Particle focusing in microfluidic devices

Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.     

Single-cell microfluidic impedance cytometry: a review

Lab-on-chip technologies are being developed for multiplexed single cell assays. Impedance offers a simple non-invasive method for counting, identifying and monitoring cellular function. A number of different microfluidic devices for single cell impedance have been developed. These have potential applications ranging from simple cell counting and label-free identification of different cell types or detecting changes in cell morphology after invasion by parasites. Devices have also been developed that trap single cells and continuously record impedance data. This technology has applications in basic research, diagnostics, or non-invasively probing cell function at the single-cell level. This review will describe the underlying principles of impedance analysis of particles. It then describes the state-of-the-art in the field of microfluidic impedance flow cytometry. Finally, future directions and challenges are discussed.     

Experimental and numerical investigation on particle–particle interaction of multi-particle separation in an alternating and repetitive microchannel

Inertial focusing plays a major role in size-based cell separation or enrichment for microfluidic applications in many medical areas such as diagnostics and treatment. These applications often deal with suspensions of different particles which cause interactions between particles with different diameters such as particle–particle collision. In this study, particle–particle interaction in a laminar flow through a low aspect ratio alternating and repetitive microchannel is investigated both numerically and experimentally. It is revealed that particle–particle collision affects high quality particle focusing. computational fluid dynamics simulations are conducted for demonstrating the effect of the flow field in the transverse cross-section on the focusing quality and position. The experiments and simulations both revealed that if the flow is seeded with a mixture of particles of 3.3 and 9.9 µm diameters, the quality of focusing intensity is degenerated compared to the focusing features obtained by particles with a diameter of 9.9 µm solely. The results clearly show that particle–particle collision between the 3.3 and 9.9 µm particles has a negative effect on particle focusing behavior of the 9.9 µm particles.

     

Development of a biological detection platform utilizing a modular microfluidic stack

The goal of this project is to build a miniaturized, user-friendly cytometry setup (Datta et al. in Microfluidic platform for education and research. COMS, Baton Rouge, 2008; Frische et al. in Development of an miniaturized flow cytometry setup for visual cell inspection and sorting. Baton Rouge, Project Report, 2008) by combining a customized, microfluidic device with visual microscope inspection to detect and extract specific cells from a continuous sample flow. We developed a cytological tool, based on the Coulter particle counter principle, using a microelectrode array patterned on a borosilicate glass chip as electrical detection set-up which is fully embedded into a polymeric multi-layer microfluidic stack. The detection takes place between pairs of coplanar Cr/Au microelectrodes by sensing an impedance change caused by particles continuously carried within a microfluidic channel across the detection area under laminar flow conditions. A wide frequency range available for counting provides information on cell size, membrane capacitance, cytoplasm conductivity and is potentially of interest for in-depth cell diagnostic e.g. to detect damaged or cancerous cells and select them for extraction and further in-depth analysis.     

A novel microfluidic high-throughput resistive pulse sensing device for cells analysis

Microfluidic impedance-based devices offer a simple method for counting and sizing particles and cells in fields of biomedical research and clinical diagnosis. In this work, we present design, fabrication and operational characteristics of a novel high throughput original MEMS-based Coulter counter. This microfluidic device possesses two sub channels including two pairs of coplanar Au/Cr electrodes in each channels which allows double detection of the particles simultaneously and increases the throughput. The present design provides minimizing the cross talk and obviating the need for hydrodynamic focusing of the sample particles by adjusting Y shape insulation obstacle in direction of flow. Moreover, reducing coincidence events and removing electrode polarization effect were purposed by applying optimum sizes for electrodes considering the ease of fabrication and low costs. The reliability of the novel device was evaluated for polystyrene particles and cancer cells in conductive solutions. Results, which were recorded as relative resistance pulses across four sensing zones, illustrate the capability of the double-channel proposed device in detecting, counting and sizing 10 and 20 µm polystyrene particles. The superiority of present design was proved by relative counting error of below 3 and 11% for the 10 µm and 20 µm particles, respectively and a throughput of hundreds particles per second. Aiming at demonstrating the functionality of the proposed device in the biomedical area, counting of SP2/0 cells was performed. The measured counting outputs for cells in the size range of 5.63–17.6 µm were validated with results of hemocytometer cell counter, with relative error less than 7%.

