Modeling and Optimization of High-Sensitivity, Low-Volume Microfluidic-Based Surface Immunoassays

Surface Reaction of Electroosmotic Flow-Driven Free Antigens With Immobilized Magnetic-Microbeads-Tagged-Antibodies in Microchannels

Immunoassays based on reactions between target pathogen (antigen; Ag) and antibody (Ab) are frequently used for Ag detection. An external magnetic field was used to immobilize magnetic microbeads-tagged-antibodies (mMB-Ab) on the surface of a microchannel in the capture zone. The mMB-Ab was subsequently used for Ag detection. The objective of this numerical study, with experimental validation, is to assess the surface reaction between mMB-Ab and Ag in the presence of electro-osmotic flow (EOF). First, immobilization of mMB-Ab complex in the wall of the capture zone was achieved. Subsequently, the Ag was transported by EOF toward the capture zone to bind with the immobilized mMB-Ab. Lastly, mMB-Ab:Ag complex was formed and immobilized in the capture zone. A finite volume solver was used to implement the above steps. The surface reaction between the mMB-Ab and Ag was investigated in the presence of electric fields (E): 150 V/cm-450 V/cm and Ag concentrations: 0.001 M-1000 M. The depletion of mMB-Ab increases with time as the E decreases. Furthermore, as the concentration of Ag decreases, the depletion of mMB-Ab increases with time. These results quantify the detection of Ag using the EOF device; thus, signifying its potential for rapid throughput screening of Ag. This platform technology can lead to the development of portable devices for the detection of target cells, pathogens, and biomolecules for testing water systems, biological fluids, and biochemicals.

Performance Optimization of a Microfluidic Virus Detection Cartridge: A Numerical and Experimental Study

Detection and imaging of viruses in a complex solution is particularly significant for virology and requires a comprehensive understanding of biosensors. While lab-on-a-chip systems are used in virus detection as biosensors, analysis and optimization of these systems are especially challenging due to the size of the system to be used in the certain application. The system of interest for virus detection is required to be cost efficient and is also needed to be able to easily operable with a simple setup. Moreover, the detailed analysis of these microfluidic systems should be made with precision in order to predict the capabilities and the efficiency of the system accurately. This paper reports on the use of a common commercial computational fluid dynamics (cfd) software for the analysis of a microfluidic lab-on-a-chip virus detection cartridge. This study evaluates the problems commonly encountered during microfluidic applications of cfd softwares particularly in the area of reaction modeling of the antigen-antibody interaction. cfd analysis is later validated and combined with experiments to optimize the amount of dilute solution used in the tests. Thereafter, the geometry of the microchannel is also optimized and optimal test conditions are set for a cost efficient and effective virus detection kit using light microscopy.

Biosensors Based on Inorganic Composite Fluorescent Hydrogels

Fluorescent hydrogels are promising candidate materials for portable biosensors to be used in point-of-care diagnosis because (1) they have a greater capacity for binding organic molecules than immunochromatographic test systems, determined by the immobilization of affinity labels within the three-dimensional hydrogel structure; (2) fluorescent detection is more sensitive than the colorimetric detection of gold nanoparticles or stained latex microparticles; (3) the properties of the gel matrix can be finely tuned for better compatibility and detection of different analytes; and (4) hydrogel biosensors can be made to be reusable and suitable for studying dynamic processes in real time. Water-soluble fluorescent nanocrystals are widely used for in vitro and in vivo biological imaging due to their unique optical properties, and hydrogels based on these allow the preservation of these properties in bulk composite macrostructures. Here we review the techniques for obtaining analyte-sensitive fluorescent hydrogels based on nanocrystals, the main methods used for detecting the fluorescent signal changes, and the approaches to the formation of inorganic fluorescent hydrogels via sol-gel phase transition using surface ligands of the nanocrystals.

Digital image analysis for biothreat detection via rapid centrifugal microfluidic orthogonal flow immunocapture

We report clear proof-of-principle for centrifugally-driven, multiplexed, paper-based orthogonal flow sandwich-style immunocapture (cOFI) and colorimetric detection of Zaire Ebola virus-like particles. Capture antibodies are immobilized onto nanoporous nitrocellulose membranes that are then laminated into polymeric microfluidic discs to yield ready-to-use analytical devices. Fluid flow is controlled solely by rotational speed, obviating the need for complex pneumatic pumping systems, and providing more precise flow control than with the capillary-driven flow used in traditional lateral flow immunoassays (LFIs). Samples containing the antigen of interest and gold nanoparticle-labeled detection antibodies are pumped centrifugally through the embedded, prefunctionalized membrane where they are subsequently captured to generate a positive, colorimetric signal. When compared to the equivalent LFI counterparts, this cOFI approach generated immunochromatographic colorimetric responses that are objectively darker (saturation), more intense (grayscale), and less variable regarding total area of the color response. We also describe an image analysis approach that enables access to rich color data and area statistics without the need for a commercial 'strip reader' or custom-written image analysis algorithms. Instead, our analytical method exploits inexpensive equipment (e.g., smart phone, flatbed scanner, etc.) and freely available software (Fiji distribution of ImageJ) to permit characterization of immunochromatographic responses that includes multiple color metrics, offering insights beyond typical grayscale analysis. The findings reported here stand as clear proof-of-principle for the feasibility of disc-based, centrifugally driven orthogonal flow through a membrane with immunocapture (cOFI) and colorimetric readout of a sandwich-type immunoassay in less than 15 minutes. Once fully developed, this cOFI platform could render a faster, more accurate diagnosis, while processing multiple samples simul-taneously.

