Microfluidic channel fabrication in dry film resist for production and prototyping of hybrid chips

On-chip light sheet illumination for nanoparticle tracking in microfluidic channels

A simple and inexpensive method is presented to efficiently integrate light sheet illumination in a microfluidic chip for dark-field microscopic tracking and sizing of nanoparticles. The basic idea is to insert an optical fiber inside a polydimethylsiloxane (PDMS) elastomer microfluidic chip and use it as a cylindrical lens. The optical fiber is in this case no longer seen as only an optical waveguide but as a ready-made micro-optical component that is inexpensive and easy to source. Upon insertion, the optical fiber stretches the PDMS microchannel walls, which has two effects. The first effect is to tone down the intrinsic ripples in the PDMS that would otherwise create inhomogeneities in the light sheet illumination. The second effect is to remove any obliqueness of the channel wall and constrain it to be strictly perpendicular to the propagation of the illumination, avoiding the formation of a prismatic diopter. Through calculations, numerical simulations and measurements, we show that the optimal configuration consists in creating a slowly converging light sheet so that its axial thickness is almost uniform along the tracked area. The corresponding thickness was estimated at 12 μm, or 10 times the depth of field of the optical system. This leads to an at least six-fold increase in the signal-to-noise ratio compared to the case without the cylindrical lens. This original light-sheet configuration is used to track and size spherical gold nanoparticles with diameters of 80 nm and 50 nm.

A model for micro-front dynamics using a ϕ4 equation

In this article, we propose a numerical model based on the ϕ4 equation to simulate the dynamics of a front inside a microchannel that features an imperfection at a sidewall to different flow rates. The micro-front displays pinning-depinning phenomena without damped oscillations in the aftermath. To model this behavior, we propose a ϕ4 model with a localized external force and a damping coefficient. Numerical simulations with a constant damping coefficient show that the front displays pinning-depinning phenomena showing damped oscillations once the imperfection is overcome. Replacing the constant damping coefficient with a parabolic spatial function, we reproduce accurately the experimental front-defect interaction.

Fully Microfabricated Surface Acoustic Wave Tweezer for Collection of Submicron Particles and Human Blood Cells

Precise manipulation of (sub)micron particles is key for the preparation, enrichment, and quality control in many biomedical applications. Surface acoustic waves (SAW) hold tremendous promise for manipulation of (bio)particles at the micron to nanoscale ranges. In commonly used SAW tweezers, particle manipulation relies on the direct acoustic radiation effect whose superior performance fades rapidly when progressing from micron to nanoscale particles due to the increasing dominance of a second order mechanism, termed acoustic streaming. Through reproducible and high-precision realization of stiff microchannels to reliably actuate the microchannel cross-section, here we introduce an approach that allows the otherwise competing acoustic streaming to complement the acoustic radiation effect. The synergetic effect of both mechanisms markedly enhances the manipulation of nanoparticles, down to 200 nm particles, even at relatively large wavelength (300 μm). Besides spherical particles ranging from 0.1 to 3 μm, we show collections of cells mixed with different sizes and shapes inherently existing in blood including erythrocytes, leukocytes, and thrombocytes.

Effect of pathological high shear exposure time on platelet activation and aggregation

Circulating platelets are sometimes exposed to high shear rate environments due to vascular stenosis, and the effect of transiently elevated pathological high shear rates on platelet activation and aggregation function has not been clarified. The aim of this study was to investigate the effect of pathological high shear rate (8302s - 1) exposure time (3.16-25.3 ms) on platelet activation and aggregation function. In addition, by adding active ingredients of antiplatelet drugs such as ASA (an active ingredient of aspirin), Ticagrelor, Tirofiban and GP1BA (platelet membrane protein GPIb inhibitor) in vitro, we studied TXA2, P2Y12-ADP, GPIIb/IIIa-fibrinogen and GPIb /IX/V-vWF receptor pathways to determine platelet activation function mediated by pathological high shear rate. In this study, we designed a set of microfluidic chips with stenosis lengths of 0.5 mm, 1 mm, 2 mm, 3 mm, and 4 mm, all with 80% stenosis, to generate pathological high shear forces that can act at different times. The whole blood flowing through the microchannels was collected by perfusion of sodium citrate anticoagulated whole blood at a physiological arterial shear rate (1500 s - 1), and the expression levels of platelet surface activation markers (P-selectin and GP IIb/IIIa) and the degree of platelet aggregation were analyzed by flow cytometry; platelet aggregation patterns were observed by microscopic examination of blood smears. The results showed that shearing significantly increased platelet activation and aggregation levels compared to un-sheared whole blood, and the activation and aggregation levels increased with increasing duration of pathological high shear rate. In vitro inhibition studies showed that ASA barely inhibited the expression of P-selectin and PAC-1 on the platelet surface; Ticagrelor effectively inhibited the expression of both P-selectin and PAC-1; Tirofiban significantly inhibited the expression of PAC-1 on the platelet surface and slightly inhibited the expression of P-selectin; GP1BA significantly inhibited the expression of both.ur results suggest that transient pathological high shear rate (8302s - 1) exposure can induce platelet activation in a time-dependent manner; however, the mechanism is more complex and may be due to the following reasons: transient elevated pathological high shear rate activates platelets through the GPIb/IX/V-vWF receptor pathway, and after platelet activation, its surface membrane protein GPIIb/IIIa receptors activate platelets through fibrinogen to form platelet-platelet aggregates, and further activation of active substances such as ADP and TXA2 released by platelet alpha particles, which contribute to the formation of irreversible platelet aggregation.

