Facile single step fabrication of microchannels with varying size

Bulk Polymer-Derived Ceramic Composites of Graphene Oxide

Bulk polymer-derived ceramic (PDC) composites of SiCO with an embedded graphene network were produced using graphene-coated poly(vinyl alcohol) (PVA) foams as templates. The pyrolysis of green bodies containing cross-linked polysiloxane, PVA foams, and graphene oxide (GO) resulted in the decomposition of PVA foams, compression of GO layers, and formation of graphitic domains adjacent to GO within the SiCO composite, leading to SiCO composites with an embedded graphene network. The SiCO/GO composite, with about 1.5% GO in the ceramic matrix, offered an increase in the electrical conductivity by more than 4 orders of magnitude compared to that of pure SiCO ceramics. Additionally, the unique graphene network in the SiCO demonstrated a drop in the observed thermal conductivity of the composite (∼0.8 W m^-1 K^-1). Young's modulus of the as-fabricated SiCO/GO composites was found to be around 210 MPa, which is notably higher than the reported values for similar composites fabricated from only ceramic precursors and PVA foams. The present approach demonstrates a facile and cost-effective method of producing bulk PDC composites with high electrical conductivity, good thermal stability, and low thermal conductivity.

A one-step strategy for ultra-fast and low-cost mass production of plastic membrane microfluidic chips

An ultra-fast, extremely cost-effective, and environmentally friendly method was developed for fabricating flexible microfluidic chips with plastic membranes. With this method, we could fabricate plastic microfluidic chips rapidly (within 12 seconds per piece) at an extremely low cost (less than $0.02 per piece). We used a heated perfluoropolymer perfluoroalkoxy (often called Teflon PFA) solid stamp to press a pile of two pieces of plastic membranes, low density polyethylene (LDPE) and polyethylene terephthalate (PET) coated with an ethylene-vinyl acetate copolymer (EVA). During the short period of contact with the heated PFA stamp, the pressed area of the membranes permanently bonded, while the LDPE membrane spontaneously rose up at the area not pressed, forming microchannels automatically. These two regions were clearly distinguishable even at the micrometer scale so we were able to fabricate microchannels with widths down to 50 microns. This method combines the two steps in the conventional strategy for microchannel fabrication, generating microchannels and sealing channels, into a single step. The production is a green process without using any solvent or generating any waste. Also, the chips showed good resistance against the absorption of Rhodamine 6G, oligonucleotides, and green fluorescent protein (GFP). We demonstrated some typical microfluidic manipulations with the flexible plastic membrane chips, including droplet formation, on-chip capillary electrophoresis, and peristaltic pumping for quantitative injection of samples and reagents. In addition, we demonstrated convenient on-chip detection of lead ions in water samples by a peristaltic-pumping design, as an example of the application of the plastic membrane chips in a resource-limited environment. Due to the high speed and low cost of the fabrication process, this single-step method will facilitate the mass production of microfluidic chips and commercialization of microfluidic technologies.

Polymer-Derived Boron Nitride: A Review on the Chemistry, Shaping and Ceramic Conversion of Borazine Derivatives

Boron nitride (BN) is a III-V compound which is the focus of important research since its discovery in the early 19th century. BN is electronic to carbon and thus, in the same way that carbon exists as graphite, BN exists in the hexagonal phase. The latter offers an unusual combination of properties that cannot be found in any other ceramics. However, these properties closely depend on the synthesis processes. This review states the recent developments in the preparation of BN through the chemistry, shaping and ceramic conversion of borazine derivatives. This concept denoted as Polymer-Derived Ceramics (PDCs) route allows tailoring the chemistry of precursors to elaborate complex BN shapes which cannot be obtained by conventional process. The effect of the chemistry of the molecular precursors, i.e., borazine and trichloroborazine, and their polymeric derivatives i.e., polyborazylene and poly[tri(methylamino)borazine], in which the specific functional groups and structural motifs determine the shaping potential by conventional liquid-phase process and plastic-forming techniques is discussed. Nanotubes, nano-fibers, coatings, monoliths and fiber-reinforced matrix composites are especially described. This leads to materials which are of significant engineering interest.

