Microfluidic platforms for lab-on-a-chip applications

We review microfluidic platforms that enable the miniaturization, integration and automation of biochemical assays. Nowadays nearly an unmanageable variety of alternative approaches exists that can do this in principle. Here we focus on those kinds of platforms only that allow performance of a set of microfluidic functions--defined as microfluidic unit operations-which can be easily combined within a well defined and consistent fabrication technology to implement application specific biochemical assays in an easy, flexible and ideally monolithically way. The microfluidic platforms discussed in the following are capillary test strips, also known as lateral flow assays, the "microfluidic large scale integration" approach, centrifugal microfluidics, the electrokinetic platform, pressure driven droplet based microfluidics, electrowetting based microfluidics, SAW driven microfluidics and, last but not least, "free scalable non-contact dispensing". The microfluidic unit operations discussed within those platforms are fluid transport, metering, mixing, switching, incubation, separation, droplet formation, droplet splitting, nL and pL dispensing, and detection.     

Lab-on-chip methodologies for the study of transport in porous media: energy applications

We present a lab-on-chip approach to the study of multiphase transport in porous media. The applicability of microfluidics to biological and chemical analysis has motivated much development in lab-on-chip methodologies. Several of these methodologies are also well suited to the study of transport in porous media. We demonstrate the application of rapid prototyping of microfluidic networks with approximately 5000 channels, controllable wettability, and fluorescence-based analysis to the study of multiphase transport phenomena in porous media. The method is applied to measure the influence of wettability relative to network regularity, and to differentiate initial percolation patterns from active flow paths. Transport phenomena in porous media are of critical importance to many fields and particularly in many energy-related applications including liquid water transport in fuel cells, oil recovery, and CO(2) sequestration.     

Electrowetting-based actuation of droplets for integrated microfluidics

The serviceability of microfluidics-based instrumentation including 'lab-on-a-chip' systems critically depends on control of fluid motion. We are reporting here an alternative approach to microfluidics based upon the micromanipulation of discrete droplets of aqueous electrolyte by electrowetting. Using a simple open structure, consisting of two sets of opposing planar electrodes fabricated on glass substrates, positional and formational control of microdroplets ranging in size from several nanoliters to several microliters has been demonstrated at voltages between 15-100 V. Since there are no permanent channels or structures between the plates, the system is highly flexible and reconfigurable. Droplet transport is rapid and efficient with average velocities exceeding 10 cm s(-1) having been observed. The dependence of the velocity on voltage is roughly independent of the droplet size within certain limits, thus the smallest droplets studied (approximately 3 nl) could be transported over 1000 times their length per second. Formation, mixing, and splitting of microdroplets was also demonstrated using the same microactuator structures. Thus, electrowetting provides a means to achieve high levels of functional integration and flexibility for microfluidic systems.     

An integrated digital microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids

Clinical diagnostics is one of the most promising applications for microfluidic lab-on-a-chip systems, especially in a point-of-care setting. Conventional microfluidic devices are usually based on continuous-flow in microchannels, and offer little flexibility in terms of reconfigurability and scalability. Handling of real physiological samples has also been a major challenge in these devices. We present an alternative paradigm--a fully integrated and reconfigurable droplet-based "digital" microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids. The microdroplets, which act as solution-phase reaction chambers, are manipulated using the electrowetting effect. Reliable and repeatable high-speed transport of microdroplets of human whole blood, serum, plasma, urine, saliva, sweat and tear, is demonstrated to establish the basic compatibility of these physiological fluids with the electrowetting platform. We further performed a colorimetric enzymatic glucose assay on serum, plasma, urine, and saliva, to show the feasibility of performing bioassays on real samples in our system. The concentrations obtained compare well with those obtained using a reference method, except for urine, where there is a significant difference due to interference by uric acid. A lab-on-a-chip architecture, integrating previously developed digital microfluidic components, is proposed for integrated and automated analysis of multiple analytes on a monolithic device. The lab-on-a-chip integrates sample injection, on-chip reservoirs, droplet formation structures, fluidic pathways, mixing areas and optical detection sites, on the same substrate. The pipelined operation of two glucose assays is shown on a prototype digital microfluidic lab-on-chip, as a proof-of-concept.     