Application of microfluidic chip technology to study the inhibitory effect of tetramethylpyrazine on platelet aggregation, activation, and phosphatidylserine exposure mediated by pathological high shear rate

OBJECTIVE

In order to study the antithrombotic effect and mechanism of tetramethylpyrazine (TMA).

METHODS

In this study, we developed a microfluidic chip model that can mimic normal arteries and stenotic arterial vessels, and studied the inhibitory effects of TMA on platelet aggregation, activation (P-selectin, GPIIb/IIIa, monocyte-platelet aggregates) and phosphatidyl serine (PS) exposure. In addition, we also investigated the effect of TMA on ADP and ristocetin-induced platelet aggregation by turbidimetry.

RESULTS

The results showed that TMA significantly inhibited the platelet aggregation, activation and PS exposure induced by pathological high shear rate. Under static conditions, TMA can inhibit ADP and ristocetin-induced platelet aggregation.

CONCLUSION

The results indicated that TMA mainly inhibited platelet aggregation, activation and PS exposure by inhibiting the binding of von Willebrand factor (vWF) to the GPIb/IX/V complex, and partially inhibited platelet aggregation through the platelet P2Y 12 -ADP receptor pathway.

Highly efficient passive Tesla valves for microfluidic applications

A multistage optimization method is developed yielding Tesla valves that are efficient even at low flow rates, characteristic, e.g., for almost all microfluidic systems, where passive valves have intrinsic advantages over active ones. We report on optimized structures that show a diodicity of up to 1.8 already at flow rates of 20 μl s^{- 1} corresponding to a Reynolds number of 36. Centerpiece of the design is a topological optimization based on the finite element method. It is set-up to yield easy-to-fabricate valve structures with a small footprint that can be directly used in microfluidic systems. Our numerical two-dimensional optimization takes into account the finite height of the channel approximately by means of a so-called shallow-channel approximation. Based on the three-dimensionally extruded optimized designs, various test structures were fabricated using standard, widely available microsystem manufacturing techniques. The manufacturing process is described in detail since it can be used for the production of similar cost-effective microfluidic systems. For the experimentally fabricated chips, the efficiency of the different valve designs, i.e., the diodicity defined as the ratio of the measured pressure drops in backward and forward flow directions, respectively, is measured and compared to theoretical predictions obtained from full 3D calculations of the Tesla valves. Good agreement is found. In addition to the direct measurement of the diodicities, the flow profiles in the fabricated test structures are determined using a two-dimensional microscopic particle image velocimetry (μPIV) method. Again, a reasonable good agreement of the measured flow profiles with simulated predictions is observed.

Multilayer Soft Photolithography Fabrication of Microfluidic Devices Using a Custom-Built Wafer-Scale PDMS Slab Aligner and Cost-Efficient Equipment

We present a robust, low-cost fabrication method for implementation in multilayer soft photolithography to create a PDMS microfluidic chip with features possessing multiple height levels. This fabrication method requires neither a cleanroom facility nor an expensive UV exposure machine. The central part of the method stays on the alignment of numerous PDMS slabs on a wafer-scale instead of applying an alignment for a photomask positioned right above a prior exposure layer using a sophisticated mask aligner. We used a manual XYZR stage attached to a vacuum tweezer to manipulate the top PDMS slab. The bottom PDMS slab sat on a rotational stage to conveniently align with the top part. The movement of the two slabs was observed by a monocular scope with a coaxial light source. As an illustration of the potential of this system for fast and low-cost multilayer microfluidic device production, we demonstrate the microfabrication of a 3D microfluidic chaotic mixer. A discussion on another alternative method for the fabrication of multiple height levels is also presented, namely the micromilling approach.

Dielectrophoretic Traps for Efficient Bead and Cell Trapping and Formation of Aggregates of Controlled Size and Composition

We present a microfluidic dielectrophoretic-actuated system designed to trap chosen single-cell and form controlled cell aggregates. A novel method is proposed to characterize the efficiency of the dielectrophoretic trapping, considering the flow speed but also the heat generated by the traps as limiting criteria in cell-safe manipulation. Two original designs with different manufacturing processes are experimentally compared. The most efficient design is selected and the cell membrane integrity is monitored by fluorescence imaging to guarantee a safe-cell trapping. Design rules are suggested to adapt the traps to multiple-cells trapping and are experimentally validated as we formed aggregates of controlled size and composition with two different types of cells. We provide hereby a simple manufactured tool allowing the controlled manipulation of particles for the composition of multicellular assemblies.

