Showing papers in "Lab on a Chip in 2016"

The upcoming 3D-printing revolution in microfluidics

Abstract: In the last two decades, the vast majority of microfluidic systems have been built in poly(dimethylsiloxane) (PDMS) by soft lithography, a technique based on PDMS micromolding. A long list of key PDMS properties have contributed to the success of soft lithography: PDMS is biocompatible, elastomeric, transparent, gas-permeable, water-impermeable, fairly inexpensive, copyright-free, and rapidly prototyped with high precision using simple procedures. However, the fabrication process typically involves substantial human labor, which tends to make PDMS devices difficult to disseminate outside of research labs, and the layered molding limits the 3D complexity of the devices that can be produced. 3D-printing has recently attracted attention as a way to fabricate microfluidic systems due to its automated, assembly-free 3D fabrication, rapidly decreasing costs, and fast-improving resolution and throughput. Resins with properties approaching those of PDMS are being developed. Here we review past and recent efforts in 3D-printing of microfluidic systems. We compare the salient features of PDMS molding with those of 3D-printing and we give an overview of the critical barriers that have prevented the adoption of 3D-printing by microfluidic developers, namely resolution, throughput, and resin biocompatibility. We also evaluate the various forces that are persuading researchers to abandon PDMS molding in favor of 3D-printing in growing numbers.

3D printed microfluidic devices: enablers and barriers

Abstract: 3D printing has the potential to significantly change the field of microfluidics. The ability to fabricate a complete microfluidic device in a single step from a computer model has obvious attractions, but it is the ability to create truly three dimensional structures that will provide new microfluidic capability that is challenging, if not impossible to make with existing approaches. This critical review covers the current state of 3D printing for microfluidics, focusing on the four most frequently used printing approaches: inkjet (i3DP), stereolithography (SLA), two photon polymerisation (2PP) and extrusion printing (focusing on fused deposition modeling). It discusses current achievements and limitations, and opportunities for advancement to reach 3D printing's full potential.

Fundamentals and applications of inertial microfluidics: a review

Abstract: In the last decade, inertial microfluidics has attracted significant attention and a wide variety of channel designs that focus, concentrate and separate particles and fluids have been demonstrated. In contrast to conventional microfluidic technologies, where fluid inertia is negligible and flow remains almost within the Stokes flow region with very low Reynolds number (Re ≪ 1), inertial microfluidics works in the intermediate Reynolds number range (~1 < Re < ~100) between Stokes and turbulent regimes. In this intermediate range, both inertia and fluid viscosity are finite and bring about several intriguing effects that form the basis of inertial microfluidics including (i) inertial migration and (ii) secondary flow. Due to the superior features of high-throughput, simplicity, precise manipulation and low cost, inertial microfluidics is a very promising candidate for cellular sample processing, especially for samples with low abundant targets. In this review, we first discuss the fundamental kinematics of particles in microchannels to familiarise readers with the mechanisms and underlying physics in inertial microfluidic systems. We then present a comprehensive review of recent developments and key applications of inertial microfluidic systems according to their microchannel structures. Finally, we discuss the perspective of employing fluid inertia in microfluidics for particle manipulation. Due to the superior benefits of inertial microfluidics, this promising technology will still be an attractive topic in the near future, with more novel designs and further applications in biology, medicine and industry on the horizon.

A microfluidic ExoSearch chip for multiplexed exosome detection towards blood-based ovarian cancer diagnosis

Abstract: Tumor-derived circulating exosomes, enriched with a group of tumor antigens, have been recognized as a promising biomarker source for cancer diagnosis via a less invasive procedure. Quantitatively pinpointing exosome tumor markers is appealing, yet challenging. In this study, we developed a simple microfluidic approach (ExoSearch) which provides enriched preparation of blood plasma exosomes for in situ, multiplexed detection using immunomagnetic beads. The ExoSearch chip offers a robust, continuous-flow design for quantitative isolation and release of blood plasma exosomes in a wide range of preparation volumes (10 μL to 10 mL). We employed the ExoSearch chip for blood-based diagnosis of ovarian cancer by multiplexed measurement of three exosomal tumor markers (CA-125, EpCAM, CD24) using a training set of ovarian cancer patient plasma, which showed significant diagnostic power (a.u.c. = 1.0, p = 0.001) and was comparable with the standard Bradford assay. This work provides an essentially needed platform for utilization of exosomes in clinical cancer diagnosis, as well as fundamental exosome research.

