Determination of dynamic contact angles within microfluidic devices

Here, we report a single-point detection method for the determination of dynamic surface conditions inside microfluidic channels. The proposed method is based on monitoring fluorescence amplitude as a function of the convolution of a laser beam with segmented flow consisting of two immiscible liquids, one containing fluorescent dye. The fluorescence amplitude is determined by the flow rate and the droplet shape, which is affected by the channel surface properties. We modeled the interaction of a droplet and a laser beam via computer-aided design software, using the laser beam location in relation to the droplet shape as a parameter. The method was applied to fused silica capillaries with both unmodified and modified surfaces, with segmented flow exhibiting water contact angles of ≈?30° and ≈?100°, respectively. The method allows discrimination between hydrophillic and hydrophobic surfaces, as well as the quality of the treatment. The results were verified using fluorescence imaging of the droplets via a stroboscopic technique. We also applied this method to the analysis of microfabricated channels with non-circular cross sections. We demonstrated that the technique enables the determination of the hydrophobicity of channel surfaces, a crucial property required for the generation of segmented flow or emulsions for applications such as digital PCR.     

Superhydrophobic,passive microvalves with controllable opening threshold: exploiting plasma nanotextured microfluidics for a programmable flow switchboard

Plasma processing is used to create passive superhydrophobic on–off valves with tailored opening pressure inside microfluidic devices. First, anisotropic O₂ plasma etching on polymeric microchannels is utilized to controllably roughen (nanotexture) the bottom of the microchannel. Second, the nanotextured surfaces are hydrophobized by means of a C₄F₈ plasma deposition step through a stencil mask creating superhydrophobic stripes or patches. The superhydrophobic patches play the role of on/off valves with predesigned opening pressure threshold (in the range 40–110 mbar), determined by the microchannel dimensions and the size of the nanotexture on the patch. These valves are integrated inside microchannel networks paving the way to autonomous microfluidic devices. To this aim, we present a novel preprogrammable flow switchboard that can split and control the liquid flow for multiple analysis purposes. The proposed valves present an example of how effectively plasma nanoscience and nanotechnology can be applied to microfluidics/nanofluidics and analytical chemistry.     

Performance of multilayered fluoropolymer surface coating for DEP surface microfluidic devices

One of the key structural features of a surface microfluidic (SMF) device is the surface coating, since it directly affects both the performance and reliability of the SMF device. This work examines and compares the performance of liquid dielectrophoresis (LDEP) SMF devices, fabricated with conventional spin-coated Teflon^? surface to those coated with a recently developed fluoropolymer composite coating, which have been shown to be superior for low-voltage electrowetting actuation. We have focused on SMF devices that leverage LDEP and utilize high AC voltages to actuate aqueous samples on hydrophobic surfaces and produce droplet arrays of controlled size and structure to facilitate rapid and large-scale combinatorial bio-assays. Our findings demonstrate the superior performance, robustness and reliability of the composite coating over the conventional spin-coated Teflon^? coating, for repeated high-voltage, high-frequency LDEP actuations for homogenous, emulsion and variable volume aqueous sample dispensing.     

A novel filtration method integrated on centrifugal microfluidic devices

A method has been developed that integrates filters directly into centrifugal microfluidic devices. This technique is suitable for both rapid prototyping and commercial applications. Commercially available filter paper was sealed into the centrifugal microfluidic device with a simple manual fabrication procedure. The method was validated using soil slurry in water and a variety of filter papers with pore sizes ranging from 0.7 to 11 μm. Filtration times of 4 s to several minutes were obtained for 100 μL samples depending on the type of filter paper and rotation rate utilized. The validity of the method was demonstrated by assessing the amount of light lost due to the scatter or absorption caused by particles in the filtered sample while the device was in motion. Filtration and sedimentation were compared and after 30 min of centrifugation, sedimentation had not removed particles as well as filtration. This technique opens up centrifugal microfluidic devices to a wide range of samples.     

