Microfluidic Channel Geometry Research Articles

Simultaneous effects of capillary number, viscosity ratio, and contraction ratio on droplet dynamics in contraction microchannel

Abstract In droplet-based microfluidic systems, fluid properties, flow conditions, and channel geometry play crucial roles in the movement and manipulation of droplets. This study employs a three-dimensional numerical simulation method to investigate droplet dynamics in a contraction microchannel under the simultaneous influence of different factors, such as capillary number (Ca), viscosity ratio (λ), and contraction ratio (C). Specifically, a theoretical model is introduced for predicting the transition of droplets from trap to squeeze, and it is observed to be 〖Ca〗_I = f(C,λ). Meanwhile, droplet dynamics are investigated for the transition from squeeze to breakup for finding critical capillary number value (CaII). In addition, the critical viscosity ratio for the process from squeeze to breakup has been figured out by numerical results for a wide range of contraction ratios. Eventually, droplet dynamics including deformation, velocity ratio, and breakup during contraction microchannel are also quantitatively studied under the factors above.

Evolution of materials, geometries, and fabrication of sweat patches and their use in high-throughput point-of-care/onsite sensing

Sweat is a non-invasive fluid used to monitor various medically relevant analytes for disease diagnosis and healthcare monitoring. Sweat patches are used to collect contaminant-free sweat from individuals in any setting. In the past 50 years, sweat patches have evolved on various fronts, like fabrication methods, materials, and microfluidic channel designs. The primary goal of this study is to explore the evolution of sweat patches over the last 50 years in terms of materials such as polydimethylsiloxane (PDMS), adhesives, and other polymers, as well as microfluidic channel geometries and fabrication approaches. In addition, modern sweat patches are integrated with electrodes and electronic systems that enable sweat collection and in-situ analysis. The sweat patches are classified into two classes: early and modern sweat patches. Further, applications of these sweat patches in monitoring hydration, performance, metabolites, and disease diagnosis have been elaborately discussed. In the end, the conclusion and future outlook are discussed to give insight into further research.

Modal Analysis Of Concentration Information In A Magnetic Micromixer

Mixing is one of the most frequently used processes in microfluidic devices and Lab-on-a-Chip applications, therefore subtantial flow field investigation efforts are spent to achieve high performance micromixers. In this work, we revisit a magnetic micromixer experiment, where velocity, concentration and interface investigations had been carried out. The magnetic micromixer exploits a phenomenon called magnetic microconvection, which appears on an interface of miscible magnetic and nonmagnetic fluids. Initial qualitative concentration measurements are improved, both in terms of accuracy and resolution. The improvements in the experimental setup, improved microfluidic channel geometry, more accurate syringe pumps and inclusion of a concentration calibration step resulted in an accurate, time resolved and quantitative description of the concentration information for magnetic microconvection. In the current work we perform a modal analysis of the concentration field using Proper Orthogonal Decomposition (POD). The modal information provides crucial details about the mixing process at the interface for the magnetic microconvection phenomenon, giving a way to characterize and improve such mixers.

Microfluidic Paper-Based Blood Plasma Separation Device as a Potential Tool for Timely Detection of Protein Biomarkers.

A current challenge regarding microfluidic paper-based analytical devices (µPAD) for blood plasma separation (BPS) and electrochemical immunodetection of protein biomarkers is how to achieve a µPAD that yields enough plasma to retain the biomarker for affinity biosensing in a functionalized electrode system. This paper describes the development of a BPS µPAD to detect and quantify the S100B biomarker from peripheral whole blood. The device uses NaCl functionalized VF2 filter paper as a sample collection pad, an MF1 filter paper for plasma retention, and an optimized microfluidic channel geometry. An inverted light microscope, scanning electron microscope (SEM), and image processing software were used for visualizing BPS efficiency. A design of experiments (DOE) assessed the device’s efficacy using an S100B ELISA Kit to measure clinically relevant S100B concentrations in plasma. The BPS device obtained 50 μL of plasma from 300 μL of whole blood after 3.5 min. The statistical correlation of S100B concentrations obtained using plasma from standard centrifugation and the BPS device was 0.98. The BPS device provides a simple manufacturing protocol, short fabrication time, and is capable of S100B detection using ELISA, making one step towards the integration of technologies aimed at low-cost POC testing of clinically relevant biomarkers.