Size-selective sliding of sessile drops on a slightly inclined plane using low-frequency AC electrowetting

When placed on an inclined solid plane, drops often stick to the solid surface due to pinning forces caused by contact angle hysteresis. When the drop size or the plane's incline angle is small, the drop is difficult to slide due to a decrease in gravitational force. Here we demonstrate that small drops (0.4-9 μL) on a slightly inclined plane (~12°, Teflon and parylene-C surface) can be mobilized through patterned electrodes by applying low-frequency ac electrowetting under 400 Hz (110-180 V(rms)), which has a mechanism different from that of the high-frequency ac method that induces sliding by reducing contact angle hysteresis. We attribute the sliding motion of our method to a combination of contact angle hysteresis and interfacial oscillation driven by ac electrowetting instead of the minimization of contact angle hysteresis at a high frequency. We investigated the effects of ac frequency on the sliding motion and terminal sliding of drops; the terminal sliding velocity is greatest at resonance frequency. Varying the electrowetting number (0.21-0.56) at a fixed frequency (40 Hz) for 5 μL drops, we found an empirical relationship between the electrowetting number and the terminal sliding velocity. Using the relationship between the drop size and ac frequency, we can selectively slide drops of a specific size or merge two drops along an inclined plane. This simple method will help with constructing microfluidic platforms with sorting, merging, transporting, and mixing of drops without a programmable control of electrical signals. Also, this method has a potential in heat transfer applications because heat removal capacity can be enhanced significantly through drop oscillation.     

Rapid prototyping polymers for microfluidic devices and high pressure injections

Multiple methods of fabrication exist for microfluidic devices, with different advantages depending on the end goal of industrial mass production or rapid prototyping for the research laboratory. Polydimethylsiloxane (PDMS) has been the mainstay for rapid prototyping in the academic microfluidics community, because of its low cost, robustness and straightforward fabrication, which are particularly advantageous in the exploratory stages of research. However, despite its many advantages and its broad use in academic laboratories, its low elastic modulus becomes a significant issue for high pressure operation as it leads to a large alteration of channel geometry. Among other consequences, such deformation makes it difficult to accurately predict the flow rates in complex microfluidic networks, change flow speed quickly for applications in stop-flow lithography, or to have predictable inertial focusing positions for cytometry applications where an accurate alignment of the optical system is critical. Recently, other polymers have been identified as complementary to PDMS, with similar fabrication procedures being characteristic of rapid prototyping but with higher rigidity and better resistance to solvents; Thermoset Polyester (TPE), Polyurethane Methacrylate (PUMA) and Norland Adhesive 81 (NOA81). In this review, we assess these different polymer alternatives to PDMS for rapid prototyping, especially in view of high pressure injections with the specific example of inertial flow conditions. These materials are compared to PDMS, for which magnitudes of deformation and dynamic characteristics are also characterized. We provide a complete and systematic analysis of these materials with side-by-side experiments conducted in our lab that also evaluate other properties, such as biocompatibility, solvent compatibility, and ease of fabrication. We emphasize that these polymer alternatives, TPE, PUMA and NOA, have some considerable strengths for rapid prototyping when bond strength, predictable operation at high pressure, or transitioning to commercialization are considered important for the application.     

Inertial microfluidics: Recent advances

Inertial microfluidics has attracted significant attentions in last decade due to its superior advantages of high throughput, label- and external field-free operation, simplicity, and low cost. A wide variety of channel geometry designs were demonstrated for focusing, concentrating, isolating, or separating of various bioparticles such as blood components, circulating tumor cells, bacteria, and microalgae. In this review, we first briefly introduce the physics of inertial migration and Dean flow for allowing the readers with diverse backgrounds to have a better understanding of the fundamental mechanisms of inertial microfluidics. Then, we present a comprehensive review of the recent advances and applications of inertial microfluidic devices according to different channel geometries ranging from straight channels, curved channels to contraction-expansion-array channels. Finally, the challenges and future perspective of inertial microfluidics are discussed. Owing to its superior benefit for particle manipulation, the inertial microfluidics will play a more important role in biology and medicine applications.     

Logic digital fluidic in miniaturized functional devices: Perspective to the next generation of microfluidic lab‐on‐chips

Microfluidics has emerged following the quest for scale reduction inherent to micro‐ and nanotechnologies. By definition, microfluidics manipulates fluids in small channels with dimensions of tens to hundreds of micrometers. Recently, microfluidics has been greatly developed and its influence extends not only the domains of chemical synthesis, bioanalysis, and medical researches but also optics and information technology. In this review article, we will shortly discuss an enlightening analogy between electrons transport in electronics and fluids transport in microfluidic channels. This analogy helps to master transport and sorting. We will present some complex microfluidic devices showing that the analogy is going a long way off toward more complex components with impressive similarities between electronics and microfluidics. We will in particular explore the vast manifold of fluidic operations with passive and active fluidic components, respectively, as well as the associated mechanisms and corresponding applications. Finally, some relevant applications and an outlook will be cited and presented.     

