Quantitative analysis of sperm rheotaxis using a microfluidic device

Quite puzzling issue in biology is how sperm cells are selected naturally where human sperm has to maintain a correct swimming behavior during the various stages of reproduction process. In nature, sperm has to compete a long journey from cervix to oocyte to stand a chance for fertilization. Although various guidance mechanisms such as chemical and thermal gradients are proposed previously, these mechanisms may only be relevant as sperm reaches very close to the oocyte. Rheotaxis, a phenomenon where sperm cells swim against the flow direction, is possibly the long-range sperm guidance mechanism for successful fertilization. A little is known quantitatively about how flow shear effects may help guide human sperm cells over long distances. Here, we have developed microfluidic devices to quantitatively investigate sperm rheotaxis at various physiological flow conditions. We observed that at certain flow rates sperm actively orient and swim against the flow. Sperm that exhibit positive rheotaxis show better motility and velocity than the control (no-flow condition), however, rheotaxis does not select sperm based on hyaluronic acid (HA) binding potential and morphology. Morphology and HA binding potential may not be a significant factor in sperm transport in natural sperm selection.     

Cell motion and recovery in a two-stream microfluidic device

The motion of cells in a two-stream microfluidic device designed to extract cryoprotective agents from cell suspensions was tested under a range of conditions. Jurkat cells (lymphoblasts) in a 10% dimethylsulfoxide solution were driven in parallel with phosphate-buffered saline solution wash streams through single rectangular channel sections and multiple sections in series. The influence of cell-stream flow rate and cell volume fraction (CVF) on cell viability and recovery were examined. The channel depth was 500 μm, and average cell stream velocity within the channels was varied from 3.6 to 8.5 mm/s corresponding with cell stream Reynolds numbers of 2.6–6.0. Cell viability measured at device outlets was high for all cases examined indicating no significant cell damage within the device. Downstream of a single stage, cell recoveries measured 90–100% for average cell stream velocities ≥6 mm/s and for CVFs up to 20%. Cell recovery downstream of multistage devices also measured 90–100% after a critical device population time. This time was found to be five times the average cell residence time within the device. The measured recovery values were significantly larger than those typically obtained using conventional cell washing methods.     

Use of a gradient-generating microfluidic device to rapidly determine a suitable glucose concentration for cell viability test

We successfully determined a suitable glucose concentration for endothelial cells (ECs) using a gradient-generating microfluidic chip and a micro-stamper that were fabricated using micro-electro-mechanical systems (MEMS) technology. Our strategy was to generate a stable concentration gradient in the observation area based on a microfluidic network and micro-mixers, which produced a concentration gradient under various flow rates. The areas for cell adhesion were delineated on a glass slide with a micro-stamper using the micro-contact printing (μCP) method. We also discuss which glucose concentration gradients are suitable for cell viability test (i.e., 0–0.2%, 0.05–0.15%, and 0.06–0.17%). After examining various concentration gradients, the suitable glucose concentration for EC’s viability test was determined to range from 0.077% (4.2 mM) to 0.147% (8.16 mM). Higher or lower concentrations caused the ECs to atrophy or die. In this study, we describe a gradient-generating microfluidic chip that can be used to produce various drug concentrations for multi-concentration tests.     

A microfluidic device for rapid concentration of particles in continuous flow by DC dielectrophoresis

This article presents a microfluidic device (so called concentrator) for rapid and efficient concentration of micro/nanoparticles using direct current dielectrophoresis (DC DEP) in continuous fluid flow. The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures to focus efficiently the electric field in the flow direction to provide high field strength and gradient. Multiple trapping regions are formed within the concentrator. The location of particle trapping depends on the strength of the electric field applied. Under the experimental conditions, both streaming movement and DEP trapping of particles simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The observation agrees with the distribution of the simulated electric field and DEP force. The performance of the device is demonstrated by successfully and effectively concentrating fluorescent nanoparticles. At the sufficiently high electric field, the device demonstrates a trapping efficiency of 100%, which means downstream DEP traps and concentrates all (100%) the incoming particles from upstream. The trapping efficiency of the device is further studied by measuring the fluorescence intensity of concentrated particles in the channel. Typically, the fluorescence intensity becomes saturated in Trap 1 by applying the voltage (400 V) for >2 min, demonstrating that rapid concentration of the nanoparticles (10⁷ particles/ml) is achieved in the device. The microfluidic concentrator described can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.     

