Microchannel Patterns Research Articles

Hybrid Fabrication of Stainless Steel Channels for Microfluidic Application

This research develops a hybrid micromanufacturing technique by combining micromilling and electrochemical micropolishing to fabricate extremely smooth surface finish, high aspect ratio, and complex microchannel patterns. Milling with coated and uncoated ball-end micromills in minimum quantity lubrication is used to remove most materials to define a channel pattern. The milled channels are then electrochemically polished to required finish. A theoretical model accurately predicts surface finish in meso-scale milling, but not in micro-scale milling due to size effect. Electrochemical polishing using an acid-based electrolyte is applied to repeatedly produce stainless steel microchannels with average surface finish of 100 nm when measuring across grain boundaries and 10 nm within a single grain.

A Novel Fabrication Procedure for the Preparation of Polymethyl Methacrylate Lab-on-a-Chip Substrates

Modification of a minicomputer numerical controller with a low cost diode laser for fabrication of polymethyl methacrylate chips has been demonstrated. The maximum power of the diode laser was 5 W at 808 nm. The scanning speed was 1.0–10.0 mm s−1 by the movement of x and y stages. The patterns of microchannels on the chip were designed using drawing software and then applied to software that controlled the operation of the controller. The parameters that affected the channel depth, width, and smoothness were studied by varying the laser power and speed of polymethyl methacrylate sheets with surface area of 4.0 × 6.0 cm2 and thickness of 0.1 cm. The optimum conditions were used to fabricate microchannels on each sheet giving different depths and widths over the range of 74 µm to 554 µm and 147 µm to 393 µm, respectively. The most appropriate conditions for polymethyl methacrylate chip fabrication were 5 W and 3 mm s−1 for the laser diode power and the speed, respectively. The optimal dimensions of the microchannel on the polymethyl methacrylate sheet were 226 µm in width and 202 µm in depth based on sensitivity, reproducibility, and low background signals. The microchannels were sealed with a polymethyl methacrylate cover plate by thermal bonding. The resulting chips were tested for iron(III) determination in water based on microreverse flow analysis.

Surface Finish of Ball-End Milled Microchannels

This study develops a hybrid micromanufacturing technique to fabricate extremely smooth surface finish, high aspect ratio, and complex microchannel patterns. Milling with coat and uncoated ball-end micromills in minimum quantity lubrication (MQL) is used to remove most materials to define a channel pattern. The milled channels are then electrochemically polished to required finish. Assessment of the fabricated microchannels is performed with optical microscopy, scanning electron microscopy, atomic force microscopy, and white-light interferometry. Theoretical models were derived for surface finish of ball-end milling. The predicted surface finish data agree with experimental data in mesoscale milling, but the calculated data are lower than microscale milling data due to size effects. Built-up-edges, being detrimental in micromilling, can be reduced with optimal coating and milling in MQL. When micromilling and then electrochemical polishing of 304, 316L stainless steels and NiTi alloy, this hybrid technique can repeatedly produce microchannels with average surface finish in the range of 100–300 nm.

Preparation of proteolytic microreactors by freeze-drying immobilization

Protease from Bacillus licheniformis was immobilized in a freeze-dried poly(vinyl alcohol) (PVA) micromonolith directly prepared in the microchannels of a microreactor. The prepared PVA micromonoliths had porous microstructures of ~10–30μm in size. Five microreactors were prepared, with microchannels consisting of interconnected straight and elbow segments or a plain straight segment. The system performance was characterized in terms of proteolytic reaction activities. The reactors demonstrated continuous proteolytic activity for 9d with considerable reaction yields. The proteolytic performance was significantly influenced by the microchannel patterning, and was closely linked to the pressure drops in the reactors. The product distributions varied depending on the flow rates of the substrate solutions and the microchannel patterns. Fractions obtained from different reactors gave different product compositions even when operating under similar conditions and producing comparable yields.

