Microfluidic sensing: state of the art fabrication and detection techniques

Battery-less long-range wireless fluidic sensing system using flexible additive manufacturing ambient energy harvester and microfluidics

Fluid sensing has been an important but missing part of the massive Internet-of-Things sensor networks due to challenges including excessive manufacturing time/cost, finite wireless interrogation range, limited immunity to ambient clutter, and excessive required power for autonomous microfluidics operability. Here, we proposed an additive manufacturing flexible system as a solution to those challenges while enabling fluid analysis from controlled labs to virtually everywhere. Energy harvesting provides all required power for the actuation of the micro-pump enabling battery-less liquid sample acquisition. Energy sources including ultra-high-frequency radio frequency identification and hand-held devices like two-way talk radio are harvested simultaneously to support energy requirements for periodic monitoring every 6.6 min and on-demand monitoring within 4.63 s. Backscattering topologies are used to significantly extend the reading range while increasing the immunity to interferences and reducing the cost to the reader. A new additive manufacturing process is proposed to reduce fabrication time and cost while enabling massive scalability of flexible microfluidics. The good flexibility makes the system suitable for working toward future wearable applications. Prototypes of a sweat sensing system are demonstrated and successfully interrogated at 3 m with more than 15 dB signal-to-noise ratio using only a 14 dBm transmitter equivalent isotropic radiated power.

Advancing Microfluidic Immunity Testing Systems: New Trends for Microbial Pathogen Detection

Pathogenic microorganisms play a crucial role in the global disease burden due to their ability to cause various diseases and spread through multiple transmission routes. Immunity tests identify antigens related to these pathogens, thereby confirming past infections and monitoring the host's immune response. Traditional pathogen detection methods, including enzyme-linked immunosorbent assays (ELISAs) and chemiluminescent immunoassays (CLIAs), are often labor-intensive, slow, and reliant on sophisticated equipment and skilled personnel, which can be limiting in resource-poor settings. In contrast, the development of microfluidic technologies presents a promising alternative, offering automation, miniaturization, and cost efficiency. These advanced methods are poised to replace traditional assays by streamlining processes and enabling rapid, high-throughput immunity testing for pathogens. This review highlights the latest advancements in microfluidic systems designed for rapid and high-throughput immunity testing, incorporating immunosensors, single molecule arrays (Simoas), a lateral flow assay (LFA), and smartphone integration. It focuses on key pathogenic microorganisms such as SARS-CoV-2, influenza, and the ZIKA virus (ZIKV). Additionally, the review discusses the challenges, commercialization prospects, and future directions to advance microfluidic systems for infectious disease detection.

Optimization of Microfluidics for Point-of-Care Blood Sensing

Blood tests are widely used in modern medicine to diagnose certain illnesses and evaluate the overall health of a patient. To enable testing in resource-limited areas, there has been increasing interest in point-of-care (PoC) testing devices. To process blood samples, liquid mixing with active pumps is usually required, making PoC blood testing expensive and bulky. We explored the possibility of processing approximately 2 μL of whole blood for image flow cytometry using capillary structures that allowed test times of a few minutes without active pumps. Capillary pump structures with five different pillar shapes were simulated using Ansys Fluent to determine which resulted in the fastest whole blood uptake. The simulation results showed a strong influence of the capillary pump pillar shape on the chip filling time. Long and thin structures with a high aspect ratio exhibited faster filling times. Microfluidic chips using the simulated pump design with the most efficient blood uptake were fabricated with polydimethylsiloxane (PDMS) and polyethylene oxide (PEO). The chip filling times were tested with 2 μL of both water and whole blood, resulting in uptake times of 24 s for water and 111 s for blood. The simulated blood plasma results deviated from the experimental filling times by about 35% without accounting for any cell-induced effects. By comparing the flow speed induced by different pump pillar geometries, this study offers insights for the design and optimization of passive microfluidic devices for inhomogenous liquids such as whole blood in sensing applications.

Optimization of Newtonian fluid pressure in microcantilever integrated flexible microfluidic channel for healthcare application

The demand for microfluidic pressure sensors is ever-increasing in various industries due to their crucial role in controlling fluid pressure within microchannels. While syringe pump setups have been traditionally used to regulate fluid pressure in microfluidic devices, they often result in larger setups that increase the cost of the device. To address this challenge and miniaturize the syringe pump setup, the researcher introduced integrated T-microcantilever-based microfluidic devices. In these devices, microcantilevers are incorporated, and their deflections correlate with the microchannel's pressure. When the relative pressure of fluid (plasma) changes, the T-microcantilever deflects, and the extent of this deflection provides information on fluid pressure within the microchannel. In this work, finite element method (FEM) based simulation was carried out to investigate the role of material, and geometric parameters of the cantilever, and the fluid viscosity on the pressure sensing capability of the T-microcantilever integrated microfluidic channel. The T-microcantilever achieves a maximum deflection of 127μm at a 5000μm/s velocity for Young's modulus(E) of 360 kPa of PDMS by employing a hinged structure. On the other hand, a minimum deflection of 4.05 × 10-5μm was attained at 5000μm/s for Young's modulus of 1 TPa for silicon. The maximum deflected angle of the T-cantilever is 20.46° for a 360 kPa Young's modulus while the minimum deflection angle of the T-cantilever is measured at 13.77° for 900 KPa at a fluid velocity of 5000μm s-1. The T-cantilever functions as a built-in microchannel that gauges the fluid pressure within the microchannel. The peak pressure, set at 8.86 Pa on the surface of the cantilever leads to a maximum deflection of 0.096μm (approximately 1μm) in the T-cantilever at a 1:1 velocity ratio. An optimized microfluidic device embedded with microchannels can optimize fluid pressure in a microchannel support cell separation.