Thermal conductivity measurement of liquids in a microfluidic device

A new microfluidic-based approach to measuring liquid thermal conductivity is developed to address the requirement in many practical applications for measurements using small (microlitre) sample size and integration into a compact device. The approach also gives the possibility of high-throughput testing. A resistance heater and temperature sensor are incorporated into a glass microfluidic chip to allow transmission and detection of a planar thermal wave crossing a thin layer of the sample. The device is designed so that heat transfer is locally one-dimensional during a short initial time period. This allows the detected temperature transient to be separated into two distinct components: a short-time, purely one-dimensional part from which sample thermal conductivity can be determined and a remaining long-time part containing the effects of three-dimensionality and of the finite size of surrounding thermal reservoirs. Identification of the one-dimensional component yields a steady temperature difference from which sample thermal conductivity can be determined. Calibration is required to give correct representation of changing heater resistance, system layer thicknesses and solid material thermal conductivities with temperature. In this preliminary study, methanol/water mixtures are measured at atmospheric pressure over the temperature range 30–50°C. The results show that the device has produced a measurement accuracy of within 2.5% over the range of thermal conductivity and temperature of the tests. A relation between measurement uncertainty and the geometric and thermal properties of the system is derived and this is used to identify ways that error could be further reduced.     

Continuous cell cross over and lysis in a microfluidic device

Complex sample preparation processes are major stumbling blocks for the development of lab-on-a-chip (LOC). We herein advance a microfluidic device for chemical cell lysis using a cell cross over (CCO) technology for the purpose of minimizing the sample preparation steps. The proposed device allows cells to continuously cross over from a cell carrier to a cell lysis solution in a CCO region and to be automatically lysed. For the successful CCO and cell lysis, microflow patterns and cell movements in the CCO region are investigated by experimental as well as numerical studies. EL-4 mammalian cells are used for the demonstration of the performance of the proposed device. The DNA sample obtained from the developed device is quantitatively and qualitatively compared with the one obtained from a conventional chemical cell lysis method by using a UV–Vis spectrophotometer and gel electrophoresis. The quantitative analysis shows that the recovered DNA is 86% compared to the one obtained from the conventional chemical cell lysis.     

Potentiometric characterisation of a dual-stream electrochemical microfluidic device

A microfluidic device is presented with off-chip electrodes residing in a reservoir and connected via micro-capillaries to the Y-shaped microfluidic channel. The device is tested by potentiometric measurements involving dual-stream laminar flow of two aqueous solutions carrying different electrolytes at various concentrations. Open circuit potentials are measured for a series of solutions of alkali metal chlorides and tetraalkylammonium chlorides as well as for dilute hydrochloric acid. The open circuit potential for the microfluidic chip was calculated by taking into account the diffusion potential at finite ionic strength as well as the potential difference introduced by the reference electrode system. The liquid junction potential developed at the boundary of the co-flowing aqueous solutions may be manipulated to have greater or lesser relative contributions to the measured open circuit potential based on use of electrolyte salts having cation and anion pairs of similar or dissimilar mobilities in solution. A reasonable agreement between theoretical and experimental values of the open circuit potential is observed for these situations. The results show that simple microfluidic structures possess a rich environment for exploration and application of the solution chemistry of ions.     

Development of sorting, aligning, and orienting motile sperm using microfluidic device operated by hydrostatic pressure

In vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are the most commonly used assisted reproductive technologies to overcome male infertility problems. One of the obstacles of IVF and ICSI procedures is separating motile sperm from non-motile sperm to select the most competent sperm population from any given sperm sample. In addition, orientation and separation of the head from the tail is another obstacle for ICSI. Using the self-movement of sperm against flow direction, motile and non-motile sperm can be separated with an inexpensive polymeric microfluidic system. In this paper, we describe the development of a microfluidic system obtained through low-cost fabrication processes. We report experimental results of sperm sorting using hydrostatic pressure of three different species: bull, mouse, and human. The movement of cells in these channels was observed under a microscope and recorded with a digital camera. It is shown that the hydrostatic pressure and self-movement of motile sperm can be used to solve separating, aligning and orienting sperm in the microchannel.     

In-situ measurement of magnetic nanoparticle quantity in a microfluidic device

Microsystem Technologies - In this article we demonstrate a method for the accurate in situ determination of the quantity of the entrapped magnetic nanoparticles in the reaction chamber of a...     

Experimental study of microbubble coalescence in a T-junction microfluidic device

This study presents the microbubble coalescence process in a confined microchannel. Triple T-junction microfluidic devices with different main channel size were designed to generate monodispersed microbubble pairs with air/n-butyl alcohol–glycerol solution as the working system. The head-on collision of microbubble pair was realized in the microfluidic devices. Three collision results including absolute coalescence, probabilistic coalescence, and non-coalescence were distinguished. The effects of liquid viscosities and two-phase superficial velocities on the coalescence behavior were determined. The results showed that microbubble coalescence process in the confined space was slightly faster than in the free space. Increasing liquid viscosity apparently prevents coalescence. In the probabilistic coalescence region, higher two-phase superficial velocity could reduce the percentage of coalescence events. Two characteristic parameters representing the bubble contact time and film drainage time have been introduced to analyze the microbubble coalescence behaviors and a linear correlation could clearly distinguish the coalescence and non-coalescence region.     