Thixoforging of Wrought Aluminum Thin Plates with Microchannels

A direct die-filling thixoforging method is designed to fabricate aluminum thin plates with a pattern of microchannels in a single forming operation. Extruded AA2024 and AA7075 wrought aluminum billets are used. A recrystallization and partial remelting process is used to prepare the semisolid slurries required for the forming process. Under a thixoforging pressure of 70 MPa, AA7075 thin plates are successfully thixoforged in a temperature range of 883 K to 893 K (610 °C to 620 °C), corresponding to liquid fractions of ~30 to 50 pct in the semisolid slurry. AA2024 thin plate requires a thixoforging temperature range of 888 K to 898 K (615 °C to 625 °C), corresponding to the liquid fractions of ~45 to 60 pct. Final microstructures of the thin plates comprise primary α-Al equiaxed globular grains in a matrix of a solidified liquid phase. With increasing thixoforging temperature, the yield strength values continuously decrease. The ultimate tensile strength (UTS) values of the thin plates initially decrease with increasing thixoforging temperature from 883 K to 888 K (610 °C to 615 °C) and from 888 K to 893 K (615 °C to 620 °C) for the AA7075 and AA2024 thin plates, respectively. The UTS values stabilize and slightly enhance when the thixoforging temperatures are further increased to 893 K and 898 K (620 °C and 625 °C) for the AA7075 and AA2024 thin plates, respectively. Very brittle behavior (elongation value of ~1 pct) is observed for the AA7075 thin plates thixoforged at 883 K and 888 K (610 °C and 615 °C). The elongation value increases to 3 pct with increasing the thixoforging temperature to 893 K (620 °C). In contrast, larger elongation values (between 4 and 6 pct) are achieved for the AA2024 thin plates. Increasing the thixoforging pressures from 70 to 100 MPa and then to 150 MPa improves the tensile properties of the thin plates. The tensile properties of the thixoforged thin plates are linked to their microstructural characteristics and processing conditions and are discussed here in detail.

Portable Microfluidic System for Rapid Genetic Testing

Polymerase chain reaction (PCR) is a key technology of genetic testing, but the long thermal cycling time has been the problem to apply the rapid diagnosis of pathogenic organisms and cancer. We describe a simple, portable, and rapid genetic testing system based on ultrafast segment-flow and fluorescent detection. Especially, we improved the flow-through PCR to accelerate the thermal cycling time and simplify the operation for the practical use. The PCR solution was injected with a small volume about several microliter, and flowed as segment-flow by an air syringe. The segment of PCR solution was moved in a serpentine microchannel on a microfluidic device over two temperature zones for denaturation and annealing/extension reactions. The base plate formed upon the serpentine microchannel pattern was made of cyclo-olefin polymer (COP) and covered with a thin transparent film. Furthermore, the specialized portable system was developed for the microfluidic device of segment-flow PCR. The heater units, a syringe pump, fluorescence detector, PC were integrated in a suitcase for portable use. Using this portable system, a model microorganism of anthrax and a point mutation of FGFR3 gene were successfully detected within 6.5 min.

Simple and low cost method for metal-based micro-capillary channels for heat exchanger use

In this work, we present an alternative, low cost method for the fabrication of a heat exchanger utilizing metal-based microchannels using the UV-LiGA technique. Lithography is used to pattern dry film negative photoresist (Ordyl P-50100) on the substrate. The resist is laminated over the substrate and exposed with a UV source. The use of dry film resist allows for simple and inexpensive microchannel patterns without requiring advanced cleanroom equipment. Following the lithography process, electrodeposition of metals is used to fill the recesses patterned in the resist. In this work, nickel has been electroplated into the bounding resist structure. After electroplating, the remaining resist is dissolved leaving free standing metal structures. The fabricated exchanger is then evaluated based on thermal absorption of simulated waste heat sources and capillary action of the metal channels themselves. Channels are fabricated to heights of 60, 70 and 90 μm respectively on copper substrate using these methods. Working fluid mass transfer rate from the heated microchannel heat exchanger (MHE) is utilized as a basic metric of operation. The mass transfer rate recorded from the nickel-based MHE is 2.19, 2.81 and 3.20 mg s−1 respectively for the different channel heights. This implies an effective thermal power consumption rate of 1.66, 2.13 and 2.42 kW m−2 respectively. By contrast, an MHE fabricated with 115 and 142 μm tall channels on silicon substrate is shown to evaporate up to 2.84 and 3.04 mg s−1 respectively, giving an effective thermal power consumption of 2.15 and 2.31 kW m−2 respectively. An investigation of working fluid contact angle with the electroplated nickel surface is also presented. The surface is found to be a porous structure stemming from the electroplating process.