Microfluidics Approach to the Mechanical Properties of Red Blood Cell Membrane and Their Effect on Blood Rheology

In this article, we describe the general features of red blood cell membranes and their effect on blood flow and blood rheology. We first present a basic description of membranes and move forward to red blood cell membranes’ characteristics and modeling. We later review the specific properties of red blood cells, presenting recent numerical and experimental microfluidics studies that elucidate the effect of the elastic properties of the red blood cell membrane on blood flow and hemorheology. Finally, we describe specific hemorheological pathologies directly related to the mechanical properties of red blood cells and their effect on microcirculation, reviewing microfluidic applications for the diagnosis and treatment of these diseases.

Polycarbonate Masters for Soft Lithography

Fabrication of microfluidic devices by soft lithography is by far the most popular approach due to its simplicity and low cost. The approach relies on casting of elastomers, such as polydimethylsiloxane (PDMS), on masters fabricated from photoresists on silicon substrates. These masters, however, can be expensive, complicated to fabricate, and fragile. Here we describe an optimized replica molding approach to preserve the original masters by heat molding of polycarbonate (PC) sheets on PDMS molds. The process is faster and simpler than previously reported methods and does not result in a loss of resolution or aspect ratio for the features. The generated PC masters were used to successfully replicate a wide range of microfluidic devices, including rectangular channels with aspect ratios from 0.025 to 7.3, large area spiral channels, and micropost arrays with 5 µm spacing. Moreover, fabrication of rounded features, such as semi-spherical microwells, was possible and easy. Quantitative analysis of the replicated features showed variability of <2%. The approach is low cost, does not require cleanroom setting or hazardous chemicals, and is rapid and simple. The fabricated masters are rigid and survive numerous replication cycles. Moreover, damaged or missing masters can be easily replaced by reproduction from previously cast PDMS replicas. All of these advantages make the PC masters highly desirable for long-term preservation of soft lithography masters for microfluidic devices.

Dry Film Resist Laminated Microfluidic System for Electrical Impedance Measurements

In micro-electrical-mechanical systems (MEMS), thick structures with high aspect ratios are often required. Dry film photoresist (DFR) in various thicknesses can be easily laminated and patterned using standard UV lithography. Here, we present a three-level DFR lamination process of SUEX for a microfluidic chip with embedded, vertically arranged microelectrodes for electrical impedance measurements. To trap and fix the object under test to the electrodes, an aperture is formed in the center of the ring-shaped electrodes in combination with a microfluidic suction channel underneath. In a proof-of-concept, the setup is characterized by electrical impedance measurements with polystyrene and ZrO2 spheres. The electrical impedance is most sensitive at approximately 2 kHz, and its magnitudes reveal around 200% higher values when a sphere is trapped. The magnitude values depend on the sizes of the spheres. Electrical equivalent circuits are applied to simulate the experimental results with a close match.

Substrate membrane bearing close-packed array of micron-level pillars incrassates air-exposed three-dimensional epidermal equivalent model

BACKGROUND

We showed previously that a thick three-dimensional epidermal equivalent can be constructed with passaged keratinocytes on a patterned surface.

MATERIAL AND METHODS

We first carried out computer simulations of a three-dimensional epidermal equivalent model built on close-packed arrays of 10 µm, 15 µm, 20 µm, 30 µm, and 60 µm diameter pillars. Based on these predictions, we evaluated epidermal equivalents built on a series of porous plastic membranes bearing arrays of pillars 15 µm, 20 µm, 25 µm, 30 µm, and 50 µm in diameter.

RESULTS

The simulations predicted that a model having near-physiological thickness would be formed on 15 ~ 30 µm pillars. In the results of in vitro study, the thickest epidermal equivalent was obtained on the 20 µm pillars. Epidermal differentiation markers, filaggrin and loricrin, were expressed at the upper layer of the epidermal equivalent model, and tight-junction proteins, claudin-1 and ZO-1, were expressed on the cell membranes. BrdU-positive cells were observed at the base and also at the top of the pillars.

CONCLUSION

The results of the study suggested that mathematical modeling might be a useful tool to guide biological studies.

Dry Film Photoresist-Based Microfabrication: A New Method to Fabricate Millimeter-Wave Waveguide Components

This paper presents a novel fabrication method based on dry film photoresists to realize waveguides and waveguide-based passive components operating at the millimeter-wave frequency (30-300 GHz). We demonstrate that the proposed fabrication method has a high potential as an alternative to other microfabrication technologies, such as silicon-based and SU8-based micromachining for realizing millimeter-wave waveguide components. Along with the nearly identical transfer of geometrical structures, the dry film photoresist offers other advantages such as fewer processing steps, lower production cost, and shorter prototyping time over the conventional micromachining technologies. To demonstrate the feasibility of the fabrication process, we use SUEX dry film to fabricate a ridge gap waveguide resonator. The resonator is designed to exhibit two resonances at 234.6 and 284 GHz. The measured attenuation at 234 GHz is 0.032 dB/mm and at 283 GHz is 0.033 dB/mm for the fabricated prototype. A comparative study among different existing technologies indicates that the reported method can give a better unloaded Q-value than other conventional processes. The measured unloaded Q-values are in good agreement with the simulated unloaded Q-values. The signal attenuation indicates that SUEX dry film photoresists can be used to fabricate passive devices operating at millimeter-wave frequencies. Moreover, this new fabrication method can offer fast and low-cost prototyping.