Organ-on-a-Chip: Design and Simulation of Various Microfluidic Channel Geometries for the Influence of Fluid Dynamic Parameters

Shear stress, pressure, and flow rate are fluid dynamic parameters that can lead to changes in the morphology, proliferation, function, and survival of many cell types and have a determinant impact on tissue function and viability. Microfluidic devices are promising tools to investigate these parameters and fluid behaviour within different microchannel geometries. This study discusses and analyses different designed microfluidic channel geometries regarding the influence of fluid dynamic parameters on their microenvironment at specified fluidic parameters. The results demonstrate that in the circular microchamber, the velocity and shear stress profiles assume a parabolic shape with a maximum velocity occurring in the centre of the chamber and a minimum velocity at the walls. The longitudinal microchannel shows a uniform velocity and shear stress profile throughout the microchannel. Simulation studies for the two geometries with three parallel microchannels showed that in proximity to the micropillars, the velocity and shear stress profiles decreased. Moreover, the pressure is inversely proportional to the width and directly proportional to the flow rate within the microfluidic channels. The simulations showed that the velocity and wall shear stress indicated different values at different flow rates. It was also found that the width and height of the microfluidic channels could affect both velocity and shear stress profiles, contributing to the control of shear stress. The study has demonstrated strategies to predict and control the effects of these forces and the potential as an alternative to conventional cell culture as well as to recapitulate the cell- and organ-specific microenvironment.

Inertial Microfluidics Enabling Clinical Research.

Fast and accurate interrogation of complex samples containing diseased cells or pathogens is important to make informed decisions on clinical and public health issues. Inertial microfluidics has been increasingly employed for such investigations to isolate target bioparticles from liquid samples with size and/or deformability-based manipulation. This phenomenon is especially useful for the clinic, owing to its rapid, label-free nature of target enrichment that enables further downstream assays. Inertial microfluidics leverages the principle of inertial focusing, which relies on the balance of inertial and viscous forces on particles to align them into size-dependent laminar streamlines. Several distinct microfluidic channel geometries (e.g., straight, curved, spiral, contraction-expansion array) have been optimized to achieve inertial focusing for a variety of purposes, including particle purification and enrichment, solution exchange, and particle alignment for on-chip assays. In this review, we will discuss how inertial microfluidics technology has contributed to improving accuracy of various assays to provide clinically relevant information. This comprehensive review expands upon studies examining both endogenous and exogenous targets from real-world samples, highlights notable hybrid devices with dual functions, and comments on the evolving outlook of the field.

Enhancing droplet transition capabilities using sloped microfluidic channel geometry for stable droplet operation.

Droplet-based microfluidics technology allows for the generation and control of droplets that function as independent chemical and biological reactors, enabling broad ranges of high-throughput assays. As more complex multi-step assays are being realized in droplet format, maintaining droplet stability throughout the assay becomes a critical requirement. Unfortunately, as droplets go through multiple manipulation steps, droplet breakage is commonly seen, especially where droplets have to go through sharp transitions in direction and shape. Standard microfabrication techniques typically result in inherent sharp geometry in Z-direction due to their two-dimensional fabrication nature. Recent advancement in micro- and nano- fabrication technology using two-photon polymerization (2PP) is enabling complex 3D microstructures with sub-micrometer resolution to be readily fabricated. Here, utilizing this microfabrication technique, we present a simple solution to the droplet stability challenge by utilizing sloped-geometry microfluidic channels to enable microdroplets to smoothly transition between microfluidic channels having two different heights without breakage. The technique and innovation demonstrated here have the potential to replace conventional droplet microfluidic device fabrication approaches and enable droplet microfluidic platforms to achieve significantly higher level of efficiency, accuracy, and stability never realized before.

Biophysical investigation of living monocytes in flow by collaborative coherent imaging techniques.

We implemented a completely label-free biophysical (morphometric and optical) property characterization of living monocytes in flow, using measurements obtained from two coherent imaging techniques: a pure light scattering approach to obtain an optical signature (OS) of cells, and a digital holography (DH) approach to achieve optical cell reconstructions in flow. A precise 3D cell alignment platform, taking advantage of viscoelastic fluid properties and microfluidic channel geometry, was used to investigate the OS of cells to achieve their refractive index, ratio of the nucleus over cytoplasm, and overall cell dimension. Further quantitative phase-contrast reconstructions by DH were employed to calculate surface area, dry mass, and biovolume of monocytes by using the OS outcomes as input parameters. The results show significantly different biophysical cell properties, confirming the possibility to differentiate monocytes from other cell classes in flow, thus avoiding chemical cell staining or labeling, which are nowadays used.