Studying the cytolytic activity of gas plasma with self-signalling phospholipid vesicles dispersed within a gelatin matrix

A synthetic biological sensor was developed to monitor the interaction of plasma with soft, hydrated biological material. It comprises phospholipid vesicles in a hydrated proteinaceous environment comprising 5% (w/v) gelatin. The vesicles contained a self-quenched dye, which was activated by vesicle destruction giving a clear fluorescent switch on. The interaction of bacterial toxins with the sensor was measured in a proof of principle experiment, then the effect of atmospheric plasma jets with the sensor, was studied in order to assess the cytolytic effect of plasma jets in biological systems. When the plasma contacted the gelatin surface perpendicular to the surface, the treatment resulted in the formation of a star-shaped pattern of microchannels that radiated out from the centre of the treatment area within the gelatin matrix, and locally damaged vesicles within the microchannels at a depth of 150 µm below the gelatin surface. Plasma jets applied in parallel to the surface of the matrix resulted in the formation of a single microchannel with damage to the vesicles only evident at the walls of the channel, and a much reduced penetration depth within the gelatin. Our data show that the effects of plasma can be deep in the gelatin material and that the angle of treatment significantly influenced the nature and level of damage to the gelatin and vesicles. Potentially this gelatin model can be used to unravel the roles of different plasma species and the direct effect of whole plasma contact, from those of primary and secondary species—i.e. primary, those emanating directly from the plasma and secondary, those species created in the ‘target’ tissue. This type of insight could be useful in the future development of safe and effective plasma medical technologies.

Fabrication of electro-microfluidic channel for single cell electroporation

The point of this paper is to demonstrate the use of a quick and cheap fabrication method to realize a laser-ablated microfluidic channel for single cell electroporation. Traditional lithography of microchannel with electrode in MEMS applications has always been complicated. Here, we introduce a new methodology of fabricating microchannel with electrical functionalities achieved through a fast and cheap process. In the present methodology, the microchannel pattern is cut out of polyimide, bonded to two ITO-coated substrates using Teflon as an adhesion layer. ITO as conductive material enables electric field in the channel and its optical transparency allows microscopy techniques to be utilized in characterizing the behavior of the microfluidic chip. The performance of the chip was tested on irreversible single-cell scale electroporation which requires relatively high voltages. CHO cells, as mammalian cells, were passed through the microchannel to experience electric field. Cells were loaded with a fluorogenic dye, Calcein AM, and the electroporation of each was individually recorded in real-time via fluorescent microscopy. The results show promising performance of the electric microchannel in electroporation. By customizing of ITO electrodes and the design of microchannel pattern, utilization and integration of the proposed electrical microchannel in variety of other MEMS-based devices are achievable.

Electroosmotic Flow Pump on Transparent Polyimide Substrate Fabricated Using Hot Embossing

Improved fabrication processes of an all-polyimide micro electroosmotic flow pump using hot embossing are described. Microchannels in the micropump were fabricated by hot embossing on a transparent polyimide substrate. A silicon micromachined mold was pressed into the transparent polyimide substrate at a temperature of 300 oC to form microchannel patterns on the substrate. The depth and width of the microchannels were 25 μm and 50 μm, respectively. A UV ozone treatment was performed to improve adhesion between the transparent polyimide substrate and film capping layer. This UV ozone treatment enhanced adhesion and resulted in the reduction of the adhesion temperature as low as 100 oC, and nearly no deformation of the microchannels was observed. As a result, the electroosmotic flow pump exhibited the flow rate of 0.7 μl/min when a voltage of 50 V was given between the electrodes separated 20 mm each other.

Skeletal Muscle Regeneration on Protein-Grafted and Microchannel-Patterned Scaffold for Hypopharyngeal Tissue Engineering

In the field of tissue engineering, polymeric materials with high biocompatibility like polylactic acid and polyglycolic acid have been widely used for fabricating living constructs. For hypopharynx tissue engineering, skeletal muscle is one important functional part of the whole organ, which assembles the unidirectionally aligned myotubes. In this study, a polyurethane (PU) scaffold with microchannel patterns was used to provide aligning guidance for the seeded human myoblasts. Due to the low hydrophilicity of PU, the scaffold was grafted with silk fibroin (PU-SF) or gelatin (PU-Gel) to improve its cell adhesion properties. Scaffolds were observed to degrade slowly over time, and their mechanical properties and hydrophilicities were improved through the surface grafting. Also, the myoblasts seeded on PU-SF had the higher proliferative rate and better differentiation compared with those on the control or PU-Gel. Our results demonstrate that polyurethane scaffolds seeded with myoblasts hold promise to guide hypopharynx muscle regeneration.

Benchtop fabrication of microfluidic systems based on curable polymers with improved solvent compatibility

This paper describes a general scheme to fabricate microchannels from curable polymers on a laboratory benchtop. Using the scheme described here, benchtop fabrication of SU-8 microfluidic systems was demonstrated for the first time, and their compatibility with organic solvents was demonstrated. The fabrication process has three major stages: 1) transferring patterns of microchannels to polymer films by molding, 2) releasing the patterned film and creating inlets and outlets for fluids, and 3) sealing two films together to create a closed channel system. Addition of a PDMS slab supporting the polymer film provided structural integrity during and after fabrication, allowing manipulation of the polymer films without fracturing or deformation. SU-8 channels fabricated according to this scheme exhibited solvent compatibility against continuous exposure to acetone and ethylacetate, which are incompatible with native PDMS. Using the SU-8 channels, continuous generation of droplets of ethylacetate, and templated synthesis of poly (lactic-co-glycolic acid) (PLGA) microparticles, both with stable size, were demonstrated continuously over 24 h, and at intervals over 75 days.