Cell refractive index for cell biology and disease diagnosis: past, present and future

Abstract: Cell refractive index is a key biophysical parameter, which has been extensively studied. It is correlated with other cell biophysical properties including mechanical, electrical and optical properties, and not only represents the intracellular mass and concentration of a cell, but also provides important insight for various biological models. Measurement techniques developed earlier only measure the effective refractive index of a cell or a cell suspension, providing only limited information on cell refractive index and hence hindering its in-depth analysis and correlation. Recently, the emergence of microfluidic, photonic and imaging technologies has enabled the manipulation of a single cell and the 3D refractive index of a single cell down to sub-micron resolution, providing powerful tools to study cells based on refractive index. In this review, we provide an overview of cell refractive index models and measurement techniques including microfluidic chip-based techniques for the last 50 years, present the applications and significance of cell refractive index in cell biology, hematology, and pathology, and discuss future research trends in the field, including 3D imaging methods, integration with microfluidics and potential applications in new and breakthrough research areas.

A review of digital microfluidics as portable platforms for lab-on a-chip applications.

Abstract: Following the development of microfluidic systems, there has been a high tendency towards developing lab-on-a-chip devices for biochemical applications. A great deal of effort has been devoted to improve and advance these devices with the goal of performing complete sets of biochemical assays on the device and possibly developing portable platforms for point of care applications. Among the different microfluidic systems used for such a purpose, digital microfluidics (DMF) shows high flexibility and capability of performing multiplex and parallel biochemical operations, and hence, has been considered as a suitable candidate for lab-on-a-chip applications. In this review, we discuss the most recent advances in the DMF platforms, and evaluate the feasibility of developing multifunctional packages for performing complete sets of processes of biochemical assays, particularly for point-of-care applications. The progress in the development of DMF systems is reviewed from eight different aspects, including device fabrication, basic fluidic operations, automation, manipulation of biological samples, advanced operations, detection, biological applications, and finally, packaging and portability of the DMF devices. Success in developing the lab-on-a-chip DMF devices will be concluded based on the advances achieved in each of these aspects.

Droplet microfluidics for microbiology: techniques, applications and challenges

Abstract: Droplet microfluidics has rapidly emerged as one of the key technologies opening up new experimental possibilities in microbiology. The ability to generate, manipulate and monitor droplets carrying single cells or small populations of bacteria in a highly parallel and high throughput manner creates new approaches for solving problems in diagnostics and for research on bacterial evolution. This review presents applications of droplet microfluidics in various fields of microbiology: i) detection and identification of pathogens, ii) antibiotic susceptibility testing, iii) studies of microbial physiology and iv) biotechnological selection and improvement of strains. We also list the challenges in the dynamically developing field and new potential uses of droplets in microbiology.

Droplet-based microfluidics in drug discovery, transcriptomics and high-throughput molecular genetics

Abstract: Droplet-based microfluidics enables assays to be carried out at very high throughput (up to thousands of samples per second) and enables researchers to work with very limited material, such as primary cells, patient's biopsies or expensive reagents. An additional strength of the technology is the possibility to perform large-scale genotypic or phenotypic screens at the single-cell level. Here we critically review the latest developments in antibody screening, drug discovery and highly multiplexed genomic applications such as targeted genetic workflows, single-cell RNAseq and single-cell ChIPseq. Starting with a comprehensive introduction for non-experts, we pinpoint current limitations, analyze how they might be overcome and give an outlook on exciting future applications.

