Interfacial instabilities affect microfluidic extraction of small molecules from non-Newtonian fluids

Nonlinear microfluidics: device physics, functions, and applications

The microfluidic flow is typically laminar due to the dominant viscous effects. At Reynolds numbers far below 1 (Re ≪ 1), the fluid inertia can be neglected. For the steady flow of incompressible Newtonian fluids, it approaches linear Stokes flow. At intermediate Re, there exists a weak-inertia flow regime where secondary flows such as Dean vortices are accessible for microfluidic manipulations. Apart from the fluid inertia, other nonlinear factors such as the non-Newtonian fluid properties, concurrent flow of dissimilar fluids, compliant fluidic structures and stimuli-responsive materials can also cause intriguing flow behaviours. Through proper designs, they can be applied for a variety of microfluidic components including mixers, valves, oscillators, stabilizers and auto-regulators etc., greatly enriching the microfluidic flow control and manipulation strategies. Due to its unique working characteristics and advantages, nonlinear microfluidics has increasingly attracted extensive attention. This review presents a systematic survey on this subject. The designs of typical nonlinear microfluidic devices, their working mechanisms, key applications, and the perspective of their future developments will be discussed. The nonlinear microfluidic techniques are believed to play an essential role in the next generation of highly-integrated, automated, and intelligent microfluidics.

Automated Pre-Analytic Processing of Whole Saliva Using Magnet-Beating for Point-of-Care Protein Biomarker Analysis

Saliva offers many advantages for point-of-care (PoC) diagnostic applications due to non-invasive, easy, and cost-effective methods of collection. However, the complex matrix with its non-Newtonian behavior and high viscosity poses handling challenges. Several tedious and long pre-analytic steps, incompatible with PoC use, are required to liquefy and homogenize saliva samples before protein analysis can be performed. We apply magnet-beating to reduce hands-on time and to simplify sample preparation. A magnet in a chamber containing the whole saliva is actuated inside a centrifugal microfluidic cartridge by the interplay of centrifugal and magnetic forces. Rigorous mixing, which homogenizes the saliva sample, is then initiated. Consequently, fewer manual steps are required to introduce the whole saliva into the cartridge. After 4 min of magnet-beating, the processed sample can be used for protein analysis. The viscosity of whole saliva has been reduced from 10.4 to 2.3 mPa s. Immunoassay results after magnet-beating for three salivary periodontal markers (MMP-8, MMP-9, TIMP-1) showed a linear correlation with a slope of 0.99 when compared to results of reference method treated samples. Conclusively, magnet-beating has been shown to be a suitable method for the pre-analytic processing of whole saliva for fully automated PoC protein analysis.

The tongue as a gripper

Frogs, chameleons and anteaters are striking examples of animals that can grab food using only their tongue. How does the soft and wet surface of a tongue grip onto objects before they are ingested? Here, we review the diversity of tongue projection methods, tongue roughnesses and tongue coatings, our goal being to highlight conditions for effective grip and mobility. A softer tongue can reach farther: the frog Rana pipiens tongue is 10 times softer than the human tongue and can extend to 130% of its length when propelled in a whip-like motion. Roughness can improve a tongue's grip: the spikes on a penguin Eudyptes chrysolophus tongue can be as large as fingernails, and help the penguin swallow fish. The saliva coating on the tongue, a non-Newtonian biofluid, can either lubricate or adhere to food. Frog saliva is 175 times more viscous than human saliva, adhering the tongue to slippery, furry or feathery food. We pay particular attention to using mathematical models such as the theory of capillarity, elasticity and friction to elucidate the parameters for effective tongue use across a variety of vertebrate species. Finally, we postulate how the use of wet and rough surfaces to simultaneously sense and grip may inspire new strategies in emerging technologies such as soft robots.

