Monitoring spatial distribution of ethanol in microfluidic channels by using a thin layer of cholesteric liquid crystal

Monitoring spatial distribution of chemicals in microfluidic devices by using traditional sensors is a challenging task. In this paper, we report utilization of a thin layer of cholesteric liquid crystal for monitoring ethanol inside microfluidic channels. This thin layer can be either a polymer dispersed cholesteric liquid crystal (PDCLC) layer or a free cholesteric liquid crystal (CLC) layer separated from the microfluidic device by using a thin film of PDMS. They both show visible colorimetric responses to 4% of ethanol solution inside the microfluidic channels. Moreover, the spatial distribution of ethanol inside the microfluidic channel can be reflected as a color map on the CLC sensing layers. By using this device, we successfully detected ethanol produced from fermentation taking place inside the microfluidic channel. These microfluidic channels with embedded PDCLC or embedded CLC offer a new sensing solution for monitoring volatile organic compounds in microfluidic devices.     

Microfluidic chip capillary electrophoresis coupled with electrochemiluminescence for enantioseparation of racemic drugs using central composite design optimization

Optimization based on central composite design (CCD) for enantioseparation of anisodamine (AN), atenolol (AT), and metoprolol (ME) in human urine was developed using a microfluidic chip‐CE device. Coupling the flexible and wide working range of microfluidic chip‐CE device to CCD for chiral separation of AN, AT, and ME in human urine, a total of 15 experiments is needed for the optimization procedure as compared to 75 experiments using the normal one variable at a time optimization. The optimum conditions obtained are found to be more robust as shown by the curvature effects of the interaction factors. The developed microfluidic chip‐CE‐ECL system with adjustable dilution ratios has been validated by satisfactory recoveries (89.5–99% for six enanotiomers) in urine sample analysis. The working range (0.3–600 μM), repeatability (3.1–4.9% RSD for peak height and 4.0–5.2% RSD for peak area), and detection limit (0.3–0.6 μM) of the method developed are found to meet the requirements for bedside monitoring of AN, AT, and ME in patients under critical conditions. In summary, the hyphenation of CCD with the microfluidic chip‐CE device is shown to offer a rapid means for optimizing the working conditions on simultaneous separation of three racemic drugs using the microfluidic chip‐CE device developed.     

Hydrogel microfluidic co‐culture device for photothermal therapy and cancer migration

We developed the photo‐crosslinkable hydrogel microfluidic co‐culture device to study photothermal therapy and cancer cell migration. To culture MCF7 human breast carcinoma cells and metastatic U87MG human glioblastoma in the microfluidic device, we used 10 w/v% gelatin methacrylate (GelMA) hydrogels as a semi‐permeable physical barrier. We demonstrated the effect of gold nanorod on photothermal therapy of cancer cells in the microfluidic co‐culture device. Interestingly, we observed that metastatic U87MG human glioblastoma largely migrated toward vascular endothelial growth factor (VEGF)‐treated GelMA hydrogel‐embedding microchannels. The main advantage of this hydrogel microfluidic co‐culture device is to simultaneously analyze the physiological migration behaviors of two cancer cells with different physiochemical motilities and study gold nanorod‐mediated photothermal therapy effect. Therefore, this hydrogel microfluidic co‐culture device could be a potentially powerful tool for photothermal therapy and cancer cell migration applications.     

Skin‐worn Soft Microfluidic Potentiometric Detection System

A flexible skin‐mounted microfluidic potentiometric device for simultaneous electrochemical monitoring of sodium and potassium in sweat is presented. The wearable device allows efficient natural sweat pumping to the potentiometric detection chamber, containing solid‐contact ion‐selective Na⁺ and K⁺ electrodes, during exercise activity. The fabricated microchip electrolyte‐sensing device displays good analytical performance and addresses sweat mixing and carry‐over issues of early epidermal potentiometric sensors. Such soft skin‐worn microchip platform integrates potentiometric measurement, microfluidic technologies with flexible electronics for real‐time wireless data transmission to mobile devices. The new fully integrated microfluidic electrolyte‐detection device paves the way for practical fitness and health monitoring applications.     