Culture and chemical-induced fusion of tobacco mesophyll protoplasts in a microfluidic device

Microfluidic systems provide a powerful platform for biological analysis and have been applied in many disciplines. However, few efforts have been devoted to plant cell study. In this article, an optimized culture of tobacco mesophyll protoplasts and their first polyethylene glycol-induced fusion in a microfluidic device are presented. Culture medium optimization and dynamics of protoplast growth including size change, organelle motion, and cell mass formation were also investigated microscopically in real-time. On-chip protoplast culture showed that the first division percentage of tobacco mesophyll protoplasts could be improved as high as up to 85.6% in 5 days using NT1 medium, and the percentage of small cell mass formation was more than 48.0% in 10 days. Meanwhile, chemical-induced fusion of tobacco mesophyll protoplasts was realized in 3–5 min and a 28.8% fusion rate was obtained, which was similar to the conventional fusion in a macro-scale environment. These results will be helpful for the development of microfluidics-based studies on manipulation and analysis of plant cells in a miniaturized environment, including cell growth and differentiation, gene isolation, and cloning.     

Enhanced-throughput production of polymersomes using a parallelized capillary microfluidic device

We report a parallelized capillary microfluidic device for enhanced production rate of monodisperse polymersomes. This device consists of four independent capillary microfluidic devices, operated in parallel; each device produces monodisperse water-in-oil-in-water (W/O/W) double-emulsion drops through a single-step emulsification. During generation of the double-emulsion drops, the innermost water drop is formed first and it triggers a breakup of the middle oil phase over wide range of flow rates; this enables robust and stable formation of the double-emulsion drops in all drop makers of the parallelized device. Double-emulsion drops are transformed to polymersomes through a dewetting of the amphiphile-laden middle oil phase on the surface of the innermost water drop, followed by the subsequent separation of the oil drop. Therefore, we can make polymersomes with a production rate enhanced by a factor given by the number of drop makers in the parallelized device.     

Electrohydrodynamics around single ion-permselective glass beads fixed in a microfluidic device

This work demonstrates by direct visualization using confocal laser scanning microscopy that the application of electrical fields to a single-fixed, ion-permselective glass bead produces a remarkable complexity in both the coupled mass and charge transport through the bead and the coupled electrokinetics and hydrodynamics in the adjoining bulk electrolyte. The visualization approach enables the acquisition of a wealth of information, forming the basis for a detailed analysis of the underlying effects (e.g., ion-permselectivity, concentration polarization, nonequilibrium electroosmotic slip) and an understanding of electrohydrodynamic phenomena at charge-selective interfaces under more general conditions. The device used for fixing single beads in a microfluidic channel is flexible and allows to investigate the electrohydrodynamics in both transient and stationary regimes under the influence of bead shape, pore size and surface charge density, mobile phase composition, and applied volume forces. This insight is relevant for the design of microfluidic/nanofluidic interconnections and addresses the ionic conductance of discrete nanochannels, as well as nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in lab-on-a-chip devices.     

Flow rate effect on droplet control in a co-flowing microfluidic device

Flow rate effect on droplet formation in a co-flowing microfluidic device is investigated numerically. Transition conditions are discovered that the droplet size is either approximately independent of or strongly dependent on the flow rate ratio. This phenomenon is explained by the relation between strain rate and droplet diameter. Regions of four drop patterns are demarcated and conditions that give polydisperse drops are described, which is helpful to assure the accuracy and efficiency in droplet production.     

Pinch-off mechanism for Taylor bubble formation in a microfluidic flow-focusing device

The present work aims at studying the nonlinear breakup mechanism for Taylor bubble formation in a microfluidic flow-focusing device by using a high-speed digital camera. Experiments were carried out in a square microchannel with cross section of 600 × 600 μm. During the nonlinear collapse process, the variation of the minimum radius of bubble neck (r ₀) with the remaining time until pinch-off (τ) can be scaled by a power–law relationship: \(r_{0} \propto \tau^{\alpha } .\) Due to the interface rearrangement around the neck, the nonlinear collapse process can be divided into two distinct stages: liquid squeezing collapse stage and free pinch-off stage. In the liquid squeezing collapse stage, the neck collapses under the constriction of the liquid flow and the exponent α approaches to 0.33 with the increase in the liquid flow rate Q _l. In the free pinch-off stage, the value of α is close to the theoretical value of 0.50 derived from the Rayleigh–Plesset equation and is independent of Q _l.     

Droplet formation behavior in a microfluidic device fabricated by hydrogel molding

We describe the behavior of droplet formation within 3D cross-junctions and 2D T-junctions with various cross-sectional geometries that were manually fabricated using the hydrogel-molding method. The method utilizes wire-shaped hydrogels as molds to construct 3D and 2D microchannel structures. We investigated the flow patterns and droplet formation within the microchannels of these microfluidic devices. Despite being fabricated manually, the microchannels with 3D cross-junctions and 2D T-junctions were reproducible and formed highly monodispersed droplets. Additionally, the sizes of the droplets formed within the microchannels could be predicted using an experimental formula. This technique of droplet formation involves the use of a device fabricated by hydrogel molding. This method is expected to facilitate studies on droplet microfluidics and promote the use of droplet-based lab-on-a-chip technologies for various applications.