Surface microfluidics fabricated by photopatternable superhydrophobic nanocomposite

Surface microfluidics can be of potential use in a variety of emerging applications, including biological and chemical analysis, cellular detection and manipulation, high-throughput pharmaceutical screening, and etc. In comparison with the conventional closed-channel microfluidic system, surface microfluidics shows the distinct advantages of simple construction, direct surface access, no cavitation or interphase obstruction, clear optical path, easy fluidic packaging, and device reusability. In this article, we first present surface microfluidic networks microfabricated by a single-step lithographic process using a novel superhydrophobic photosensitive nanocomposite formula. The photopatternable superhydrophobic nanocomposite (PSN) incorporates PTFE nanoparticles into a SU-8 matrix, in which superhydrophobicity (contact angle of above 160°) is primarily contributed by the extremely low chemical energy and nano-topology of PTFE nanoparticles, while the SU-8 polymer matrix offers photopatternability (lithographic resolution of 10 μm) and substrate adhesion. Moreover, an additive intermediate layer with hydrophilic sidewall considerably reduces flow resistance while improving the substrate adhesion, as a crucial improvement from the previous surface flow configuration. Furthermore, self-propelled microfluidic networks driven by surface tension-induced pressure gradient have been fabricated and characterized to demonstrate the applicability of the novel nanocomposite fabrication approach.     

One-step rapid fabrication of paper-based microfluidic devices using fluorocarbon plasma polymerization

In this work, we demonstrated an all-dry, top-down, and one-step rapid process to fabricate paper-based microfluidic devices using fluorocarbon plasma polymerization. This process is able to create fluorocarbon-coated hydrophobic patterns on filter paper substrates while maintaining the trench and detection regions intact and free of contamination after the fabrication process, as confirmed by attenuated total reflectance–Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. We have shown that the processing time is one critical factor that influences the device performance. For the device fabricated with a sufficiently long processing time (180 s), the sample fluid flow can be well confined in the patterned trenches. By testing the device with an 800 μm channel width, a sample solution amount as small as 4.5 μL is sufficient to perform the test. NO₂ ^? assay is also performed and shows that such a device is capable for biochemical analysis.     

Scale-up and control of droplet production in coupled microfluidic flow-focusing geometries

A single microfluidic chip consisting of six microfluidic flow-focusing devices operating in parallel was developed to investigate the feasibility of scaling microfluidic droplet generation up to production rates of hundreds of milliliters per hour. The design utilizes a single inlet channel for both the dispersed aqueous phase and the continuous oil phase from which the fluids were distributed to all six flow-focusing devices. The exit tubing for each of the six flow-focusing devices is separate and individually plumbed to each device. Within each flow-focusing device, the droplet size was monodisperse, but some droplet size variations were observed across devices. We show that by modifying the flow resistance in the outlet channel of an individual flow-focusing device it is possible to control both the droplet size and frequency of droplet production. This can be achieved through the use of valves or, as is done in this study, by changing the length of the exit tubing plumbed to the outlet of the each device. Longer exit tubing and larger flow resistance is found to lead to larger droplets and higher production frequencies. The devices can thus be individually tuned to create a monodisperse emulsion or an emulsion with a specific drop size distribution.     

Integrated liquid and droplet dielectrophoresis for biochemical assays

Surface microfluidic systems have emerged as an attractive alternative to conventional closed-channel microfluidic devices. In many such systems, electric fields are leveraged for the manipulation and transport of discrete nanoliter droplets on open planar surfaces. The present research work discusses dielectrophoretic liquid and droplet actuations, which provide an attractive methodology for dispensing and manipulating nanoliter and picoliter droplets on planar surfaces. We demonstrate the integration of two independent sample actuation schemes, namely liquid dielectrophoresis (L-DEP) and droplet dielectrophoresis, and furthermore validate its applicability through model biochemical assays (DNA-PicoGreen^® assay and DNA FRET assay). We also describe and present ‘tapering L-DEP’ actuation scheme, whereby we demonstrate how to simultaneously create multiple droplets of different sizes and volumes in the range of nanoliter and picoliters, from a given larger parent sample droplet.     