A pressure-tolerant polymer microfluidic device fabricated by the simultaneous solidification-bonding method and flash chemistry application

Pressure-tolerant polymer-glass microfluidic reactors with excellent bonding strength have been fabricated by the simultaneous solidification-bonding (SSB) method, in which a viscous and reactive matrix polymer was cast on the glass substrate with pre-patterned wax as a sacrificial template. Elaborate interfacial chemistry between the matrix polymer and the functionalized glass surface was designed to achieve simultaneous solidification and chemical bonding under UV or/and mild thermal conditions (<200 °C with no pressure). Highly pressure-tolerant microchannels were obtained by complete removal of the liquid wax template at 80 °C. Versatility was demonstrated by fabricating microreactors from various polymers with different interfacial chemistry, which were all stable at 1000 psi with the highest burst pressure of 2000 psi. In particular, the fluoropolymer-glass microreactor can withstand a burst pressure that is two orders of magnitude higher than that of the microchannel made by the conventional method. Finally, the polymer-glass microfluidic device was used for the synthesis of a natural product, tryptanthrin, by flash chemistry under high pressure induced conditions (synthetic yield: 90%, flow rate: 10.5 mL min(-1), reaction time: 14 ms). The transparent microfluidic device can be used as a useful platform for miniaturizing spectroscopic tools for chemical analysis studies under high pressure conditions.

Foil assisted replica molding for fabrication of microfluidic devices and their application in vitro

We present a simple, rapid, benchtop, Foil Assisted Rapid Molding (FARM) method for the fabrication of microfluidic devices. This novel technique involves the use of aluminium foil, pen and an X-Y plotter to create semi-circular or plano-concave, shallow microchannels. It is an easy do-it-yourself (DIY) technique for creating a microfluidic device in three simple steps: (1) create a channel design using the CAD software, (2) plot the patterns on aluminium foil and (3) use the reverse of the engraved foil as a mold to create microfluidic devices. In this report, we present a detailed study of the proposed method by varying a range of parameters such as foil thickness, tip material, and tip sizes and by investigating their effect on the creation of channels with varying geometry. Furthermore, we demonstrated the cytocompatibility of these devices in vitro.

Whole ceramic-like microreactors from inorganic polymers for high temperature or/and high pressure chemical syntheses

Two types of whole ceramic-like microreactors were fabricated from inorganic polymers, polysilsesquioxane (POSS) and polyvinylsilazane (PVSZ), that were embedded with either perfluoroalkoxy (PFA) tube or polystyrene (PS) film templates, and subsequently the templates were removed by physical removal (PFA tube) or thermal decomposition (PS). A POSS derived ceramic-like microreactor with a 10 cm long serpentine channel was obtained by an additional "selective blocking of microchannel" step and subsequent annealing at 300 °C for 1 h, while a PVSZ derived ceramic-like microreactor with a 14 cm long channel was yielded by a co-firing process of the PVSZ-PS composite at 500 °C for 2 h that led to complete decomposition of the film template leaving a microchannel behind. The obtained whole ceramic-like microfluidic devices revealed excellent chemical and thermal stabilities in various solvents, and they were able to demonstrate unique chemical performance at high temperature or/and high pressure conditions such as Michaelis-Arbuzov rearrangement at 150-170 °C, Wolff-Kishner reduction at 200 °C, synthesis of super-paramagnetic Fe3O4 nanoparticles at 320 °C and isomerisation of allyloxybenzene to 2-allylphenol (250 °C and 400 psi). These economic ceramic-like microreactors fabricated by a facile non-lithographic method displayed excellent utility under challenging conditions that is superior to any plastic microreactors and comparable to glass and metal microreactors with high cost.

Screen printing of solder resist as master substrates for fabrication of multi-level microfluidic channels and flask-shaped microstructures for cell-based applications

Although silicon technology can be adopted for the fabrication of microfluidic devices with high precision, the capital and operating costs for such technology is often prohibitively expensive. In recent years, many alternative methods have been advocated to reduce the cost of microfabrication but often with reduced qualities in many important features, such as channel resolution, surface smoothness and aspect ratio. In this study, we have developed a microfabrication method that retains high channel quality and aspect ratio by exploring a rarely used solder resist material in combination with screen printing technique to generate masters where PDMS-based microfluidic devices could be fabricated by replica molding from the masters. Using screen printing, different channel heights from 5 to 60 μm on the master were prepared by varying mesh density, controlling solder resist viscosity, and/or adjusting the off-contact gap between a mesh and a substrate, while the entire master fabrication process was completed within 3 h. This simple, low-cost method could generate fine channel features (50 μm) and high aspect ratio (2:1) structures. Microfluidic devices with multi-level structure could be fabricated by multi-steps photolithography using this approach. Moreover, the properties of solder resist enabled the fabrication of flask-shaped well structures by controlled partial exposure and development in a single-step of photolithography, which was potentially used as cell holding reservoirs for cell quantification and cell culture. We believe this fabrication method can be easily adopted by other laboratories to conduct microfluidic researches without specialized equipment.