A Microstrip Transmission Line Biosensor to Measure the Interaction between Microliter Aqueous Solutions and 1.0-17.0 GHz Radio Frequencies

Radio frequency (RF) biosensors are an expanding field of interest because of the ability to design noninvasive, label-free, low-production-cost sensing devices. Previous works identified the need for smaller experimental devices, requiring nanoliter to milliliter sampling volumes and increased capability of repeatable and sensitive measurement capability. The following work aims to verify a millimeter-sized, microstrip transmission line biosensor design with a microliter well operating on a broadband radio frequency range of 1.0-17.0 GHz. Three successive experiments were performed to provide evidence for (1) repeatability of measurements after loading/unloading the well, (2) sensitivity of measurement sets, and (3) methodology verification. Materials under test (MUTs) loaded into the well included deionized water, Tris-EDTA buffer, and lambda DNA. S-parameters were measured to determine interaction levels between the radio frequencies and MUTs during the broadband sweep. MUTs increasing in concentration were repeatably detected and demonstrated high measurement sensitivity, with the highest error value observed being 0.36%. Comparing Tris-EDTA buffer versus lambda DNA suspended in Tris-EDTA buffer suggests that introducing lambda DNA into the Tris-EDTA buffer repeatably alters S-parameters. The innovative aspect of this biosensor is that it can measure interactions of electromagnetic energy and MUTs in microliter quantities with high repeatability and sensitivity.

Craft-and-Stick Xurographic Manufacturing of Integrated Microfluidic Electrochemical Sensing Platform

An innovative modular approach for facile design and construction of flexible microfluidic biosensor platforms based on a dry manufacturing "craft-and-stick" approach is developed. The design and fabrication of the flexible graphene paper electrode (GPE) unit and polyethylene tetraphthalate sheet (PET)6/adhesive fluidic unit are completed by an economic and generic xurographic craft approach. The GPE widths and the microfluidic channels can be constructed down to 300 μm and 200 μm, respectively. Both units were assembled by simple double-sided adhesive tapes into a microfluidic integrated GPE (MF-iGPE) that are flexible, thin (<0.5 mm), and lightweight (0.4 g). We further functionalized the iGPE with Prussian blue and glucose oxidase for the fabrication of MF-iGPE glucose biosensors. With a closed-channel PET fluidic pattern, the MF-iGPE glucose biosensors were packaged and sealed to protect the integrated device from moisture for storage and could easily open with scissors for sample loading. Our glucose biosensors showed 2 linear dynamic regions of 0.05-1.0 and 1.0-5.5 mmol L^-1 glucose. The MF-iGPE showed good reproducibility for glucose detection (RSD < 6.1%, n = 6) and required only 10 μL of the analyte. This modular craft-and-stick manufacturing approach could potentially further develop along the concept of paper-crafted model assembly kits suitable for low-resource laboratories or classroom settings.

Photonic Microfluidic Technologies for Phytoplankton Research

Phytoplankton is a crucial component for the correct functioning of different ecosystems, climate regulation and carbon reduction. Being at least a quarter of the biomass of the world's vegetation, they produce approximately 50% of atmospheric O2 and remove nearly a third of the anthropogenic carbon released into the atmosphere through photosynthesis. In addition, they support directly or indirectly all the animals of the ocean and freshwater ecosystems, being the base of the food web. The importance of their measurement and identification has increased in the last years, becoming an essential consideration for marine management. The gold standard process used to identify and quantify phytoplankton is manual sample collection and microscopy-based identification, which is a tedious and time-consuming task and requires highly trained professionals. Microfluidic Lab-on-a-Chip technology represents a potential technical solution for environmental monitoring, for example, in situ quantifying toxic phytoplankton. Its main advantages are miniaturisation, portability, reduced reagent/sample consumption and cost reduction. In particular, photonic microfluidic chips that rely on optical sensing have emerged as powerful tools that can be used to identify and analyse phytoplankton with high specificity, sensitivity and throughput. In this review, we focus on recent advances in photonic microfluidic technologies for phytoplankton research. Different optical properties of phytoplankton, fabrication and sensing technologies will be reviewed. To conclude, current challenges and possible future directions will be discussed.

Label-Free Sensing with Metal Nanostructure-Based Surface-Enhanced Raman Spectroscopy for Cancer Diagnosis

Surface-Enhanced Raman Spectroscopy (SERS) is a powerful analytical technique for the detection of small analytes with great potential for medical diagnostic applications. Its high sensitivity and excellent molecular specificity, which stems from the unique fingerprint of molecular species, have been applied toward the detection of different types of cancer. The noninvasive and rapid detection offered by SERS highlights its applicability for point-of-care (PoC) deployment for cancer diagnosis, screening, and staging, as well as for predicting tumor recurrence and treatment monitoring. This review provides an overview of the progress in label-free (direct) SERS-based chemical detection for cancer diagnosis with the main focus on the advances in the design and preparation of SERS substrates on the basis of metal nanoparticle structures formed via bottom-up strategies. It begins by introducing a synopsis of the working principles of SERS, including key chemometric approaches for spectroscopic data analysis. Then it introduces the advances of label-free sensing with SERS in cancer diagnosis using biofluids (blood, urine, saliva, sweat) and breath as the detection media. In the end, an outlook of the advances and challenges in cancer diagnosis via SERS is provided.

Vision-Based Performance Analysis of an Active Microfluidic Droplet Generation System Using Droplet Images

This paper discusses an active droplet generation system, and the presented droplet generator successfully performs droplet generation using two fluid phases: continuous phase fluid and dispersed phase fluid. The performance of an active droplet generation system is analysed based on the droplet morphology using vision sensing and digital image processing. The proposed system in the study includes a droplet generator, camera module with image pre-processing and identification algorithm, and controller and control algorithm with a workstation computer. The overall system is able to control, sense, and analyse the generation of droplets. The main controller consists of a microcontroller, motor controller, voltage regulator, and power supply. Among the morphological features of droplets, the diameter is extracted from the images to observe the system performance. The MATLAB-based image processing algorithm consists of image acquisition, image enhancement, droplet identification, feature extraction, and analysis. RGB band filtering, thresholding, and opening are used in image pre-processing. After the image enhancement, droplet identification is performed by tracing the boundary of the droplets. The average droplet diameter varied from ~3.05 mm to ~4.04 mm in the experiments, and the average droplet diameter decrement presented a relationship of a second-order polynomial with the droplet generation time.