Beating heart on a chip: a novel microfluidic platform to generate functional 3D cardiac microtissues

Abstract: In the past few years, microfluidic-based technology has developed microscale models recapitulating key physical and biological cues typical of the native myocardium. However, the application of controlled physiological uniaxial cyclic strains on a defined three-dimension cellular environment is not yet possible. Two-dimension mechanical stimulation was particularly investigated, neglecting the complex three-dimensional cell-cell and cell-matrix interactions. For this purpose, we developed a heart-on-a-chip platform, which recapitulates the physiologic mechanical environment experienced by cells in the native myocardium. The device includes an array of hanging posts to confine cell-laden gels, and a pneumatic actuation system to induce homogeneous uniaxial cyclic strains to the 3D cell constructs during culture. The device was used to generate mature and highly functional micro-engineered cardiac tissues (μECTs), from both neonatal rat and human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM), strongly suggesting the robustness of our engineered cardiac micro-niche. Our results demonstrated that the cyclic strain was effectively highly uniaxial and uniformly transferred to cells in culture. As compared to control, stimulated μECTs showed superior cardiac differentiation, as well as electrical and mechanical coupling, owing to a remarkable increase in junction complexes. Mechanical stimulation also promoted early spontaneous synchronous beating and better contractile capability in response to electric pacing. Pacing analyses of hiPSC-CM constructs upon controlled administration of isoprenaline showed further promising applications of our platform in drug discovery, delivery and toxicology fields. The proposed heart-on-a-chip device represents a relevant step forward in the field, providing a standard functional three-dimensional cardiac model to possibly predict signs of hypertrophic changes in cardiac phenotype by mechanical and biochemical co-stimulation.

Ultrasensitive microfluidic analysis of circulating exosomes using a nanostructured graphene oxide/polydopamine coating

Abstract: Exosomes are cell-derived nano-sized vesicles that have been recently recognized as new mediators for many cellular processes and potential biomarkers for non-invasive disease diagnosis and the monitoring of treatment response. To better elucidate the biology and clinical value of exosomes, there is a pressing need for new analytical technologies capable of the efficient isolation and sensitive analysis of such small and molecularly diverse vesicles. Herein, we developed a microfluidic exosome analysis platform based on a new graphene oxide/polydopamine (GO/PDA) nano-interface. To the best of our best knowledge, we report for the first time, the GO-induced formation of a 3D nanoporous PDA surface coating enabled by the microfluidic layer-by-layer deposition of GO and PDA. It was demonstrated that this nanostructured GO/PDA interface greatly improves the efficiency of exosome immuno-capture, while at the same time effectively suppressing non-specific exosome adsorption. Based on this nano-interface, an ultrasensitive exosome ELISA assay was developed to afford a very low detection limit of 50 μL−1 with a 4 log dynamic range, which is substantially better than the existing methods. As a proof of concept for clinical applications, we adapted this platform to discriminate ovarian cancer patients from healthy controls by the quantitative detection of exosomes directly from 2 μL plasma without sample processing. Thus, this platform could provide a useful tool to facilitate basic and clinical investigations of exosomes for non-invasive disease diagnosis and to aid precision treatment.

One-step fabrication of an organ-on-a-chip with spatial heterogeneity using a 3D bioprinting technology.

Abstract: Although various types of organs-on-chips have been introduced recently as tools for drug discovery, the current studies are limited in terms of fabrication methods. The fabrication methods currently available not only need a secondary cell-seeding process and result in severe protein absorption due to the material used, but also have difficulties in providing various cell types and extracellular matrix (ECM) environments for spatial heterogeneity in the organs-on-chips. Therefore, in this research, we introduce a novel 3D bioprinting method for organ-on-a-chip applications. With our novel 3D bioprinting method, it was possible to prepare an organ-on-a-chip in a simple one-step fabrication process. Furthermore, protein absorption on the printed platform was very low, which will lead to accurate measurement of metabolism and drug sensitivity. Moreover, heterotypic cell types and biomaterials were successfully used and positioned at the desired position for various organ-on-a-chip applications, which will promote full mimicry of the natural conditions of the organs. The liver organ was selected for the evaluation of the developed method, and liver function was shown to be significantly enhanced on the liver-on-a-chip, which was prepared by 3D bioprinting. Consequently, the results demonstrate that the suggested 3D bioprinting method is easier and more versatile for production of organs-on-chips.