Micropatterning of planar metal electrodes by vacuum filling microfluidic channel geometries

We present a simple, facile method to micropattern planar metal electrodes defined by the geometry of a microfluidic channel network template. By introducing aqueous solutions of metal into reversibly adhered PDMS devices by desiccation instead of flow, we are able to produce difficult to pattern “dead end” or discontinuous features with ease. We characterize electrodes fabricated using this method and perform electrical lysis of mammalian cancer cells and demonstrate their use as part of an antibody capture assay for GFP. Cell lysis in microwell arrays is achieved using the electrodes and the protein released is detected using an antibody microarray. We show how the template channels used as part of the workflow for patterning the electrodes may be produced using photolithography-free methods, such as laser micromachining and PDMS master moulding, and demonstrate how the use of an immiscible phase may be employed to create electrode spacings on the order of 25–50 μm, that overcome the current resolution limits of such methods. This work demonstrates how the rapid prototyping of electrodes for use in total analysis systems can be achieved on the bench with little or no need for centralized facilities.

Effect of Channel Geometry on Ionic Current Signal of a Bridge Circuit Based Microfluidic Channel

Bridge circuit based ionic current sensing in a microfluidic channel has attracted attention as a highly sensitive analytical method for bio-related molecules and particles. However, channel geometry which greatly influences the detected ionic current has not been investigated. Here, we investigate experimentally and theoretically the effect of differences in the microfluidic channel geometry on shapes and amplitude of signals in ionic current sensing. Our results clarify the geometrical effect of the channel in the bridge circuit based ionic current sensing method.

Microfluidic System for In-Flow Reversible Photoswitching of Near-Infrared Fluorescent Proteins.

We have developed a microfluidic flow cytometry system to screen reversibly photoswitchable fluorescent proteins for contrast and stability of reversible photoconversion between high- and low-fluorescent states. A two-color array of 20 excitation and deactivation beams generated with diffractive optics was combined with a serpentine microfluidic channel geometry designed to provide five cycles of photoswitching with real-time calculation of photoconversion fluorescence contrast. The characteristics of photoswitching in-flow as a function of excitation and deactivation beam fluence, flow speed, and protein concentration were studied with droplets of the bacterial phytochrome from Deinococcus radiodurans (DrBphP), which is weakly fluorescent in the near-infrared (NIR) spectral range. In agreement with measurements on stationary droplets and HeLa S3 mammalian cells expressing DrBphP, optimized operation of the flow system provided up to 50% photoconversion contrast in-flow at a droplet rate of few hertz and a coefficient of variation (CV) of up to 2% over 10 000 events. The methods for calibrating the brightness and photoswitching measurements in microfluidic flow established here provide a basis for screening of cell-based libraries of reversibly switchable NIR fluorescent proteins.

The effect of dilution on the dispersion with respect to microfluidic channel geometries

In most microfluidic devices, single or multiple dilutions of reagents are required to perform reactions or measurements over a range of concentrations using a set of sample solutions to fill the inlets. In this paper, we discuss the results of a study to understand the effects of different dilution schemes on the Taylor dispersion of sample plugs in microchannels. Taylor dispersion arises due to axial spreading of the solute plug due to variations of fluid velocity in the transverse direction. Dilution ratios (DR), channel dimensions, and channel layouts are varied. The results show that the one-sided dilution scheme provides a wider plug of 1.9%, 1.2% and 0.7% when DR=0.5, DR=0.25 and DR=0.1, than its two-side dilution counterpart, independent of dilution channel angle and shift. This deviation increases by increasing the Péclet number, where σ1ch2-σ2ch2∼Pe2. Moreover, we discussed the effects of the compression ratio of the plug, the width ratio between plug and dilution channel, the angle of the dilution channel, the staggered dilution channel, and the 3-dimensional case having different width to height ratios. The compression and width ratios change the final length of the plug dramatically, whereas the channel geometry, i.e. channel angle and shift variation do not. For the 3-dimensional extension, the converged Taylor dispersion values increase with decreasing aspect ratios. These physical understandings and findings for fundamental fluid flow are important for the design of microfluidic devices.