Behaviour and design considerations for continuous flow closed-open-closed liquid microchannels

This paper introduces a method of combining open and closed microchannels in a single component in a novel way which couples the benefits of both open and closed microfluidic systems and introduces interesting on-chip microfluidic behaviour. Fluid behaviour in such a component, based on continuous pressure driven flow and surface tension, is discussed in terms of cross sectional flow behaviour, robustness, flow-pressure performance, and its application to microfluidic interfacing. The closed-open-closed microchannel possesses the versatility of upstream and downstream closed microfluidics along with open fluidic direct access. The device has the advantage of eliminating gas bubbles present upstream when these enter the open channel section. The unique behaviour of this device opens the door to applications including direct liquid sample interfacing without the need for additional and bulky sample tubing.     

Fabrication of circular microfluidic channels by combining mechanical micromilling and soft lithography

The fabrication of microfluidic channels with complex three-dimensional (3D) geometries presents a major challenge to the field of microfluidics, because conventional lithography methods are mainly limited to rectangular cross-sections. In this paper, we demonstrate the use of mechanical micromachining to fabricate microfluidic channels with complex cross-sectional geometries. Micro-scale milling tools are first used to fabricate semi-circular patterns on planar metallic surfaces to create a master mold. The micromilled pattern is then transferred to polydimethylsiloxane (PDMS) through a two-step reverse molding process. Using these semi-circular PDMS channels, circular cross-sectioned microchannels are created by aligning and adhering two channels face-to-face. Straight and serpentine-shaped microchannels were fabricated, and the channel geometry and precision of the metallic master and PDMS molds were assessed through scanning electron microscopy and non-contact profilometry. Channel functionality was tested by perfusion of liquid through the channels. This work demonstrates that micromachining enabled soft lithography is capable of fabricating non-rectangular cross-section channels for microfluidic applications. We believe that this approach will be important for many fields from biomimetics and vascular engineering to microfabrication and microreactor technologies.     

Synergistic microfluidic and electrochemical strategy to activate and pattern surfaces selectively with ligands and cells

We show a straightforward, flexible synergistic approach that combines microfluidics, electrochemistry, and a general immobilization strategy to activate regions of a substrate selectively for the precise immobilization of ligands and cells in patterns for a variety of cell-based assays and cell migration and cell adhesion studies. We develop microfluidic microchips to control the delivery of electrolyte solution to select regions of an electroactive hydroquinone SAM. Once an electrical potential is applied to the substrate, only the hydroquinone exposed to electrolyte solution within the microfluidic channels oxidizes to the corresponding quinone. The quinone form can then react chemoselectively with oxyamine-tethered ligands to pattern the surface. Therefore, this microfluidic/electrochemistry strategy selectively activates the surface for ligand patterning that exactly matches the channel design of the microfluidic channel. We demonstrate the ease of this system by first quantitatively characterizing the electrochemical activation and immobilization of ligands on the surface. Second, we immobilize a fluorescent dye to show the fidelity of the methodology, and third, we show the immobilization of biospecific cell adhesive peptide ligands to pattern cells. This is the first report that combines microfluidics/electrochemistry and a general electroactive immobilization strategy to pattern ligands and cells. We believe that this strategy will be of broad utility for applications ranging from fundamental studies of cell behavior to patterning molecules on a variety of materials for molecular electronic devices.     

Digital microfluidics using soft lithography

Although microfluidic chips have demonstrated basic functionality for single applications, performing varied and complex experiments on a single device is still technically challenging. While many groups have implemented control software to drive the pumps, valves, and electrodes used to manipulate fluids in microfluidic devices, a new level of programmability is needed for end users to orchestrate their own unique experiments on a given device. This paper presents an approach for programmable and scalable control of discrete fluid samples in a polydimethylsiloxane (PDMS) microfluidic system using multiphase flows. An immiscible fluid phase is utilized to separate aqueous samples from one another, and a novel "microfluidic latch" is used to precisely align a sample after it has been transported a long distance through the flow channels. To demonstrate the scalability of the approach, this paper introduces a "general-purpose" microfluidic chip containing a rotary mixer and addressable storage cells. The system is general purpose in that all operations on the chip operate in terms of unit-sized aqueous samples; using the underlying mechanisms for sample transport and storage, additional sensors and actuators can be integrated in a scalable manner. A novel high-level software library allows users to specify experiments in terms of variables (i.e., fluids) and operations (i.e., mixes) without the need for detailed knowledge about the underlying device architecture. This research represents a first step to provide a programmable interface to the microfluidic realm, with the aim of enabling a new level of scalability and flexibility for lab-on-a-chip experiments.     