Direct enrichment of pathogens from physiological samples of high conductivity and viscosity using H-filter and positive dielectrophoresis

The full potential of microfluidic techniques as rapid and accurate methods for the detection of disease-causing agents and foodborne pathogens is critically limited by the complex sample preparation process, which commonly comprises the enrichment of bacterial cells to detectable levels. In this manuscript, we describe a microfluidic device which integrates H-filter desalination with positive dielectrophoresis (pDEP) for direct enrichment of bacterial cells from physiological samples of high conductivity and viscosity, such as cow's milk and whole human blood. The device contained a winding channel in which electrolytes in the samples continuously diffused into deionized (DI) water (desalination), while the bacterial cells remained in the samples. The length of the main channel was optimized by numerical simulation and experimentally evaluated by the diffusion of fluorescein into DI water. The effects of another three factors on H-filter desalination were also investigated, including (a) the flow rate ratio between the sample and DI water, (b) sample viscosity, and (c) non-Newtonian fluids. After H-filter desalination, the samples were withdrawn into the dielectrophoresis chamber in which the bacterial cells were captured by pDEP. The feasibility of the device was demonstrated by the direct capture of the bacterial cells in 1× PBS buffer, cow's milk, and whole human blood after H-filter desalination, with the capture efficiencies of 70.7%, 90.0%, and 80.2%, respectively. We believe that this simple method can be easily integrated into portable microfluidic diagnosis devices for rapid and accurate detection of disease-causing agents and foodborne pathogens.

Synthetic ligand-coated magnetic nanoparticles for microfluidic bacterial separation from blood

Bacterial sepsis is a serious clinical condition that can lead to multiple organ dysfunction and death despite timely treatment with antibiotics and fluid resuscitation. We have developed an approach to clearing bacteria and endotoxin from the bloodstream, using magnetic nanoparticles (MNPs) modified with bis-Zn-DPA, a synthetic ligand that binds to both Gram-positive and Gram-negative bacteria. Magnetic microfluidic devices were used to remove MNPs bound to Escherichia coli , a Gram-negative bacterium commonly implicated in bacterial sepsis, from bovine whole blood at flows as high as 60 mL/h, resulting in almost 100% clearance. Such devices could be adapted to clear bacteria from septicemic patients.

Microfluidic droplet-based liquid-liquid extraction and on-chip IR spectroscopy detection of cocaine in human saliva

We present a portable microsystem to quantitatively detect cocaine in human saliva. In this system, we combine a microfluidic-based multiphase liquid-liquid extraction method to transfer cocaine continuously from IR-light-absorbing saliva to an IR-transparent solvent (tetrachloroethylene) with waveguide IR spectroscopy (QC-laser, waveguide, detector) to detect the cocaine on-chip. For the fabrication of the low-cost polymer microfluidic chips a simple rapid prototyping technique based on Scotch-tape masters was further developed and applied. To perform the droplet-based liquid-liquid extraction, we designed and integrated a simple and robust droplet generation method based on the capillary focusing effect within the device. Compared to well-characterized and commonly used microfluidic H-filters, our system showed at least two times higher extraction efficiencies with potential for further improvements. The current liquid-liquid extraction method alone can efficiently extract cocaine and pre-concentrate the analytes in a new solvent. Our fully integrated optofluidic system successfully detected cocaine in real saliva samples spiked with the drug (500 μg/mL) and allowed real time measurements, which makes this approach suitable for point-of-care applications.

Size selective DNA transport through a nanoporous membrane in a PDMS microfluidic device

A microfluidic counter current dialysis device for size based purification of DNA is described. The device consists of two polydimethylsiloxane (PDMS) channels separated by a track etched polycarbonate membrane with a 50 nm pore size. Recovery of fluorescein across the membrane was compared with 10 and 80 nucleotide (nt) ssDNA to characterize the device. Recovery of all three analytes improved with decreasing flow rate. Size selectivity was observed. Greater than 2-fold selectivity between 10 nt and 80 nt ssDNA was observed at linear velocities less than 3mm s(-1). Increasing the ionic strength of the buffer increased transport across the membrane. Recovery of 80 nt ssDNA increased over 4-fold by adding 30 mM NaCl to the buffer. The effect was size dependent as 10 nt showed a smaller increase while the recovery of fluorescein was largely unaffected by increasing the ionic strength of the buffer.

