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Abstract: Abstract Microfluidic devices have been employed in micro-analytical systems and microelectronics using inexpensive, customisable fluid-handling automation at the microliter scale. Here we utilise a well-established fibre drawing technique, which offers a range of materials and capillary conformations, that can be utilized within microfluidic devices to control fluid movement via electroosmotic processes to produce a simple electroosmotic pump (EOP). Single capillary EOPs were fabricated from drawn PU capillary fibres with internal diameters ranging from 73 to 200 µm and were shown to be capable of actively transporting a buffer solution using an external driving electric potential. A maximum flow rate of 0.8 ± 0.1 μL/min was achieved for a 73 ± 2 µm diameter PU capillary fibre at an applied potential of 750 V/cm. This flow rate was successfully increased up to 5.3 ± 0.3 μL/min by drawing a multi-capillary array consisting of 4, 5 and 7 capillaries. PubDate: 2022-05-22
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Abstract: Abstract This study reports the influence of the dead zone (DZ) formed around obstacles due to the viscoelastic fluid flow on the flow behavior and properties. It is known that when a viscoelastic fluid, which is a semi-dilute polymer solution, is infused into a channel with structures resembling a pillar, a DZ is formed around the structure owing to the shear-variation-induced viscosity changes. In this study, hydrolyzed polyacrylamide solutions were infused into a microchannel with a triangle-shaped pillar array. Furthermore, the continuous contraction and expansion flow around the pillars were investigated using a micro-particle image velocimetry. The flow measurement results showed that the shape of DZ changes depending on the Weissenberg number, and accordingly, the flow behavior around and inside the DZ changes. In particular, it was confirmed that the velocity fluctuations trend in the entire flow field increased with the growth in DZ. Moreover, it was found that the shear caused by the velocity difference between the DZ and the other region induces a non-uniform viscosity distribution in the DZ, resulting in the formation of a complex flow containing both extension and rotation behavior inside the DZ. PubDate: 2022-05-21
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Abstract: Abstract Aptamers are synthetic single-stranded nucleic acid molecules that bind to biochemical targets with high affinity and specificity. The method of systematic evolution of ligands by exponential enrichment (SELEX) is widely used to isolate aptamers from randomized oligonucleotides. Recently, microfluidic technology has been applied to improve the efficiency and reduce the cost in SELEX processes. In this work, we present an approach that exploits surface acoustic waves to improve the affinity selection process in microfluidic SELEX. Acoustic streaming is used to enhance the interactions of the solution-based oligonucleotide molecules with microbead-immobilized target molecules, allowing the identification of high-affinity aptamer candidates in a more efficient manner. For demonstration, a DNA aptamer is isolated within three rounds of selection in 5 h to specifically bind to immunoglobulin E, a representative target protein, with an equilibrium dissociation constant of approximately 22.6 nM. PubDate: 2022-05-18
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Abstract: Abstract In this study, we proposed a simple microfluidic device with expansion–contraction structures to obtain a single focusing and filtration of microparticles in different sizes. The presented device comprised of three different regions, the straight rectangular pre-focusing channel, the second focusing channel with expansion–contraction structures, called notches, and the observing region. The randomly distributed microparticles align to the two-equilibrium position in the first region with the assistance of inertial lift forces. In the second region, the uneven structures induce the dean drag force in the microchannel. The inertial lift forces in combination with the dean drag force let the microparticles all align to one equilibrium position gradually. We investigated different geometry of notches with the different length ratios (L/C) to analysis the focusing efficiency and the filtration efficiency of microparticles with different sizes. The highest focusing efficiency of microparticles with 6 and 12 μm diameter are 94 and 99% respectively in the presented device. The filtration efficiency of microparticles with 12 μm diameter exceeds 99% from the mixture of 1 and 12 μm microparticles. With small L/C value, the dean flow strength is higher in the entire expansion region, and the smaller microparticles are relatively easy to follow the high strength dean flow which caused dramatically decrease in focusing efficiency. The notch height also affects the focusing efficiency even with the same L/C value. The optimized notch height with highest focusing efficiency is positive related to the microparticles size. In this study, we found the design rule of the expansion–contraction structures in microchannel. This device has the ability to focus and filter the specific size of microparticles from the mixture of microparticles with different sizes effectively, which benefits the performance of flow cytometry and other biomedical applications. PubDate: 2022-05-14