An integrated paper-based sample-to-answer biosensor for nucleic acid testing at the point of care

Abstract: With advances in point-of-care testing (POCT), lateral flow assays (LFAs) have been explored for nucleic acid detection. However, biological samples generally contain complex compositions and low amounts of target nucleic acids, and currently require laborious off-chip nucleic acid extraction and amplification processes (e.g., tube-based extraction and polymerase chain reaction (PCR)) prior to detection. To the best of our knowledge, even though the integration of DNA extraction and amplification into a paper-based biosensor has been reported, a combination of LFA with the aforementioned steps for simple colorimetric readout has not yet been demonstrated. Here, we demonstrate for the first time an integrated paper-based biosensor incorporating nucleic acid extraction, amplification and visual detection or quantification using a smartphone. A handheld battery-powered heating device was specially developed for nucleic acid amplification in POC settings, which is coupled with this simple assay for rapid target detection. The biosensor can successfully detect Escherichia coli (as a model analyte) in spiked drinking water, milk, blood, and spinach with a detection limit of as low as 10-1000 CFU mL(-1), and Streptococcus pneumonia in clinical blood samples, highlighting its potential use in medical diagnostics, food safety analysis and environmental monitoring. As compared to the lengthy conventional assay, which requires more than 5 hours for the entire sample-to-answer process, it takes about 1 hour for our integrated biosensor. The integrated biosensor holds great potential for detection of various target analytes for wide applications in the near future.

Polymerase chain reaction in microfluidic devices

Abstract: The invention of the polymerase chain reaction (PCR) has caused a revolution in molecular biology, giving access to a method of amplifying deoxyribonucleic acid (DNA) molecules across several orders of magnitude. Since the first application of PCR in a microfluidic device was developed in 1998, an increasing number of researchers have continued the development of microfluidic PCR systems. In this review, we introduce recent developments in microfluidic-based space and time domain devices as well as discuss various designs integrated with multiple functions for sample preparation and detection. The development of isothermal nucleic acid amplification and digital PCR microfluidic devices within the last five years is also highlighted. Furthermore, we introduce various commercial microfluidic PCR devices.

3D-printing of transparent bio-microfluidic devices in PEG-DA

Abstract: The vast majority of microfluidic systems are molded in poly(dimethylsiloxane) (PDMS) by soft lithography due to the favorable properties of PDMS: biocompatible, elastomeric, transparent, gas-permeable, inexpensive, and copyright-free. However, PDMS molding involves tedious manual labor, which makes PDMS devices prone to assembly failures and difficult to disseminate to research and clinical settings. Furthermore, the fabrication procedures limit the 3D complexity of the devices to layered designs. Stereolithography (SL), a form of 3D-printing, has recently attracted attention as a way to customize the fabrication of biomedical devices due to its automated, assembly-free 3D fabrication, rapidly decreasing costs, and fast-improving resolution and throughput. However, existing SL resins are not biocompatible and patterning transparent resins at high resolution remains difficult. Here we report procedures for the preparation and patterning of a transparent resin based on low-MW poly(ethylene glycol) diacrylate (MW 250) (PEG-DA-250). The 3D-printed devices are highly transparent and cells can be cultured on PEG-DA-250 prints for several days. This biocompatible SL resin and printing process solves some of the main drawbacks of 3D-printed microfluidic devices: biocompatibility and transparency. In addition, it should also enable the production of non-microfluidic biomedical devices.

High density 3D printed microfluidic valves, pumps, and multiplexers

Abstract: In this paper we demonstrate that 3D printing with a digital light processor stereolithographic (DLP-SLA) 3D printer can be used to create high density microfluidic devices with active components such as valves and pumps. Leveraging our previous work on optical formulation of inexpensive resins (RSC Adv., 2015, 5, 106621), we demonstrate valves with only 10% of the volume of our original 3D printed valves (Biomicrofluidics, 2015, 9, 016501), which were already the smallest that have been reported. Moreover, we show that incorporation of a thermal initiator in the resin formulation along with a post-print bake can dramatically improve the durability of 3D printed valves up to 1 million actuations. Using two valves and a valve-like displacement chamber (DC), we also create compact 3D printed pumps. With 5-phase actuation and a 15 ms phase interval, we obtain pump flow rates as high as 40 μL min−1. We also characterize maximum pump back pressure (i.e., maximum pressure the pump can work against), maximum flow rate (flow rate when there is zero back pressure), and flow rate as a function of the height of the pump outlet. We further demonstrate combining 5 valves and one DC to create a 3-to-2 multiplexer with integrated pump. In addition to serial multiplexing, we also show that the device can operate as a mixer. Importantly, we illustrate the rapid fabrication and test cycles that 3D printing makes possible by implementing a new multiplexer design to improve mixing, and fabricate and test it within one day.