Pen microfluidics: rapid desktop manufacturing of sealed thermoplastic microchannels.

A unique technique for the rapid fabrication of thermoplastic microfluidic chips is described. The method enables the realization of fully-sealed microchannels in around one hour while requiring only minimal infrastructure by taking advantage of a solvent swelling mechanism that allows raised features to be patterned on the surface of homogeneous thermoplastic materials. Patterning is achieved without photolithography by simply drawing the desired microchannel pattern onto the polymer surface using a suitable ink as a masking layer, either manually or under robotic control, followed by timed exposure to solvent vapor to yield a desired depth for the masked channel features. The channels are then permanently sealed through solvent bonding of the microchannel chip to a mating thermoplastic substrate. The process is demonstrated using cyclic olefin copolymer as a thermoplastic material, with fully operational microfluidic devices fabricated following a true desktop manufacturing model suitable for rapid prototyping.

Rapid prototyping of microfluidic modules with a water-developable dry-film photoresist bondable to PDMS

A new, rapid, environmentally friendly prototyping method is presented. This method is based on the use of a negative, water-developable dry-film photoresist as the structural material. In this method, the photomask is prepared by printing microchannel patterns to either a sheet of Vellum paper or a transparency by an office laser printer. It follows by transferring the patterns to the dry-film photoresist by low-dose UV exposure. The patterned photoresist is developed by spraying with pressurized warm water, which makes this method unique and environmentally friendly. The developed SR-3000 with engraved open patterns can be sealed, after plasma-oxidation, to poly(dimethylsiloxane) (PDMS) with a good strength of bonding as high as the 340–410 KPa (50–60 psi) range, strong enough to hold pressure for typical microfluidic applications. The turn-around can be accomplished in 3 h with no cleanroom facility. Microchannel designs were prototyped to demonstrate three typical applications in microfluidics.

Fabrication of Electroosmotic Flow Pump on Polymethylemethacrylate Substrate Using Hot Embossing

Improved fabrication processes of a micro electroosmotic flow pump using hot embossing are described. The microchannels in the micropump were fabricated by hot embossing on a polymethylmethacrylate (PMMA) substrate. A silicon micromachined mold was pressed into the PMMA substrate at a temperature of 145 °C to form microchannel patterns on the substrate. The depth and width of the microchannels were 50 μm and 100 μm, respectively. Aluminum electrodes were deposited using thermal vacuum deposition. A UV ozone treatment was performed to improve adhesion between the PMMA substrate and a PMMA capping layer. This UV ozone treatment enhanced adhesion and resulted in the reduction of the adhesion temperature as low as 70 °C, and nearly no deformation of the microchannels was observed. As a result, the electroosmotic flow pump exhibited the flow rate of 0.5 μl/min when a voltage of 50 V was given between the electrodes separated 8 mm each other.

Pool Boiling Heat Transfer Enhancement Through Nanostructures on Silicon Microchannels

Uniform silicon nanowires (SiNW) were successfully fabricated on the top, bottom, and sidewall surfaces of silicon microchannels by using a two-step electroless etching process. Different microchannel patterns with the channel width from 100 to 300 μm were first fabricated in a 10 mm × 10 mm silicon chip and then covered by SiNW with an average height of 10–20 μm. The effects of the microchannel geometry, micro/nano-hierarchical structures on pool boiling were studied and the bubble dynamics on different sample surfaces were compared. It was found that the combination of the micro/nanostructures promoted microbubble emission boiling under moderate heat fluxes, and yielded superior boiling heat transfer performance. At given wall superheats, the maximum heat flux of the microchannel with SiNW was improved by 120% over the microchannel-only surface, and more than 400% over a plain silicon surface. These results provide a new insight into the boiling mechanism for micro/nano-hierarchical structures and demonstrate their potential in improving pool boiling performance for microchannels.