Microfluidic Overhauser DNP chip for signal-enhanced compact NMR

Nuclear magnetic resonance at low field strength is an insensitive spectroscopic technique, precluding portable applications with small sample volumes, such as needed for biomarker detection in body fluids. Here we report a compact double resonant chip stack system that implements in situ dynamic nuclear polarisation of a 130 nL sample volume, achieving signal enhancements of up to - 60 w.r.t. the thermal equilibrium level at a microwave power level of 0.5 W. This work overcomes instrumental barriers to the use of NMR detection for point-of-care applications.

Image-based sorting and negative dielectrophoresis for high purity cell and particle separation

Microelectrode arrays are used to sort single fluorescently labeled cells and particles as they flow through a microfluidic channel using dielectrophoresis. Negative dielectrophoresis is used to create a "Dielectrophoretic virtual channel" that runs along the center of the microfluidic channel. By switching the polarity of the electrodes, the virtual channel can be dynamically reconfigured to direct particles along a different path. This is demonstrated by sorting particles into two microfluidic outlets, controlled by an automated system that interprets video data from a color camera and makes complex sorting decisions based on color, intensity, size, and shape. This enables the rejection of particle aggregates and other impurities, and the system is optimized to isolate high purity populations from a heterogeneous sample. Green beads are isolated from an excess of red beads with 100% purity at a rate of up to 0.9 particles per second, in addition application to the sorting of osteosarcoma and human bone marrow cells is evidenced. The extension of Dielectrophoretic Virtual Channels to an arbitrary number of sorting outputs is examined, with design, simulation, and experimental verification of two alternate geometries presented and compared.

Rapid Prototyping of Soft Lithography Masters for Microfluidic Devices Using Dry Film Photoresist in a Non-Cleanroom Setting

Fabrication of microfluidic devices by soft lithography is by far the most popular approach due to simplicity and low cost. In this approach PDMS (polydimethylsiloxane) is cast on a photoresist master to generate replicas that are then sealed against glass slides using oxygen plasma. In this work, we demonstrated fabrication of soft photolithography masters using lamination of ADEX dry film as an alternative to the now classic SU-8 resist masters formed by spin coating. Advantages of using ADEX dry film include the easily-achievable uniform thickness without edge bead; simplicity of the process with significant time savings due to non-sticky nature of the film; and fewer health concerns due to less toxic developing solution and antimony-free composition. As we demonstrate, the process can be performed in a low-cost improvised fabrication room in ambient light, in place of a conventional yellow-light cleanroom environment. We believe this approach holds the promise of delivering state-of-the-art microfluidic techniques to the broad field of biomedical and pharmaceutical research.

High-Throughput Fabrication of Flexible and Transparent All-Carbon Nanotube Electronics

This study reports a simple and effective technique for the high-throughput fabrication of flexible all-carbon nanotube (CNT) electronics using a photosensitive dry film instead of traditional liquid photoresists. A 10 in. sized photosensitive dry film is laminated onto a flexible substrate by a roll-to-roll technology, and a 5 µm pattern resolution of the resulting CNT films is achieved for the construction of flexible and transparent all-CNT thin-film transistors (TFTs) and integrated circuits. The fabricated TFTs exhibit a desirable electrical performance including an on-off current ratio of more than 105, a carrier mobility of 33 cm2 V-1 s-1, and a small hysteresis. The standard deviations of on-current and mobility are, respectively, 5% and 2% of the average value, demonstrating the excellent reproducibility and uniformity of the devices, which allows constructing a large noise margin inverter circuit with a voltage gain of 30. This study indicates that a photosensitive dry film is very promising for the low-cost, fast, reliable, and scalable fabrication of flexible and transparent CNT-based integrated circuits, and opens up opportunities for future high-throughput CNT-based printed electronics.

The lab-on-PCB approach: tackling the μTAS commercial upscaling bottleneck

Commercialization of lab-on-a-chip devices is currently the "holy grail" within the μTAS research community. While a wide variety of highly sophisticated chips which could potentially revolutionize healthcare, biology, chemistry and all related disciplines are increasingly being demonstrated, very few chips are or can be adopted by the market and reach the end-users. The major inhibition factor lies in the lack of an established commercial manufacturing technology. The lab-on-printed circuit board (lab-on-PCB) approach, while suggested many years ago, only recently has re-emerged as a very strong candidate, owing to its inherent upscaling potential: the PCB industry is well established all around the world, with standardized fabrication facilities and processes, but commercially exploited currently only for electronics. Owing to these characteristics, complex μTASs integrating microfluidics, sensors, and electronics on the same PCB platform can easily be upscaled, provided more processes and prototypes adapted to the PCB industry are proposed. In this article, we will be reviewing for the first time the PCB-based prototypes presented in the literature to date, highlighting the upscaling potential of this technology. The authors believe that further evolution of this technology has the potential to become a much sought-after standardized industrial fabrication technology for low-cost μTASs, which could in turn trigger the projected exponential market growth of μTASs, in a fashion analogous to the revolution of Si microchips via the CMOS industry establishment.