In Vitro Flow Chamber Design for the Study of Endothelial Cell (Patho)Physiology

In the native vasculature, flowing blood produces a frictional force on vessel walls that affects endothelial cell function and phenotype. In the arterial system, the vasculature's local geometry directly influences variations in flow profiles and shear stress magnitudes. Straight arterial sections with pulsatile shear stress have been shown to promote an athero-protective endothelial phenotype. Conversely, areas with more complex geometry, such as arterial bifurcations and branch points with disturbed flow patterns and lower, oscillatory shear stress, typically lead to endothelial dysfunction and the pathogenesis of cardiovascular diseases. Many studies have investigated the regulation of endothelial responses to various shear stress environments. Importantly, the accurate in vitro simulation of in vivo hemodynamics is critical to the deeper understanding of mechanotransduction through the proper design and use of flow chamber devices. In this review, we describe several flow chamber apparatuses and their fluid mechanics design parameters, including parallel-plate flow chambers, cone-and-plate devices, and microfluidic devices. In addition, chamber-specific design criteria and relevant equations are defined in detail for the accurate simulation of shear stress environments to study endothelial cell responses.

Effect of Controlled Microtopography on Osteogenic Differentiation of Mesenchymal Stem Cells

Various kinds of controlled microtopographies can promote osteogenic differentiation of mesenchymal stem cells (MSCs), such as microgrooves, micropillars, and micropits. However, the optimal shape, size, and mechanism remain unclear. In this review, we summarize the relationship between the parameters of different microtopographies and the behavior of MSCs. Then, we try to reveal the potential mechanism between them. The results showed that the microgrooves with a width of 4–60 μm and ridge width <10 μm, micropillars with parameters less than 10 μm, and square micropits had the full potential to promote osteogenic differentiation of MSCs, while the micromorphology of the same size could induce larger focal adhesions (FAs), well-organized cytoskeleton, and superior cell areas. Therefore, such events are possibly mediated by microtopography-induced mechanotransduction pathways.

Label-Free Physical Techniques and Methodologies for Proteins Detection in Microfluidic Biosensor Structures

Proteins in biological fluids (blood, urine, cerebrospinal fluid) are important biomarkers of various pathological conditions. Protein biomarkers detection and quantification have been proven to be an indispensable diagnostic tool in clinical practice. There is a growing tendency towards using portable diagnostic biosensor devices for point-of-care (POC) analysis based on microfluidic technology as an alternative to conventional laboratory protein assays. In contrast to universally accepted analytical methods involving protein labeling, label-free approaches often allow the development of biosensors with minimal requirements for sample preparation by omitting expensive labelling reagents. The aim of the present work is to review the variety of physical label-free techniques of protein detection and characterization which are suitable for application in micro-fluidic structures and analyze the technological and material aspects of label-free biosensors that implement these methods. The most widely used optical and impedance spectroscopy techniques: absorption, fluorescence, surface plasmon resonance, Raman scattering, and interferometry, as well as new trends in photonics are reviewed. The challenges of materials selection, surfaces tailoring in microfluidic structures, and enhancement of the sensitivity and miniaturization of biosensor systems are discussed. The review provides an overview for current advances and future trends in microfluidics integrated technologies for label-free protein biomarkers detection and discusses existing challenges and a way towards novel solutions.

Nanotechnology-Based Strategies for Effective and Rapid Detection of SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic has gained worldwide attention and has prompted the development of innovative diagnostics, therapeutics, and vaccines to mitigate the pandemic. Diagnostic methods based on reverse transcriptase-polymerase chain reaction (RT-PCR) technology are the gold standard in the fight against COVID-19. However, this test might not be easily accessible in low-resource settings for the early detection and diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The lack of access to well-equipped clinical laboratories, requirement for the high level of technical competence, and the cost of the RT-PCR test are the major limitations. Moreover, RT-PCR is unsuitable for application at the point-of-care testing (PoCT) as it is time-consuming and lab-based. Due to emerging mutations of the virus and the burden it has placed on the health care systems, there is a growing urgency to develop sensitive, selective, and rapid diagnostic devices for COVID-19. Nanotechnology has emerged as a versatile technology in the production of reliable diagnostic tools for various diseases and offers new opportunities for the development of COVID-19 diagnostic systems. This review summarizes some of the nano-enabled diagnostic systems that were explored for the detection of SARS-CoV-2. It highlights how the unique physicochemical properties of nanoparticles were exploited in the development of novel colorimetric assays and biosensors for COVID-19 at the PoCT. The potential to improve the efficiency of the current assays, as well as the challenges associated with the development of these innovative diagnostic tools, are also discussed.

Wireless bipolar electrode-based textile electrofluidics: towards novel micro-total-analysis systems

Point of care testing using micro-total-analysis systems (μTAS) is critical to emergent healthcare devices with rapid and robust responses. However, two major barriers to the success of this approach are the prohibitive cost of microchip fabrication and poor sensitivity due to small sample volumes in a microfluidic format. Here, we aimed to replace the complex microchip format with a low-cost textile substrate with inherently built microchannels using the fibers' spaces. Secondly, by integrating this textile-based microfluidics with electrophoresis and wireless bipolar electrochemistry, we can significantly improve solute detection by focusing and concentrating the analytes of interest. Herein, we demonstrated that an in situ metal electrode simply inserted inside the textile-based electrophoretic system can act as a wireless bipolar electrode (BPE) that generates localized electric field and pH gradients adjacent to the BPE and extended along the length of the textile construct. As a result, charged analytes were not only separated electrophoretically but also focused where their electrophoretic migration and counter flow (EOF) balances due to redox reactions proceeding at the BPE edges. The developed wireless redox focusing technique on textile constructs was shown to achieve a 242-fold enrichment of anionically charged solute over an extended time of 3000 s. These findings suggest a simple route that achieves separation and analyte focusing on low-cost surface-accessible inverted substrates, which is far simpler than the more complex ITP on conventional closed and inaccessible capillary channels.

DNA Studies: Latest Spectroscopic and Structural Approaches

This review looks at the different approaches, techniques, and materials devoted to DNA studies. In the past few decades, DNA nanotechnology, micro-fabrication, imaging, and spectroscopies have been tailored and combined for a broad range of medical-oriented applications. The continuous advancements in miniaturization of the devices, as well as the continuous need to study biological material structures and interactions, down to single molecules, have increase the interdisciplinarity of emerging technologies. In the following paragraphs, we will focus on recent sensing approaches, with a particular effort attributed to cutting-edge techniques for structural and mechanical studies of nucleic acids.