Numerical optimization of microfluidic biosensor detection time for the SARS-CoV-2 using the Taguchi method

The performance of microfluidic biosensor of the SARS-Cov-2 was numerically analyzed through finite element method. The calculation results have been validated with comparison with experimental data reported in the literature. The novelty of this study is the use of the Taguchi method in the optimization analysis, and an L8(25) orthogonal table of five critical parameters-Reynolds number (Re), Damköhler number (Da), relative adsorption capacity (σ), equilibrium dissociation constant (KD), and Schmidt number (Sc), with two levels was designed. ANOVA methods are used to obtain the significance of key parameters. The optimal combination of the key parameters is Re = 10-2, Da = 1000, σ = 0.2, KD = 5, and Sc 104 to achieve the minimum response time (0.15). Among the selected key parameters, the relative adsorption capacity (σ) has the highest contribution (42.17%) to the reduction of the response time, while the Schmidt number (Sc) has the lowest contribution (5.19%). The presented simulation results are useful in designing microfluidic biosensors in order to reduce their response time.

Numerical simulation and optimization of AC electrothermal microfluidic biosensor for COVID-19 detection through Taguchi method and artificial network

Microfluidic biosensors have played an important and challenging role for the rapid detection of the new severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Previous studies have shown that the kinetic binding reaction of the target antigen is strongly affected by process parameters. The purpose of this research was to optimize the performance of a microfluidic biosensor using two different approaches: Taguchi optimization and artificial neural network (ANN) optimization. Taguchi L8(2⁵) orthogonal array involving eight groups of experiments for five key parameters, which are microchannel shape, biosensor position, applied alternating current voltage, adsorption constant, and average inlet flow velocity, at two levels each, are performed to minimize the detection time of a biosensor excited by an alternating current electrothermal force. Signal to noise ratio ( S / N ) and analysis of variance were used to reach the optimal levels of process parameters and to demonstrate their percentage contributions, in terms of improved device response time. The principal results of this study showed that the Taguchi method was able to identify that the kinetic adsorption rate is the most influential parameter at 93% contribution, and the reaction surface position is the least influential parameter at 0.07% contribution. Also, the ANN model was able to accurately predict the optimal input values with a very low prediction error. Overall, the major conclusion of this study is both the Taguchi and ANN approaches can be effectively utilized to optimize the performance of a microfluidic biosensor. These advances have the potential to revolutionize the field of biosensing.

Multiplexed microfluidic chip for cell co-culture

Paracrine signaling is challenging to study in vitro, as conventional culture tools dilute soluble factors and offer little to no spatiotemporal control over signaling. Microfluidic chips offer potential to address both of these issues. However, few solutions offer both control over onset and duration of cell-cell communication, and high throughput. We have developed a microfluidic chip designed to culture cells in adjacent chambers, separated by valves to selectively allow or prevent exchange of paracrine signals. The chip features 16 fluidic inputs and 128 individually-addressable chambers arranged in 32 sets of 4 chambers. Media can be continuously perfused or delivered by diffusion, which we model under different culture conditions to ensure normal cell viability. Immunocytochemistry assays can be performed in the chip, which we modeled and fine-tuned to reduce total assay time to 1 h. Finally, we validate the use of the chip for co-culture studies by showing that HEK293Ta cells respond to signals secreted by RAW 264.7 immune cells in adjacent chambers, only when the valve between the chambers is opened.

Centrifugal disc liquid reciprocation flow considerations for antibody binding to COVID antigen array during microfluidic integration

Heterogeneous immunoassays (HI) are an invaluable tool for biomarker detection and remain an ideal candidate for microfluidic point-of-care diagnostics. However, automating and controlling sustained fluid flow from benchtop to microfluidics for the HI reaction during the extended sample incubation step, remains difficult to implement; this leads to challenges for assay integration and assay result interpretation. To address these issues, we investigated the liquid reciprocation process on a microfluidic centrifugal disc (CD) to generate continuous, bidirectional fluid flow using only a rotating motor. Large volumetric flow rates (μL s^-1) through the HI reaction chamber were sustained for extended durations (up to 1 h). The CD liquid reciprocation operating behavior was characterized experimentally and simulated to determine fluid flow shear rates through our HI reaction chamber. We demonstrated the continuous CD liquid reciprocation for target molecule incubation for a microarray HI and that higher fluid shear rates negatively influenced our fluorescence intensity. We highlight the importance of proper fluid flow considerations when integrating HIs with microfluidics.

Development of an Aptamer-Based Electrochemical Microfluidic Device for Viral Vaccine Quantitation

Global deployment of vaccines poses significant challenges in the distribution and use of the accompanying immunoassays, one of the standard methods for quality control of vaccines, particularly when establishing assays in countries worldwide to support testing/release upon importation. This work describes our effort toward developing an integrated, portable device to carry out affinity assays for viral particles quantification in viral vaccines by incorporating (i) aptamers, (ii) microfluidic devices, and (iii) electrochemical detection. We generated and characterized more than eight aptamers against multiple membrane proteins of cytomegalovirus (CMV), which we used as a model system and designed and fabricated electrochemical microfluidic devices to measure CMV concentrations in a candidate vaccine under development. The aptamer-based assays provided a half maximal effective concentration, EC₅₀, of 12 U/mL, comparable to that of an ELISA using a pair of antibodies (EC₅₀ 60 U/mL). The device measured relative CMV concentrations accurately (within ±10% bias) and precisely (11%, percent relative standard deviation). This work represents the critical first steps toward developing simple, affordable, and robust affinity assays for global deployment without the need for sensitive equipment and extensive analyst training.