Capillary-Driven Microfluidic Chips for Miniaturized Immunoassays: Efficient Fabrication and Sealing of Chips Using a "Chip-Olate" Process

The fabrication of silicon-based microfluidic chips is invaluable in supporting the development of many microfluidic concepts for research in the life sciences and in vitro diagnostic applications such as the realization of miniaturized immunoassays using capillary-driven chips. While being extremely abundant, the literature covering microfluidic chip fabrication and assay development might not have addressed properly the challenge of fabricating microfluidic chips on a wafer level or the need for dicing wafers to release chips that need then to be further processed, cleaned, rinsed, and dried one by one. Here, we describe the "chip-olate" process wherein microfluidic structures are formed on a silicon wafer, followed by partial dicing, cleaning, and drying steps. Then, integration of reagents (if any) can be done, followed by lamination of a sealing cover. Breaking by hand the partially diced wafer yields individual chips ready for use.

Capillary-Driven Microfluidic Chips for Miniaturized Immunoassays: Patterning Capture Antibodies Using Microcontact Printing and Dry-Film Resists

The miniaturization of immunoassays using microfluidic devices is attractive for many applications, but an important challenge remains the patterning of capture antibodies (cAbs) on the surface of microfluidic structures. Here, we describe how to pattern cAbs on planar poly(dimethylsiloxane) (PDMS) stamps and how to microcontact print the cAbs on a dry-film resist (DFR). DFRs are new types of photoresists having excellent chemical resistance and good mechanical, adhesive, and optical properties. Instead of being liquid photoresists, DFRs are thin layers that are easy to handle, cut, photo-pattern, and laminate over surfaces. We show how to perform a simple fluorescence immunoassay using anti-biotin cAbs patterned on a 50-μm-thick DF-1050 DFR, Atto 647N-biotin analytes, and capillary-driven chips fabricated in silicon.

Optofluidic laser scanner based on a rotating liquid prism

We demonstrate an electrowetting-actuated optofluidic system based on a rotatable liquid prism implemented as a two-dimensional laser scanner. The system is fabricated through a novel technology using a patterned flexible polymeric foil on which a high density of electrodes is structured and which is subsequently inserted into a cylindrical housing. The resulting radial electrode array is used for electrowetting actuation of two fluids filled into the cylinder, which allows a controllable tilt and orientation of the planar liquid interface and thus represents a tunable rotating prism. Finite element simulations and subsequent experimental verification show that this highly planar and precisely positionable liquid/liquid interface may be actuated to a deflection angle of ±6.4°, with a standard deviation of ±0.18°, and rotated 360° about the vertical axis. Power consumption is limited to several microwatts, and switching times of several hundred milliseconds were determined.

Microfluidic structures for large-scale manufacture combining photo-patternable materials

Novel use of SiNR, a robust wafer bonding dry adhesive, for industrial and automatable fabrication of microfluidics compatible with DNA analysis.

A Versatile Microarray Platform for Capturing Rare Cells

Analyses of rare events occurring at extremely low frequencies in body fluids are still challenging. We established a versatile microarray-based platform able to capture single target cells from large background populations. As use case we chose the challenging application of detecting circulating tumor cells (CTCs) – about one cell in a billion normal blood cells. After incubation with an antibody cocktail, targeted cells are extracted on a microarray in a microfluidic chip. The accessibility of our platform allows for subsequent recovery of targets for further analysis. The microarray facilitates exclusion of false positive capture events by co-localization allowing for detection without fluorescent labelling. Analyzing blood samples from cancer patients with our platform reached and partly outreached gold standard performance, demonstrating feasibility for clinical application. Clinical researchers free choice of antibody cocktail without need for altered chip manufacturing or incubation protocol, allows virtual arbitrary targeting of capture species and therefore wide spread applications in biomedical sciences.

Fabrication of glass-based microfluidic devices with dry film photoresists as pattern transfer masks for wet etching

A simple, cheap and rapid method is developed to fabricate glass microfluidic devices with dry film photoresist as pattern transfer masks for wet etching, which provides an efficient approach for mass-production of glass microchips.

Phaseguide assisted liquid lamination for magnetic particle-based assays

We have developed a magnetic particle-based assay platform in which functionalised magnetic particles are transferred sequentially through laminated volumes of reagents and washing buffers. Lamination of aqueous liquids is achieved via the use of phaseguide technology; microstructures that control the advancing air-liquid interface of solutions as they enter a microfluidic chamber. This allows manual filling of the device, eliminating the need for external pumping systems, and preparation of the system requires only a few minutes. Here, we apply the platform to two on-chip strategies: (i) a one-step streptavidin-biotin binding assay, and (ii) a two-step C-reactive protein immunoassay. With these, we demonstrate how condensing multiple reaction and washing processes into a single step significantly reduces procedural times, with both assay procedures requiring less than 8 seconds.