All-inkjet-printed “lab-on-paper” with Electrodes and Microchannels

A facile and novel fabrication method for paper-based microchips, usually called μPADs (microfluidic paper-based analytical devices) or “lab-on-paper”, has been developed. Through inkjet-printing and green chemical integration technology, lab-on-paper was successfully fabricated. Conductive electrodes were printed on paper substrates prepared from highly-beaten cotton linter pulps. For patterning of microchannels, volatile organic compounds (VOC)-free UV-curable ink was printed as surrounding microchannel patterns followed by UV curing. For enzymatic electrochemical detection, poly(dopamine) (PDA) were used as an immobilization support of enzyme. By applying this method to determination of glucose concentrations, electrochemical response was successfully enhanced.

Optoelectronic reconfigurable microchannels

This paper reports a new optoelectronic reconfigurable microchannel (OERM) technology which uses low-power light to create and reconfigure microchannels within seconds. This technology thereby removes the need for complicated, on-chip fluidic interconnects and enables real-time modification of microchannel networks. In essence, OERM technology is based on the local thawing of a frozen solution using an optoelectronic heating effect powered by an external voltage supplier; a light pattern of microchannels is projected on an OERM chip containing a thin layer of frozen liquid, which produces microchannels corresponding to the light pattern via melting. When the pattern is modified, specific regions freeze and others melt according to the new light pattern, which causes the microchannel network to reconfigure. The versatility of this platform is illustrated by performing several experiments: single-phase flows are used to draw a smiley-face pattern, and multi-phase flow operations, such as generating and merging droplets and bubbles are demonstrated. Due to its low optical power requirement and its fast reconfiguration speed, OERM can dynamically alter microfluidic circuits while benefiting from the flexibility of light patterns and thereby offers a myriad of possible microfluidic applications.

Micropatterned agarose scaffolds covalently modified with collagen for culture of normal and neoplastic hepatocytes

Anchorage-dependent cells including hepatocytes, the main functional cellular constituent comprising liver tissue, require a substrate for cell adhesion when cultured outside their native tissue. The challenge with hepatocyte culture is that material substrates and designs supporting hepatocyte attachment, phenotype, and function are not readily available. Our laboratory previously published that type I collagen found in the liver extracellular matrix supports hepatocyte culture. We hypothesized that micropatterned agarose with a coating of collagen covalently bound to the surface would facilitate hepatocyte adhesion and phenotype. To test this hypothesis, primary canine hepatocytes and neoplastic human HepG2 hepatocellular carcinoma cells were cultured on these substrates. Hepatocyte adhesion was dependent on the cell type and also the micropattern design. Viable normal and neoplastic hepatocytes attached to the microchannel troughs rather than on the ridges. In contrast, hepatocyte adhesion on the microcircular patterns was similar to control agarose as cells did not sense differences in surface topology on these substrates. Neoplastic cells exhibited a distinct difference in growth behavior following 7 days in culture on the microchannel patterns, exhibiting aberrant proliferation relative to normal hepatocytes which did not proliferate. Our results suggest that patterned microchannel agarose may be useful to evaluate hepatoprotective and noxious agents.

Multiphysics simulation of a novel concept of MEMS-based solid propellant thruster for space propulsion

This work analyzes a novel MEMS-based architecture of submillimeter size thruster for the propulsion of small spacecrafts, addressing its preliminary characterization of performance. The architecture of microthruster comprises a setup of miniaturized channels surrounding the solid-propellant reservoir filled up with a high-energetic polymer. These channels guide the hot gases from the combustion region towards the nozzle entrance located at the opposite side of the thruster. Numerical simulations of the transient response of the combustion gases and wafer heating in thruster firings have been conducted with FLUENT under a multiphysics modelling that fully couples the gas and solid parts involved. The approach includes the gas-wafer and gas-polymer thermal exchange, burnback of the polymer with a simplified non-reacting gas pyrolysis model at its front, and a slip-model inside the nozzle portion to incorporate the effect of gas-surface and rarefaction onto the gas expansion. Besides, accurate characterization of thruster operation requires the inclusion of the receding front of the polymer and heat transfer in the moving gas-solid interfaces. The study stresses the improvement attained in thermal management by the inclusion of lateral micro-channels in the device. In particular, the temperature maps reveal the significant dependence of the thermal loss on the instantaneous surface of the reservoir wall exposed to the heat flux of hot gases. Specifically, the simulations stress the benefit of implementing such a pattern of micro-channels connecting the exit of the combustion reservoir with the nozzle. The results prove that hot gases flowing along the micro-channels exert a sealing action upon the heat flux at the reservoir wall and partly mitigate the overall thermal loss at the inner-wall vicinity during the burnback. The analysis shows that propellant decomposition rate is accelerated due to surface preheating and it suggests that a delay of the flame extinction into the reservoir is possible. The simulated operation of the thruster concept shows encouraging performance.