Paper-based sensors and assays: a success of the engineering design and the convergence of knowledge areas

Abstract: This review shows the recent advances and state of the art in paper-based analytical devices (PADs) through the analysis of their integration with microfluidics and LOC micro- and nanotechnologies, electrochemical/optical detection and electronic devices as the convergence of various knowledge areas. The important role of the paper design/architecture in the improvement of the performance of sensor devices is discussed. The discussion is fundamentally based on μPADs as the new generation of paper-based (bio)sensors. Data about the scientific publication ranking of PADs, illustrating their increase as an experimental research topic in the past years, are supplied. In addition, an analysis of the simultaneous evolution of PADs in academic lab research and industrial commercialization highlighting the parallelism of the technological transfer from academia to industry is displayed. A general overview of the market behaviour, the leading industries in the sector and their commercialized devices is given. Finally, personal opinions of the authors about future perspectives and tendencies in the design and fabrication technology of PADs are disclosed.

Engineering anastomosis between living capillary networks and endothelial cell-lined microfluidic channels

Abstract: This paper reports a method for generating an intact and perfusable microvascular network that connects to microfluidic channels without appreciable leakage. This platform incorporates different stages of vascular development including vasculogenesis, endothelial cell (EC) lining, sprouting angiogenesis, and anastomosis in sequential order. After formation of a capillary network inside the tissue chamber via vasculogenesis, the adjacent microfluidic channels are lined with a monolayer of ECs, which then serve as the high-pressure input ("artery") and low pressure output ("vein") conduits. To promote a tight interconnection between the artery/vein and the capillary network, sprouting angiogenesis is induced, which promotes anastomosis of the vasculature inside the tissue chamber with the EC lining along the microfluidic channels. Flow of fluorescent microparticles confirms the perfusability of the lumenized microvascular network, and minimal leakage of 70 kDa FITC-dextran confirms physiologic tightness of the EC junctions and completeness of the interconnections between artery/vein and the capillary network. This versatile device design and its robust construction methodology establish a physiological transport model of interconnected perfused vessels from artery to vascularized tissue to vein. The system has utility in a wide range of organ-on-a-chip applications as it enables the physiological vascular interconnection of multiple on-chip tissue constructs that can serve as disease models for drug screening.

A fully integrated paperfluidic molecular diagnostic chip for the extraction, amplification, and detection of nucleic acids from clinical samples

Abstract: Paper diagnostics have successfully been employed to detect the presence of antigens or small molecules in clinical samples through immunoassays; however, the detection of many disease targets relies on the much higher sensitivity and specificity achieved via nucleic acid amplification tests (NAAT). The steps involved in NAAT have recently begun to be explored in paper matrices, and our group, among others, has reported on paper-based extraction, amplification, and detection of DNA and RNA targets. Here, we integrate these paper-based NAAT steps into a single paperfluidic chip in a modular, foldable system that allows for fully integrated fluidic handling from sample to result. We showcase the functionality of the chip by combining nucleic acid isolation, isothermal amplification, and lateral flow detection of human papillomavirus (HPV) 16 DNA directly from crude cervical specimens in less than 1 hour for rapid, early detection of cervical cancer. The chip is made entirely of paper and adhesive sheets, making it low-cost, portable, and disposable, and offering the potential for a point-of-care molecular diagnostic platform even in remote and resource-limited settings.

Active droplet generation in microfluidics

Abstract: The reliable generation of micron-sized droplets is an important process for various applications in droplet-based microfluidics. The generated droplets work as a self-contained reaction platform in droplet-based lab-on-a-chip systems. With the maturity of this platform technology, sophisticated and delicate control of the droplet generation process is needed to address increasingly complex applications. This review presents the state of the art of active droplet generation concepts, which are categorized according to the nature of the induced energy. At the liquid/liquid interface, an energy imbalance leads to instability and droplet breakup.