Droplet-based microfluidic lab-on-a-chip for glucose detection

A microfluidic lab-on-a-chip (LoC) platform for in vitro measurement of glucose for clinical diagnostic applications is presented in this paper. The LoC uses a discrete droplet format in contrast to conventional continuous flow microfluidic systems. The droplets act as solution-phase reaction chambers and are manipulated using the electrowetting effect. Glucose is measured using a colorimetric enzyme-kinetic method based on Trinder’s reaction. The color change is detected using an absorbance measurement system consisting of a light emitting diode and a photodiode. The linear range of the assay is 9-100 mg/dl using a sample dilution factor of 2 and 15-300 mg/dl using a sample dilution factor of 3. The results obtained on the electrowetting system compare favorably with conventional measurements done on a spectrophotometer, indicating that there is no change in enzyme activity under electrowetting conditions.     

Droplet-on-a-wristband: chip-to-chip digital microfluidic interfaces between replaceable and flexible electrowetting modules

We present a long (204 mm), curved (curvature of 0.04 mm(-1)), and closed droplet pathway in "droplet-on-a-wristband" (DOW) with the designed digital microfluidic modular interfaces for electric signal and droplet connections based on the study of electrowetting-on-dielectric (EWOD) in inclined and curved devices. Instead of using sealed and leakage-proof pipes to transmit liquid and pumping pressure, the demonstrated modular interface for electrowetting-driven digital microfluidics provides simply electric and fluidic connections between two adjacent parallel-plate modules which are easy-to-attach/detach, showing the advantages of using droplets for microfluidic connections between modules. With the previously reported digital-to-channel interfaces (Abdelgawad et al., Lab Chip, 2009, 9, 1046-1051), the chip-to-chip interface presented here would be further applied to continuous microfluidics. Droplet pumping across a single top plate gap and through a modular interface with two gaps between overlapping plates are investigated. To ensure the droplet transportation in the DOW, we actuate droplets against gravity in an inclined or curved device fabricated on flexible PET substrates prepared by a special razor blade cutter and low temperature processes. Pumping a 2.5 μl droplet at a speed above 105 mm s(-1) is achieved by sequentially switching the entire 136 driving electrodes (1.5 mm × 1.5 mm) along the four flexible modules of the DOW fabricated by 4-inch wafer facilities.     

Programmable fluidic production of microparticles with configurable anisotropy

We report a technique for continuous production of microparticles of variable size with new forms of anisotropy including alternating bond angles, configurable patchiness, and uniform roughness. The sequence and shape of the anisotropic particles are configured by exploiting a combination of confinement effects and microfluidics to pack precursor colloids with different properties into a narrow, terminal channel. The width and length of the channel relative to the particle size fully specify the configuration of the anisotropic particle that will be produced. The precursor spheres packed in the production zone are then permanently bonded into particles by thermal fusing. The flow in the production zone is reversed to release the particles for collection and use. Particles produced have linear chain structure with precisely configured, repeatable bond angles. With software programmable microfluidics, sequence and shape anisotropy are combined to yield synthesized homogeneous (type "A"), surfactantlike (type "A-B") or triblock (type "A-B-A") internal sequences in a single device. By controlling the dimensions of the microfluidic production zone, triangular prisms and particles with controlled roughness and patchiness are produced. The fabrication method is performed with precursors spheres with diameter as small as 3.0 microm.     

Integrated cell manipulation system--CMOS/microfluidic hybrid

Manipulation of biological cells using a CMOS/microfluidic hybrid system is demonstrated. The hybrid system starts with a custom-designed CMOS (complementary metal-oxide semiconductor) chip fabricated in a semiconductor foundry. A microfluidic channel is post-fabricated on top of the CMOS chip to provide biocompatible environments. The motion of individual biological cells that are tagged with magnetic beads is directly controlled by the CMOS chip that generates microscopic magnetic field patterns using an on-chip array of micro-electromagnets. Furthermore, the CMOS chip allows high-speed and programmable reconfiguration of the magnetic fields, substantially increasing the manipulation capability of the hybrid system. Extending from previous work that verified the concept of the hybrid system, this paper reports a set of manipulation experiments with biological cells, which further confirms the advantage of the hybrid approach. To enhance the biocompatibility of the system, the microfluidic channel is redesigned and the temperature of the device is monitored by on-chip sensors. Combining microelectronics and microfluidics, the CMOS/microfluidic hybrid system presents a new model for a cell manipulation platform in biological and biomedical applications.     