     

Inertially focused diamagnetic particle separation in ferrofluids

Continuous flow separation of target particles from a mixture is essential to many chemical and biomedical applications. There has recently been an increasing interest in the integration of active and passive particle separation techniques for enhanced sensitivity and flexibility. We demonstrate herein the proof-of-concept of a ferrofluid-based hybrid microfluidic technique that combines passive inertial focusing with active magnetic deflection to separate diamagnetic particles by size. The two operations take place in series in a continuous flow through a straight rectangular microchannel with a nearby permanent magnet. We also develop a three-dimensional numerical model to simulate the transport of diamagnetic particles during their inertial focusing and magnetic separation processes in the entire microchannel. The predicted particle trajectories are found to be approximately consistent with the experimental observations at different ferrofluid flow rates and ferrofluid concentrations.     

Integrated CMOS optical sensor for cell detection and analysis

To circumvent the complexity of the detection systems of microfluidic devices, Hartley et al. recently reported on a CMOS optical active pixel sensor (APS) for near-field detection and counting of microscopic particles. To further enhance the digital cytometric capabilities of the original sensor, we modified and utilized a dual APS-array scheme to facilitate the determination of the velocity and size of particles flowing in microfluidic channels. Our findings indicate that the prototype dual-APS sensor is capable of detecting particle velocities up to ～500 μm/s and particles with diameter in the range of 5–15 μm. The dual APS CMOS sensor, as a result of the hybrid integration with a microfluidic, provides a low cost and practical means of noninvasively monitoring the contents of microfluidic and lab-on-a-chip devices.     

Diamagnetic particle focusing using ferromicrofluidics with a single magnet

Focusing particles into a tight stream is critical to many applications such as microfluidic flow cytometry and particle sorting. Current magnetic field-induced particle focusing techniques rely on the use of a pair of repulsive magnets, which makes the device integration and operation difficult. We develop herein a new approach to focusing nonmagnetic particles in ferrofluid flow through a T-microchannel using a single permanent magnet. Particles are deflected across the suspending ferrofluid by negative magnetophoresis and confined by a water flow to the center plane of the microchannel, leading to a focused particle stream flowing near the bottom channel wall. Such three-dimensional diamagnetic particle focusing is demonstrated in a sufficiently diluted ferrofluid through both the top and side views of the microchannel. As the suspended particles can be visualized in bright field, this magnetic focusing method is expected to find applications to label-free (i.e., no magnetic or fluorescent labeling) cellular focusing in lab-on-a-chip devices.     

A Nanoparticle Doped Micro-Geiger Counter for Multispecies Radiation Detection

This paper reports on a multichannel radiation detection platform enabled with nanoparticles that is capable of detecting and discriminating all types of radiation emitted from fissionable bomb making materials. Typical Geiger counters are limited to detecting only beta and gamma radiation. The micro-Geiger counter reported here detects all species of radiation including beta particles, gamma/X rays, alpha particles, and neutrons. The multispecies detecting micro-Geiger counter contains a hermetically sealed and electrically biased fill gas. Impinging radiation interacts with tailored nanoparticles to release secondary charged particles that ionize the fill gas. The ionized particles collect on respectively biased electrodes resulting in a characteristic electrical pulse. Pulse height spectroscopy and radiation energy binning techniques can then be used to analyze the pulses to determine the specific radiation isotope. The ideal voltage range of operation for energy discrimination was found to be in the proportional region at 1000 V_dc. In this region, specific pulse heights for different radiation species resulted. The amplification region strength which determines the device sensitivity to radiation energy can be tuned with the electrode separation distance. An electrode separation of 0.8 mm produced a count rate of 530 cpm for a ⁹⁰Sr beta source when compared to an off-the-shelf Geiger counter which produced 1500 cpm. Count rates as high as 15 300 were observed for the same radiation source with electrodes spaced closer than 0.5 mm. By using a novel microinjection ceramic molding and silver paste metallizing process, the batch fabrication of essentially disposable devices can be achieved.     

无透镜微流控成像流动细胞检测与计数系统

在未来面向个人化的生物医疗诊断中,实时的细胞检测与计数具有重要需求.现有的细胞检测和计数系统例如流式细胞仪和血细胞计数器不适用于小型化流动细胞实时检测和计数.通过将CMOS图像传感器芯片和微流控芯片结合,提出了一种用于流动细胞检测和计数的无透镜微流控成像系统,与用于计数静态细胞的其它无透镜微流控成像系统不同,该系统可以通过基于时域差分的运动检测算法检测和计数微流体通道中连续流动的细胞样本.测试结果表明:该系统可以对微流控通道中流动的人体骨髓基质细胞实现自动检测和计数,并具有-6.53％的低统计错误率.该系统提供了面向未来即时应用的细胞检测和计数解决方案.     