Automating fluid delivery in a capillary microfluidic device using low-voltage electrowetting valves

A multilayer capillary polymeric microfluidic device integrated with three normally closed electrowetting valves for timed fluidic delivery was developed. The microfluidic channel consisted two flexible layers of poly (ethylene terephthalate) bonded by a pressure-sensitive adhesive spacer tape. Channels were patterned in the spacer tape using laser ablation. Each valve contained two inkjet-printed silver electrodes in series. Capillary flow within the microchannel was stopped at the second electrode which was modified with a hydrophobic monolayer (valve closed). When a potential was applied across the electrodes, the hydrophobic monolayer became hydrophilic and allowed flow to continue (valve opened). The relationship between the actuation voltage, the actuation time, and the distance between two electrodes was performed using a microfluidic chip containing a single microchannel design. The results showed that a low voltage (4.5 V) was able to open the valve within 1 s when the distance between two electrodes was 1 mm. Increased voltages were needed to open the valves when the distance between two electrodes was increased. Additionally, the actuation time required to open the valve increased when voltage was decreased. A multichannel device was fabricated to demonstrate timed fluid delivery between three solutions. Our electrowetting valve system was fabricated using low-cost materials and techniques, can be actuated by a battery, and can be integrated into portable microfluidic devices suitable for point-of-care analysis in resource-limited settings.     

Superhydrophobic, hierarchical, plasma-nanotextured polymeric microchannels sustaining high-pressure flows

We fabricated superhydrophilic and superhydrophobic polymeric microfluidic devices with controlled hierarchical, random roughness, using plasma processing. We implemented a dye staining technique to visually demonstrate the persistence of the superhydrophobic state under flow for pressures in excess of 2.5 bar inside the microchannel. We further confirmed the stability of superhydrophobicity by pressure drop measurements, friction factor and slip length calculations under laminar flow conditions. We also compared identical rough superhydrophilic and superhydrophobic microchannels showing reduced pressure drop in the latter by as much as 22 %. Plasma etching and simultaneous nanotexturing (followed by optional fluorocarbon plasma deposition) are thus shown as an easy-to-implement method for attaining robust Cassie-state against high-pressure microchannel flows.     

Laminated paper-based analytical devices (LPAD): fabrication, characterization, and assays

Paper-based microfluidic devices have recently garnered an increasing interest in the literature. The majority of these devices were produced by patterning hydrophobic zones in hydrophilic paper via photoresist or wax. Others were created by cutting paper using a laser. Here, we present a fabrication method for producing devices by simple craft-cutting and lamination, in a way similar to making an identification (ID) card. The method employs a digital craft cutter and roll laminator to produce laminated paper-based analytical devices (LPAD). Lamination with a plastic backing provides the mechanical strength for a paper device. The approach of using a craft cutter and laminator makes it possible to rapid-prototype LPAD with no more difficulty than producing a typical ID card, at very low cost. Devices constructed using this method have been exploited for simultaneous detection of bovine serum albumin (BSA) and glucose in synthetic urine with colorimetric assays. Both BSA and glucose are detectable at clinically relevant concentrations, with the detection limit at 2.5 μM for BSA and 0.5 mM for glucose.     

Time-resolved electrochemical measurement device for microscopic liquid interfaces during droplet formation

An electrochemical measurement system with a high-speed camera for observation of dynamic behavior of ionic molecules at a water-in-oil interface during microfluidic droplet formation is described. In order to demonstrate the usefulness of the system, a liquid interface between 1 M sodium chloride aqueous solution and 0.02 M tetrabutylammonium tetraphenylborate 1,2-dichloroethane solution was investigated. During aqueous droplet formation in a microfluidic device, averaged and dynamic currents between the two phases were measured under potential control. The measured current corresponded to the transport of electrolyte ions to form the electrical double layer at the liquid interface. When an 18-μm-sized droplet was formed in each 1.2 ms, the amount of charge on each droplet was measured to be 20 pC at 0.4 V and negligible at the potential of zero charge (0.19 V). In addition, the high-speed camera observations revealed that the charge affects the stability of the droplet during and/or just after the generation process. This measurement system is expected to facilitate a fuller understanding of the droplet formation process.     

Microchannel geometry design for rapid and uniform reagent distribution

Rapid and uniform reagent distribution is critical to the performance of a high-throughput microfluidic system, and its geometric design of the microchannels dominates the accuracy and distribution uniformity of the daughter droplets. This research’s purpose is to optimize the geometry of the T-junction to achieve a uniform distribution of two daughter droplets from a single liquid droplet. Models of gas–liquid flow were realized in the transient numerical simulations to investigate the geometry-dependent pressure distributions and the flowing velocities inside the droplet during the splitting process that leads to an improved design of the T-junction that can increase the stability of the droplet splitting process. To validate that increasing the stability of the splitting process can help improve the distribution uniformity of the daughter droplets, microfluidic devices were manufactured on poly(methyl methacrylate) substrates with micromilling and thermal bonding for experiments. In the multiple experiments, 2 μl of reagent was loaded into the microfluidic device and a uniform pneumatic pressure was applied to push the droplet into the T-junction for splitting. The experimental results, after statistical analysis, show that the improved T-junction can achieve better distribution uniformity of the daughter droplets with a higher reliability and a less reagent loss during the splitting process.     