Microfluidics: an enabling screening technology for enhanced oil recovery (EOR)

Abstract: Oil production is a critical industrial process that affects the entire world population and any improvements in its efficiency while reducing its environmental impact are of utmost societal importance. The paper reviews recent applications of microfluidics and microtechnology to study processes of oil extraction and recovery. It shows that microfluidic devices can be useful tools in investigation and visualization of such processes used in the oil & gas industry as fluid propagation, flooding, fracturing, emulsification and many others. Critical macro-scale processes that define oil extraction and recovery are controlled by the micro-scale processes based on wetting, adhesion, surface tension, colloids and other concepts of microfluidics. A growing number of research efforts demonstrates that microfluidics is becoming, albeit slowly, an accepted methodology in this area. We propose several areas of development where implementation of microfluidics may bring about deeper understanding and hence better control over the processes of oil recovery based on fluid propagation, droplet generation, wettability control. Studies of processes such as hydraulic fracturing, sand particle propagation in porous networks, high throughput screening of chemicals (for example, emulsifiers and surfactants) in microfluidic devices that simulate oil reservoirs are proposed to improve our understanding of these complicated physico-chemical systems. We also discuss why methods of additive manufacturing (3D printing) should be evaluated for quick prototyping and modification of the three-dimensional structures replicating natural oil-bearing rock formations for studies accessible to a wider audience of researchers.

3D printed microfluidic circuitry via multijet-based additive manufacturing

Abstract: The miniaturization of integrated fluidic processors affords extensive benefits for chemical and biological fields, yet traditional, monolithic methods of microfabrication present numerous obstacles for the scaling of fluidic operators. Recently, researchers have investigated the use of additive manufacturing or “three-dimensional (3D) printing” technologies – predominantly stereolithography – as a promising alternative for the construction of submillimeter-scale fluidic components. One challenge, however, is that current stereolithography methods lack the ability to simultaneously print sacrificial support materials, which limits the geometric versatility of such approaches. In this work, we investigate the use of multijet modelling (alternatively, polyjet printing) – a layer-by-layer, multi-material inkjetting process – for 3D printing geometrically complex, yet functionally advantageous fluidic components comprised of both static and dynamic physical elements. We examine a fundamental class of 3D printed microfluidic operators, including fluidic capacitors, fluidic diodes, and fluidic transistors. In addition, we evaluate the potential to advance on-chip automation of integrated fluidic systems via geometric modification of component parameters. Theoretical and experimental results for 3D fluidic capacitors demonstrated that transitioning from planar to non-planar diaphragm architectures improved component performance. Flow rectification experiments for 3D printed fluidic diodes revealed a diodicity of 80.6 ± 1.8. Geometry-based gain enhancement for 3D printed fluidic transistors yielded pressure gain of 3.01 ± 0.78. Consistent with additional additive manufacturing methodologies, the use of digitally-transferrable 3D models of fluidic components combined with commercially-available 3D printers could extend the fluidic routing capabilities presented here to researchers in fields beyond the core engineering community.

Bioprinted thrombosis-on-a-chip

Abstract: Pathologic thrombosis kills more people than cancer and trauma combined; it is associated with significant disability and morbidity, and represents a major healthcare burden. Despite advancements in medical therapies and imaging, there is often incomplete resolution of the thrombus. The residual thrombus can undergo fibrotic changes over time through infiltration of fibroblasts from the surrounding tissues and eventually transform into a permanent clot often associated with post-thrombotic syndrome. In order to understand the importance of cellular interactions and the impact of potential therapeutics to treat thrombosis, an in vitro platform using human cells and blood components would be beneficial. Towards achieving this aim, there have been studies utilizing the capabilities of microdevices to study the hemodynamics associated with thrombosis. In this work, we further exploited the utilization of 3D bioprinting technology, for the construction of a highly biomimetic thrombosis-on-a-chip model. The model consisted of microchannels coated with a layer of confluent human endothelium embedded in a gelatin methacryloyl (GelMA) hydrogel, where human whole blood was infused and induced to form thrombi. Continuous perfusion with tissue plasmin activator led to dissolution of non-fibrotic clots, revealing clinical relevance of the model. Further encapsulating fibroblasts in the GelMA matrix demonstrated the potential migration of these cells into the clot and subsequent deposition of collagen type I over time, facilitating fibrosis remodeling that resembled the in vivo scenario. Our study suggests that in vitro 3D bioprinted blood coagulation models can be used to study the pathology of fibrosis, and particularly, in thrombosis. This versatile platform may be conveniently extended to other vascularized fibrotic disease models.