Taguchi optimization of integrated flow microfluidic biosensor for COVID-19 detection

In this research, Taguchi's method was employed to optimize the performance of a microfluidic biosensor with an integrated flow confinement for rapid detection of the SARS-CoV-2. The finite element method was used to solve the physical model which has been first validated by comparison with experimental results. The novelty of this study is the use of the Taguchi approach in the optimization analysis. An L 8 2 7 orthogonal array of seven critical parameters-Reynolds number (Re), Damköhler number (Da), relative adsorption capacity ( σ ), equilibrium dissociation constant (K_D), Schmidt number (Sc), confinement coefficient (α) and dimensionless confinement position (X), with two levels was designed. Analysis of variance (ANOVA) methods are also used to calculate the contribution of each parameter. The optimal combination of these key parameters was Re = 10^-2, Da = 1000, σ = 0.5, K _D  = 5, Sc = 10⁵, α = 2 and X = 2 to achieve the lowest dimensionless response time (0.11). Among the all-optimization factors, the relative adsorption capacity ( σ ) has the highest contribution (37%) to the reduction of the response time, while the Schmidt number (Sc) has the lowest contribution (7%).

3D simulation of microfluidic biosensor for SARS-CoV-2 S protein binding kinetics using new reaction surface design

In this study, we performed 3D finite element simulations on the binding reaction kinetics of SARS-CoV-2 S protein (target analyte) and its corresponding immobilized antibody (ligand) in a heterogeneous microfluidic immunoassay. Two types of biosensors with two different shapes and geometries of the reaction surface and electrodes were studied. Alternating current electrothermal (ACET) force was applied to improve the binding efficiency of the biomolecular pairs by accelerating the transport of analytes to the binding surface. The ACET force stirs the flow field, thereby reducing the thickness of the diffusion boundary layer, often developed on the reaction surface due to the slow flow velocity, low analyte diffusion coefficient, and surface reaction high rate. The results showed that the detection time of one of the biosensors can be improved by 69% under an applied voltage of 10 Vrms and an operating frequency of 100 kHz. Certain control factors such as the thermal boundary conditions as well as the electrical conductivity of the buffer solution were analyzed in order to find the appropriate values to improve the efficiency of the biosensor.

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Antibody Surface Coverage Drives Matrix Interference in Microfluidic Capillary Immunoassays

The performance of biosensors is often optimized in buffers, which brings inconsistencies during applications with biological samples. Current strategies for minimizing sample (matrix) interference are complex to automate and miniaturize, involving, e.g., sample dilution or recovery of serum/plasma. This study shows the first systematic analysis using hundreds of actual microfluidic immunoassay fluoropolymer strips to understand matrix interference in microflow systems. As many interfering factors are assay-specific, we have explored matrix interference for a range of enzymatic immunoassays, including a direct mIgG/anti-mIgG, a sandwich cancer biomarker PSA, and a sandwich inflammatory cytokine IL-1β. Serum matrix interference was significantly affected by capillary antibody surface coverage, suggesting for the first time that the main cause of the serum matrix effect is low-affinity serum components (e.g., autoantibodies) competing with high-affinity antigens for the immobilized antibody. Additional experiments carried out with different capillary diameters confirmed the importance of antibody surface coverage in managing matrix interference. Building on these findings, we propose a novel analytical approach where antibody surface coverage and sample incubation times are key for eliminating and/or minimizing serum matrix interference, consisting in bioassay optimization carried out in serum instead of buffer, without compromising the performance of the bioassay or adding extra cost or steps. This will help establishing a new route toward faster development of modern point-of-care tests and effective biosensor development.

Microfluidic devices for glycobiomarker detection in cancer

During oncogenesis, several alterations occur within cells, one of them being the abnormal glycosylation of proteins, resulting in the formation of glycoproteins with aberrant glycan structures, which can be secreted into the blood stream. Their specific association to tumour cells makes them useful indicators (biomarkers) of the oncogenic process and their detection in blood can be employed in different stages of tumour development for early detection, prognosis and therapeutic drug monitoring. Due to the importance of detecting cancer-associated glycoproteins with aberrant glycosylation in blood or serum, analytical methodologies with improved performance are required to ameliorate the laboratorial tests currently used for the detection of these analytes. Microfluidics was created to facilitate the implementation of simple and point-of-care analysis, away from a centralized laboratory. The massive use of microfluidic systems in clinical settings can be seen in pregnancy tests and diabetes control, for example. But what about other clinical domains, such as the detection of glycoproteins with aberrant glycans secreted by tumour cells? Are microfluidic systems helpful in this case? This review analyses the requirements of a microfluidic assay for the detection of low-abundant blood/serum cancer-associated glycoproteins with abnormal glycans and the progresses that have been made in the last years to develop integrated microfluidic devices for this particular application. The diverse microfluidic systems found in literature present, in general, the same analytical performance as the conventional assays but have additional advantages, namely a reduction in assay times, a decrease of sample and reagent consumption and lower costs. The review will also focus on the improvements that are still needed for better biosensing of this type of cancer biomarkers using microfluidic devices.