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Abstract: Abstract Proper oxygen supply with controlled gradients, mimicking physiological conditions, is an important feature for getting reliable results from in vitro cell models in medical and pharmaceutical sciences. In this paper, microfluidic chips made from thiol-ene (TE) polymers for the generation of controlled oxygen gradients were employed, and kinetics studies of TE-induced oxygen depletion over time were performed. Dissolved oxygen concentrations inside microchannels under different flow conditions were imaged and measured with a system relying on an oxygen-sensitive dye immobilized in a foil that constituted the bottom of the microchannels. These studies revealed that at least two different processes are involved in the oxygen scavenging process, and that physisorbed water on the channel surfaces and in the immediate bulk is likely playing an important role. The results also showed that flow rate can be used to tune oxygen concentration gradients along a microchannel, with a higher flow rate (2.0 µL/min) generating a shallower gradient while a lower flow rate (0.7 µL/min) generated a steeper gradient. The obtained results provide further insight into the underlying mechanisms behind the scavenging process, but much remains still poorly understood. This includes the long-term behavior of the scavenging properties, which requires flow modulation strategies to stabilize gradients for extended experimental durations. Still, we show how TE polymers provide a practically applicable venue to generate oxygen gradients for cell-based studies in, e.g., microphysiological systems. PubDate: 2022-05-14
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Abstract: Abstract For several decades, there has been a strong research interest in microfluidic systems and their applications. To bring these systems to market, a high development effort is often necessary for conceptualization and fabrication of such systems as well as the implementation of automated biological analysis processes. In this context, the simulation of microfluidic processes and entire microfluidic networks is becoming increasingly important, as this allows a significant reduction in development efforts, as well as easy adaptation of existing systems to specific requirements. This work presents an analytical model for elastomeric membrane-based micropumps as well as guidelines on how to apply this model to calculate microfluidic networks. The model is derived from the Young–Laplace equation for a non-prestressed elastomeric membrane with a purely nonlinear deflection as a function of applied pressure. The resulting cubic pressure–volume relation is validated by static measurements of the membrane deflection and the transported volume. The model is further used to calculate transient volume flows induced by a membrane micropump in a microfluidic network by adding Hagen Poiseuille’s law. Pressure measurements in the pump chamber confirm the basic assumptions of the model and allow definition of its validity scope. This work lays the foundation for designing elastomeric membrane-based micropumps together with microfluidic networks, estimating maximum flow rates in the system and optimizing pumping frequencies for different microfluidic configurations. PubDate: 2022-05-10
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Abstract: Abstract Decentralized bioanalytical testing in resource-poor settings ranks among the most common applications of microfluidic systems. The high operational autonomy in such point-of-care/point-of-use scenarios requires long-term onboard storage of liquid reagents, which also need to be safely contained during transport and handling, and then reliably released just prior to their introduction to an assay protocol. Over the recent decades, centrifugal microfluidic technologies have demonstrated the capability of integrated, automated and parallelized sample preparation and detection of bioanalytical protocols. This paper presents a novel technique for onboard storage of liquid reagents which can be issued by a rotational stimulus of the system-innate spindle motor, while still aligning with the conceptual simplicity of such “Lab-on-a-Disc” (LoaD) systems. In this work, this highly configurable reagent storage technology is captured by a digital twin, which permits complex performance analysis and algorithmic design optimization according to objectives as expressed by target metrics. PubDate: 2022-04-26
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Abstract: Abstract We use theory, simulation, and experiment to quantify the dielectrophoretic force produced on spherical colloidal particles exposed to an interdigitated electrode array in a microfluidic environment. Our analytical predictions are based on a simplified two-dimensional model and agree with numerical solutions based on a finite difference scheme. Theoretical predictions align with experimental results without any fitting parameters over a wide range of frequencies and applied voltages. A frequency–response function for negative-dielectrophoresis instruments is derived by developing an equivalent electrical circuit model to understand the system power requirements better. Finally, we investigate the role of electrode width and array spacing on the magnitude and distance dependence of the negative dielectrophoresis force. Our analyses show that the electrode width controls the lateral position of the particles, whereas the voltage controls the vertical position. The strongest forces are achieved when the array spacing is matched to the particle size and when the electrode width is ~1/3 of the array spacing. These results can improve the design and optimization of negative-dielectrophoresis microfluidic instruments for applications in the separation and purification of colloidal microparticles in microelectromechanical systems. PubDate: 2022-04-21