An Overview on Microfluidic Systems for Nucleic Acids Extraction from Human Raw Samples

Nucleic acid (NA) extraction is a basic step for genetic analysis, from scientific research to diagnostic and forensic applications. It aims at preparing samples for its application with biomolecular technologies such as isothermal and non-isothermal amplification, hybridization, electrophoresis, Sanger sequencing and next-generation sequencing. Multiple steps are involved in NA collection from raw samples, including cell separation from the rest of the specimen, cell lysis, NA isolation and release. Typically, this process needs molecular biology facilities, specialized instrumentation and labor-intensive operations. Microfluidic devices have been developed to analyze NA samples with high efficacy and sensitivity. In this context, the integration within the chip of the sample preparation phase is crucial to leverage the promise of portable, fast, user-friendly and economic point-of-care solutions. This review presents an overview of existing lab-on-a-chip (LOC) solutions designed to provide automated NA extraction from human raw biological fluids, such as whole blood, excreta (urine and feces), saliva. It mainly focuses on LOC implementation aspects, aiming to describe a detailed panorama of strategies implemented for different human raw sample preparations.

Glioma-on-a-Chip Models

Glioma, as an aggressive type of cancer, accounts for virtually 80% of malignant brain tumors. Despite advances in therapeutic approaches, the long-term survival of glioma patients is poor (it is usually fatal within 12-14 months). Glioma-on-chip platforms, with continuous perfusion, mimic in vivo metabolic functions of cancer cells for analytical purposes. This offers an unprecedented opportunity for understanding the underlying reasons that arise glioma, determining the most effective radiotherapy approach, testing different drug combinations, and screening conceivable side effects of drugs on other organs. Glioma-on-chip technologies can ultimately enhance the efficacy of treatments, promote the survival rate of patients, and pave a path for personalized medicine. In this perspective paper, we briefly review the latest developments of glioma-on-chip technologies, such as therapy applications, drug screening, and cell behavior studies, and discuss the current challenges as well as future research directions in this field.

Microfluidic-based virus detection methods for respiratory diseases

With the recent SARS-CoV-2 outbreak, the importance of rapid and direct detection of respiratory disease viruses has been well recognized. The detection of these viruses with novel technologies is vital in timely prevention and treatment strategies for epidemics and pandemics. Respiratory viruses can be detected from saliva, swab samples, nasal fluid, and blood, and collected samples can be analyzed by various techniques. Conventional methods for virus detection are based on techniques relying on cell culture, antigen-antibody interactions, and nucleic acids. However, these methods require trained personnel as well as expensive equipment. Microfluidic technologies, on the other hand, are one of the most accurate and specific methods to directly detect respiratory tract viruses. During viral infections, the production of detectable amounts of relevant antibodies takes a few days to weeks, hampering the aim of prevention. Alternatively, nucleic acid-based methods can directly detect the virus-specific RNA or DNA region, even before the immune response. There are numerous methods to detect respiratory viruses, but direct detection techniques have higher specificity and sensitivity than other techniques. This review aims to summarize the methods and technologies developed for microfluidic-based direct detection of viruses that cause respiratory infection using different detection techniques. Microfluidics enables the use of minimal sample volumes and thereby leading to a time, cost, and labor effective operation. Microfluidic-based detection technologies provide affordable, portable, rapid, and sensitive analysis of intact virus or virus genetic material, which is very important in pandemic and epidemic events to control outbreaks with an effective diagnosis.

Fluorescence-activated droplet sorting for enhanced pyruvic acid accumulation by Candida glabrata

One of the goals of metabolic engineering is to engineer strains that can optimally produce target metabolites. However, the current workflow for rational engineering of the metabolic pathway is sometimes time-consuming and labor-intensive. Here, we have established a cost-effective approach for screening for variants secreting metabolites. Different surface display systems were adopted and verified, which anchored pHluorin to the Candida glabrata cell surface to associate pyruvic acid detection with the read out of this reporter. A generalizable simulation approach based on computational fluid dynamics and regularity of generated droplet dimension was presented, which was found to be an efficient design tool to explore microfluidic characteristics or optimization. Finally, a microfluidic platform based on simulation coupled with surface display system was constructed. A mutant exhibiting a 73.6% increase in pyruvic acid production was identified. This ultrahigh-throughput screening pattern offers a practical guide for identifying microbial strains with many traits of interest.

Fabrication routes via projection stereolithography for 3D-printing of microfluidic geometries for nucleic acid amplification

Digital Light Processing (DLP) stereolithography (SLA) as a high-resolution 3D printing process offers a low-cost alternative for prototyping of microfluidic geometries, compared to traditional clean-room and workshop-based methods. Here, we investigate DLP-SLA printing performance for the production of micro-chamber chip geometries suitable for Polymerase Chain Reaction (PCR), a key process in molecular diagnostics to amplify nucleic acid sequences. A DLP-SLA fabrication protocol for printed micro-chamber devices with monolithic micro-channels is developed and evaluated. Printed devices were post-processed with ultraviolet (UV) light and solvent baths to reduce PCR inhibiting residuals and further treated with silane coupling agents to passivate the surface, thereby limiting biomolecular adsorption occurences during the reaction. The printed devices were evaluated on a purpose-built infrared (IR) mediated PCR thermocycler. Amplification of 75 base pair long target sequences from genomic DNA templates on fluorosilane and glass modified chips produced amplicons consistent with the control reactions, unlike the non-silanized chips that produced faint or no amplicon. The results indicated good functionality of the IR thermocycler and good PCR compatibility of the printed and silanized SLA polymer. Based on the proposed methods, various microfluidic designs and ideas can be validated in-house at negligible costs without the requirement of tool manufacturing and workshop or clean-room access. Additionally, the versatile chemistry of 3D printing resins enables customised surface properties adding significant value to the printed prototypes. Considering the low setup and unit cost, design flexibility and flexible resin chemistries, DLP-SLA is anticipated to play a key role in future prototyping of microfluidics, particularly in the fields of research biology and molecular diagnostics. From a system point-of-view, the proposed method of thermocycling shows promise for portability and modular integration of funcitonalitites for diagnostic or research applications that utilize nucleic acid amplification technology.