Critical Comparison between Large and Mini Vertical Flow Immunoassay Platforms for Yersinia Pestis Detection

Yersinia pestis is a Gram-negative bacterium that is the causative agent of plague and is widely recognized as a potential biological weapon. Due to the high fatality rate of plague when diagnosis is delayed, the development of rapid, sensitive, specific, and cost-effective methods is needed for its diagnosis. The Y. pestis low calcium response V (LcrV) protein has been identified as a potential microbial biomarker for the diagnosis of plague. In this paper, we present a highly sensitive, paper-based, vertical flow immunoassay (VFI) prototype for the detection of LcrV and the diagnosis of plague. An antigen-capture assay using monoclonal antibodies is employed to capture and detect the LcrV protein, using a colorimetric approach. In addition, the effect of miniaturizing the VFI device is explored based on two different sizes of VFI platforms, denoted as "large VFI" and "mini VFI." Also, a comparative analysis is performed between the VFI platform and a lateral flow immunoassay (LFI) platform to exhibit the improved assay sensitivity suitable for point-of-care (POC) diagnostics. The analytical sensitivity or limit of detection (LOD) in the mini VFI is approximately 0.025 ng/mL, that is, 10 times better than that of the large VFI platform or 80 times over a standard lateral flow configuration. The low LOD of the LcrV VFI appears to be highly suitable for testing clinical samples and potentially diagnosing plague at earlier time points. In addition, optimization of the gold nanoparticle (AuNP) concentration, nanomaterial plasmonic properties, and flow velocity analysis could improve the performance of the VFI. Furthermore, we developed automated image analysis software that shows potential for integrating the diagnostic system into a smartphone. These methods and findings demonstrate that the VFI platform is a highly sensitive device for detecting the LcrV and potentially many other biomarkers.

Numerical Modeling for Sensitive and Rapid Molecular Detection by Membrane-Based Immunosensors

Rapid, simple, and sensitive point-of-care testing (POCT) has attracted attention in recent years due to its excellent potential for early disease diagnosis and health monitoring. The flow-through biosensor design is a candidate for POCT that utilizes the small-sized pores of a porous membrane as a recognition space where it emits a signal comparable to that of a conventional enzyme-linked immunosorbent assay within 35 min of detection time. In this paper, we present a numerical model for this immunosensing technology to systematically design an improved recognition system. The model considers mass transfer into the pore (convection and diffusion), the kinetics between the immobilized receptor and the target molecule, and the flow conditions, successfully leading to a bottleneck step (capture of secondary antibody) in sandwich-type detection. Our simulation results also show that this problem can be solved by adopting both appropriate receptors and analytical conditions. Eventually, the requirements to achieve the sensitivity required for POCT were fulfilled, which will allow for further development of immunosensing devices for disease detection.

Controlled protein adsorption on a silica microparticle

In the present study, controlled protein adsorption on a rigid silica microparticle is investigated numerically using classical Langmuir and two-state models under electrokinetic flow conditions. The instantaneous particle locations are simulated along a straight microchannel using an arbitrary Lagrangian-Eulerian framework in the finite element method for the electrophoretic motion of the charged particle. Within the scope of the parametric study, the strength of the external electric field (E), particle diameter (D_p ), the zeta potential of the particle (ζ_p ), and the location of the microparticle away from the channel wall (H) are systematically varied. The results are also compared to the data of pressure-driven flow having a parabolic flow profile at the inlet whose maximum magnitude is set to the particle's electrophoretic velocity magnitude. The validation studies reveal that the code developed for the particle motion in the present simulations agrees well with the experimental results. It is observed that protein adsorption can be controlled using electrokinetic phenomena. The plug-like flow profile in electrokinetics is beneficial for a microparticle at every spatial location in the microchannel, whereas it is not valid for the pressure-driven flow. The electric field strength and the zeta potential of the particle accelerate the protein adsorption. The wall shear stress and shear rate are good indicators to predict the adsorption process for electrokinetic flow.

Enhancement of COVID-19 detection time by means of electrothermal force

The rapid spread and quick transmission of the new ongoing pandemic coronavirus disease 2019 (COVID-19) has urged the scientific community to looking for strong technology to understand its pathogenicity, transmission, and infectivity, which helps in the development of effective vaccines and therapies. Furthermore, there was a great effort to improve the performance of biosensors so that they can detect the pathogenic virus quickly, in reliable and precise way. In this context, we propose a numerical simulation to highlight the important role of the design parameters that can significantly improve the performance of the biosensor, in particular the sensitivity as well as the detection limit. Applied alternating current electrothermal (ACET) force can generate swirling patterns in the fluid within the microfluidic channel, which improve the transport of target molecule toward the reaction surface and, thus, enhance the response time of the biosensor. In this work, the ACET effect on the SARS-CoV-2 S protein binding reaction kinetics and on the detection time of the biosensor was analyzed. Appropriate choice of electrodes location on the walls of the microchannel and suitable values of the dissociation and association rates of the binding reaction, while maintaining the same affinity, with and without ACET effect, are also, discussed to enhance the total performance of the biosensor and reduce its response time. The two-dimensional equations system is solved by the finite element approach. The best performance of the biosensor is obtained in the case where the response time decreased by 61% with AC applying voltage.