Print-to-Pattern Dry Film Photoresist Lithography

Here we present facile microfabrication processes, referred to as Print-to-Pattern dry film photoresist (DFP) lithography, that utilize the combined advantages of wax printing and DFP to produce micropatterned substrates with high resolution over a large surface area in a non-cleanroom setting. The Print-to-Pattern methods can be performed in an out-of-cleanroom environment making microfabrication much more accessible to minimally equipped laboratories. Two different approaches employing either wax photomasks or wax etchmasks from a solid ink desktop printer have been demonstrated that allow the DFP to be processed in a negative tone or positive tone fashion, respectively, with resolutions of 100 μm. The effect of wax melting on resolution and as a bonding material was also characterized. In addition, solid ink printers have the capacity to pattern large areas with high resolution which was demonstrated by stacking DFP layers in a 50 mm × 50 mm woven pattern with 1 mm features. By using an office printer to generate the masking patterns, the mask designs can be easily altered in a graphic user interface to enable rapid prototyping.

Microfluidic titer plate for stratified 3D cell culture

Human tissues and organs are inherently heterogeneous. Their functionality is determined by the interplay between different cell types, their secondary architecture, vascular system and gradients of signaling molecules and metabolites. Here we propose a stratified 3D cell culture platform, in which adjacent lanes of gels and liquids are patterned by phaseguides to capture this tissue heterogeneity. We demonstrate 3D cell culture of HepG2 hepatocytes under continuous perfusion, a rifampicin toxicity assay and co-culture with fibroblasts. 4T1 breast cancer cells are used to demonstrate invasion and aggregation models. The platform is incorporated in a microtiter plate format that renders it fully compatible with automation and high-content screening equipment. The extended functionality, ease of handling and full compatibility to standard equipment is an important step towards adoption of Organ-on-a-Chip technology for screening in an industrial setting.

Bubble-free electrode actuation for micro-preparative scale electrophoresis of RNA

A microfluidic chip is presented for lysis and one-step RNA purification from bacteria. Bacteria are lysed by joule-heating followed by a gel electrophoresis step for clean-up and subsequent elution of small RNA. Bubble formation during electrophoresis at constant current is suppressed through the use of a silver chloride cathode and a silver anode. To prevent silver chloride sediment in the bulk solution, the anode was immersed in a saturated chloride solution. Salt bridges in the form of polyacrylamide gels are used that could be precisely patterned with the help of phaseguides. Bubble-free actuation could be performed for more than 20 min under a constant current. For longer actuation times, cathodic silver-chloride became depleted and a silver-chloride sediment formed in the anodic microchamber at increasing distance from the anode with time. The chip functioning was verified by extraction of transfer-messenger RNA from Escherichia coli and subsequent amplification using reverse transcription real-time PCR. Incorporation of salt bridges enables effective bubble free actuation of Ag/AgCl electrodes in a microfluidic chip. This opens up new possibilities in a surge towards fully integrated diagnostic cartridges that are miniaturized and disposable.

Detection of Dissolved Lactose Employing an Optofluidic Micro-System

In this work, a novel optofluidic sensor principle is employed for a non-invasive and label-free characterization of lactose containing liquid samples. Especially for medicine and food industry, a simple, fast and accurate determination of the amount of lactose in various products is highly desirable. The presented system exploits the impact of dissolved molecules on the refractive index for sample characterization. On the optofluidic chip, a microfluidic channel filled with the analyte is hit by slightly diverging laser light. The center incident angle of the beam on-chip is set close to the critical angle for total internal reflection. Both the reflected and the transmitted light signals are recorded at the solid-liquid interface. The ratio of those two signals is then used as representative value for the analyte. Using this principle, lactose containing samples were differentiated based on their concentrations at a step size of 10 mmol/L. The use of the signals ratio instead of a single signal approach improves the stability of the system significantly, allowing for higher resolutions to be achieved. Furthermore, the fabrication of the devices in PDMS ensures biocompatibility and provides low absorbance of light in the visible range.

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.

Impedance measurement technique for high-sensitivity cell detection in microstructures with non-uniform conductivity distribution

Particle detection in microstructures is a key procedure required by modern lab-on-a-chip devices. Unfortunately, state of the art approaches to impedance measuring as applied to cell detection do not perform well in regions characterized by non-homogeneous physical parameters due, for example, to the presence of air-liquid interfaces or when the particle-electrode distance is relatively high. This paper presents a robust impedance measurement technique and a circuit for detecting cells flowing in microstructures such as microchannels and microwells. Our solution makes use of an innovative three-electrode measurement scheme with asymmetric polarization in order to increase cell detection ability in microstructures featuring large electrode distances of up to 100 μm as well as to limit signal loss due to cell position relative to the electrodes. Compared to standard techniques, numerical simulations show that, with the proposed approach, the cell detection sensitivity is increased by more than 40%. In addition, we propose a custom circuit based on division instead of difference between signals, as in standard differential circuits, so as to reduce the baseline signal drift induced by non-homogeneous conductivity. A simplified analytical model shows an increase in the signal-to-noise-ratio comprised in the range 3.9-5.9. Experimental results, carried out using an open-microwell device made with flexible printed circuit board technology, are in agreement with simulations, suggesting a six-fold increase of the signal-to-noise ratio compared to the differential measurement technique. We were thus able to successfully monitor the process of isolating K562 leukemia cells inside open-microwells determining all single-cell events with no false positive detection.