Microfluidic production of multiple emulsions and functional microcapsules

Abstract: Recent advances in microfluidics have enabled the controlled production of multiple-emulsion drops with onion-like topology. The multiple-emulsion drops possess an intrinsic core-shell geometry, which makes them useful as templates to create microcapsules with a solid membrane. High flexibility in the selection of materials and hierarchical order, achieved by microfluidic technologies, has provided versatility in the membrane properties and microcapsule functions. The microcapsules are now designed not just for storage and release of encapsulants but for sensing microenvironments, developing structural colours, and many other uses. This article reviews the current state of the art in the microfluidic-based production of multiple-emulsion drops and functional microcapsules. The three main sections of this paper discuss distinct microfluidic techniques developed for the generation of multiple emulsions, four representative methods used for solid membrane formation, and various applications of functional microcapsules. Finally, we outline the current limitations and future perspectives of microfluidics and microcapsules.

3D printing of liquid metals as fugitive inks for fabrication of 3D microfluidic channels

Abstract: This paper demonstrates a simple method to fabricate 3D microchannels and microvasculature at room temperature by direct-writing liquid metal as a sacrificial template. The formation of a surface oxide skin on the low-viscosity liquid metal stabilizes the shape of the printed metal for planar and out-of-plane structures. The printed structures can be embedded in a variety of soft (e.g. elastomeric) and rigid (e.g. thermoset) polymers. Both acid and electrochemical reduction are capable of removing the oxide skin that forms on the metal, which destabilizes the ink so that it withdraws from the encapsulating material due to capillary forces, resulting in nearly full recovery of the fugitive ink at room temperature. Whereas conventional fabrication procedures typically confine microchannels to 2D planes, the geometry of the printed microchannels can be varied from a simple 2D network to complex 3D architectures without using lithography. The method produces robust monolithic structures without the need for any bonding or assembling techniques that often limit the materials of construction of conventional microchannels. Removing select portions of the metal leaves behind 3D metal features that can be used as antennas, interconnects, or electrodes for interfacing with lab-on-a-chip devices. This paper describes the capabilities and limitations of this simple process.

Robust scalable high throughput production of monodisperse drops

Abstract: Monodisperse drops with diameters between 20 μm and 200 μm can be used to produce particles or capsules for many applications such as for cosmetics, food, and biotechnology. Drops composed of low viscosity fluids can be conveniently made using microfluidic devices. However, the throughput of microfluidic devices is limited and scale-up, achieved by increasing the number of devices run in parallel, can compromise the narrow drop-size distribution. In this paper, we present a microfluidic device, the millipede device, which forms drops through a static instability such that the fluid volume that is pinched off is the same every time a drop forms. As a result, the drops are highly monodisperse because their size is solely determined by the device geometry. This makes the operation of the device very robust. Therefore, the device can be scaled to a large number of nozzles operating simultaneously on the same chip; we demonstrate the operation of more than 500 nozzles on a single chip that produces up to 150 mL h−1 of highly monodisperse drops.

Controlled assembly of heterotypic cells in a core–shell scaffold: organ in a droplet

Abstract: This paper reports a droplet-based microfluidic approach to fabricate a large number of monodisperse, portable microtissues, each in an individual drop. We use water–water–oil double emulsions as templates and spatially assemble hepatocytes in the core and fibroblasts in the shell, forming a 3D liver model in a drop.