Rapid Multianalyte Microfluidic Homogeneous Immunoassay on Electrokinetically Driven Beads

The simplicity of homogeneous immunoassays makes them suitable for diagnostics of acute conditions. Indeed, the absence of washing steps reduces the binding reaction duration and favors a rapid and compact device, a critical asset for patients experiencing life-threatening diseases. In order to maximize analytical performance, standard systems employed in clinical laboratories rely largely on the use of high surface-to-volume ratio suspended moieties, such as microbeads, which provide at the same time a fast and efficient collection of analytes from the sample and controlled aggregation of collected material for improved readout. Here, we introduce an integrated microfluidic system that can perform analyte detection on antibody-decorated beads and their accumulation in confined regions within 15 min. We employed the system to the concomitant analysis of clinical concentrations of Neutrophil Gelatinase-Associated Lipocalin (NGAL) and Cystatin C in serum, two acute kidney injury (AKI) biomarkers. To this end, high-aspect-ratio, three-dimensional electrodes were integrated within a microfluidic channel to impart a controlled trajectory to antibody-decorated microbeads through the application of dielectrophoretic (DEP) forces. Beads were efficiently retained against the fluid flow of reagents, granting an efficient on-chip analyte-to-bead binding. Electrokinetic forces specific to the beads' size were generated in the same channel, leading differently decorated beads to different readout regions of the chip. Therefore, this microfluidic multianalyte immunoassay was demonstrated as a powerful tool for the rapid detection of acute life-threatening conditions.

Microfluidic platform enables live-cell imaging of signaling and transcription combined with multiplexed secretion measurements in the same single cells

Innate immune cells, including macrophages and dendritic cells, protect the host from pathogenic assaults in part through secretion of a program of cytokines and chemokines (C/Cs). Cell-to-cell variability in C/C secretion appears to contribute to the regulation of the immune response, but the sources of secretion variability are largely unknown. To begin to track the biological sources that control secretion variability, we developed and validated a microfluidic device to integrate live-cell imaging of fluorescent reporter proteins with a single-cell assay of protein secretion. We used this device to image NF-κB RelA nuclear translocation dynamics and Tnf transcription dynamics in macrophages in response to stimulation with the bacterial component lipopolysaccharide (LPS), followed by quantification of secretion of TNF, CCL2, CCL3, and CCL5. We found that the timing of the initial peak of RelA signaling in part determined the relative level of TNF and CCL3 secretion, but not CCL2 and CCL5 secretion. Our results support evidence that differences in timing across cell processes partly account for cell-to-cell variability in downstream responses, but that other factors introduce variability at each biological step.

On-chip plasmonic immunoassay based on targeted assembly of gold nanoplasmonic particles

An on-chip, non-enzymatic immunoassay was developed via the targeted assemblies of gold nanoparticles with target proteins in degassing-driven microfluidic devices and simply quantified at the single particle level.

Paper-based Vertical Flow Immunoassay (VFI) for detection of bio-threat pathogens

Currently, the standard method for identifying biological agents of potential threats to national security and public health, such as pathogens, virus, and toxins, mainly rely on microbiological cultivation. This method is time-consuming and it requires sophisticated equipment and well-trained personnel, which are often unavailable in remote areas or at point-of-need. Therefore, an alternative rapid, simple, and sensitive method for detecting bio-threat agents is in crucial need. We report a paper-based Vertical Flow Immunoassay (VFI) device that can overcome these limitations. The VFI device utilizes a nanoporous nitrocellulose membrane encapsulated in a stainless steel filter holder. As the sample is pushed through the membrane, which is pre-functionalized with capture antibody, a sandwich assay is formed and colorimetric signal is generated to reflect the presence of target antigens. Through theoretical analyses of antigen-antibody binding process inside a porous membrane, we identified two critical factors - membrane pore size and sample flow rate that can be optimized to improve the assay sensitivity. Then, the effects were demonstrated through experimental studies using Burkholderia pseudomallei (the causative agent of melioidosis) as a model pathogen. The B. pseudomallei VFI was based on an immunoassay targeting the B. pseudomallei surface capsular polysaccharide (CPS). The experimental results agreed well with the theory showing that increasing the flow speed (up to 1.06 mm/s) and reducing the membrane pore size (down to 0.1 µm) could improve the sensitivity by at least 5 times. The VFI's limit-of-detection for CPS spiked in buffer solution was determined to be 0.02 ng/mL. The developed VFI shows great potential as a point-of-care tool for detection of bio-threat agents in a variety of clinical and resource-restricted conditions.

Total Capture, Convection-Limited Nanofluidic Immunoassays Exhibiting Nanoconfinement Effects

Understanding nanoconfinement phenomena is necessary to develop nanofluidic technology platforms. One example of nanoconfinement phenomena is shifts in reaction equilibria toward reaction products in nanoconfined systems, which have been predicted theoretically and observed experimentally in DNA hybridization. Here we demonstrate a convection-limited nanofluidic immunoassay that achieves total capture of a target analyte and an apparent shift in the antibody-antigen reaction equilibrium due to nanoconfinement. The system exhibits wavefronts of the target analyte that propagate along the length of the nanochannel at a velocity much slower than that of the carrier fluid. We apply an analytical model describing the propagation of these wavefronts to determine the density of capture antibody binding sites in the enclosed nanochannel for a known concentration of the target analyte. We then use this binding site density to estimate the concentration of solutions with 5× and 10× less analyte. Our analysis suggests that nanoconfinement results in a preference toward binding of the target analyte with the surface-grafted capture antibody, as evidenced by an apparent reduction in the equilibrium dissociation constant. Our findings motivate the advancement of new biomedical and chemical synthesis technologies by leveraging nanoconfinement effects, and demonstrate a useful platform for studying the effect of nanoconfinement on chemical systems.

A capillary flow-driven microfluidic system for microparticle-labeled immunoassays

A simple and sensitive capillary-driven microfluidic platform is designed and demonstrated for direct and sandwich microparticle-labeled immunoassays.

