays. Commercial cotton cloth fabric is proposed as a new inexpensive, lightweight, and flexible platform for fabricating two- (2D) and three-dimensional (3D) microfluidic systems. We demonstrated that the wicking property of the cotton microfluidic channel can be improved by scouring in soda ash (Na(2)CO(3)) solution which will remove the natural surface wax and expose the underlying texture of the cellulose fiber. After this treatment, we fabricated narrow hydrophilic channels with hydrophobic barriers made from patterned wax to define the 2D microfluidic devices. The designed pattern is carved on wax-impregnated paper, and subsequently transferred to attached cotton cloth by heat treatment. To further obtain 3D microfluidic devices having multiple layers of pattern, a single layer of wax patterned cloth can be folded along a predefined folding line and subsequently pressed using mechanical force. All the fabrication steps are simple and low cost since no special equipment is required. Diagnostic application of cloth-based devices is shown by the development of simple devices that wick and distribute microvolumes of simulated body fluids along the hydrophilic channels into reaction zones to react with analytical reagents. Colorimetric detection of bovine serum albumin (BSA) in artificial urine is carried out by direct visual observation of bromophenol blue (BPB) colour change in the reaction zones. Finally, we show the flexibility of the novel microfluidic platform by conducting a similar reaction in a bent pinned μCAD.     

A simplified method for capillary embedment into microfluidic devices - exemplified by sol-gel-based preconcentration

We here describe an alternative method of embedding functionalized capillaries into microdevices fabricated in PDMS. The capillaries have square-shaped outer dimensions and fit into elastic PDMS channel networks of similar dimensions. By modifying the capillary off-chip, the technique makes it possible to integrate a new chip function without risking contamination of already existing chemically patterned areas because of new reagent solutions. Leak-free insertion of these capillaries has earlier been reported, where a thin layer of uncured PDMS bonded the capillary to the microchannel and the lid structure. In this new approach, oxygen plasma is used to bond the square capillary to the PDMS, eliminating the risk of clogging the microsystem with uncured prepolymer. The new embedding technique was exemplified and evaluated by inserting a square capillary piece containing monolithic sol-gel for sample preconcentration application. The assembled microdevice was run with mass spectrometric detection, showing that peptides were preconcentrated without leakage from either the sol-gel itself or around the inserted capillary. Repeated preconcentration runs showed migration times better than 3% for all peptides. We believe that the presented microchip assembling technique greatly simplifies the insertion of functionalized capillary pieces, e.g., an initial preconcentrator to a PDMS device containing other downstream modules.     

Inorganic polymer photoresist for direct ceramic patterning by photolithography

A novel negative, inorganic polymer photoresist was demonstrated to be suitable for simple and direct fabrication of tribological SiCN-based ceramic microstructures via UV photolithography and subsequent pyrolysis at 800 degrees C.     

A microfluidic photolithography for controlled encapsulation of single cells inside hydrogel microstructures

A microfluidic approach to generate hydrogel microstructures inside microchannels for controlled encapsulation of single cells was developed. The method was based on a modified microscope projection photolithography which allowed for the photopolymerization of poly(ethylene glycol) diacrylate (PEG-DA) inside microchannels. Uniform-sized hydrogel microstructures (~50 μm in diameter) were generated one by one with determined positions to encapsulate single cells without losing the viability. Cells of interest could be identified by any kinds of visible labels to be selectively encapsulated inside the formed hydrogel microstructures. Large-scale encapsulation of single cells was achieved with a relatively high efficiency of 80% and the viability of encapsulated cells could be guaranteed by removing the dead cells identified with Trypan blue. This method is simple, fast and convenient to pattern the microchannels with single cells for a wide range of cell-based applications. For demonstration, two intracellular enzyme assays of carboxylesterase were performed to investigate the distribution of enzyme concentrations and the kinetic information within the encapsulated single HepG2 cells.     

A passive pumping method for microfluidic devices

The surface energy present in a small drop of liquid is used to pump the liquid through a microchannel. The flow rate is determined by the volume of the drop present on the pumping port of the microchannel. A flow rate of 1.25 microL s(-1) is demonstrated using 0.5 microL drops of water. Two other fluid manipulations are demonstrated using the passive pumping method: pumping liquid to a higher gravitational potential energy and creating a plug within a microchannel.     

Comparison of monodisperse droplet generation in flow-focusing devices with hydrophilic and hydrophobic surfaces

A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.     

PDMS-based microfluidic device with multi-height structures fabricated by single-step photolithography using printed circuit board as masters

We have developed a method for fabricating microfluidic devices with multi-height structures using single step photolithography. The whole fabrication process is executed by conventional printed circuit board (PCB) technology without the need of having access to clean room facilities. Specifically designed "windows" and "rims" architectures were printed on films that were used as photomasks. Different levels of protruding features on the PCB master were produced by exposing a photomask followed by chemical wet etching. Poly(dimethylsiloxane) (PDMS) was then moulded against the positive relief master to generate microfluidic structures. In this report, we described the fabrication of a microfluidic device featured with a multi-height "sandbag" structure for particle entrapment and peripheral microchannels. Controlled immobilization of biological cells and immunocytochemcial staining assays were performed to demonstrate the applicability of the microfluidic device for cellular analysis. The integrity of the microdevice remained stable under applied pressure, indicating the robustness of the elastic PDMS structures for analytical operation. The simple microfabrication process requires only low-cost materials and minimal specialized equipment and can reproducibly produce mask lines of about 20 microm in width, which is sufficient for most microfluidic applications.     