Novel Approach toward Electrofluidic Substrates Utilizing Textile-Based Braided Structure

Electrofluidics is the unique combination of electrophoresis and microfluidics, which has opened up broad opportunities for bioanalysis and multiplexed assay. These systems typically comprise inaccessible and fully enclosed microcapillary or microchannels, with limited sample loading capacities and no direct access to the solutes within. Here, we investigate the application of multiyarn textile assemblies which provides an open and surface accessible electrophoretic separation platform. Three-dimensional (3D) textile structures have been produced using conventional knitting and braiding techniques from a range of commercially available yarns. Capillary zone electrophoresis separation studies have been carried out on these substrates using fluorescent anionic (fluorescence, FL) and cationic (rhodamine-B, Rh-B) markers. The effects of different yarn surface chemistry, textile fabrication technique, and electrolyte ionic strength on the electrophoretic mobility of the test analytes have been studied. From the broad range of yarns investigated, polyester was shown to have the highest electrophoretic mobility for Rh-B (6 × 10^-4 cm² V^-1 s^-1) and for FL (4 × 10^-4 cm² V^-1 s^-1). The braiding approach, being simple and versatile, was found to be the most effective route to produce 3D textile-based structures and offered the potential for selective movement and targeted delivery to different channels. Composite braids made with yarns of differential surface chemistries further revealed a unique behavior of separation and parallel movement of oppositely charged ionic species. We also demonstrate the feasibility to apply isotachophoresis (ITP) on these braided textile substrates to rapidly focus dispersed FL sample bands. Here, we demonstrate the focusing of FL from a dispersed band into narrow band with a 400 times reduction in sample width over 90 s. Owing to the simplicity and reproducibility of the developed approach, textile-based inverted microfluidic applications are expected to enable opportunities in bioanalysis, proteomics, and rapid clinical diagnostics.

3D printed self-supporting elastomeric structures for multifunctional microfluidics

Microfluidic devices fabricated via soft lithography have demonstrated compelling applications such as lab-on-a-chip diagnostics, DNA microarrays, and cell-based assays. These technologies could be further developed by directly integrating microfluidics with electronic sensors and curvilinear substrates as well as improved automation for higher throughput. Current additive manufacturing methods, such as stereolithography and multi-jet printing, tend to contaminate substrates with uncured resins or supporting materials during printing. Here, we present a printing methodology based on precisely extruding viscoelastic inks into self-supporting microchannels and chambers without requiring sacrificial materials. We demonstrate that, in the submillimeter regime, the yield strength of the as-extruded silicone ink is sufficient to prevent creep within a certain angular range. Printing toolpaths are specifically designed to realize leakage-free connections between channels and chambers, T-shaped intersections, and overlapping channels. The self-supporting microfluidic structures enable the automatable fabrication of multifunctional devices, including multimaterial mixers, microfluidic-integrated sensors, automation components, and 3D microfluidics.

3D Printed Microfluidic Devices for Solid-Phase Extraction and On-Chip Fluorescent Labeling of Preterm Birth Risk Biomarkers

Solid-phase extraction (SPE) is a general preconcentration method for sample preparation that can be performed on a variety of specimens. The miniaturization of SPE within a 3D printed microfluidic device further allows for fast and simple extraction of analytes while also enabling integration of SPE with other sample preparation and separation methods. Here, we present the development and application of a reversed-phase lauryl methacrylate-based monolith, formed in 3D printed microfluidic devices, which can selectively retain peptides and proteins. The effectiveness of these SPE monoliths and 3D printed microfluidic devices was tested using a panel of nine preterm birth biomarkers of varying hydrophobicities and ranging in mass from 2 to 470 kDa. The biomarkers were selectively retained, fluorescently labeled, and eluted separately from the excess fluorescent label in 3D printed microfluidic systems. These are the first results demonstrating microfluidic analysis processes on a complete panel of preterm birth biomarkers, an important step toward developing a miniaturized, fully integrated analysis system.

Thermophoretic Micron-Scale Devices: Practical Approach and Review

In recent years, there has been increasing interest in the development of micron-scale devices utilizing thermal gradients to manipulate molecules and colloids, and to measure their thermophoretic properties quantitatively. Various devices have been realized, such as on-chip implements, micro-thermogravitational columns and other micron-scale thermophoretic cells. The advantage of the miniaturized devices lies in the reduced sample volume. Often, a direct observation of particles using various microscopic techniques is possible. On the other hand, the small dimensions lead to some technical problems, such as a precise temperature measurement on small length scale with high spatial resolution. In this review, we will focus on the "state of the art" thermophoretic micron-scale devices, covering various aspects such as generating temperature gradients, temperature measurement, and the analysis of the current micron-scale devices. We want to give researchers an orientation for their development of thermophoretic micron-scale devices for biological, chemical, analytical, and medical applications.

Advanced Fabrication Techniques of Microengineered Physiological Systems

The field of organs-on-chips (OOCs) has experienced tremendous growth over the last decade. However, the current main limiting factor for further growth lies in the fabrication techniques utilized to reproducibly create multiscale and multifunctional devices. Conventional methods of photolithography and etching remain less useful to complex geometric conditions with high precision needed to manufacture the devices, while laser-induced methods have become an alternative for higher precision engineering yet remain costly. Meanwhile, soft lithography has become the foundation upon which OOCs are fabricated and newer methods including 3D printing and injection molding show great promise to innovate the way OOCs are fabricated. This review is focused on the advantages and disadvantages associated with the commonly used fabrication techniques applied to these microengineered physiological systems (MPS) and the obstacles that remain in the way of further innovation in the field.