Evaluation of Cell‐Penetrating Peptides Using Microfluidic In Vitro 3D Brain Endothelial Barrier

In drug delivery to the human brain, blood vessels are a significant hurdle because they restrict the entry of most solutes to protect brain. To overcome this hurdle, an in vitro 3D model for brain endothelial barrier is developed using a microfluidic device with hydrogel providing a 3D extracellular matrix scaffold. Using the model, peptides known to utilize receptor‐mediated transcytosis are verified, which has been one of the most promising mechanisms for brain‐specific penetration. The cytotoxicity and cellular damage to the peptide are investigated and the receptor‐mediated transcytosis and brain endothelial specific penetrating abilities of the peptides in a quantitative manner are demonstrated. As a preclinical test, applying the quantification assays conducted in this study are suggested, including the penetrating ability, cytotoxicity, endothelial damage, and receptor specificity. Using this microfluidic device as an in vitro platform for evaluating various brain targeting drugs and drug carrier candidates is also proposed.     

A novel microfluidic capture and monitoring method for assessing physiological damage of C. elegans under microgravity

Spatial microgravity is a significant factor affecting and causing physiological changes of organisms in space environment. On‐site assessment of the damage associated to microgravity is very important for future long‐term space exploration of mankind. In this paper, a new microfluidic device for analyzing the damage of microgravity on Caenorhabditis elegans (C. elegans) has been developed. This device is mainly composed of a microfluidic chip, a dual imaging module, and an imaging acquisition and processing module, which are integrated into a compact system. The microfluidic chip is designed as a platform for monitoring C. elegans, which is captured in an imaging region through a suction structure in the microfluidic chip. A dual imaging module is designed to obtain the images of bright field and fluorescence of C. elegans. The behaviors of C. elegans are analyzed based on the dual‐mode imaging of bright field and fluorescence to assess the degree of damage due to microgravity. A comparative study using a commercial microscope is also conducted to demonstrate the unique advantage of the developed system under the simulated microgravity. The results show that the developed system can evaluate the damage of C. elegans under microgravity accurately and conveniently. Furthermore, this device has compact size and weight, easy operation, and low‐cost, which could be highly advantageous for on‐site evaluation of the damage to microorganisms under microgravity in a space station.     

Epithelial‐to‐mesenchymal transition of human lung alveolar epithelial cells in a microfluidic gradient device

Epithelial‐to‐mesenchymal transition (EMT), a process in which epithelial cells undergo phenotypic transitions to fibrotic cells, is induced by stimulants including transforming growth factor‐beta1 (TGF‐β1). In the present study, we developed a microfluidic gradient device to reproduce EMT in A549 human lung alveolar epithelial cells in response to TGF‐β1 gradients. The device was directly mounted on the cells that had grown in cell culture plates and produced a stable concentration gradient of TGF‐β1 with negligible shear stress, thereby providing a favorable environment for the anchorage‐dependent cells. A549 cells elongated with the characteristic spindle‐shaped morphological changes with upregulation of alpha‐smooth muscle actin, a mesenchyme marker, in a gradient‐dependent manner, suggestive of EMT progression. We observed that at higher TGF‐β1 concentrations ranging from 5 to 10 ng/mL, the cultures in the microfluidic device allowed to quantitatively pick up subtle differences in the EMT cellular response as compared with plate cultures. These results suggest that the microfluidic gradient device would accurately determine the optimal concentrations of TGF‐β1, given that epithelial cells of different tissue origins greatly vary their responses to TGF‐β1. Therefore, this microfluidic device could be a powerful tool to monitor EMT induced by a variety of environmental stresses including cigarette smoke with high sensitivity.     

Microdevice for separation and quantitative fraction collection

A new microfluidic concept for quantitative whole-column fraction collection of electrophoretically separated zones was developed. The prototype device, fabricated on a polycarbonate disk by injection molding, integrated electrophoretic separation channels with fraction collection reservoirs distributed along the separation channel. The microdevice was designed in a CD-like format to use the centrifugal force for moving the liquid in the microchannels. A serpentine shape of the separation channel was selected to create segments for quantitative whole-column fraction collection. The operation was tested with visual monitoring of isotachophoretic separation and collection of cationic dyes.     