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Abstract: Abstract Circulating tumor cells (CTCs) known as tumor cells dissociate from primary tumor sites and travel to secondary tumor sites via vasculature during cancer metastasis. Accumulated clinical evidence has shown that the presence of CTCs may be considered as a major prognostic marker and may increase the risk of head and neck cancer metastasis. Moreover, partial CTCs might consist of cancer stem cells (CSCs) which play critical roles in tumor recurrence and chemo-resistance. However, the connection between CTCs and CSCs remains uncertain. In this study, a physiologically bio-mimicking fluid shear stress (FSS) circulatory system was developed to precisely study the relationship between CTCs and CSCs. The properties of human head and neck cancer cells (HNCs) were studied under a continuous circulatory FSS microenvironment. The results revealed that the cell viability of HNCs was inversely correlated to FSS magnitude and time. The level of intracellular reactive oxygen species in HNCs was increased after circulating the cells under FSS. Moreover, cancer stemness related gene expression and aldehyde dehydrogenase (ALDH) activity were significantly elevated in the FSS survived HNCs. Also, results from chemo-resistance assay showed that the FSS survived HNCs expressed higher resistance to anti-cancer drugs. Our approach is the first report to provide an in vitro fully continuous FSS circulatory system on modulating cancer stemness property of HNCs. Better understanding of the modulation of cancer stemness property in CTCs is expected to advancement of effective therapeutic strategies for cancer metastasis. PubDate: 2022-04-20
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Abstract: Abstract Precise measurement of flow velocity in microfluidic channels is of importance in microfluidic applications, such as quantitative chemical analysis, sample preparation and drug synthesis. However, simple approaches for quickly and precisely measuring the flow velocity in microchannels are still lacking. Herein, we propose a deep neural networks assisted scalar imaging velocimetry (DNN-SIV) for quick and precise extraction of fluid velocity in a shallow microfluidic channel with a high aspect ratio, which is a basic geometry for cell culture, from a dye concentration field with spatiotemporal gradients. DNN-SIV is built on physics-informed neural networks and residual neural networks that integrate data of scalar field and physics laws to determine the height-averaged velocity. The underlying enforcing physics laws are derived from the Navier–Stokes equation and the scalar transport equation. Apart from this, dynamic concentration boundary condition is adopted to improve the velocity measurement of laminar flow with small Reynolds number in microchannels. The proposed DNN-SIV is validated and analyzed by numerical simulations. Compared to integral minimization algorithm used in conventional SIV, DNN-SIV is robust to noise in the measured scalar field and more efficiently allowing real-time flow visualization. Furthermore, the fundamental significance of rational construction of concentration field in microchannels is also underscored. The proposed DNN-SIV in this paper is agnostic to initial and boundary conditions that can be a promising velocity measurement approach for many potential applications in microfluidic chips, although its performance remains to be experimentally validated. PubDate: 2022-04-12
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Abstract: Abstract A new strategy to stimulate vorticity by chaotic advection in microchannels has been investigated. In a microfluidic pattern study, the enhancement of the vorticity still represents a burden for many researchers. The reason is that diffusion transport prevails over the convective forces in laminar regimes, contributing to a no chaotic flow performance. Most of the Computational Fluid Dynamics (CFD) analyses investigate the influence of grooving and obstacles to enhance mixing. This study investigates the formation of vortices in a flow by varying the geometry of an entrance chamber using CFD. The principles applied in the design of cyclones are considered. The narrowing of the microchannel has contributed considerably to create vortices. Likewise, the flattening of the bumps has provided a slight increment in both the vorticity field and the pressure gradient. Lastly, the lateral feeding in the conical chamber has created zones of recirculation. The numerical findings provide evidence that the side feeding in a conical chamber enhances the vorticity. PubDate: 2022-04-09
