A clip-on electroosmotic pump for oscillating flow in microfluidic cell culture devices

Recent advances in microfluidic devices put a high demand on small, robust and reliable pumps suitable for high-throughput applications. Here we demonstrate a compact, low-cost, directly attachable (clip-on) electroosmotic pump that couples with standard Luer connectors on a microfluidic device. The pump is easy to make and consists of a porous polycarbonate membrane and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes. The soft electrode and membrane materials make it possible to incorporate the pump into a standard syringe filter holder, which in turn can be attached to commercial chips. The pump is less than half the size of the microscope slide used for many commercial lab-on-a-chip devices, meaning that these pumps can be used to control fluid flow in individual reactors in highly parallelized chemistry and biology experiments. Flow rates at various electric current and device dimensions are reported. We demonstrate the feasibility and safety of the pump for biological experiments by exposing endothelial cells to oscillating shear stress (up to 5 dyn/cm²) and by controlling the movement of both micro- and macroparticles, generating steady or oscillatory flow rates up to ± 400 μL/min.     

Coupled acoustic-shell model for experimental study of cell stiffness under acoustophoresis

Under the influence of acoustic radiation force, particles can be trapped and deformed at the pressure node in a microfluidic channel. Based on this principle, the elastic modulus of biological cells can be estimated. In this study, a numerical framework, consisting of a boundary element model for acoustic field and an axisymmetric shell model, is developed to simulate the cell deformation under acoustic radiation force. The boundary element model is used to calculate the radiation traction exerted on the cell surface. The cell membrane deformation due to this traction is simulated by using the axisymmetric shell model. The Young’s moduli of algae and red blood cell membranes are then estimated by comparing the experimental observation with the simulated membrane deformation. It is found that the value of Young’s modulus of the red blood cell membrane is lower than that of algae cell membrane. Furthermore, for both cells, the estimated Young’s moduli are negligible compared to the bulk moduli of the cells reported in the previous studies.     

Mathematical analysis of oxygen transfer through polydimethylsiloxane membrane between double layers of cell culture channel and gas chamber in microfluidic oxygenator

For successful cell culture in microfluidic devices, precise control of the microenvironment, including gas transfer between the cells and the surrounding medium, is exceptionally important. The work is motivated by a polydimethylsiloxane (PDMS) microfluidic oxygenator chip for mammalian cell culture suggesting that the speed of the oxygen transfer may vary depending on the thickness of a PDMS membrane or the height of a fluid channel. In this paper, a model is presented to describe the oxygen transfer dynamics in the PDMS microfluidic oxygenator chip for mammalian cell culture. Theoretical studies were carried out to evaluate the oxygen profile within the multilayer device, consisting of a gas reservoir, a PDMS membrane, a fluid channel containing growth media, and a cell culture layer. The corresponding semi-analytical solution was derived to evaluate dissolved oxygen concentration within the heterogeneous materials, and was found to be in good agreement with the numerical solution. In addition, a separate analytical solution was obtained to investigate the oxygen pressure drop (OPD) along the cell layer due to oxygen uptake of cells, with experimental validation of the OPD model carried out using human umbilical vein endothelial cells cultured in a PDMS microfluidic oxygenator. Within the theoretical framework, the effects of several microfluidic oxygenator design parameters were studied, including cell type and critical device dimensions.     

Electrokinetic mixing in microfluidic systems

The applications of electrokinetics in the development of microfluidic devices have been widely attractive in the past decade. Electrokinetic devices generally require no external mechanical moving parts and can be made portable by replacing the power supply by small battery. Therefore, electrokinetic-based microfluidic systems can serve as a viable tool in creating a lab-on-a-chip (LOC) or micro-total analysis system (μTAS) for use in biological and chemical assays. Mixing of analytes and reagents is a critical step in realizing lab-on-a-chip. This step is difficult due to the low Reynolds numbers flows in microscale devices. Hence, various schemes to enhance micro-mixing have been proposed in the past years. This review reports recent developments in the micro-mixing schemes based on DC and AC electrokinetics, including electrowetting-on-dielectric (EWOD), dielectrophoresis (DEP), and electroosmosis (EO). These electrokinetic-based mixing approaches are generally categorized as either active or passive in nature. Active mixers either use time-dependent (AC or DC field switching) or time-independent (DC field) external electric fields to achieve mixing, while passive mixers achieve mixing in DC fields simply by virtue of their geometric topology and surface properties, or electrokinetic instability flows. Typically, chaotic mixing can be achieved in some ways and is helpful to mixing under large Péclet number regimes. The overview given in this article provides a potential user or researcher of electrokinetic-based technology to select the most favorable mixing scheme for applications in the field of micro-total analysis systems.     