A smart and portable micropump for stable liquid delivery

Precise and reliable liquid delivery is vital for microfluidic applications. Here, we illustrate the design, fabrication, characterization, and application of a portable, low cost, and robust micropump, which brings solution to stable liquid delivery in microfluidic environment. The pump is designed with three optional speeds of different pumping flow rates, and it can be simply actuated by spring‐driven mechanism. The different flow rates of the pump are realized via passive microvalves in a compact microfluidic chip, which is installed in the pump. Importantly, the membrane structures of the microvalves allow accurate liquid control, and stable flow rates can be achieved via a spring setup. The proposed pump is applied to continuously and stably infuse microbead suspension into an inertial microfluidic chip, and good particle focusing is realized in the spiral channel of the inertial microfluidic chip. The proposed portable, self‐powered, and cost‐efficient pump is crucial for microfluidic lab‐on‐a‐chip system integration, which may facilitate microfluidic application for precise liquid delivery, control, measurement, and analysis.     

Recent strategies toward microfluidic‐based surface‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy (SERS) is an extremely powerful analytical tool, which not only yields information about the molecular structure of the analyte in the form of characteristic vibrational spectrum but also gives sensitivities approaching those in fluorescence spectroscopy. The SERS measurement on the microfluidic platform provides possibility to manufacture the device with design perfectly fulfilling the needs of the application with minimal sample consumption. This review aims at describing basic strategies for SERS measurement in microfluidic devices published in the last decade and covers current trends in microfluidics with SERS detection in the field of bioanalysis and approaches toward on‐line coupling of liquid‐based separation techniques with SERS detection.     

Design of a microfluidic device for comprehensive spatial two‐dimensional liquid chromatography

This study discusses the design aspects for the construction of a microfluidic device for comprehensive spatial two‐dimensional liquid chromatography. In spatial two‐dimensional liquid chromatography each peak is characterized by its coordinates in the plane. After completing the first‐dimension separation all fractions are analyzed in parallel second‐dimension separations. Hence, spatial two‐dimensional liquid chromatography potentially provides much higher peak‐production rates than a coupled column multi‐dimensional liquid chromatography approach in which the second‐dimension analyses are performed sequentially. A chip for spatial two‐dimensional liquid chromatography has been manufactured from cyclic olefin copolymer and features a first‐dimension separation channel and 21 parallel second‐dimension separation channels oriented perpendicularly to the former. Compartmentalization of first‐ and second‐dimension developments by physical barriers allowed for a preferential flow path with a minimal dispersion into the second‐dimension separation channels. To generate a homogenous flow across all the parallel second‐dimension channels, a radially interconnected flow distributor containing two zones of diamond‐shaped pillars was integrated on‐chip. A methacrylate ester based monolithic stationary phase with optimized macroporous structure was created in situ in the confines of the microfluidic chip. In addition, the use of a photomask was explored to localize monolith formation in the parallel second‐dimension channels. Finally, to connect the spatial chip to the liquid chromatography instrument, connector ports were integrated allowing the use of Viper fittings. As an alternative, a chip holder with adjustable clasp locks was designed that allows the clamping force to be adjusted.     

Circumventing air bubbles in microfluidic systems and quantitative continuous-flow PCR applications

Polymerase chain reaction (PCR) is an essential part of research based on genomics or cell analysis. The development of a microfluidic device that would be suitable for high-temperature-based reactions therefore becomes an important contribution towards the integration of micro-total analysis systems (μTAS). However, problems associated with the generation of air bubbles in the microchannels before the introduction of the assay liquid, which we call the “initial start-up” in this study, made the flow irregular and unstable. In this report, we have tried to address these problems by adapting a novel liquid-flow method for high-temperature-based reactions. A PDMS-based microfluidic device was fabricated by soft-lithography techniques and placed on a cartridge heater. The generation of the air bubbles was prevented by introducing the fluorinated oil, an inert and highly viscous liquid, as the cap just before the introduction of the sample solutions into the microchannels. The technique was applied for continuous-flow PCR, which could perform PCR on-chip in a microfluidic system. For the evaluation of practical accuracy, plasmid DNA that serves as a reference molecule for the quantification of genetically modified (GM) maize was used as the template DNA for continuous-flow PCR. After PCR, the products were collected in a vial and analyzed by gel electrophoresis to confirm the accuracy of the results. Additionally, quantitative continuous-flow PCR was performed using TaqMan technology on our PCR device. A laser detection system was also used for the quantitative PCR method. We observed a linear relationship between the threshold cycle (Ct) and the initial DNA concentration. These results showed that it would be possible to quantify the initial copies of the template DNA on our microfluidic device. Accurate quantitative DNA analysis in microfluidic systems is required for the integration of PCR with μTAS, thus we anticipate that our device would have promising potential for applications in a wide range of research.     