Sucrose-based fabrication of 3D-networked, cylindrical microfluidic channels for rapid prototyping of lab-on-a-chip and vaso-mimetic devices

We present a new fabrication scheme for 3D-networked, cylindrical microfluidic (MF) channels based on shaping, bonding, and assembly of sucrose fibers. It is a simple, cleanroom-free, and environment-friendly method, ideal for rapid prototyping of lab-on-a-chip devices. Despite its simplicity, it can realize complex 3D MF channel architectures such as cylindrical tapers, internal loops, end-to-side junctions, tapered junctions, and stenosis. The last two will be of special use for realizing vaso-mimetic MF structures. It also enables molding with polymers incompatible with high-temperature processing.

Rapid and cost-effective fabrication of selectively permeable calcium-alginate microfluidic device using "modified" embedded template method

In this paper, we have presented a non-lithographic embedded template method for rapid and cost-effective fabrication of a selectively permeable calcium-alginate (Ca-alginate) based microfluidic device with long serpentine delay channel. To demonstrate the versatility of the presented method, we have demonstrated two different strategies to fabricate serpentine long delay channels without using any sophisticated microfabrication techniques, in formal lab atmosphere. The procedure presented here, also, enables the preparation of a multilayered microfluidic device with channels of varying dimensions, in a single device without using any sophisticated micromachining instrumentation. In addition, we have also qualitatively studied the diffusion of small and large molecules from a Ca-alginate based microfluidic device and proposed a method to effectively control the out-flow of macro biomolecules from the crosslinked Ca-alginate matrix to create a selectively permeable matrix required for various biological and biomimetic applications, as mentioned in the Introduction section of this work.

Efficient and continuous monoacylation with superior selectivity of symmetrical diamines in microreactors

Efficient and continuous monoacylation of symmetrical diamines performed in microreactors yielded superior selectivity to that predicted by statistical considerations. It is highly valuable that the kinetically controlled product in high yields was achieved without any special catalyst at ambient temperature.

Traffic of leukocytes in microfluidic channels with rectangular and rounded cross-sections

Traffic of leukocytes in microvascular networks (particularly through arteriolar bifurcations and venular convergences) affects the dynamics of capillary blood flow, initiation of leukocyte adhesion during inflammation, and localization and development of atherosclerotic plaques in vivo. Recently, a growing research effort has been focused on fabricating microvascular networks comprising artificial vessels with more realistic, rounded cross-sections. This paper investigated the impact of the cross-sectional geometry of microchannels on the traffic of leukocytes flowing with human whole blood through a non-symmetrical bifurcation that consisted of a 50 μm mother channel bifurcating into 30 μm and 50 μm daughter branches. Two versions of the same bifurcation comprising microchannels with rectangular and rounded cross-sections were fabricated using conventional multi-layer photolithography to produce rectangular microchannles that were then rounded in situ using a recently developed method of liquid PDMS/air bubble injection. For microchannels with rounded cross-sections, about two-thirds of marginated leukocytes traveling along a path in the top plane of the bifurcation entered the smallest 30 μm daughter branch. This distribution was reversed in microchannels with rectangular cross-sections--the majority of leukocytes traveling along a similar path continued to follow the 50 μm microchannels after the bifurcation. This dramatic difference in the distribution of leukocyte traffic among the branches of the bifurcation can be explained by preferential margination of leukocytes towards the corners of the 50 μm mother microchannels with rectangular cross-sections, and by the additional hindrance to leukocyte entry created by the sharp transition from the 50 μm mother microchannel to the 30 μm daughter branch at the intersection. The results of this study suggest that the trajectories of marginated leukocytes passing through non-symmetrical bifurcations are significantly affected by the cross-sectional geometry of microchannels and emphasize the importance of using microfludic systems with geometrical configurations closely matching physiological configurations when modeling the dynamics of whole blood flow in the microcirculation.

Parallel sub-micrometre channels with different dimensions for laser scattering detection

A novel and simple approach for the realization of polymer sub-micrometre channels is introduced by exploiting replica molding of Pt wires deposited by focused ion beam. We fabricate arrays of parallel channels with typical dimensions down to 600 nm and with variable height. We characterize the pressure-driven transport of polymer colloids through the channels in terms of the translocation frequency, amplitude and duration by implementing a laser scattering detection technique. We propose a prototype application of the presented platform such as the in situ sizing and sensing of populations of particles with different dimensions down to 50 nm.