From hydrophilic to hydrophobic wood using direct fluorination: A localized treatment

When controlled by the treatment duration or the quantity of reactive molecules, direct fluorination using F₂ gas is efficient to decrease the hydrophilicity of wood both in the form of massive piece and powder. To prove that silver fir pieces and wood flour (mix of spruce and silver fir species) were investigated as representative examples. In both the cases, hydroxyl groups are converted into CF bonds resulting in the decrease in affinity for water. Fluorination is mainly located in the outer parts of the wood cell, where the content of lignin is the highest, maintaining the inner ones nonmodified. Because the microstructure is maintained by this location of fluorination, the mechanical properties are conserved for silver fir pieces. The mechanical properties are even enhanced for composite containing fluorinated wood flour because of a better compatibility between the fluorinated fillers and the polyester matrix.     

Direct immobilization of DNA probes on non-modified plastics by UV irradiation and integration in microfluidic devices for rapid bioassay

DNA microarrays have become one of the most powerful tools in the field of genomics and medical diagnosis. Recently, there has been increased interest in combining microfluidics with microarrays since this approach offers advantages in terms of portability, reduced analysis time, low consumption of reagents, and increased system integration. Polymers are widely used for microfluidic systems, but fabrication of microarrays on such materials often requires complicated chemical surface modifications, which hinders the integration of microarrays into microfluidic systems. In this paper, we demonstrate that simple UV irradiation can be used to directly immobilize poly(T)poly(C)-tagged DNA oligonucleotide probes on many different types of plastics without any surface modification. On average, five- and fourfold improvement in immobilization and hybridization efficiency have been achieved compared to surface-modified slides with aminated DNA probes. Moreover, the TC tag only costs 30% of the commonly used amino group modifications. Using this microarray fabrication technique, a portable cyclic olefin copolymer biochip containing eight individually addressable microfluidic channels was developed and used for rapid and parallel identification of Avian Influenza Virus by DNA hybridization. The one-step, cost-effective DNA-linking method on non-modified polymers significantly simplifies microarray fabrication procedures and permits great flexibility to plastic material selection, thus making it convenient to integrate microarrays into plastic microfluidic systems.     

A review on recent developments for biomolecule separation at analytical scale using microfluidic devices

Microfluidic devices with their inherent advantages like the ability to handle 10⁻⁹ to 10⁻¹⁸ L volume, multiplexing of microchannels, rapid analysis and on-chip detection are proving to be efficient systems in various fields of life sciences. This review highlights articles published since 2010 that reports the use of microfluidic devices to separate biomolecules (DNA, RNA and proteins) using chromatography principles (size, charge, hydrophobicity and affinity) along with microchip capillary electrophoresis, isotachophoresis etc. A detailed overview of stationary phase materials and the approaches to incorporate them within the microchannels of microchips is provided as well as a brief overview of chemical methods to immobilize ligand(s). Furthermore, we review research articles that deal with microfluidic devices as analytical tools for biomolecule (DNA, RNA and protein) separation.     

Quantitative determination of surface energy using atomic force microscopy: the case of hydrophobic/hydrophobic contact and hydrophilic/hydrophilic contact

Atomic force microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. In order to achieve this aim, it was necessary to graft both the AFM tip and the substrate with the same chemical functional groups. Two different organothiols terminated either by hydrophilic or hydrophobic chemical functionalities were used. Grafting process classically reported shows that after UV/ozone treatment for 30 min, the tip is coated by thermal deposition with 4‐5‐nm‐thick titanium layer followed by a 30‐nm‐thick gold layer. Finally, the tip is grafted by organothiols. The thickness of the layer deposited on the tip is of the same order of magnitude as the tip radius. To avoid the use of Ti and to decrease the thickness of the gold layer, we have developed a new way of grafting by using organic molecules like (3‐mercaptopropyl)triethoxysilane (MPS) as a linkage agent. Then this way of grafting was checked. Finally, AFM force‐distance curves, between grafted tips and chemically modified surface, were carried out in contact mode. Calibration of the various parts of the apparatus and especially of the cantilever (spring constant and tip radius) is of major importance to reach quantitative data. Finally, by applying a suitable theory of contact, we were able to determine the surface energy of our system. Copyright © 2005 John Wiley & Sons, Ltd.     

Dissolution-guided wetting for microarray and microfluidic devices

The trapping of air bubbles presents a substantial impediment for the user in the increasingly widespread use of lab-on-a-chip products having microcavities in the forms of microwells, traps, dead ends and corners. Here we demonstrate a simple, effective, and passive method to eliminate air bubbles by coating hydrophilized microarray and microfluidic devices with a monosaccharide such as d-glucose or D-sorbitol, where the microcavities are filled with a conformal, elliptical, cone-shaped monosaccharide solid. These devices were stored in air for up to 6 months with a complete rewetting of the microcavities by dissolution of the monosaccharide with an aqueous solution.     