Electrokinetically driven concentration of particles and cells by dielectrophoresis with DC-offset AC electric field

Insulator-based dielectrophoresis (iDEP) has been successfully used for on-chip manipulations of biological samples. Despite its effectiveness, iDEP typically requires high DC voltages to achieve sufficient electric field; this is mainly due to the coupled phenomena among linear electrokinetics: electroosmosis (EO) and electrophoresis (EP) and nonlinear electrokinetics: dielectrophoresis (DEP). This paper presents a microfluidic technique using DC-offset AC electric field for electrokinetic concentration of particles and cells by repulsive iDEP. This technique introduces AC electric field for producing iDEP which is decoupled from electroosmosis (EO) and electrophoresis (EP). The repulsive iDEP is generated in a PDMS tapered contraction channel that induces non-uniform electric field. The benefits of introducing AC electric field component are threefold: (i) it contributes to DEP force acting on particles, (ii) it suppresses EO flow and (iii) it does not cause any EP motion. As a result, the required DC field component that is mainly used to transport particles on the basis of EO and EP can be significantly reduced. Experimental results supported by numerical simulations showed that the total DC-offset AC electric field strength required to concentrate 15-μm particles is significantly reduced up to 85.9% as compared to using sole DC electric field. Parametric experimental studies showed that the higher buffer concentration, larger particle size and higher ratio of AC-to-DC electric field are favorable for particle concentration. In addition, the proposed technique was demonstrated for concentration of yeast cells.     

A cell electrofusion microfluidic chip with micro-cavity microelectrode array

A new cell electrofusion microfluidic chip with 19,000 pairs of micro-cavity structures patterned on vertical sidewalls of a serpentine-shaped microchannel has been designed and fabricated. In each micro-cavity structure, the two sidewalls perpendicular to the microchannel are made of SiO₂ insulator, and that parallel to the microchannel is made of silicon as the microelectrode. One purpose of the design with micro-cavity microelectrode array is to obtain high membrane voltage occurring at the contact point of two paired cells, where cell fusion takes place. The device was tested to electrofuse NIH3T3 and myoblast cells under a relatively low voltage (~9 V). Under an AC electric field applied between the pair of microelectrodes positioned in the opposite micro-cavities, about 85–90 % micro-cavities captured cells, and about 60 % micro-cavities are effectively capable of trapping the desired two-cell pairs. DC electric pulses of low voltage (~9 V) were subsequently applied between the micro-cavity microelectrode arrays to induce electrofusion. Due to the concentration of the local electric field near the micro-cavity structure, fusion efficiency reaches about 50 % of total cells loaded into the device. Multi-cell electrofusion and membrane rupture at the end of cell chains are eliminated through the present novel design.     

Microfluidic devices in superconducting magnets: on-chip free-flow diamagnetophoresis of polymer particles and bubbles

Superconducting magnets enable the study of high magnetic fields on materials and objects, for example in material synthesis, self-assembly or levitation experiments. The setups employed often lack in precise spatial control of the object of interest within the bore of the magnet. Microfluidic technology enables accurate manipulation of fluidic surroundings and we have investigated the integration of microfluidic devices into superconducting magnets to enable controlled studies of objects in high magnetic fields. Polymeric microparticles similar in size to biological cells were manipulated via diamagnetic repulsion. The particles were suspended in an aqueous paramagnetic medium of manganese (II) chloride and pumped into a microfluidic chip, where they were repelled in continuous flow by the high magnetic field. The extent of deflection was studied as a function of increasing (1) particle size, (2) paramagnetic salt concentration, and (3) magnetic field strength. Optimizing these parameters allowed for the spatial separation of two particle populations via on-chip free-flow diamagnetophoresis. Finally, preliminary findings on the repulsion of air bubbles are shown.     