Sensing and structure analysis byin situIR spectroscopy: from mL flow cells to microfluidic applications

In situmid-infrared (MIR) spectroscopy in liquids is an emerging field for the analysis of functional surfaces and chemical reactions. Different basic geometries exist forin situMIR spectroscopy in milliliter (mL) and microfluidic flow cells, such as attenuated total reflection (ATR), simple reflection, transmission and fiber waveguides. After a general introduction of linear opticalin situMIR techniques, the methodology of ATR, ellipsometric and microfluidic applications in single-reflection geometries is presented. Selected examples focusing on thin layers relevant to optical, electronical, polymer, biomedical, sensing and silicon technology are discussed. The development of an optofluidic platform translates IR spectroscopy to the world of micro- and nanofluidics. With the implementation of SEIRA (surface enhanced infrared absorption) interfaces, the sensitivity of optofluidic analyses of biomolecules can be improved significantly. A large variety of enhancement surfaces ranging from tailored nanostructures to metal-island film substrates are promising for this purpose. Meanwhile, time-resolved studies, such as sub-monolayer formation of organic molecules in nL volumes, become available in microscopic or laser-based set-ups. With the adaption of modern brilliant IR sources, such as tunable and broadband IR lasers as well as frequency comb sources, possible applications of far-field IR spectroscopy inin situsensing with high lateral (sub-mm) and time (sub-s) resolution are considerably extended.

Microfluidics in Haemostasis: A Review

Haemostatic disorders are both complex and costly in relation to both their treatment and subsequent management. As leading causes of mortality worldwide, there is an ever-increasing drive to improve the diagnosis and prevention of haemostatic disorders. The field of microfluidic and Lab on a Chip (LOC) technologies is rapidly advancing and the important role of miniaturised diagnostics is becoming more evident in the healthcare system, with particular importance in near patient testing (NPT) and point of care (POC) settings. Microfluidic technologies present innovative solutions to diagnostic and clinical challenges which have the knock-on effect of improving health care and quality of life. In this review, both advanced microfluidic devices (R&D) and commercially available devices for the diagnosis and monitoring of haemostasis-related disorders and antithrombotic therapies, respectively, are discussed. Innovative design specifications, fabrication techniques, and modes of detection in addition to the materials used in developing micro-channels are reviewed in the context of application to the field of haemostasis.

Smartphone-Based Point-of-Care Microfluidic Platform Fabricated with a ZnO Nanorod Template for Colorimetric Virus Detection

Viruses pose serious infectious disease threats to humans and animals. To significantly decrease the mortality and morbidity caused by virus infections, there is an urgent need of sensitive and rapid point-of-care platforms for virus detection, especially in low-resource settings. Herein, we developed a smartphone-based point-of-care platform for highly sensitive and selective detection of the avian influenza virus based on nanomaterial-enabled colorimetric detection. The 3D nanostructures, which serve as a scaffold for antibody conjugation to capture the avian influenza virus, are made on PDMS herringbone structures with a ZnO nanorod template. After virus capture, the on-chip gold nanoparticle-based colorimetric reaction allows virus detection by naked eyes with a detection limit of 2.7 × 10⁴ EID₅₀/mL, which is one order of magnitude better than that of conventional fluorescence-based ELISA. Furthermore, a smartphone imaging system with data processing capability further improves the detection limit, reaching down to 8 × 10³ EID₅₀/mL. The entire virus capture and detection process can be completed in 1.5 h. We envision that this point-of-care microfluidic system integrated with smartphone imaging and colorimetric detection would provide a fast, cheap, sensitive, and user-friendly platform for virus detection in low-resource settings.

Novel PDMS-Based Sensor System for MPWM Measurements of Picoliter Volumes in Microfluidic Devices

In order for automatic microinjection to serve biomedical and genetic research, we have designed and manufactured a PDMS-based sensor with a circular section channel using the microwire molding technique. For the very precise control of microfluidic transport, we developed a microfluidic pulse width modulation system (MPWM) for automatic microinjections at a picoliter level. By adding a computer-aided detection and tracking of fluid-specific elements in the microfluidic circuit, the PDMS microchannel sensor became the basic element in the automatic control of the microinjection sensor. With the PDMS microinjection sensor, we precise measured microfluidic volumes under visual detection, assisted by very precise computer equipment (with precision below 1 μm) based on image processing. The calibration of the MPWM system was performed to increase the reproducibility of the results and to detect and measure microfluidic volumes. The novel PDMS-based sensor system for MPWM measurements of microfluidic volumes contributes to the advancement of intelligent control methods and techniques, which could lead to new developments in the design, control, and in applications of real-time intelligent sensor system control.

Recent advances in microfluidics for drug screening

With ever increasing drug resistance and emergence of new diseases, demand for new drug development is at an unprecedented urgency. This fact has led to extensive recent efforts to develop new drugs and novel techniques for efficient drug screening. However, new drug development is commonly hindered by cost and time span. Thus, developing more accessible, cost-effective methods for drug screening is necessary. Compared with conventional drug screening methods, a microfluidic-based system has superior advantages in sample consumption, reaction time, and cost of the operation. In this paper, the advantages of microfluidic technology in drug screening as well as the critical factors for device design are described. The strategies and applications of microfluidics for drug screening are reviewed. Moreover, current limitations and future prospects for a drug screening microdevice are also discussed.

Optical sizing of nanoparticles in thin films of nonabsorbing nanocolloids

We study an optical method to infer the size of nanoparticles in a thin film of a dilute nonabsorbing nanocolloid. It is based on determining the contribution of the nanoparticles to the complex effective refractive index of a suspension from reflectivity versus the angle of incidence curves in an internal reflection configuration. The method requires knowing only approximately the particles' refractive index and volume fraction. The error margin in the refractive index used to illustrate this technique was 2%. The method is applicable to sizing nanoparticles from a few tens of nanometers to about 200 nm in radius and requires a small volume of the sample, in the range of a few microliters. The method could be used to sense nanoparticle aggregation and is suitable to be integrated into microfluidic devices.