Microfluidic integration for electrochemical biosensor applications

Electrochemical biosensors are particularly suitable for miniaturization and integration in microfluidic devices. Applications include the detection of whole cells, cell components, proteins, and small molecules to address tasks in the fields of diagnostics and food and environmental control. Microfluidic setups range from simple channels for sample transport to channels with integrated sensing electrodes to highly sophisticated platforms with additional elements for sample preparation. The design of the microfluidics depends on both the type of detection and on the application and sample material. This review summarizes recent work on electrochemical biosensors with integrated microfluidics with the focus on developments for real sample applications, particularly those including measurements with real sample media.     

Thermoplastic elastomers for microfluidics: towards a high-throughput fabrication method of multilayered microfluidic devices

Multilayer soft lithography of polydimethylsiloxane (PDMS) is a well-known method for the fabrication of complex fluidic functions. With advantages and drawbacks, this technique allows fabrication of valves, pumps and micro-mixers. However, the process is inadequate for industrial applications. Here, we report a rapid prototyping technique for the fabrication of multilayer microfluidic devices, using a different and promising class of polymers. Using styrenic thermoplastic elastomers (TPE), we demonstrate a rapid technique for the fabrication and assembly of pneumatically driven valves in a multilayer microfluidic device made completely from thermoplastics. This material solution is transparent, biocompatible and as flexible as PDMS, and has high throughput thermoforming processing characteristics. We established a proof of principle for valving and mixing with three different grades of TPE using an SU-8 master mold. Specific viscoelastic properties of each grade allow us to report enhanced bonding capabilities from room temperature bonding to free pressure thermally assisted bonding. In terms of microfabrication, beyond classically embossing means, we demonstrate a high-throughput thermoforming method, where TPE molding experiments have been carried out without applied pressure and vacuum assistance within an overall cycle time of 180 s. The quality of the obtained thermoplastic systems show robust behavior and an opening/closing frequency of 5 Hz.     

Optical imaging techniques in microfluidics and their applications

Microfluidic devices have undergone rapid development in recent years and provide a lab-on-a-chip solution for many biomedical and chemical applications. Optical imaging techniques are essential in microfluidics for observing and extracting information from biological or chemical samples. Traditionally, imaging in microfluidics is achieved by bench-top conventional microscopes or other bulky imaging systems. More recently, many novel compact microscopic techniques have been developed to provide a low-cost and portable solution. In this review, we provide an overview of optical imaging techniques used in microfluidics followed with their applications. We first discuss bulky imaging systems including microscopes and interferometer-based techniques, then we focus on compact imaging systems that can be better integrated with microfluidic devices, including digital in-line holography and scanning-based imaging techniques. The applications in biomedicine or chemistry are also discussed along with the specific imaging techniques.     

Three-dimensional (3D) hydrodynamic focusing for continuous sampling and analysis of adherent cells

A simple three-dimensional (3D) hydrodynamic focusing microfluidic device integrated with continuous sampling, rapid dynamic lysis, capillary electrophoretic (CE) separation and detection of intracellular content is presented. One of the major difficulties in microfluidic cell analysis for adherent cells is that the cells are prone to attaching to the channel surface. To solve this problem, a cross microfluidic chip with three sheath-flow channels located on both sides of and below the sampling channel was developed. With the three sheath flows around the sample solution-containing cells, the formed soft fluid wall prevents the cells from adhering to the channel surface. Labeled cells were 3D hydrodynamically focused by the sheath-flow streams and smoothly introduced into the cross-section one by one. The introduction of sheath-flow streams not only ensured single-cell sampling but avoided blockage of the sampling channel by adherent cells as well. The maximum rate for introduction of individual cells into the separation channel was about 151 cells min(-1). With electric field applied on the separation channel, the aligned cells were driven into the separation channel and rapidly lysed within 400 ms at the entry of the channel by sodium dodecylsulfate (SDS) added in the sheath-flow solution. The microfluidic system was evaluated by analysis of reduced glutathione (GSH) and reactive oxygen species (ROS) in single HepG2 cells. The average analysis throughput of ROS and GSH in single cells was 16-18 cells min(-1).