Sample pretreatment and nucleic acid-based detection for fast diagnosis utilizing microfluidic systems

Recently, micro-electro-mechanical-systems (MEMS) technology and micromachining techniques have enabled miniaturization of biomedical devices and systems. Not only do these techniques facilitate the development of miniaturized instrumentation for biomedical analysis, but they also open a new era for integration of microdevices for performing accurate and sensitive diagnostic assays. A so-called “micro-total-analysis-system”, which integrates sample pretreatment, transport, reaction, and detection on a small chip in an automatic format, can be realized by combining functional microfluidic components manufactured by specific MEMS technologies. Among the promising applications using microfluidic technologies, nucleic acid-based detection has shown considerable potential recently. For instance, micro-polymerase chain reaction chips for rapid DNA amplification have attracted considerable interest. In addition, microfluidic devices for rapid sample pretreatment prior to nucleic acid-based detection have also achieved significant progress in the recent years. In this review paper, microfluidic systems for sample preparation, nucleic acid amplification and detection for fast diagnosis will be reviewed. These microfluidic devices and systems have several advantages over their large-scale counterparts, including lower sample/reagent consumption, lower power consumption, compact size, faster analysis, and lower per unit cost. The development of these microfluidic devices and systems may provide a revolutionary platform technology for fast sample pretreatment and accurate, sensitive diagnosis.

Abatement of mixing in shear-free elongationally unstable viscoelastic microflows

The addition of minute amounts of chemically inert polyacrylamide polymer to liquids results in large instabilities under steady electro-osmotic pumping through 2 : 1 constrictions, demonstrating that laminar flow conditions can be broken in electro-osmotic flow of viscoelastic material. By excluding shear and imposing symmetry we create a platform where only elongational viscoelastic instabilities, and diffusion, affect mixing. In contrast to earlier studies with significant shear that found up to orders of magnitude increase in mixing we find that inclusion of polymers excites large viscoelastic instabilities yet mixing is reduced relative to polymer-free liquids. The absolute decrease in mixing we find is consistent with the understanding that adding polymer increases viscosity while viscoelastic flows progress towards elastic turbulence, a type of mild (Batchelor) turbulence, and indicates that electro-osmotic pumped devices are an ideal platform for studying viscoelastic instabilities without supplementary factors.

Nano/Microfluidics for diagnosis of infectious diseases in developing countries

Nano/Microfluidic technologies are emerging as powerful enabling tools for diagnosis and monitoring of infectious diseases in both developed and developing countries. Miniaturized nano/microfluidic platforms that precisely manipulate small fluid volumes can be used to enable medical diagnosis in a more rapid and accurate manner. In particular, these nano/microfluidic diagnostic technologies are potentially applicable to global health applications, since they are disposable, inexpensive, portable, and easy-to-use for detection of infectious diseases. In this paper, we review recent advances in nano/microfluidic technologies for clinical point-of-care applications at resource-limited settings in developing countries.

Extensional instability in electro-osmotic microflows of polymer solutions

Fluid transport in microfluidic systems typically is laminar due to the low Reynolds number characteristic of the flow. The inclusion of suspended polymers imparts elasticity to fluids, allowing instabilities to be excited when substantial polymer stretching occurs. For high molecular weight polymer chains we find that flow velocities achievable by standard electro-osmotic pumping are sufficient to excite extensional instabilities in dilute polymer solutions. We observe a dependence in measured fluctuations on polymer concentration which plateaus at a threshold corresponding to the onset of significant molecular crowding in macromolecular solutions; plateauing occurs well below the overlap concentration. Our results show that electro-osmotic flows of complex fluids are disturbed from the steady regime, suggesting potential for enhanced mixing and requiring care in modeling the flow of complex liquids such as biopolymer suspensions.

Dynamic bioprocessing and microfluidic transport control with smart magnetic nanoparticles in laminar-flow devices

In the absence of applied forces, the transport of molecules and particulate reagents across laminar flowstreams in microfluidic devices is dominated by the diffusivities of the transported species. While the differential diffusional properties between smaller and larger diagnostic targets and reagents have been exploited for bioseparation and assay applications, there are limitations to methods that depend on these intrinsic size differences. Here a new strategy is described for exploiting the sharply reversible change in size and magnetophoretic mobility of "smart" magnetic nanoparticles (mNPs) to perform bioseparation and target isolation under continuous flow processing conditions. The isolated 5 nm mNPs do not exhibit significant magnetophoretic velocities, but do exhibit high magnetophoretic velocities when aggregated by the action of a pH-responsive polymer coating. A simple external magnet is used to magnetophorese the aggregated mNPs that have captured a diagnostic target from a lower pH laminar flowstream (pH 7.3) to a second higher pH flowstream (pH 8.4) that induces rapid mNP disaggregation. In this second dis-aggregated state and flowstream, the mNPs continue to flow past the magnet rather than being immobilized at the channel surface near the magnet. This stimuli-responsive reagent system has been shown to transfer 81% of a model protein target from an input flowstream to a second flowstream in a continuous flow H-filter device.