Enhancing the biocompatibility of microfluidics-assisted fabrication of cell-laden microgels with channel geometry

Microfluidic flow-focusing devices (FFD) are widely used to generate monodisperse droplets and microgels with controllable size, shape and composition for various biomedical applications. However, highly inconsistent and often low viability of cells encapsulated within the microgels prepared via microfluidic FFD has been a major concern, and yet this aspect has not been systematically explored. In this study, we demonstrate that the biocompatibility of microfluidic FFD to fabricate cell-laden microgels can be significantly enhanced by controlling the channel geometry. When a single emulsion (“single”) microfluidic FFD is used to fabricate cell-laden microgels, there is a significant decrease and batch-to-batch variability in the cell viability, regardless of their size and composition. It is determined that during droplet generation, some of the cells are exposed to the oil phase which is shown to have a cytotoxic effect. Therefore, a microfluidic device with a sequential (‘double’) flow-focusing channels is employed instead, in which a secondary aqueous phase containing cells enters the primary aqueous phase, so the cells’ exposure to the oil phase is minimized by directing them to the center of droplets. This microfluidic channel geometry significantly enhances the biocompatibility of cell-laden microgels, while maintaining the benefits of a typical microfluidic process. This study therefore provides a simple and yet highly effective strategy to improve the biocompatibility of microfluidic fabrication of cell-laden microgels.

Flow-Through Electroporation of HL-60 White Blood Cell Suspensions using Nanoporous Membrane Electrodes.

A flow-through electroporation system, based on a novel nanoporous membrane/electrode design, for the delivery of cell wall-impermeant molecules into model leukocytes, HL-60 promyelocytes, was demonstrated. The ability to apply low voltages to cell populations, with nm-scale concentrated electric field in a periodic array, contributes to high cell viability. With applied biases of 1-4V, delivery of target molecules was achieved with 90% viability and up to 65% transfection efficiency. More importantly, the system allowed electrophoretic pumping of molecules from a microscale reservoir across the membrane/electrode system into a microfluidic flow channel for transfection of cells, a design that can reduce reagent amount by eightfold compared to current strategies. The flow-through system, which forces intimate membrane/electrode contact by using a 10μm channel height, can be easily scaled-up by adjusting the microfluidic channel geometry and/or the applied voltage pulse frequency to control cell residence times at the cell membrane/electrode interface. The demonstrated system shows promise in clinical applications where low-cost, high cell viability and high volume transfection methods are needed without the risk of viral vectors. In particular genetic modification of freely mobile white blood cells to either target disease cells or to express desired protein/enzyme biomolecules is an important target platform enabled by this device system.

Passive Feed Network Designs for Microfluidic Beam-Scanning Focal Plane Arrays and Their Performance Evaluation

Microfluidic focal plane arrays (MFPAs) have been recently introduced to implement compact high-gain beam-scanning antennas without resorting to active RF devices. This beam-scanning technique relies on a patch antenna element that can be microfluidically repositioned at the focal plane of a microwave lens. The feed network is strategically designed to be passive and accommodate the position variation in the antenna element. This paper, for the first time, considers the design details and performance evaluation of three different passive network layouts that can potentially be utilized to excite MFPAs. Specifically, resonant corporate, resonant straight, and nonresonant straight microstrip line feed networks are introduced and their loss/bandwidth performances are investigated using the transmission line theory. In addition, a method of moments and ray tracing-based hybrid analysis is utilized to demonstrate the impact of the proposed feed networks on the radiation properties of the MFPA in terms of realized gain and side lobe levels (SLLs). It is shown that the resonant and nonresonant straight microstrip line feed networks reduce the SLL by more than 10 dB relative to the resonant corporate feed network utilized in the prior work. The performance improvements are experimentally verified through an eight-element extended hemispherical dielectric lens-based MFPA prototype. Different than the recent work that relied on liquid metal, the antenna element of this MFPA is implemented from a metalized plate by carrying out flow characterizations on various microfluidic channel geometries. This metalized plate approach paves the way for reliable liquid-metal-free microfluidic reconfigurable devices with higher efficiency and power-handling capabilities.

Interference Modeling and Capacity Analysis for Microfluidic Molecular Communication Channels

The impact of the interference on the molecular communication (MC) between a transmitter and receiver pair which are connected through a microfluidic channel containing fluid flow is investigated. The interference modeling and the capacity analysis is performed based on the microfluidic channel geometry, the flow velocity, and the distance. During the analysis, time-scale of biological oscillators is specifically targeted, which is in the range of several minutes to a few hours. The signal-to-interference and noise ratio is shown to be constant with respect to the location of the interfering transmitter. The capacity of the MC link between the designated transmitter and the corresponding receiver is shown to be upper bounded by 1 bit/per channel use when exposed to a single interfering transmitter. For the multiple, i.e., N , transmitters, the decay of pairwise MC capacity is also studied as a factor of N . Finally, placement of the two transmitter and receiver pairs on the opposite sides of the microfluidic channel is studied. Three different microfluidic interference channel configurations, i.e., both-sided interference (microfluidic X channel), one-sided interference (microfluidic Z channel), and interference-free, are proposed based on the distance of the receiver from the interfering transmitter, microfluidic channel cross section, and the fluid flow velocity. For large-scale integration of chemical analysis systems on a microfluidic chip, the provided information-theoretic analysis and the capacity expressions for the MIC can be utilized to analyze the throughput of the chip, which can lead to improvement in efficiency and optimization of the design.