Optimization of microfluidic biosensor efficiency by means of fluid flow engineering

Binding reaction kinetics of analyte-ligand at the level of a sensitive membrane into a microchannel of a biosensor has been limited by the formation of the boundary diffusion layer. Therefore, the response time increases and affects the overall performance of a biosensor. In the present work, we develop an approach to engineer fluid streams into a complex configuration in order to improve the binding efficiency. We investigate numerically the flow deformations around a parallelepiped with square cross-section inside the microfluidic channel and exploit these deformations to simulate the analyte transport to the sensitive membrane and enhance both association and dissociation processes. The effect of several parameters on the binding reaction is provided such as: the obstacle location from the inlet of the microchannel, the average flow velocity, and the inlet analyte concentration. The optimal position of the obstacle is determined. An appropriate choice of the inlet flow velocity and inlet analyte concentration may reduce significantly the response time.

Protein-Ligand Interaction Detection with a Novel Method of Transient Induced Molecular Electronic Spectroscopy (TIMES): Experimental and Theoretical Studies

Protein-ligand interaction detection without disturbances (e.g., surface immobilization, fluorescent labeling, and crystallization) presents a key question in protein chemistry and drug discovery. The emergent technology of transient induced molecular electronic spectroscopy (TIMES), which incorporates a unique design of microfluidic platform and integrated sensing electrodes, is designed to operate in a label-free and immobilization-free manner to provide crucial information for protein-ligand interactions in relevant physiological conditions. Through experiments and theoretical simulations, we demonstrate that the TIMES technique actually detects protein-ligand binding through signals generated by surface electric polarization. The accuracy and sensitivity of experiments were demonstrated by precise measurements of dissociation constant of lysozyme and N-acetyl-d-glucosamine (NAG) ligand and its trimer, NAG₃. Computational fluid dynamics (CFD) computation is performed to demonstrate that the surface's electric polarization signal originates from the induced image charges during the transition state of surface mass transport, which is governed by the overall effects of protein concentration, hydraulic forces, and surface fouling due to protein adsorption. Hybrid atomistic molecular dynamics (MD) simulations and free energy computation show that ligand binding affects lysozyme structure and stability, producing different adsorption orientation and surface polarization to give the characteristic TIMES signals. Although the current work is focused on protein-ligand interactions, the TIMES method is a general technique that can be applied to study signals from reactions between many kinds of molecules.

High sensitivity, high surface area Enzyme-linked Immunosorbent Assay (ELISA)

BACKGROUND

Enzyme-linked immunosorbent assays (ELISA) are considered the gold standard in the demonstration of various immunological reactions with an application in the detection of infectious diseases such as during outbreaks or in patient care.

OBJECTIVE

This study aimed to produce an ELISA-based diagnostic with an increased sensitivity of detection compared to the standard 96-well method in the immunologic diagnosis of infectious diseases.

METHODS

A '3DStack' was developed using readily available, low cost fabrication technologies namely nanoimprinting and press stamping with an increased surface area of 4 to 6 times more compared to 96-well plates. This was achieved by stacking multiple nanoimprinted polymer sheets. The flow of analytes between the sheets was enhanced by rotating the 3DStack and confirmed by Finite-Element (FE) simulation. An Immunoglobulin G (IgG) ELISA for the detection of antibodies in human serum raised against Rubella virus was performed for validation.

RESULTS

An improved sensitivity of up to 1.9 folds higher was observed using the 3DStack compared to the standard method.

CONCLUSIONS

The increased surface area of the 3DStack developed using nanoimprinting and press stamping technologies, and the flow pattern between sheets generated by rotating the 3DStack were potential contributors to a more sensitive ELISA-based diagnostic device.

Lab-on-chip systems for integrated bioanalyses

Biomolecular detection systems based on microfluidics are often called lab-on-chip systems. To fully benefit from the miniaturization resulting from microfluidics, one aims to develop 'from sample-to-answer' analytical systems, in which the input is a raw or minimally processed biological, food/feed or environmental sample and the output is a quantitative or qualitative assessment of one or more analytes of interest. In general, such systems will require the integration of several steps or operations to perform their function. This review will discuss these stages of operation, including fluidic handling, which assures that the desired fluid arrives at a specific location at the right time and under the appropriate flow conditions; molecular recognition, which allows the capture of specific analytes at precise locations on the chip; transduction of the molecular recognition event into a measurable signal; sample preparation upstream from analyte capture; and signal amplification procedures to increase sensitivity. Seamless integration of the different stages is required to achieve a point-of-care/point-of-use lab-on-chip device that allows analyte detection at the relevant sensitivity ranges, with a competitive analysis time and cost.

Multiplexed immunosensing and kinetics monitoring in nanofluidic devices with highly enhanced target capture efficiency

Nanofluidic devices promise high reaction efficiency and fast kinetic responses due to the spatial constriction of transported biomolecules with confined molecular diffusion. However, parallel detection of multiple biomolecules, particularly proteins, in highly confined space remains challenging. This study integrates extended nanofluidics with embedded protein microarray to achieve multiplexed real-time biosensing and kinetics monitoring. Implementation of embedded standard-sized antibody microarray is attained by epoxy-silane surface modification and a room-temperature low-aspect-ratio bonding technique. An effective sample transport is achieved by electrokinetic pumping via electroosmotic flow. Through the nanoslit-based spatial confinement, the antigen-antibody binding reaction is enhanced with ∼100% efficiency and may be directly observed with fluorescence microscopy without the requirement of intermediate washing steps. The image-based data provide numerous spatially distributed reaction kinetic curves and are collectively modeled using a simple one-dimensional convection-reaction model. This study represents an integrated nanofluidic solution for real-time multiplexed immunosensing and kinetics monitoring, starting from device fabrication, protein immobilization, device bonding, sample transport, to data analysis at Péclet number less than 1.