Real‐time Monitoring of Discrete Synaptic Release Events and Excitatory Potentials within Self‐reconstructed Neuromuscular Junctions

Chemical synaptic transmission is central to the brain functions. In this regard, real‐time monitoring of chemical synaptic transmission during neuronal communication remains a great challenge. In this work, in vivo‐like oriented neural networks between superior cervical ganglion (SCG) neurons and their effector smooth muscle cells (SMC) were assembled in a microfluidic device. This allowed amperometric detection of individual neurotransmitter release events inside functional SCG‐SMC synapse with carbon fiber nanoelectrodes as well as recording of postsynaptic potential using glass nanopipette electrodes. The high vesicular release activities essentially involved complex events arising from flickering fusion pores as quantitatively established based on simulations. This work allowed for the first time monitoring in situ chemical synaptic transmission under conditions close to those found in vivo, which may yield important and new insights into the nature of neuronal communications.     

A rapid,maskless 3D prototyping for fabrication of capillary circuits: Toward urinary protein detection

Proteinuria is an established risk marker for progressive renal function loss and patients would significantly benefit from a point‐of‐care testing. Although extensive work has been done to develop the microfluidic devices for the detection of urinary protein, they need the complicated operation and bulky peripherals. Here, we present a rapid, maskless 3D prototyping for fabrication of capillary fluidic circuits using laser engraving. The capillary circuits can be fabricated in a short amount of time (<10 min) without the requirements of clean‐room facilities and photomasks. The advanced capillary components (e.g., trigger valves, retention valves and retention bursting valves) were fabricated, enabling the sequential liquid delivery and sample‐reagent mixing. With the integration of smartphone‐based detection platform, the microfluidic device can quantify the urinary protein via a colorimetric analysis. By eliminating the bulky and expensive equipment, this smartphone‐based detection platform is portable for on‐site quantitative detection.     

Circular‐shaped microfluidic device to study the effect of shear stress on cellular orientation

Understanding the effects of shear stress on mammalian cells is a crucial factor for understanding a number of biological processes and diseases. Here, we show the development of a circular‐shaped microfluidic device for the facile generation of shear stress gradients. With this microfluidic device, the effect of shear stress on orientation of human umbilical vein endothelial cells was studied. This microfluidic device, which enables to control the alignment of human umbilical vein endothelial cells within a microchannel, can be a valuable tool to mimic blood vessels.     

Generation of concentration gradient by controlled flow distribution and diffusive mixing in a microfluidic chip

We have developed a simple method to generate a concentration gradient in a microfluidic device. This method is based on the combination of controlled fluid distribution at each intersection of a microfluidic network by liquid pressure and subsequent diffusion between laminas in the downstream microchannel. A fluid dynamic model taking into account the diffusion coefficient was established to simulate the on-chip flow distribution and diffusion. Concentration gradients along a distance of a few hundred micrometers were generated in a series of microchannels. The gradients could be varied by carefully regulating the liquid pressure applied to the sample injection vials. The observed concentration gradients of fluorescent dyes generated on the microfluidic channel are consistent with the theoretically predicted results. The microfluidic design described in this study may provide a new tool for applications based on concentration gradients, including many biological and chemical analyses such as cellular reaction monitoring and drug screening.     