Enrichment of viable bacteria in a micro-volume by free-flow electrophoresis

Macro- to micro-volume concentration of viable bacteria is performed in a microfluidic chip. The enrichment principle is based on free flow electrophoresis and is demonstrated for Gram positive bacteria. Bacteria from a suspension flow are trapped on a gel interface that separates the trapping location from integrated actuation electrodes in order to enable non-destructive trapping. The microfluidic chip contains integrated electrolytic gas expulsion structures and phaseguides for gel and liquid handling. Trapping efficiency is systematically optimized to reach 25 times the initial concentration from a theoretical maximum of 30. Finally, enrichment from analytically relevant concentrations down to 3 × 10(2) colony forming units per millilitre is demonstrated with a trapping efficiency of 80% which represents the most important parameter in enrichment.

Rapid prototyping polymers for microfluidic devices and high pressure injections

Multiple methods of fabrication exist for microfluidic devices, with different advantages depending on the end goal of industrial mass production or rapid prototyping for the research laboratory. Polydimethylsiloxane (PDMS) has been the mainstay for rapid prototyping in the academic microfluidics community, because of its low cost, robustness and straightforward fabrication, which are particularly advantageous in the exploratory stages of research. However, despite its many advantages and its broad use in academic laboratories, its low elastic modulus becomes a significant issue for high pressure operation as it leads to a large alteration of channel geometry. Among other consequences, such deformation makes it difficult to accurately predict the flow rates in complex microfluidic networks, change flow speed quickly for applications in stop-flow lithography, or to have predictable inertial focusing positions for cytometry applications where an accurate alignment of the optical system is critical. Recently, other polymers have been identified as complementary to PDMS, with similar fabrication procedures being characteristic of rapid prototyping but with higher rigidity and better resistance to solvents; Thermoset Polyester (TPE), Polyurethane Methacrylate (PUMA) and Norland Adhesive 81 (NOA81). In this review, we assess these different polymer alternatives to PDMS for rapid prototyping, especially in view of high pressure injections with the specific example of inertial flow conditions. These materials are compared to PDMS, for which magnitudes of deformation and dynamic characteristics are also characterized. We provide a complete and systematic analysis of these materials with side-by-side experiments conducted in our lab that also evaluate other properties, such as biocompatibility, solvent compatibility, and ease of fabrication. We emphasize that these polymer alternatives, TPE, PUMA and NOA, have some considerable strengths for rapid prototyping when bond strength, predictable operation at high pressure, or transitioning to commercialization are considered important for the application.

Multilayer soft lithography of perfluoropolyether based elastomer for microfluidic device fabrication

The compatibility of microfluidic devices with solvents and other chemicals is extremely important for many applications such as organic synthesis in microreactors and drug screening. We report the successful fabrication of microfluidic devices from a novel perfluoropolyether based polymer utilizing the Multilayer Soft Lithography™ (MSL) technique with simple, straightforward processing. The perfluorinated polymer SIFEL X-71 8115 is a highly chemically resistant elastomeric material. We demonstrate fabrication of a microfluidic device using an off-ratio bonding technique to bond multiple SIFEL layers, each patterned lithographically. The mechanical properties of the SIFEL MSL valves (including actuation pressures) are similar to PDMS MSL valves of the same geometry. Chemical compatibility tests highlight SIFEL's remarkable resistance to organic solvents, acids and alkalis.

From cleanroom to desktop: emerging micro-nanofabrication technology for biomedical applications

This review is motivated by the growing demand for low-cost, easy-to-use, compact-size yet powerful micro-nanofabrication technology to address emerging challenges of fundamental biology and translational medicine in regular laboratory settings. Recent advancements in the field benefit considerably from rapidly expanding material selections, ranging from inorganics to organics and from nanoparticles to self-assembled molecules. Meanwhile a great number of novel methodologies, employing off-the-shelf consumer electronics, intriguing interfacial phenomena, bottom-up self-assembly principles, etc., have been implemented to transit micro-nanofabrication from a cleanroom environment to a desktop setup. Furthermore, the latest application of micro-nanofabrication to emerging biomedical research will be presented in detail, which includes point-of-care diagnostics, on-chip cell culture as well as bio-manipulation. While significant progresses have been made in the rapidly growing field, both apparent and unrevealed roadblocks will need to be addressed in the future. We conclude this review by offering our perspectives on the current technical challenges and future research opportunities.

Stereomask lithography (SML): a universal multi-object micro-patterning technique for biological applications

The advent of biological micro-patterning techniques has given new impetus to many areas of biological research, including quantitative biochemical analysis, tissue engineering, biosensing, and regenerative medicine. Derived from photolithography or soft lithography, current bio-patterning approaches have yet to completely address the needs of out-of-cleanroom, universal applicability, high feature resolution, as well as multi-object placement, though many have shown great promise to precisely pattern one specific biomaterial. In this paper, we present a novel versatile biological lithography technique to achieve integrated multi-object patterning with high feature resolution and high adaptability to various biomaterials, referred to as stereomask lithography (SML). Successive patterning of multiple objects is enabled by using unique three-dimensional masks (i.e., the stereomasks), which lay out current micropatterns while protecting pre-existing biological features on the substrate. Furthermore, high-precision reversible alignment among multiple bio-objects is achieved by adopting a peg-in-hole design between the substrate and stereomasks. We demonstrate that the SML technique is capable of constructing a complex biological microenvironment with various bio-functional components at the single-cell resolution, which to the best of our knowledge has not been realized before.