Automated cell viability assessment using a microfluidics based portable imaging flow analyzer.

In this work, we report a system-level integration of portable microscopy and microfluidics for the realization of optofluidic imaging flow analyzer with a throughput of 450 cells/s. With the use of a cellphone augmented with off-the-shelf optical components and custom designed microfluidics, we demonstrate a portable optofluidic imaging flow analyzer. A multiple microfluidic channel geometry was employed to demonstrate the enhancement of throughput in the context of low frame-rate imaging systems. Using the cell-phone based digital imaging flow analyzer, we have imaged yeast cells present in a suspension. By digitally processing the recorded videos of the flow stream on the cellphone, we demonstrated an automated cell viability assessment of the yeast cell population. In addition, we also demonstrate the suitability of the system for blood cell counting.

Efficient gas-liquid contact using microfluidic membrane devices with staggered herringbone mixers.

We describe a novel membrane based gas-liquid-contacting device with increased mass transport and reduced pressure loss by combining a membrane with a staggered herringbone static mixer. Herringbone structures are imposed on the microfluidic channel geometry via soft lithography, acting as mixers which introduce secondary flows at the membrane interface. Such flows include Dean vortices and Taylor flows generating effective mixing while improving mass transport and preventing concentration polarization in microfluidic channels. Furthermore, our static herringbone mixer membranes effectively reduce pressure losses leading to devices with enhanced transfer properties for microfluidic gas-liquid contact. We investigate the red blood cell distribution to tailor our devices towards miniaturised extracorporeal membrane oxygenation and improved comfort of patients with lung insufficiencies.

End-to-End Propagation Noise and Memory Analysis for Molecular Communication over Microfluidic Channels

Molecular communication (MC) between a transmitter and a receiver placed in the chambers attached to a microfluidic channel is investigated. A linear end-to-end channel model is developed capturing the effects of the diffusion and the junction transition at the chambers, as well as the microfluidic channel shapes and the fluid flow. The spectral density of the propagation noise is studied, and the flat frequency bands are identified for the chambers and the microfluidic channel. This suggests that in certain microfluidic design choices, the spectral density of noise may end up naturally being flat. Motivated by this result, the additive white Gaussian noise (AWGN) model is developed based on the chamber, the microfluidic channel, and the fluid flow parameters for the end-to-end propagation noise. Furthermore, the molecular memory is modeled due to inter-diffusion among transmitted molecular signals. The effect of the molecular memory on the end-to-end propagation noise is also analyzed. To substantiate our analytical results, the ranges of physical parameters that yield a linear end-to-end MC channel are investigated. These results show the validity of the AWGN model for MC over microfluidic channels and characterize the impact of the microfluidic channel and chamber geometry on the propagation noise and memory.

Surface Textured Polymer Fibers for Microfluidics

This article introduces surface textured polymer fibers as a new platform for the fabrication of affordable microfluidic devices. Fibers are produced tens of meters‐long at a time and comprise 20 continuous and ordered channels (equilateral triangle grooves with side lengths as small as 30 micrometers) on their surfaces. Extreme anisotropic spreading behavior due to capillary action along the grooves of fibers is observed after surface modification with polydopamine (PDA). These flexible fibers can be fixed on any surface—independent of its material and shape—to form three‐dimensional arrays, which spontaneously spread liquid on predefined paths without the need for external pumps or actuators. Surface textured fibers offer high‐throughput fabrication of complex open microfluidic channel geometries, which is challenging to achieve using current photolithography‐based techniques. Several microfluidic systems are designed and prepared on either planar or 3D surfaces to demonstrate outstanding capability of the fiber arrays in control of fluid flow in both vertical and lateral directions. Surface textured fibers are well suited to the fabrication of flexible, robust, lightweight, and affordable microfluidic devices, which expand the role of microfluidics in a scope of fields including drug discovery, medical diagnostics, and monitoring food and water quality.