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Abstract: Abstract To simulate the mass separation or mixture for chemical or biological fluids with viscoelastic characteristics, the rotating electroosmotic flow (EOF) of two-layer fluid in a microchannel with parallel plates is studied. This system is composed of Newtonian fluid and Caputo fractional Oldroyd-B fluid. Maxwell stress is introduced to describe the interaction of two fluids at the interface as well as the shear stress. Based on L1 approximation, numerical solutions are obtained by the finite difference method. The results show that mainstream velocity will oscillate first and then reach a stable state as there is enough time. Due to the existence of centrifugal force, mainstream velocity will increase with the increasing rotating angular velocity. Furthermore, the minimum value of velocity does not lie at the middle of channel because of viscoelastic effects, and the position where reverse flow appears also is pushed to the right side of the center when the rotating angular velocity is large enough. Moreover, with the increase of the interfacial zeta potential difference, the velocity distributions of two-layer fluid have different trends. PubDate: 2022-04-09
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Abstract: Abstract In this paper, a theoretical model is developed to describe the comprehensive influences of surface roughness, fluid rarefication and nonlocal effect on the instability and dynamic behaviors of rough nanotubes conveying nanoflow. Correction factors for fluid are utilized to characterize the effects of the surface roughness and Knudsen number on the internal fluid. The results demonstrate that the surface roughness of nanotube and rarefication effect of nanoflow have opposite influences on the stability and natural frequencies of the system. For fixed–fixed nanotubes, as the roughness height increases, the critical flow velocity increases. On the other hand, as the Knudsen number increases, which indicates the rarefication effect dominates, the critical velocity decreases. In addition, with the increasing of roughness height or the decreasing of Knudsen number, the natural frequency of the first mode increases. For cantilevered nanotubes, the surface roughness makes the curve, which describes the relationship between the critical velocity and the mass ratio, move to the top right of the critical velocity-mass ratio plane while the rarefication effect induces the curve shifting to the bottom left. In addition, the influences of nonlocal effect are also analyzed and discussed. The material length scale parameter can enhance the stiffness of nanotube and increase the critical velocity. PubDate: 2022-04-02
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Abstract: Abstract Materials with a mechanical response to an external stimulus are promising for application in miniaturized cargo and fluid manipulation in microfluidic (lab-on-a-chip) systems and microsystems in general. One of the main challenges in droplet microfluidics is the precise control of the droplet motion, and existing technologies have drawbacks that can compromise the droplet contents. Here, we demonstrate how an on–off switchable ratchet topography combined with a simple actuation strategy can be exploited to accurately manipulate mm-sized droplets. Because of the mechanowetting principle, the three-phase line dynamically attaches to these deforming ratchets, affecting the droplet displacement in a controlled matter. We show that such topographies are capable of transporting droplets over a surface in a stepwise fashion. We calculate the forces generated by the surface using both a theoretical description of the three-phase line and fluid simulations, and we identify the window of applicability in terms of the droplet size relative to the sawtooth dimensions. Our results enable the design of microfluidic systems with deforming wall topographies for controlled droplet manipulation. PubDate: 2022-03-30
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Abstract: Abstract Chemical reaction monitoring based on integrated optofluidic systems is highly desirable because it is simpler, more flexible and efficient compared to traditional optical spectroscopy technique. This paper reports a novel chemical microreactor which is developed with a tunable optofluidic Y-branch waveguide, and this work demonstrates the monitoring of the sucrose hydrolysis reaction process by detecting the ratio of the two output intensities of light. The optofluidic Y-branch waveguide is formed by a liquid-core/liquid-cladding configuration with counter-flows. With the sucrose hydrolyzed into glucose and fructose in the waveguide core, the average molecular size of the product changes, which leads to the change of diffusion coefficient and the refractive index distribution profile. Therefore, the two output intensities of light change accordingly. Experimental studies have well demonstrated that the optofluidic Y-branch waveguide can monitor the sucrose hydrolysis in the concentration range from 0 to 2.1 mol/L with a limit of detection (LOD) of 250 μmol/L. This integrated optofluidic device can be used for various sensing applications in chemical reaction monitoring and quantification of molecular interactions with shortened test time and small sample volume. PubDate: 2022-03-26
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Abstract: Abstract A new and simple analytical method for creatinine determination in human serum and urine samples was developed. The method is based on the reaction of creatinine and N-(1-naphthyl) ethylenediamine (NED) in acidic conditions. A colorimetric reaction based on NED was tested for measuring creatinine concentration with high sensitivity by the microfluidic paper-based analytical device (µPAD). The intensity of the color was checked by observation or by capturing an image and quantification of the signal with ImageJ 1.46r software or color picker app. Under the optimal conditions, the microfluidic device was enabled to determine creatinine in the 0.03–0.50 mM analytical range with a 0.022 mM limit of detection. The strategy proposed in this study could be performed in many clinical laboratories. PubDate: 2022-03-23