Fabrication of polystyrene microfluidic devices using a pulsed CO<Subscript>2</Subscript> laser system

In this article, we described a simple and rapid method for fabrication of droplet microfluidic devices on polystyrene substrate using a CO₂ laser system. The effects of the laser power and the cutting speed on the depth, width and aspect ratio of the microchannels fabricated on polystyrene were investigated. The polystyrene microfluidic channels were encapsulated using a hot press bonding technique. The experimental results showed that both discrete droplets and laminar flows could be obtained in the device.     

A two-stage electrophoretic microfluidic device for nucleic acid collection and enrichment

Sample purification and enrichment is an important and usually time-consuming step for on-chip nucleic acid detection and analysis. This paper presents an electrophoretic DNA focusing method in microfluidic devices to enrich nucleic acid concentration by around 2700-fold. The electrical waveforms applied to five individual electrodes are such designed that DNAs move successively to the collection electrodes at high speed, while the interferences from bubbles due to electrohydrolysis are minimized. In a spiral channel with a total length of 48 cm, 1 ml DNA sample is purified and enriched by 57 times at a flow rate of 30 μl/min at first. The captured DNAs are then released and transported to the second microfluidic chamber where DNAs are collected and concentrated by 49 times. Thus, in about 40 min, the two-stage device can extract DNAs from 1 ml sample volume and enrich its concentration by 2790-fold. A trade-off exists between the process throughput and the DNA collection efficiency. A DNA capture efficiency of 99.7 % is reached when the flow rate is 1 μl/min, and the maximum DNA capture throughput is achieved at a flow rate of 30 μl/min. As a platform technology, the device can be integrated into bio-sensing and genetic analysis assays for DNA extraction and pre-concentration.     

Characteristics and impact of <Emphasis Type="Italic">Taq</Emphasis> enzyme adsorption on surfaces in microfluidic devices

We investigated the adsorption of Taq enzyme, using sessile droplets, on different microfluidic materials. In propagating adsorption materials, the contact angle (CA) of a sessile Taq droplet continually recedes and collapses due to adsorption. Contrastingly, in contained adsorption materials it exhibits an initial reduced CA due to an instantaneous adsorption, however remains time-invariant. Spectrophotometer analysis on SU8, a propagating adsorption material, reveals a gradual loss of Taq from the droplet onto the surface during droplet collapsing, as opposed to a rapid saturated adsorption in Teflon, a contained adsorption material. AFM micrographs of the adsorbed surfaces suggest a network-like structure in SU8 and distinctly different pillar-like structures in Teflon. With this understanding, we have successfully applied a SU8-Teflon coating to impart a time-invariant contact angle with minimal loss of Taq in surface microfluidic devices.     

Highly integrated polymeric microliquid flow controller for droplet microfluidics

Microfluidic applications demand accurate control and measurement of small fluid flows and volumes, and the majority of approaches found in the literature involve materials and fabrication methods not suitable for a monolithic integration of different microcomponents needed to make a complex Lab-on-a-Chip (LoC) system. The present work leads to a design and manufacturing approach for problem-free monolithic integration of components on thermoplastics, allowing the production of excellent quality devices either as stand-alone components or combined in a complex structures. In particular, a polymeric liquid flow controlling system (LFCS) at microscale is presented, which is composed of a pneumatic microvalve and an on-chip microflow sensor. It enables flow regulation between 30 and 230 μl/min with excellent reproducibility and accuracy (error lower than 5%). The device is made of a single Cyclic Olefin Polymer (COP) piece, where the channels and cavities are hot-embossed, sealed with a single COP membrane by solvent bonding and metalized, after sealing, to render a fully functional microfluidic control system that features on-chip flow sensing. In contrast with commercially available flow control systems, the device can be used for high-quality flow modulation in disposable LoC devices, since the microfluidic chip is low cost and replaceable from the external electronic and pneumatic actuators box. Functionality of the LFCS is tested by connecting it to a microfluidic droplet generator, rendering highly stable flow rates and allowing generation of monodisperse droplets over a wide range of flow rates. The results indicate the successful performance of the LFCS with significant improvements over existing LFCS devices, facing the possibility of using the system for biological applications such as generating distinct perfusion modes in cell culture, novel digital microfluidics. Moreover, the integration capabilities and the reproducible fabrication method enable straightforward transition from prototype to product in a way that is lean, cost-effective and with reduced risk.     