Investigation of Reagent Delivery Formats in a Multivalent Malaria Sandwich Immunoassay and Implications for Assay Performance

Conventional lateral flow tests (LFTs), the current standard bioassay format used in low-resource point-of-care (POC) settings, have limitations that have held back their application in the testing of low concentration analytes requiring high sensitivity and low limits of detection. LFTs use a premix format for a rapid one-step delivery of premixed sample and labeled antibody to the detection region. We have compared the signal characteristics of two types of reagent delivery formats in a model system of a sandwich immunoassay for malarial protein detection. The premix format produced a uniform binding profile within the detection region. In contrast, decoupling the delivery of sample and labeled antibody to the detection region in a sequential format produced a nonuniform binding profile in which the majority of the signal was localized to the upstream edge of the detection region. The assay response was characterized in both the sequential and premix formats. The sequential format had a 4- to 10-fold lower limit of detection than the premix format, depending on assay conjugate concentration. A mathematical model of the assay quantitatively reproduced the experimental binding profiles for a set of rate constants that were consistent with surface plasmon resonance measurements and absorbance measurements of the experimental multivalent malaria system.

Correlation of Capture Efficiency with the Geometry, Transport, and Reaction Parameters in Heterogeneous Immunosensors

Higher capture efficiency of biomarkers in heterogeneous immunosensors would enable early detection of diseases. Several strategies are used to improve the capture efficiency of these immunosensors including the geometry of the system along with the transport and reaction parameters. Having a prior knowledge of the behavior of the above parameters would facilitate the design of an efficient immunosensor. While the contributions of the transport and reaction parameters toward understanding of the mechanism involved in capture have been well studied in the literature, their effect in combination with the geometry of the sensors has not been explored until now. In this work, we have experimentally demonstrated that the capture efficiency of the antigen-antibody systems is inversely related to the size of the sensor patch. The experimental system was simulated in order to get an in-depth understanding of the mechanism behind the experimental observation. Further, the extent of heterogeneity in the system was analyzed using the Sips isotherm to obtain the heterogeneity index (α) and the reaction rate constant (K(D)) as fitted parameters for a sensor patch of 1.5 mm radius. The experimental kinetic data obtained for the same sensor patch matched reasonably with the simulation results by considering K(D) as the global affinity constant, which indicated that our system can be considered to be homogeneous. Our simulation results associated with the size dependency of the capture efficiency were in agreement with the trends obtained in our experimental observations where an inverse relation was observed owing to the fact that the mass-transfer limitation decreases with the decrease in the size of the sensor patch. The possible underlying mechanism associated with size dependency of capture efficiency was discussed based on the time-dependent radial variation of captured antigens obtained from our simulation results. A study on the parametric variation was further conducted for the nonmixed and mixed systems on the transport (Deff), reaction (K(D)), and geometric parameters (R). Two different correlations were established for the nonmixed and mixed systems between the capture efficiency (f) and a nondimensional number (t(D)/t(R)) consisting of the above-mentioned parameters. Such unified relations will be useful in designing heterogeneous immunosensors and can be extended to microfluidic immunosensors.

Ultrafast immunoassays by coupling dielectrophoretic biomarker enrichment in nanoslit channel with electrochemical detection on graphene

Heterogeneous immunoassays usually require long incubation times to promote specific target binding and several wash steps to eliminate non-specific binding. Hence, signal saturation is rarely achieved at detection limit levels of analyte, leading to significant errors in analyte quantification due to extreme sensitivity of the signals to incubation time and methodology. The poor binding kinetics of immunoassays at detection limit levels can be alleviated through creating an enriched analyte plug in the vicinity of immobilized capture probes to enable signal saturation at higher levels and at earlier times, due to higher analyte association and its faster replenishment at the binding surface. Herein, we achieve this by coupling frequency-selective dielectrophoretic molecular dam enrichment of the target biomarker in physiological media to capture probes immobilized on graphene-modified surfaces in a nanoslit to enable ultrafast immunoassays with near-instantaneous (<2 minutes) signal saturation at dilute biomarker levels (picomolar) within ultra-low sample volumes (picoliters). This methodology is applied to the detection of Prostate Specific Antigen (PSA) diluted in serum samples, followed by validation against a standard two-step immunoassay using three de-identified patient samples. Based on the ability of dielectrophoretic molecular dam analyte enrichment methods to enable the detection of PSA at 1-5 pg mL(-1) levels within a minute, and the relative insensitivity of the signals to incubation time after the first two minutes, we envision its application for improving the sensitivity of immunoassays and their accuracy at detection limit levels.

Enhancing conjugation rate of antibodies to carboxylates: Numerical modeling of conjugation kinetics in microfluidic channels and characterization of chemical over-exposure in conventional protocols by quartz crystal microbalance

This research reports an improved conjugation process for immobilization of antibodies on carboxyl ended self-assembled monolayers (SAMs). The kinetics of antibody/SAM binding in microfluidic heterogeneous immunoassays has been studied through numerical simulation and experiments. Through numerical simulations, the mass transport of reacting species, namely, antibodies and crosslinking reagent, is related to the available surface concentration of carboxyl ended SAMs in a microchannel. In the bulk flow, the mass transport equation (diffusion and convection) is coupled to the surface reaction between the antibodies and SAM. The model developed is employed to study the effect of the flow rate, conjugating reagents concentration, and height of the microchannel. Dimensionless groups, such as the Damköhler number, are used to compare the reaction and fluidic phenomena present and justify the kinetic trends observed. Based on the model predictions, the conventional conjugation protocol is modified to increase the yield of conjugation reaction. A quartz crystal microbalance device is implemented to examine the resulting surface density of antibodies. As a result, an increase in surface density from 321 ng/cm(2), in the conventional protocol, to 617 ng/cm(2) in the modified protocol is observed, which is quite promising for (bio-) sensing applications.