Geometry design of herringbone structures for cancer cell capture in a microfluidic device

Cancer cell detection with high capture efficiency is important for its extensive clinical applications. Herringbone structures in microfluidic devices have been widely adopted to increase the cell capture performance due to its chaotic effect. Given the fact of laminar flow in microfluidic devices, geometry-based optimization acting as a design strategy is effective and can help researchers reduce repetitive trial experiments. In this work, we presented a computational model to track the cell motion and used normalized capture efficiency to evaluate the tumor cell capture performance under various geometry settings. Cell adhesion probability was implemented in the model to consider the nature of ligand–receptor formation and breakage during cell–surface interactions. A facile approach was introduced to determine the two lumped coefficients of cell adhesion probability through two microfluidic experiments. A comprehensive geometric study was then performed by using this model, and results were explained from the fluid dynamics. Although most of the geometric guides agree with the general criterion concluded in the literature, we found herringbone structures with symmetric arms rather than a short arm–long arm ratio of 1/3 are optimal. This difference mainly comes from the fact that our model considers the particulate nature of cells while most studies in the literature optimize the geometry merely relying on mixing effects. Thus, our computational model implemented with cell adhesion probability can serve as a more accurate and reliable approach to optimize microfluidic devices for cancer cell capture.     

Microvalve and liquid membrane double-controlled integrated microfluidics for observing the interaction of breast cancer cells

The methodical development of cell biology has resulted in significant advancements in the study of breast tumors. However, the dynamic investigation of the self-seeding process remains largely out of reach. In the present study, we describe a microvalve and liquid membrane double-controlled integrated microfluidic device that provides for the versatile assessment of breast tumor cell invasion dynamics. The liquid membrane formation was first optimized to obtain a high level of control, and was then applied to different types of homotypic and heterotypic cell seeding with precise selective positioning for monoculture and coculture. Using this device, the interaction between breast cancer cells MDA231-LM2 and MDA-MB231 was successfully observed to investigate self-seeding dynamics, including migration, infiltration, and coexistence. The results quantitatively demonstrate the mutual signal-induced attraction between MDA-MB231 and MDA231-LM2 cells, as well as the progressive infiltration of MDA231-LM2 cells into the MDA-MB231 cell population. These results are valuable in the development of spatiotemporal-controlled microfluidic systems and to many microscale-based biological and diagnostic studies involving cell growth, cell differentiation, cell interaction, and cell signal.     

A Multilayer MEMS Platform for Single-Cell Electric Impedance Spectroscopy and Electrochemical Analysis

The fabrication and characterization of a microchamber electrode array for electrical and electrochemical studies of individual biological cells are presented. The geometry was tailored specifically for measurements from sensory hair cells isolated from the cochlea of the mammalian inner ear. Conventional microelectromechanical system (MEMS) fabrication techniques were combined with a heat-sealing technique and polydimethylsiloxane micromolding to achieve a multilayered microfluidic system that facilitates cell manipulation and selection. The system allowed for electrical stimulation of individual living cells and interrogation of excitable cell membrane dielectric properties as a function of space and time. A three-electrode impedimetric system was incorporated to provide the additional ability to record the time-dependent concentrations of specific biochemicals in microdomain volumes near identified regions of the cell membrane. The design and fabrication of a robust fluidic and electrical interface are also described. The interface provided the flexibility and simplicity of a “cartridge-based” approach in connecting to the MEMS devices. Cytometric measurement capabilities were characterized by using electric impedance spectroscopy (1 kHz–10 MHz) of isolated outer hair cells. Chemical sensing capability within the microchannel recording chamber was characterized by using cyclic voltammetry with varying concentrations of potassium ferricyanide $(hbox{K}_{3}hbox{Fe}(hbox{CN})_{6})$. Chronoamperometric recordings of electrically stimulated PC12 cells highlight the ability of the platform to resolve exocytosis events from individual cells.$hfill$[2007-0105]      

Analytical formulation of electric field and dielectrophoretic force for moving dielectrophoresis using Fourier series

Electrokinetics manipulation and separation of living cells employing microfluidic devices require good knowledge of the strength and distribution of electric field in such devices. AC dielectrophoresis is performed by generating non-uniform electric field using microsize electrodes. Among the several applications of dielectrophoretic phenomenon, this present study considers the recently introduced phenomenon of moving dielectrophoresis. An analytical solution using Fourier series is presented for the electric field distribution and dielectrophoretic force generated inside a microchannel. The potential at the upper part of the microchannel has been found by solving the governing equation of the electric potential with specific boundary conditions. The solutions for the electric field and dielectrophoretic force show excellent agreement with the numerical results. Microdevices were fabricated and experiments were carried out with living cells confirming and validating the analytical solutions.     