Norland optical adhesive (NOA81) microchannels with adjustable wetting behavior and high chemical resistance against a range of mid-infrared-transparent organic solvents

Different methods to adjust the wetting behavior of surfaces of the UV-curable adhesive NOA81 were investigated and quantitatively characterized by dynamic contact angle measurements with an optical goniometer. A new method to make NOA81 surfaces hydrophobic by mixing an additive in the uncured polymer was presented. The effect was confirmed by surface roughness studies using atomic force microscopy and X-ray photoelectron spectroscopy measurements. The chemical resistance of NOA81 microfluidic channels was evaluated by flowing organic solvents therein. Emphasis was placed on IR-transparent organic solvents. A simple, low-cost method to fabricate chemically resistant, hydrophilic, hydrophobic and hybrid (hydrophilic and hydrophobic), all-polymer microfluidic channels made of NOA81 was reported. Applications like oil-in-water and water-in-oil droplet generation or handling of a multi-phase flow were presented to demonstrate the chemical resistance and the control over the wetting behavior of NOA81 microfluidic chips.     

Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation

We demonstrate controlled guiding of nanoliter emulsion droplets of polar liquids suspended in oil along shallow hydrophilic tracks fabricated at the base of microchannels located within microfluidic chips. The tracks for droplet guiding are generated by exposing the glass surface of polydimethylsiloxane (PDMS)-coated microscope slides via femtosecond laser ablation. The difference in wettability of glass and PDMS surfaces together with the shallow step-like transverse topographical profile of the ablated tracks allows polar droplets wetting preferentially the glass surface to follow the track. In this study, we investigate guiding of droplets of two different polar liquids (water/ethylene glycol) with and without surfactant suspended in an oil medium along surface tracks of different depths of 1, 1.5, and 2 \(\upmu\)m. The results of experiments are also verified with computational fluid dynamics simulations. Guiding of droplets along the tracks as a function of the droplet composition and size and the surface profile depth is evaluated by analyzing the trajectories of moving droplets with respect to the track central axis, and conditions for stable guiding are identified. The experiments and numerical simulations indicate that while the track topography plays a role in droplet guiding using 1.5- and 2-\(\upmu\)m deep tracks, for the case of the smallest track depth of 1 \(\upmu\)m, droplet guiding is mainly caused by surface energy modification along the track rather than the presence of a topographical step on the surface. Our results can be exploited to sort passively different microdroplets mixed in the same microfluidic chip, based on their inherent wetting properties, and they can also pave the way for guiding of droplets along reconfigurable tracks defined by surface energy modifications obtained using other external control mechanisms such as electric field or light.     

Desktop micromilled microfluidics

Micromilling is a proven method for prototyping microfluidic devices; however, high overhead costs, large machine footprints, an esoteric software stack, and nonstandard device bonding protocols may be hampering the widespread adoption of micromilling in the greater microfluidics community. This research exploits a free design-to-device software chain and uses it to explore the applicability of a new class of inexpensive, desktop micromills for fabricating microfluidic devices out of polycarbonate. We present an analysis framework for stratifying micromill’s spatial accuracy and surface quality. Utilizing this we concluded milling geometries directly on the substrate is advantageous to making molds out of the substrate, in terms of accuracy and minimum feature size. Moreover, we proposed a general procedure to calculate feedrate and spindle-speed for any sub-millimeter endmill based on a recommended load percentage. We also established stepover is the major parameter in determining the surface quality rather than spindle-speed and feedrate, showing low-cost mills are able to deliver high-quality surface finishes. Ultimately, we clarified the suitability of low-cost micromills and a cost-efficient assembly method in the field of microfluidics by demonstrating rate- and size-controlled microfluidic droplet generation.