Photothermally generated bubble on fiber (BoF) for precise sensing and control of liquid flow along a microfluidic channel

The recent development of liquid-phase chemical analyses, drug delivery, and flow cytometry requires precise sensing and control of the liquid flow in a microfluidic chip environment. The channel in microfluidic chips is getting narrower to cope with complex liquid controls on a single chip, where small-footprint sensors and actuators are in urgent demand for accurate flow management. In this study, a unique microscopic bubble-on-fiber (BoF) device that can be readily integrated to current microfluidic chips was proposed and demonstrated for in situ sensing and control of microfluidic flow rate. The single microbubble was optically generated on the gold-deposited facet of an optical fiber by the local heating due to optical absorption. The BoF is a microscopic Fabry-Perot cavity, which serves as a thermal flow sensor precisely detecting the flow-induced temperature changes in the optical frequency domain. Experimentally we achieved the minimum detectable flow rate of ~0.06 mm/s in a single microfluidic channel, which is equivalent to a volume flow rate of 22 nL/s, and a response time of ~6 s. We also demonstrated that the BoF functioned as a microfluidic valve to regulate the flow rate in a Y-shape microfluidic chip by optically varying the bubble diameter. In addition to advantages of highly integrated functionalities and microscopic form factor, the proposed BoF can obviate the usage of chemical tracer such as dyes and can provide a high sensitivity over repeated flow cycles in a highly consistent manner. The BoF is promising for the timely development of high-density lab-on-a-chip devices using its efficient liquid flow management capability.

Rapid and Inexpensive Fabrication of Multi-Depth Microfluidic Device using High-Resolution LCD Stereolithographic 3D Printing

With the dramatic increment of complexity, more microfluidic devices require 3D structures, such as multi-depth and -layer channels. The traditional multi-step photolithography is time-consuming and labor-intensive and also requires precise alignment during the fabrication of microfluidic devices. Here, we present an inexpensive, single-step, and rapid fabrication method for multi-depth microfluidic devices using a high-resolution liquid crystal display (LCD) stereolithographic (SLA) three-dimensional (3D) printing system. With the pixel size down to 47.25 μm, the feature resolutions in the horizontal and vertical directions are 150 μm and 50 μm, respectively. The multi-depth molds were successfully printed at the same time and the multi-depth features were transferred properly to the polydimethylsiloxane (PDMS) having multi-depth channels via soft lithography. A flow-focusing droplet generator with a multi-depth channel was fabricated using the presented 3D printing method. Experimental results show that the multi-depth channel could manipulate the morphology and size of droplets, which is desired for many engineering applications. Taken together, LCD SLA 3D printing is an excellent alternative method to the multi-step photolithography for the fabrication of multi-depth microfluidic devices. Taking the advantages of its controllability, cost-effectiveness, and acceptable resolution, LCD SLA 3D printing can have a great potential to fabricate 3D microfluidic devices.

Separation and Detection of Escherichia coli and Saccharomyces cerevisiae Using a Microfluidic Device Integrated with an Optical Fibre

This paper describes the development of an integrated system using a dry film resistant (DFR) microfluidic channel consisting of pulsed field dielectrophoretic field-flow-fractionation (DEP-FFF) separation and optical detection. The prototype chip employs the pulse DEP-FFF concept to separate the cells (Escherichia coli and Saccharomyces cerevisiae) from a continuous flow, and the rate of release of the cells was measured. The separation experiments were conducted by changing the pulsing time over a pulsing time range of 2⁻24 s and a flow rate range of 1.2⁻9.6 μ L min - 1 . The frequency and voltage were set to a constant value of 1 M Hz and 14 V pk-pk, respectively. After cell sorting, the particles pass the optical fibre, and the incident light is scattered (or absorbed), thus, reducing the intensity of the transmitted light. The change in light level is measured by a spectrophotometer and recorded as an absorbance spectrum. The results revealed that, generally, the flow rate and pulsing time influenced the separation of E. coli and S. cerevisiae. It was found that E. coli had the highest rate of release, followed by S. cerevisiae. In this investigation, the developed integrated chip-in-a lab has enabled two microorganisms of different cell dielectric properties and particle size to be separated and subsequently detected using unique optical properties. Optimum separation between these two microorganisms could be obtained using a longer pulsing time of 12 s and a faster flow rate of 9.6 μ L min - 1 at a constant frequency, voltage, and a low conductivity.

A microfluidic sensor for detecting chlorophenols using cross-linked enzyme aggregates (CLEAs)

Chlorophenols have a strong medicinal smell and can be detected by the human nose at parts-per-million levels. Therefore, continuous monitoring of chlorophenols in water supplies is highly important. Herein, we reported a microfluidic sensor which can be used to detect 2,4-dichlorophenol (2,4-DCP) in real time with a limit of detection of around 0.1 ppm. The microfluidic sensor is a membrane-less galvanic cell which consists of two laminar flows running in parallel inside a straight channel. The sensor measures the potential difference between a solution containing 2,4-DCP and a reference solution containing acetate buffer. In a continuous-flow mode, the cell potential is proportional to the concentration of 2,4-DCP. To render specificity for the sensor, we incorporate a pre-treatment section where the incoming solution containing 2,4-DCP is split into two streams. One of the streams is brought into contact with cross-linked laccase aggregates (which catalyzes the hydrolysis of 2,4-DCP) and the second stream is taken as a reference solution. By comparing the potential difference between the two streams, we can determine the concentration of 2,4-DCP with high specificity. The microfluidic sensor platform is potentially useful for real-time detection of micropollutants present in aquatic systems with high sensitivity and specificity.

An Interdigital Capacitor for Microwave Heating at 25 GHz and Wideband Dielectric Sensing of nL Volumes in Continuous Microfluidics

This paper proposes a miniature microwave-microfluidic chip based on continuous microfluidics and a miniature interdigital capacitor (IDC). The novel chip consists of three individually accessible heaters, three platinum temperature sensors and two liquid cooling and mixing zones. The IDC is designed to achieve localized, fast and uniform heating of nanoliter volumes flowing through the microfluidic channel. The heating performance of the IDC located on the novel chip was evaluated using a fluorescent dye (Rhodamine B) diluted in demineralized water on a novel microwave-optical-fluidic (MOF) measurement setup. The MOF setup allows simultaneous microwave excitation of the IDC by means of a custom-made printed circuit board (connected to microwave equipment) placed in a top stage of a microscope, manipulation of liquid flowing through the channel located over the IDC with a pump and optical inspection of the same liquid flowing over the IDC using a fast camera, a light source and the microscope. The designed IDC brings a liquid volume of around 1.2 nL from room temperature to 100 °C in 21 ms with 1.58 W at 25 GHz. Next to the heating capability, the designed IDC can dielectrically sense the flowing liquid. Liquid sensing was evaluated on different concentration of water-isopropanol mixtures, and a reflection coefficient magnitude change of 6 dB was recorded around 8.1 GHz, while the minimum of the reflection coefficient magnitude shifted in the same frequency range for 60 MHz.