M³: Microscope-based maskless micropatterning with dry film photoresist

We present a maskless micropatterning system that utilizes a fluorescence microscope with programmable X-Y stage and dry film photoresist to realize feature sizes in the sub-millimeter range (40-700 μm). The method allows for flexible in-house maskless photolithography without a dedicated microfabrication facility and is well-suited for rapid prototyping of microfluidic channels, scaffold templates for protein/cell patterning or optically-guided cell encapsulation for biomedical applications.

Review article-dielectrophoresis: status of the theory, technology, and applications

A review is presented of the present status of the theory, the developed technology and the current applications of dielectrophoresis (DEP). Over the past 10 years around 2000 publications have addressed these three aspects, and current trends suggest that the theory and technology have matured sufficiently for most effort to now be directed towards applying DEP to unmet needs in such areas as biosensors, cell therapeutics, drug discovery, medical diagnostics, microfluidics, nanoassembly, and particle filtration. The dipole approximation to describe the DEP force acting on a particle subjected to a nonuniform electric field has evolved to include multipole contributions, the perturbing effects arising from interactions with other cells and boundary surfaces, and the influence of electrical double-layer polarizations that must be considered for nanoparticles. Theoretical modelling of the electric field gradients generated by different electrode designs has also reached an advanced state. Advances in the technology include the development of sophisticated electrode designs, along with the introduction of new materials (e.g., silicone polymers, dry film resist) and methods for fabricating the electrodes and microfluidics of DEP devices (photo and electron beam lithography, laser ablation, thin film techniques, CMOS technology). Around three-quarters of the 300 or so scientific publications now being published each year on DEP are directed towards practical applications, and this is matched with an increasing number of patent applications. A summary of the US patents granted since January 2005 is given, along with an outline of the small number of perceived industrial applications (e.g., mineral separation, micropolishing, manipulation and dispensing of fluid droplets, manipulation and assembly of micro components). The technology has also advanced sufficiently for DEP to be used as a tool to manipulate nanoparticles (e.g., carbon nanotubes, nano wires, gold and metal oxide nanoparticles) for the fabrication of devices and sensors. Most efforts are now being directed towards biomedical applications, such as the spatial manipulation and selective separationenrichment of target cells or bacteria, high-throughput molecular screening, biosensors, immunoassays, and the artificial engineering of three-dimensional cell constructs. DEP is able to manipulate and sort cells without the need for biochemical labels or other bioengineered tags, and without contact to any surfaces. This opens up potentially important applications of DEP as a tool to address an unmet need in stem cell research and therapy.

Trapping single human osteoblast-like cells from a heterogeneous population using a dielectrophoretic microfluidic device

We describe a system for the isolation, concentration, separation, and recovery of human osteoblast-like cells from a heterogeneous population using dielectrophoretic ring traps. Cells flowing in a microfluidic channel are immobilized inside an electric field cage using negative dielectrophoresis. A planar ring electrode creates a closed trap while repelling surrounding cells. Target cells are identified by fluorescent labeling, and are trapped as they pass across a ring electrode by an automated system. We demonstrate recovery of small populations of human osteoblast-like cells with a purity of 100%, which in turn demonstrates the potential of such a device for cell selection from a heterogeneous population.

Horizontal nDEP cages within open microwell arrays for precise positioning of cells and particles

We present the structure of an open microwell, i.e. a microwell open at both the top and bottom ends, which enables single-cells to be handled, processed and recovered after the experiment. The microwell has a novel architecture which allows particles to be trapped and forced to interact by means of a cylindrical negative dielectrophoretic cage. Particles are aligned along a horizontal axis where the electric field minimum is placed. Arrays of open microwells are fabricated using flexible printed circuit board (PCB) technology providing cheap and disposable devices. Levitation and precise positioning of both polystyrene beads and K562 cells were experimented, confirming the results of physical simulations. Assessment of cell viability after 20 min exposure to the electric field was performed through a standard calcein-release assay.

A rapid microfluidic switching system for analysis at the single cellular level

Analysis of cellular responses to chemicals at high spatiotemporal resolution is required for precise understanding of intracellular signal transduction. Here, we demonstrated a novel method for applying different solutions to a part of or all of a cell at high spatiotemporal resolution. We fabricated a microfluidic device using polydimethylsiloxane, and the sharp interface between the two solution streams flowing in the channel was used for the application of different solutions. We constructed a computer-controlled system to control the interface movement precisely, rapidly, and reproducibly during positioning, and spatial and temporal resolutions attained were 1.6 mum and 189 ms, respectively. We then applied the present system to the analysis of intracellular responses to chemicals. We were able to measure [Ca (2+)] (i) increases within 500 ms, when one laminar stream covered a part of the cell. This method can be used as a generic platform to investigate responses against drugs at the single cell level.