Particle/cell separation on microfluidic platforms based on centrifugation effect: a review

Particle/cell separation in heterogeneous mixtures including biological samples is a standard sample preparation step for various biomedical assays. A wide range of microfluidic-based methods have been proposed for particle/cell sorting and isolation. Two promising microfluidic platforms for this task are microfluidic chips and centrifugal microfluidic disks. In this review, we focus on particle/cell isolation methods that are based on liquid centrifugation phenomena. Under this category, we reviewed particle/cell sorting methods which have been performed on centrifugal microfluidic platforms, and inertial microfluidic platforms that contain spiral channels and multi-orifice channels. All of these platforms implement a form of centrifuge-based particle/cell separation: either physical platform centrifugation in the case of centrifugal microfluidic platforms or liquid centrifugation due to Dean drag force in the case of inertial microfluidics. Centrifugal microfluidic platforms are suitable for cases where the preparation step of a raw sample is required to be integrated on the same platform. However, the limited available space on the platform is the main disadvantage, especially when high sample volume is required. On the other hand, inertial microfluidics (spiral and multi-orifice) showed various advantages such as simple design and fabrication, the ability to process large sample volume, high throughput, high recovery rate, and the ability for multiplexing for improved performance. However, the utilization of syringe pump can reduce the portability options of the platform. In conclusion, the requirement of each application should be carefully considered prior to platform selection.     

No more bonding,no more clamping,magnetically assisted membrane integration in microfluidic devices

The integration of porous membranes with microfluidic devices allows a simple but high-throughput mass transport control for numerous microfluidic applications, such as single-cell separation, sample analysis, and purification. In this study, we demonstrate a novel integration process of porous membranes into microfluidic devices by applying a magnetic field and hydrodynamically stabilizing them. This new approach simplifies the integration process by removing physicochemical bonding between membranes and microfluidic devices, but overcomes many practical issues observed in current methods, such as device leakage, membrane replacement, and membrane material selection. More importantly, our approach allows us to install membranes with diverse physicochemical features and spatial configurations into a single microfluidic device. This additional ability can significantly improve its performance and capability in applications. Finally, we successfully demonstrate the utilization of our membrane device for simple particle separation.     

Carbon nanotubes in microfluidic lab-on-a-chip technology: current trends and future perspectives

Advanced nanomaterials such as carbon nanotubes (CNTs) display unprecedented properties such as strength, electrical conductance, thermal stability, and intriguing optical properties. These properties of CNT allow construction of small microfluidic devices leading to miniaturization of analyses previously conducted on a laboratory bench. With dimensions of only millimeters to a few square centimeters, these devices are called lab-on-a-chip (LOC). A LOC device requires a multidisciplinary contribution from different fields and offers automation, portability, and high-throughput screening along with a significant reduction in reagent consumption. Today, CNT can play a vital role in many parts of a LOC such as membrane channels, sensors and channel walls. This review paper provides an overview of recent trends in the use of CNT in LOC devices and covers challenges and recent advances in the field. CNTs are also reviewed in terms of synthesis, integration techniques, functionalization and superhydrophobicity. In addition, the toxicity of these nanomaterials is reviewed as a major challenge and recent approaches addressing this issue are discussed.     

Study of high-throughput cell electrofusion in a microelectrode-array chip

A microfabricated high-throughput cell electrofusion chip with 1,368 pairs of high aspect ratio silicon microelectrodes is presented. These microelectrodes, which were distributed in six individual microscale cell-fusion chambers, were covered with titanium and gold thin film to improve their electric conductivity as well as surface hydrophobility. Six chambers having different electrode distances make the chip highly suitable for fusing cells with different sizes. A microfluidic platform was set up for flowing control, cell manipulation and also experimental observation. Cells for electrofusion were first aligned at the prearranged locations by the dielectrophoretic force between two counter-electrodes, which benefits the traverse of electric pulse through the cell–cell contacting point for electroporation. Several on-chip cell electrofusion experiments have been carried out on different kinds of animal cells and plant protoplasts. Compared with conventional electrofusion methods, higher fusion efficiency was achieved on this device for precisely forming micropores on the proximate membranes of two contacting cells, and high throughput was also obtained due to the use of a large number of microelectrodes for cell manipulation and fusion. Moreover, a much lower power supply was required for the shorter distance between two counter-electrodes.     