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Abstract: Abstract In this paper, a flexible rod type micro-swimmer is proposed which achieves swimming direction reversal on the fly by forming a chiral helix-like geometry through external magnetic excitation. Furthermore an accompanying low Reynolds number flow-structure interaction analysis framework is developed which effectively combines a geometrically non-linear shear deformable beam model with regularized Stokeslet method in a monolithic implicit solution algorithm. This framework is used to investigate the basic characteristics of the proposed micro-swimmer in terms of dimensionless groups reflecting the interplay between different forces involved. PubDate: 2022-03-22
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Abstract: Abstract We report an effective surface-enhanced Raman scattering (SERS) substrate enabled by synthesis of gold nanobipyramids (Au NBPs) in a microfluidic chip. Improved seed-mediated method was implemented for consecutive synthesis of Au NBPs in an S-shaped micromixer. Under precise control of flow rates of reactants, Au NBPs with various morphologies can be prepared in microchannels to serve as effective SERS substrates. With the assistance of this substrate, Raman signals of rhodamine 6G (R6G) probe molecules were significantly enhanced with high sensitivity and reproducibility, the dependence between SERS spectra and Au NBPs morphologies was investigated. Owing to the control mechanism involved in the reaction process in microchannels, these flexible, stable and high-performance SERS substrates can have potential application in diverse fields, particularly in biosensing areas. PubDate: 2022-03-15
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Abstract: Abstract To meet the expanding demands of current cross-disciplinary research, this article presents a uniaxial tensile tester that is capable of high forces (500 N), large displacements (100 mm), bidirectional and symmetric stretching, and high spatial precision (< ± 0.5 µm) while being compatible with optical microscopes for real-time bright-field and fluorescent imaging. The design and characterization of this computer-controlled, stepper–motor-based tensile tester (i.e., stretcher) is presented. Its performance is demonstrated using it to stretch and form cracks in an h-PDMS nanoscale film on top of a more compliant PDMS substrate. The width of the cracks can be nano or micro scale and the width is controlled by the strain applied by the stretcher. These cracks are then used as crack-valves to provide femto-liter fluid delivery between traditional microchannels. The fabrication of these crack-valves is a new process that utilizes dilute solutions of h-PDMS that are spin coated, which allows h-PDMS films with nanoscale thickness to be achieved. The capabilities of the tensile tester are explored by controlling the stretching and relaxation of the PDMS device with crack-valves and, thereby, opening and closing the variable width crack-valves and thus controlling fluid flow. The flow rate through these crack-valves was controlled with a high degree of precision between 0 and 2600 fL s−1. Computer control of the tensile tester allowed for cyclic actuation, and the stability of the crack-valves was demonstrated for over 100 cycles of open/close operations in two scenarios; one between a flow rate of 280 fL s−1 and fully stopped flow and the second between 2600 fL s−1 and fully stopped flow. PubDate: 2022-03-10 DOI: 10.1007/s10404-022-02533-3
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Abstract: Abstract Designing a model to predict the main non-dimensional parameters related to the fluids, i.e., the Capillary number, the Reynolds number, flow rates ratio, and viscosities ratio of two fluids, to achieve a desired droplet size is so important. We use a soft computing method, i.e., a multi-layer perceptron artificial neural network (MLP-ANN), to extract this model. The model is trained by both experimental and validated simulation data in the COMSOL environment. To optimize the structure of the MLP-ANN, the swarm-based metaheuristic algorithms are used, i.e., Particle Swarm Optimization, Firefly Algorithm (FA), Grey wolf optimizer, and Grasshopper Optimization Algorithm. The results show that the FA algorithm has the best results. The optimized network has two hidden layers, with 6 and 14 neurons in its first and second hidden layers, and the network's transfer functions in its hidden layers are in the type of logsig. The RMSE and \({R}^{2}\) values for the optimized MLP network are equal to 4.0076 and 0.9900, respectively. Then, the inverse model is used to calculate the optimum parameters related to the fluids to achieve a desired droplet size. This problem is solved as an optimization problem. A 3D diagram of these optimal parameters is plotted for five desired droplet sizes. It can be seen that these optimal points create different zones related to each droplet size. In other words, for a desired droplet size, there is a confined zone in the 3D space of the capillary number, flow rates ratio, and viscosities ratio. PubDate: 2022-03-10 DOI: 10.1007/s10404-022-02529-z
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