A dry film technology for the manufacturing of 3-D multi-layered microstructures and buried channels for lab-on-chip

The development of innovative and reliable techniques for devices miniaturization are enabling the massive growth of lab on chip (LOC) applications. In this article, we briefly review the technological options for LOC microfabrication, then we present the optimization of a process for the realization of tridimensional multi-layered structures and buried channels in a microfluidic network using a photo-patternable dry film, with a potential for LOC manufacturing. The tuning of all the fabrication parameters is widely discussed and micrographs and optical profiler images are reported to show fabrication results. The fabrication process is used for a Split-flow-thin (SPLITT) fractionation cell configuration. SPLITT is a particle fractionation technique based on the combined effect of two laminar streams (the sample containing the particles and a carrier) flowing inside a thin microchannel and the action of a vertical driving force for particle displacement. Since the SPLITT implemented in this work is electrically driven, patterned electrodes (thickness: 100 nm) are also integrated in the flow cell walls. The functionality of the cell was tested first verifying the presence of proper flow conditions for microfluidic SPLITT (absence of mixing between the streams) and then proving electrical fractionation with two different proteins (BSA and β-lactoglobulin) at different levels of ionic strength. The flow of the streams within the microfluidic channel was also simulated by a numerical 2-D model exactly reproducing the cell geometry, with a good accordance with experimental results.

     

Inertial particle focusing and spacing control in microfluidic devices

Focusing particles into a tight stream is usually a necessary step prior to counting, detecting and sorting them. Meanwhile, particle spacing control in microfluidic devices could also be applied in the field of accurate cell detection, material synthesis and chemical reaction. To achieve simultaneous particle focusing and spacing control, a novel microchannel composed by Dean and sheath flow section was proposed and fabricated according to the elaborated design principle with its manipulating performance in situ visualized. Using microspheres with a few microns as a template, the trajectory of the particles was discovered to follow lateral migration and reach certain equilibrium positions at the end of the designed Dean section. After being focused, the streamline was further concentrated and centralized with a controllable interparticle distance in sheath flow section. For sheath flow section, the angle between symmetrical tributaries and the mainstream channel and abrupt constriction/expansion structure of mainstream channel as important channel geometric features were investigated to minimize the focusing streamline width and optimize spacing control. An modified analytical model for sheath flow with different tributary angles was derived and proved to well describe the microsphere spacing control process.     

Magnetic particle dosing and size separation in a microfluidic channel

Separation of functional magnetic particles or magnetically labeled entities is a key feature for bioanalytical or biomedical applications and therefore also an important component of lab-on-a-chip devices for biological applications. We present a novel integrated microfluidic magnetic bead manipulation device, comprising dosing of magnetic particles, controlled release and subsequent magnetophoretic size separation with high resolution. The system is designed to meet the requirements of specific bioassays, in particular of on-chip agglutination assays for the detection of rare analytes by particle coupling as doublets. Integrated soft-magnetic microtips with different shapes provide the magnetic driving force of the bead manipulation protocol. The magnetic tips that serve as field concentrators of an external electromagnetic field, are positioned in close contact to a microfluidic channel in order to generate high magnetic actuation forces. Mixtures of 1.0 μm and 2.8 μm superparamagnetic beads have been used to characterize the system. Magnetophoretic size separation with high resolution was performed in static conditions and in continuous flow mode. In particular, we could demonstrate the separation of 1.0 μm single beads and doublets in a sample flow.     

Inertial microfluidics for continuous particle filtration and extraction

In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.     

Microfluidic counterflow centrifugal elutriation system for sedimentation-based cell separation

We present a centrifugal microfluidic system for precise cell/particle sorting using the concept of counterflow centrifugal elutriation (CCE). A conventional CCE system uses a rotor device incorporating a flow-through separation chamber, in which the balance of centrifugal and counterflow drag forces exerted on particles is gradually shifted by changing the flow rate and/or the rotation speed. In the present system, both the centrifugal and the fluid forces are generated through microdevice rotation in order to significantly simplify the setup of the conventional CCE. In addition, the density gradient of the medium is employed to elute particles/cells of different sedimentation velocities stepwise from the separation chamber instead of changing the rotation speed. We successfully separated polymer particles with diameters of 1.0–5.0 μm using a branched loading channel for focusing particles to the center of the separation chamber. We also demonstrated the sorting of blood cells for biological applications. This system may provide a versatile means for cell/particle sorting in a general biological laboratory and function as a unit operation in various centrifugal microfluidic platforms for biochemical experiments and clinical diagnosis.     