Bromo-oxidation reaction in enzyme-entrapped alginate hollow microfibers

In this article, the authors present the fabrication of an enzyme-entrapped alginate hollow fiber using a microfluidic device. Further use of enzyme-entrapped alginate hollow fibers as a biocatalytic microchemical reactor for chemical synthesis is also deliberated in this article. To ensure that there is no enzyme leaching from the fiber, fiber surfaces were coated with chitosan. To confine the mobility of reactants and products within the porous hollow fibers the entire fibers were embedded into a transparent polydimethylsiloxane (PDMS) matrix which also works as a support matrix. A vanadium-containing bromoperoxidase enzyme isolated from Corallina confusa was used as a model enzyme to demonstrate the use of these alginate hollow-fiber reactors in bromo-oxidation of phenol red to bromophenol blue at different dye flow rates. Stability of the entrapped enzyme at different temperatures and the effect of the chitosan coating on the reaction conversion were also studied. It was observed that molecules as big as 27 kDa can be retained in the matrix after coating with chitosan while molecules with molecular-weight of around 378 Da can still diffuse in and out of the matrix. The kinetic conversion rate in this microfluidic bioreactor was more than 41-fold faster when compared with the standard test-tube procedure.

A fast and simple method to fabricate circular microchannels in polydimethylsiloxane (PDMS)

A simple method to fabricate circular microchannels in polydimethylsiloxane (PDMS) is presented. A coating of liquid PDMS is applied on the walls of rectangular microchannels, fabricated using standard soft-lithography, by introducing a pressurized air stream inside the PDMS filled microchannels. Surface tension of the liquid PDMS forces the coating to take a circular cross-section which is preserved by baking the device to cure the coated layer. Diameters ranging from a few micrometres to a few hundreds of micrometres were achieved. The method was verified to work on microchannel networks as well as in straight channels. Different coating conditions were systematically tested. Design curves are reported for one to choose appropriate coating conditions for obtaining a desired diameter. A comparison between the performance of square and circular microchannels in trapping SiHa cells (cervical cancer cell line) is shown.

Droplet synthesis of well-defined block copolymers using solvent-resistant microfluidic device

Well-defined diblock copolymers were synthesized via an exothermic RAFT route by a droplet microfluidic process using a solvent-resistant and thermally stable fluoropolymer microreactor fabricated by a non-lithographic embedded template method. The resulting polymers were compared to products obtained from continuous flow capillary reactor and conventional bulk synthesis. The droplet based microreactor demonstrated superior molecular weight distribution control by synthesizing a higher molecular weight product with higher conversion and narrow polydispersity in a much shorter reaction time. The high quality of the as-synthesized block copolymer PMMA-b-PS led to a generation of micelles with a narrow size distribution that could be used as a template for well-ordered mesoporous silica with regular frameworks and high surface areas.

Quantum dot based immunosensor using 3D circular microchannels fabricated in PDMS

Microchannel is basic functional component of microfluidic chip and every step-forward of its construction technique has been receiving concern all over the world. The present work describes a novel, rapid and simple fabrication technique for building 3D microchannels in poly(dimethyl siloxane) (PDMS) elastomer. These microchannels were used for rapid detection of antigens (E. coli) by quantum dot (QD) based approach. Luminescent QD (CdTe) were synthesized by aqueous method and characterized using high resolution transmission electron microscopy (HRTEM), fluorescence spectroscopy and X-ray diffraction (XRD). The QDs were functionalized with anti-E. coli antibodies for immuno-detection. The reported process allowed easier and faster method of fabrication of circular 3D micochannels and demonstrated their potential use in an immuno-biosensor device.

Print-and-peel fabrication for microfluidics: what's in it for biomedical applications?

This article reviews the development and the advances of print-and-peel (PAP) microfabrication. PAP techniques provide means for facile and expedient prototyping of microfluidic devices. Therefore, PAP has the potential for broadening the microfluidics technology by bringing it to researchers who lack regular or any accesses to specialized fabrication facilities and equipment. Microfluidics have, indeed, proven to be an indispensable toolkit for biological and biomedical research and development. Through accessibility to such methodologies for relatively fast and easy prototyping, PAP has the potential to considerably accelerate the impacts of microfluidics on the biological sciences and engineering. In summary, PAP encompasses: (1) direct printing of the masters for casting polymer device components; and (2) adding three-dimensional elements onto the masters for single-molding-step formation of channels and cavities within the bulk of the polymer slabs. Comparative discussions of the different PAP techniques, along with the current challenges and approaches for addressing them, outline the perspectives for PAP and how it can be readily adopted by a broad range of scientists and engineers.