Paper‐Based Electrochemiluminescent 3D Immunodevice for Lab‐on‐Paper,Specific, and Sensitive Point‐of‐Care Testing

Recent research on microfluidic paper‐based analytical devices (μPADs) has shown that paper has great potential for the fabrication of low‐cost diagnostic devices for healthcare and environmental monitoring applications. Herein, electrochemiluminescence (ECL) was introduced for the first time into μPADs that were based on screen‐printed paper‐electrodes. To further perform high‐specificity, high‐performance, and high‐sensitivity ECL on μPADs for point‐of‐care testing (POCT), ECL immunoassay capabilities were introduced into a wax‐patterned 3D paper‐based ECL device, which was characterized by SEM, contact‐angle measurement, and electrochemical impedance spectroscopy. With the aid of a home‐made device‐holder, the ECL reaction was triggered at room temperature. By using a typical tris(bipyridine)ruthenium–tri‐n‐propylamine ECL system, this paper‐based ECL 3D immunodevice was applied to the diagnosis of carcinoembryonic antigens in real clinical serum samples. This contribution further expands the number of sensitive and specific detection modes of μPADs.     

Electrokinetic Size and Mobility Traps for On‐site Therapeutic Drug Monitoring

The extraction of target analytes from biological samples is a bottleneck in analysis. A microfluidic device featuring an electrokinetic size and mobility trap was formed by two nanojunctions of different pore size to extract and concentrate analytical targets from complex samples. The trap was seamlessly coupled with electrophoretic separation for quantitative analysis. The device was applied to the analysis of ampicillin levels in blood within 5 min and a linear response over the range of 2.5–20 μg mL^?1. This covers the recommended levels for treating sepsis, a critical condition with 30 to 50 % mortality and unpredicted drug levels. The device provides a new opportunity for on‐site therapeutic drug monitoring, which should enable quick and accurate dosing and may save lives in such critical conditions.     

Simultaneous analysis of multiple urinary biomarkers for the evaluation of oxidative stress by automated online in‐tube solid‐phase microextraction coupled with negative/positive ion‐switching mode liquid chromatography–tandem mass spectrometry

This study described an automated online method for the simultaneous determination of 8‐isoprostane, 8‐hydroxy‐2′‐deoxyguanosine, and 3‐nitro‐l ‐tyrosine in human urine. The method involves in‐tube solid‐phase microextraction using a Carboxen 1006 PLOT capillary column as an extraction device, followed by liquid chromatography with tandem mass spectrometry using a CX column and detection in the negative/positive switching ion‐mode by multiple reaction monitoring. Using their stable isotope‐labeled internal standards, each of these oxidative stress biomarkers showed good linearity from 0.02 to 2.0 ng/mL. Their detection limits (S/N = 3) were 3.4–21.5 pg/mL, and their intra‐ and inter‐day precisions (relative standard deviations) were >3.9 and 6.5% (n = 5), respectively. This method was applied successfully to the analysis of urine samples, without any other pretreatment and interference peaks.     

Cotton thread modified with ionic liquid copolymerized polymer for online in‐tube solid‐phase microextraction and HPLC analysis of nonsteroidal anti‐inflammatory drugs

Cotton fiber is a biodegradable material that possesses properties such as high specific area, adjustable shape, and hygroscopicity. In this work, organic polymer was directly in situ grown on the surface of cotton thread and packed into a poly(ether ether ketone) tube for online in‐tube solid‐phase microextraction. The novel strategy solves the problems like high backpressure and tedious optimization process of normal monolithic polymer‐based in‐tube solid‐phase microextraction capillary. The quaternary ammonium typed ionic liquid of 1‐allyl‐methylimidazolium chloride, 4‐vinylbiphenyl, and ethylene dimethacrylate were co‐polymerized and in situ grown on the surface of cotton thread as extraction phase. The solid‐phase microextraction tube showed excellent performance for the extraction of three nonsteroidal anti‐inflammatory drugs including ketoprofen, naproxen, and flurbiprofen due to the strong ion exchange and hydrophobic interactions. After online coupling with a high‐performance liquid chromatography system by six‐port valve, the method was applied for the quantitative analysis of nonsteroidal anti‐inflammatory drugs in human plasma samples showing good enrichment performance (enrichment factor between 263 and 279), high sensitivity, good linearity, and good reproducibility.