Utilization of nanoparticles in microfluidic systems for optical detection

An increasing interest has been shown in microfluidic systems due to their properties including low consumption of reagents, short analysis time and easy integration. However, despite of these advantages over conventional methods, some limitations in sensitivity and selectivity still exist in microfluidic systems. Recently advancements in nanotechnology offer some new approaches for the detection of target analytes with high sensitivity and selectivity. As a result, it is an appropriate method to enhance the detection sensitivity through a combination between microfluidic system and nanotechnology. Optical detection is a dominant technique in microfluidics because of its noninvasive nature and easy coupling. Numerous studies that integrate optical microfluidic system with nanotechnology have been reported in recent years. Therefore, optical microfluidic systems in combination with nanomaterials (NMs) are reviewed in our work. We illustrate the functions of different NMs in optical microfluidic systems and the efforts of different researchers to improve the performance of devices. After the introduction of different nanoparticle-based optical detection methods, challenges and future directions in the development of nanoparticle-based optical detection schemes in microfluidics have also been discussed.     

EWOD microfluidic systems for biomedical applications

As the technology advances, a growing number of biomedical microelectromechanical systems (bio-MEMS) research involves development of lab-on-a-chip devices and micrototal analysis systems. For example, a portable instrument capable of biomedical analyses (e.g., blood sample analysis) and immediate recording, whether the patients are in the hospital or home, would be a considerable benefit to human health with an excellent commercial viability. Digital microfluidic (DMF) system based on the electrowetting-on-dielectric (EWOD) mechanism is an especially promising candidate for such point-of-care systems. The EWOD-based DMF system processes droplets in a thin space or on an open surface, unlike the usual microfluidic systems that process liquids by pumping them in microchannels. Droplets can be generated and manipulated on EWOD chip only with electric signals without the use of pumps or valves, simplifying the chip fabrication and the system construction. Microfluidic operations by EWOD actuation feature precise droplet actuation, less contamination risk, reduced reagents volume, better reagents mixing efficiency, shorter reaction time, and flexibility for integration with other elements. In addition, the simplicity and portability make the EWOD-based DMF system widely popular in biomedical or chemical fields as a powerful sample preparation platform. Many chemical and biomedical researches, such as DNA assays, proteomics, cell assays, and immunoassays, have been reported using the technology. In this paper, we have reviewed the recent developments and studies of EWOD-based DMF systems for biomedical applications published mostly during the last 5 years.     

Lattice Boltzmann method for microfluidics: models and applications

The lattice Boltzmann method (LBM) has experienced tremendous advances and has been well accepted as a useful method to simulate various fluid behaviors. For computational microfluidics, LBM may present some advantages, including the physical representation of microscopic interactions, the uniform algorithm for multiphase flows, and the easiness in dealing with complex boundary. In addition, LBM-like algorithms have been developed to solve microfluidics-related processes and phenomena, such as heat transfer, electric/magnetic field, and diffusion. This article provides a practical overview of these LBM models and implementation details for external force, initial condition, and boundary condition. Moreover, recent LBM applications in various microfluidic situations have been reviewed, including microscopic gaseous flows, surface wettability and solid–liquid interfacial slip, multiphase flows in microchannels, electrokinetic flows, interface deformation in electric/magnetic field, flows through porous structures, and biological microflows. These simulations show some examples of the capability and efficiency of LBM in computational microfluidics.     

Effect of channel geometry on solute dispersion in pressure-driven microfluidic systems

Pressure-driven transport of fluid and solute samples is often desirable in microfluidic devices, particularly where sufficient electroosmotic flow rates cannot be realized or the use of an electric field is restricted. Unfortunately, this mode of actuation also leads to hydrodynamic dispersion due to the inherent fluid shear in the system. While such dispersivity is known to scale with the square of the Peclet number based on the narrower dimension of the conduit (often the channel depth), the proportionality constant can vary significantly depending on its actual cross section. In this article, we review previous studies to understand the effect of commonly microfabricated channel cross sections on the Taylor–Aris dispersion of solute slugs in simple pressure-driven flow systems. We also analyze some recently proposed optimum designs which can reduce the contribution to this band broadening arising from the presence of the channel sidewalls. Finally, new simulation results have been presented in the last section of this paper which describe solutal spreading due to bowing of microchannels that can occur from stresses developed during their fabrication or operation under high-pressure conditions.     

基于无纺布检测浓度的微流控装置

目前微流控器件主要由玻璃或聚合物材料制成,而无纺布具有成本低、易加工、一次性等优点,为微流体材料提供了一种新的选择。本研究把蜡印染在无纺布上制备微流控器件,可以形成有效的蜡印通道,控制流体在内部通道扩散和流动。并把不同浓度的红色水溶性染料滴到通道中,利用图像处理算法检测其灰度值做浓度比对,发现在特定浓度下,灰度值呈现近似线性规律,可将其推广到生物医学现场检测等领域。