Patterned superhydrophobic paper for microfluidic devices obtained by writing and printing

This work outlines inexpensive patterning methodologies to create open-air microfluidic paper-based devices. A phase-separation methodology was used to obtain biomimetic superhydrophobic paper, hierarchically composed by micro and nano topographies. Writing and printing are simple actions that can be used to pattern flat superhydrophobic paper with more wettable channels. In particular, inkjet printing permits controlling the wettability of the surface by changing the darkness of the printed regions. The difference between capillary forces provides the possibility to control and drive liquid flows through the open path lines, just by titling the piece of paper. Additionally, maintaining a continuous flow, it is possible to direct the liquid at different volumetric rates in a horizontal position along non-linear channel paths printed/written over the surface.     

Fabrication of microfluidic devices using dry film photoresist for microchip capillary electrophoresis

An inexpensive, disposable microfluidic device was fabricated from a dry film photoresist using a combination of photolithographic and hot roll lamination techniques. A microfluidic flow pattern was prefabricated in a dry film photoresist tape using traditional photolithographic methods. This tape became bonded to a poly(methyl methacrylate) (PMMA) sheet with prepouched holes when passed through a hot roll laminator. A copper working electrode and platinum decoupler was readily incorporated within this microchip. The integrated microchip device was then fixed in a laboratory-built Plexiglas holder prior to its use in microchip capillary electrophoresis. The performance of this device with amperometric detection for the separation of dopamine and catechol was examined. The separation was complete within 50 s at an applied potential of 200 V/cm. The relative standard deviations (RSD) of analyte migration times were less than 0.71%, and the theoretical plate numbers for dopamine and catechol were 3.2 x 10(4) and 4.1 x 10(4), respectively, based on a 65 mm separation channel.     

Synthesis of agar microparticles using temperature-controlled microfluidic devices for Cordyceps militaris cultivation

A temperature-controlled microfluidic approach was developed for fabricating monodispersed agar beads with the potential to be a brand-new strategy for cultivating Cordyceps militaris. The proposed microfluidic system features a circulating water bath with precise temperature control (temperature deviation ?T<0.1°C). This device holds the promise of allowing us to develop a temperature-controlled system, characterized as simple, low cost, and easy to set up and use. The size-controllable agar beads were achieved by utilizing microfluidic emulsification in the cross-junction channel under temperature-controlled conditions. The flow conditions of the dispersed/continuous phases were adjusted to generate various sizes of agar beads. Our results show that the microparticles produced are as small as 176 μm with a 95% particle size distribution within 5 μm. The prepared agar microparticles performed well as a substrate for the cultivation of C. militaris. The proposed method could also be applied for encapsulating biomaterials, enzymes, drugs, catalysts, and nanoparticles into agar beads for biomedical applications.     

A lattice Boltzmann algorithm for electro-osmotic flows in microfluidic devices

In this paper, a finite-difference-based lattice Boltzmann (LB) algorithm is proposed to simulate electro-osmotic flows (EOF) with the effect of Joule heating. This new algorithm enables a nonuniform mesh to be adapted, which is desirable for handling the extremely thin electrical double layer in EOF. The LB algorithm has been validated by simulating a problem with an available analytical solution and it is found that the numerical results predicted by the algorithm are in good agreement with the analytical solution. The LB algorithm is also applied to modeling a mixed electro-osmotic/pressure driven flow in a channel. The numerical results show that Joule heating plays an important role in EOF.     

A fast and highly sensitive detection of cholesterol using polymer microfluidic devices and amperometric system

In this work, the rapid detection of cholesterol using poly(dimethylsiloxane) microchip capillary electrophoresis, based on the coupling of enzymatic assays and electrochemical detection, was developed. Direct amperometric detection for poly(dimethylsiloxane) (PDMS) microchip capillary electrophoresis was successfully applied to quantify cholesterol levels. Factors influencing the performance of the method (such as the concentration and pH value of buffer electrolyte, concentration of cholesterol oxidase enzyme (ChOx), effect of solvent on the cholesterol solubility, and interferences) were carefully investigated and optimized. The migration time of hydrogen peroxide, product of the reaction, was less than 100 s when using 40 mM phosphate buffer at pH 7.0 as the running buffer, a concentration of 0.68 U/mL of the ChOx, a separation voltage of +1.6 kV, an injection time of 20 s, and a detection potential of +0.5 V. PDMS microchip capillary electrophoresis showed linearity between 38.7 μg/dL (1 μM) and 270.6 mg/dL (7 mM) for the cholesterol standard; the detection limit was determined as 38.7 ng/dL (1 nM). To demonstrate the potential of this assay, the proposed method was applied to quantify cholesterol in bovine serum. The percentages of recoveries were assessed over the range of 98.9-101.8%. The sample throughput was found to be 60 samples per hour. Therefore, PDMS microchip capillary electrophoresis, based on the coupling of enzymatic assays and electrochemical detection, is very rapid, accurate and sensitive method for the determination of cholesterol levels.