Micromagnetic-microfluidic blood cleansing device

Sepsis is a lethal disease caused by a systemic microbial infection that spreads via the bloodstream to overwhelm the body's defenses. Current therapeutic approaches are often suboptimal, in part, because they do not fully eliminate the pathogen, and hence the source of deadly toxins. Here we describe an extracorporeal blood cleansing device to selectively remove pathogens from contaminated blood and thereby enhance the patient's response to antibiotic therapy. Immunomagnetic microbeads were modified to create magnetic opsonins that were used to cleanse flowing human whole blood of Candida albicans fungi, a leading cause of sepsis-related deaths. The micromagnetic-microfluidic blood cleansing device generates magnetic field gradients across vertically stacked channels to enable continuous and high throughput separation of fungi from flowing whole blood. A multiplexed version of the device containing four parallel channels achieved over 80% clearance of fungi from contaminated blood at a flow rate of 20 mL/h in a single pass, a rate 1000 times faster than a previously described prototype micromagnetic-microfluidic cell separation system. These results provide the first proof-of-principle that a multiplexed micromagnetic-microfluidic separation system can be used to cleanse pathogens from flowing human blood at a rate and separation efficiency that is relevant for clinical applications.

Determination of the asphaltene and carboxylic acid content of a heavy oil using a microfluidic device

Heavy oil utilisation is set to increase over the coming decades as reserves of conventional oil decline. Heavy oil differs from conventional oil in containing relatively large quantities of asphaltene and carboxylic acids. The proportions of these compounds greatly influence how oil behaves during production and its utilisation as a fuel or feedstock. We report the development of a microfluidic technique, based on a H-cell, that can extract the carboxylic acid components of an oil and assess its asphaltene content. Ultimately this technology could yield a field-deployable device capable of performing measurements that facilitate improved resource management at the point of resource-extraction.

Conditioning saliva for use in a microfluidic biosensor

This report details an approach to saliva conditioning for compatibility of raw patient samples with microfluidic immunoassay components, principally biosensor surfaces susceptible to fouling. Stimulated whole human saliva spiked with a small molecule analyte (phenytoin, 252 Da) was first depleted of cells, debris and high molecular weight glycoproteins (mucins) using membrane filtration. This process significantly reduced but did not eliminate fouling of biosensor surfaces exposed to the sample. An H-filter, which separates solutes from mixed samples based on their diffusion in laminar flow, was used to extract the analyte from the remaining large molecular weight species in the filtered saliva sample. Patient samples treated in this way retained 23% of the analyte with 97% and 92% reduction in glycoproteins and proteins, respectively, and resulted in 3.6 times less surface fouling than either untreated or filtered saliva alone. These sample conditioning steps will enable the use of fouling-sensitive detection techniques in future studies using clinical saliva samples.

Effect of viscoelasticity on the flow pattern and the volumetric flow rate in electroosmotic flows through a microchannel

Many lab-on-a-chip based microsystems process biofluids such as blood and DNA solutions. These fluids are viscoelastic and show extraordinary flow behaviors, not existing in Newtonian fluids. Adopting appropriate constitutive equations these exotic flow behaviors can be modeled and predicted reasonably using various numerical methods. In the present paper, we investigate viscoelastic electroosmotic flows through a rectangular straight microchannel with and without pressure gradient. It is shown that the volumetric flow rates of viscoelastic fluids are significantly different from those of Newtonian fluids under the same external electric field and pressure gradient. Moreover, when pressure gradient is imposed on the microchannel there appear appreciable secondary flows in the viscoelastic fluids, which is never possible for Newtonian laminar flows through straight microchannels. The retarded or enhanced volumetric flow rates and secondary flows affect dispersion of solutes in the microchannel nontrivially.