A microphysiological model of the human placental barrier

Abstract: During human pregnancy, the fetal circulation is separated from maternal blood in the placenta by two cell layers – the fetal capillary endothelium and placental trophoblast. This placental barrier plays an essential role in fetal development and health by tightly regulating the exchange of endogenous and exogenous materials between the mother and the fetus. Here we present a microengineered device that provides a novel platform to mimic the structural and functional complexity of this specialized tissue in vitro. Our model is created in a multilayered microfluidic system that enables co-culture of human trophoblast cells and human fetal endothelial cells in a physiologically relevant spatial arrangement to replicate the characteristic architecture of the human placental barrier. We have engineered this co-culture model to induce progressive fusion of trophoblast cells and to form a syncytialized epithelium that resembles the syncytiotrophoblast in vivo. Our system also allows the cultured trophoblasts to form dense microvilli under dynamic flow conditions and to reconstitute expression and physiological localization of membrane transport proteins, such as glucose transporters (GLUTs), critical to the barrier function of the placenta. To provide a proof-of-principle for using this microdevice to recapitulate native function of the placental barrier, we demonstrated physiological transport of glucose across the microengineered maternal–fetal interface. Importantly, the rate of maternal-to-fetal glucose transfer in this system closely approximated that measured in ex vivo perfused human placentas. Our “placenta-on-a-chip” platform represents an important advance in the development of new technologies to model and study the physiological complexity of the human placenta for a wide variety of applications.

Modular, pumpless body-on-a-chip platform for the co-culture of GI tract epithelium and 3D primary liver tissue

Abstract: We have developed an expandable modular body-on-a-chip system that allows for a plug-and-play approach with several in vitro tissues. The design consists of single-organ chips that are combined with each other to yield a multi-organ body-on-a-chip system. Fluidic flow through the organ chips is driven via gravity and controlled passively via hydraulic resistances of the microfluidic channel network. Such pumpless body-on-a-chip devices are inexpensive and easy to use. We tested the device by culturing GI tract tissue and liver tissue within the device. Integrated Ag/AgCl electrodes were used to measure the resistance across the GI tract cell layer. The transepithelial resistance (TEER) reached values between 250 to 650 Ω cm2 throughout the 14 day co-culture period. These data indicate that the GI tract cells retained their viability and the GI tract layer as a whole retained its barrier function. Throughout the 14 day co-culture period we measured low amounts of aspartate aminotransferase (AST, ∼10–17.5 U L−1), indicating low rates of liver cell death. Metabolic rates of hepatocytes were comparable to those of hepatocytes in single-organ fluidic cell culture systems (albumin production ranged between 3–6 μg per day per million hepatocytes and urea production ranged between 150–200 μg per day per million hepatocytes). Induced CYP activities were higher than previously measured with microfluidic liver only systems.

Novel developments in mobile sensing based on the integration of microfluidic devices and smartphones

Abstract: Portable electronic devices and wireless communication systems enable a broad range of applications such as environmental and food safety monitoring, personalized medicine and healthcare management. Particularly, hybrid smartphone and microfluidic devices provide an integrated solution for the new generation of mobile sensing applications. Such mobile sensing based on microfluidic devices (broadly defined) and smartphones (MS(2)) offers a mobile laboratory for performing a wide range of bio-chemical detection and analysis functions such as water and food quality analysis, routine health tests and disease diagnosis. MS(2) offers significant advantages over traditional platforms in terms of test speed and control, low cost, mobility, ease-of-operation and data management. These improvements put MS(2) in a promising position in the fields of interdisciplinary basic and applied research. In particular, MS(2) enables applications to remote in-field testing, homecare, and healthcare in low-resource areas. The marriage of smartphones and microfluidic devices offers a powerful on-chip operating platform to enable various bio-chemical tests, remote sensing, data analysis and management in a mobile fashion. The implications of such integration are beyond telecommunication and microfluidic-related research and technology development. In this review, we will first provide the general background of microfluidic-based sensing, smartphone-based sensing, and their integration. Then, we will focus on several key application areas of MS(2) by systematically reviewing the important literature in each area. We will conclude by discussing our perspectives on the opportunities, issues and future directions of this emerging novel field.

Review: imaging technologies for flow cytometry

Abstract: High-throughput single cell imaging is a critical enabling and driving technology in molecular and cellular biology, biotechnology, medicine and related areas Imaging flow cytometry combines the single-cell imaging capabilities of microscopy with the high-throughput capabilities of conventional flow cytometry Recent advances in imaging flow cytometry are remarkably revolutionizing single-cell analysis This article describes recent imaging flow cytometry technologies and their challenges