Cluster formation and growth in microchannel flow of dilute particle suspensions

The lifetime of microfluidic devices depends on their ability to maintain flow without interruption. Certain applications require microdevices for transport of liquids containing particles. However, microchannels are susceptible to blockage by solid particles. Therefore, in this study, the phenomenon of interest is the formation and growth of clusters on a microchannel surface in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flows in microchannels with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel are presented. Although the cluster size seems to be independent of location, more clusters are found at the inlet/outlet regions than in the microchannel center. Similarly as for an individual cluster, as long as particle–cluster interaction is the dominant mode, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size, and concentration are also reported.     

On the direct employment of dipolar particle interaction in microfluidic systems

This review article will summarize recent developments in the employment of dipolar coupled magnetic particle structures. We will discuss the basics of magnetic dipolar particle interaction in static and rotating magnetic fields. In dependence on the magnetic fields employed, agglomerates of different dimensionality may form within the carrier liquid. The stability and formation dynamics of these particle structures will be presented. Furthermore, we will review recent microfluidic applications based on the interaction of magnetic particles and present methods for surface patterning with micron-sized and nano-sized particles which employ dipolar particle coupling.     

Oscillations in light-triggered logic microfluidic circuit

Control of droplets in microfluidic environments has numerous applications ranging from analysis and sample preparation for biomaterials synthesis (Mann and Ozin Nature 382:313–318, 1996) and medical diagnostics (Pipper et al. Nat Med 13:1259–1263, 2007) to photonics (Schmidt and Hawkins Nat Photonics 5:598–604, 2011). Here we study the oscillations present in a microfluidic circuit capable of sorting curable droplets on demand by triggering the circuit with UV-light. Prior to this paper we showed that a simple circuit can self-sort particles and produce a binary output, sorted or rejected stream of particles, based on the hydrodynamic resistance induced by the particles as they flow through the microfluidic channels. We showed that the cross-linking of droplets can modulate the resistance, and demonstrated particle switching by sorting of otherwise identical droplets of uncured and cured photocurable solution immersed in mineral oil solution. Before arriving at the sorting circuit, droplets made of a photocurable solution were illuminated by a UV-light from a mercury lamp, curing them. By tuning the outlet pressures, the switching threshold could be tuned so that uncured droplets were rejected while cured droplets were switched (Raafat et al. μTAS Proc 1826–1828, 2010; Cartas-Ayala et al. Small 9:375–381, 2013). Here we use this system to study the oscillations in this circuit due to particle–particle interactions in the circuit. The circuit oscillation can be used as a counter with a light ON/OFF switch. The circuit behavior agrees well with theoretical predictions of droplet oscillations. Furthermore, the circuit oscillations can be switched on or off by UV-light illumination. This experiment demonstrates switching of particles based on deformability, illustrates the switching of particles by using light, and the possibility of creating new managing schemes for droplets by combining light control with droplet generation-rate control.     

Micro-injection moulding of polymer microfluidic devices

Microfluidic devices have several applications in different fields, such as chemistry, medicine and biotechnology. Many research activities are currently investigating the manufacturing of integrated microfluidic devices on a mass-production scale with relatively low costs. This is especially important for applications where disposable devices are used for medical analysis. Micromoulding of thermoplastic polymers is a developing process with great potential for producing low-cost microfluidic devices. Among different micromoulding techniques, micro-injection moulding is one of the most promising processes suitable for manufacturing polymeric disposable microfluidic devices. This review paper aims at presenting the main significant developments that have been achieved in different aspects of micro-injection moulding of microfluidic devices. Aspects covered include device design, machine capabilities, mould manufacturing, material selection and process parameters. Problems, challenges and potential areas for research are highlighted.     

Enhancing microcantilever capability with integrated AC electroosmotic trapping

Microcantilevers are finding wide applications in detecting biochemical agents. However, their usage has been limited to highly concentrated samples to ensure sufficient deposition of agents onto cantilevers. A pre-concentration or enrichment step will expand their application range to more dilute, practical samples and real-time detection. This paper reports the integration of in-situ particle concentrators on microcantilevers. Only a thin metal layer on microcantilevers is required to generate microfluidic convection of particles from solution bulk onto microcantilever surfaces, greatly enriching local particle counts and enhancing sensitivity of the system. A working prototype is presented in the paper. Preliminary experiments concentrating latex particles were conducted and the particle concentration effect has been experimentally verified using AFM probes as microcantilevers. As ACEO concentrator has no dependence on particle properties, the method is expected to be applicable to bio-particles collection.