A Point-of-Care Device for Molecular Diagnosis Based on CMOS SPAD Detectors with Integrated Microfluidics

We describe the integration of techniques and technologies to develop a Point-of-Care for molecular diagnosis PoC-MD, based on a fluorescence lifetime measurement. Our PoC-MD is a low-cost, simple, fast, and easy-to-use general-purpose platform, aimed at carrying out fast diagnostics test through label detection of a variety of biomarkers. It is based on a 1-D array of 10 ultra-sensitive Single-Photon Avalanche Diode (SPAD) detectors made in a 0.18 μm High-Voltage Complementary Metal Oxide Semiconductor (HV-CMOS) technology. A custom microfluidic polydimethylsiloxane cartridge to insert the sample is straightforwardly positioned on top of the SPAD array without any alignment procedure with the SPAD array. Moreover, the proximity between the sample and the gate-operated SPAD sensor makes unnecessary any lens or optical filters to detect the fluorescence for long lifetime fluorescent dyes, such as quantum dots. Additionally, the use of a low-cost laser diode as pulsed excitation source and a Field-Programmable Gate Array (FPGA) to implement the control and processing electronics, makes the device flexible and easy to adapt to the target label molecule by only changing the laser diode. Using this device, reliable and sensitive real-time proof-of-concept fluorescence lifetime measurement of quantum dot Qdot^TM 605 streptavidin conjugate is demonstrated.

A Review of State-of-the-Art Microfluidic Technologies for Environmental Applications: Detection and Remediation

Microfluidic systems have advanced beyond natural and life science applications and lab-on-a-chip uses. A growing trend of employing microfluidic technologies for environmental detection has emerged thanks to the precision, time-effectiveness, and cost-effectiveness of advanced microfluidic systems. This paper reviews state-of-the-art microfluidic technologies for environmental applications, such as on-site environmental monitoring and detection. Microdevices are extensively used in collecting environmental samples as a means to facilitate detection and quantification of targeted components with minimal quantities of samples. Likewise, microfluidic-inspired approaches for separation and treatment of contaminated water and air, such as the removal of heavy metals and waterborne pathogens from wastewater and carbon capture are also investigated.

Label-free real-time optical monitoring of DNA hybridization using SiN Mach-Zehnder interferometer-based integrated biosensing platform

We report on the label-free real-time optical monitoring of DNA hybridization upon exposure to a flow of complementary DNA at different concentrations. The biosensor is composed of a silicon nitride integrated unbalanced Mach-Zehnder interferometer (MZI), with an integrated arrayed waveguide grating as a spectral filter. This MZI has been shown to have both sufficient multiplexing capability and limit of detection on the order of 10  -  6 RIU. Probe DNA, consisting of a 36-mer fragment is covalently immobilized on the silicon nitride integrated biosensor. The wavelength shift is monitored upon complementary DNA targets being flown over the sensor. Concentrations of 1 pM can be easily detected. Also, an alternative route to modify the sensor surface with carboxylic groups using the photochemical reaction of fatty acids is proposed and preliminary XPS results are presented. Moreover, preliminary results for DNA obtained from a rolling circle amplification (RCA-DNA) process and spiked in a realistic amplification buffer are presented.

A Review of Current Methods in Microfluidic Device Fabrication and Future Commercialization Prospects

Microfluidic devices currently play an important role in many biological, chemical, and engineering applications, and there are many ways to fabricate the necessary channel and feature dimensions. In this review, we provide an overview of microfabrication techniques that are relevant to both research and commercial use. A special emphasis on both the most practical and the recently developed methods for microfluidic device fabrication is applied, and it leads us to specifically address laminate, molding, 3D printing, and high resolution nanofabrication techniques. The methods are compared for their relative costs and benefits, with special attention paid to the commercialization prospects of the various technologies.

Optical bio-chemical sensors based on whispering gallery mode resonators

Whispering gallery mode (WGM) resonators have attracted extensive attention and their unique characteristics have led to some remarkable achievements. In particular, when combined with optical sensing technology, the WGM reonator-based sensor offers the advantages of small size, high sensitivity and a real-time dynamic response. At present, this type of sensor is widely applied in the bio-chemical sensing field. In this paper, we briefly review the sensing principle, the structures and the sensing applications of optical bio-chemical sensors based on the WGM resonator, with particular focuses on their sensing properties and their advantages and disadvantages. In addition, the existing problems and future development trends of WGM resonator-based optical bio-chemical sensors are discussed.

On-Chip Spyhole Nanoelectrospray Ionization Mass Spectrometry for Sensitive Biomarker Detection in Small Volumes

A polyimide microfluidic chip with a microhole emitter (Ø 10-12 μm) created on top of a microchannel by scanning laser ablation has been designed for nanoelectrospray ionization (spyhole-nanoESI) to couple microfluidics with mass spectrometry. The spyhole-nanoESI showed higher sensitivity compared to standard ESI and microESI from the end of the microchannel. The limits of detection (LOD) for peptide with the spyhole-nanoESI MS reached 50 pM, which was 600 times lower than that with standard ESI. The present microchip emitter allows the analysis of small volumes of samples. As an example, a small cell lung cancer biomarker, neuron-specific enolase (NSE), was detected by monitoring the transition of its unique peptide with the spyhole-nanoESI MS/MS. NSE at 0.2 nM could be well identified with a signal to noise ratio (S/N) of 50, and thereby its LOD was estimated to be 12 pM. The potential application of the spyhole-nanoESI MS/MS in cancer diagnosis was further demonstrated with the successful detection of 2 nM NSE from 1 μL of human serum. Before the detection, the serum sample spiked with NSE was first depleted with immune spin column, then desalted by centrifugal filter device, and finally digested by trypsin, without any other complicated preparation steps. The concentration matched the real condition of clinical samples. In addition, the microchips can be disposable to avoid any cross contamination. The present technique provides a highly efficient way to couple microfluidics with MS, which brings additional values to various microfluidics and MS-based analysis. Graphical Abstract ᅟ.