Diagnosing dengue virus infection: rapid tests and the role of micro/nanotechnologies

Due to the progressive spread of the dengue virus and a rising incidence of dengue disease, its rapid diagnosis is important for developing countries and of increasing relevance for countries in temperate climates. Recent advances in bioelectronics, micro- and nanofabrication technologies have led to new miniaturized point-of-care devices and analytical platforms suited for rapid detection of infections. Starting from the available tests for dengue diagnosis, this review examines emerging rapid, micro/nanotechnologies-based tools, including label-free biosensor methods, microarray and microfluidic platforms, which hold significant potential, but still need further development and evaluation. The epidemiological and clinical setting as key determinants for selecting the best analytical strategy in patients presenting with fever is then discussed. This review is aimed at the clinicians and microbiologists to deepen understanding and enhance application of dengue diagnostics, and also serves as knowledge base for researchers and test developers to overcome the challenges posed by this disease.

FROM THE CLINICAL EDITOR

Dengue disease remains a significant problem in many developing countries. Unfortunately rapid diagnosis with easy and low cost tests for this disease is currently still not realized. In this comprehensive review, the authors highlighted recent advances in nanotechnology which would enable development in this field, which would result in beneficial outcomes to the population.

Development of a low-volume, highly sensitive microimmunoassay using computational fluid dynamics-driven multiobjective optimization

Immunoassays are one of the most versatile and widely performed biochemical assays and, given their selectivity and specificity, are used in both clinical and research settings. However, the high cost of reagents and relatively large sample volumes constrain the integration of immunoassays into many applications. Scaling the assay down within microfluidic devices can alleviate issues associated with reagent and sample consumption. However, in many cases a new device is designed and empirically optimized for each specific analyte, a costly and time consuming approach. In this paper, we report the development of a microfluidic bead-based immunoassay which, using antibody coated microbeads, can potentially detect any analyte or combination of analytes for which antibody coated microbeads can be generated. We also developed a computational reaction model and optimization algorithm that can be used to optimize the device for any analyte. We applied this technique to develop a low volume IL-6 immunoassay with high sensitivity (358 fM, 10 pg/mL) and a large dynamic range (4 orders of magnitude). This device design and optimization technique can be used to design assays for any protein with an available antibody and can be used with a large number of applications including biomarker discovery, temporal in vitro studies using a reduced number of cells and reagents, and analysis of scarce biological samples in animal studies and clinical research settings.

Mass transfer in the biomolecular binding of a target against probe molecules on the surface of microbeads sequestered in wells in a microfluidic cell

Diagnostic tools which screen the binding interactions of a protein target against a display of biomolecular probes to identify molecules which bind the target are central to cell proteomic studies, and to diagnostic assays. Here, we study a microfluidic design for screening interactions in which the probe molecules are hosted on microbeads sequestered in wells arranged at the bottom of a microfluidic flow channel. Assays are undertaken by streaming an analyte solution with a fluorescently labelled target through the cell, and identifying the fluorescing beads. Numerical simulations are first constructed for the analyte flow over the microbeads in the well array, and the increase in the target concentration on the microbead surface. The binding profile is expressed as a function of the ratio of the convective to the diffusive transport rates (Peclet number or Pe), and the ratio of the kinetic to the diffusive rates (Damkohler number, Da). For any Pe, as Da becomes small enough, the transport is determined by the intrinsic kinetic binding rate. As Pe increases, a thin concentration boundary layer develops over the top surface of the microbead because of the convective flow, and target binds more rapidly. However, the relatively stagnant layers of liquid in the well provide a diffusion barrier which slows the target transport, and for any Da and Pe the transport is slower than equivalent patches of probes arranged on the channel wall. Experiments are also undertaken at high Pe, using the binding of fluorescently labelled NeutrAvidin as a target to probes of its binding partner, biotin, on the microbead surface. The binding profile is compared to the simulations to measure the kinetic rate constant, and this comparison shows that the transport in the cell is not kinetically limited because of the diffusion barriers created by the stagnant liquid layer in the well. Simulations and experiments on microbeads which are only partially recessed in the well demonstrate an increase in the mass transfer rate as more of the microbead surface intersects the flow and the diffusion limitation due to the stagnant layer of liquid surrounding the bottom part of the microbead is minimized.

Rapid, sensitive and real-time multiplexing platform for the analysis of protein and nucleic-acid biomarkers

We describe a multiplexing technology, named Evalution, based on novel digitally encoded microparticles in microfluidic channels. Quantitative multiplexing is becoming increasingly important for research and routine clinical diagnostics, but fast, easy-to-use, flexible and highly reproducible technologies are needed to leverage the advantages of multiplexing. The presented technology has been tailored to ensure (i) short assay times and high reproducibility thanks to reaction-limited binding regime, (ii) dynamic control of assay conditions and real-time binding monitoring allowing optimization of multiple parameters within a single assay run, (iii) compatibility with various immunoassay formats such as coflowing the samples and detection antibodies simultaneously and hence simplifying workflows, (iv) analyte quantification based on initial binding rates leading to increased system dynamic range and (v) high sensitivity via enhanced fluorescence collection. These key features are demonstrated with assays for proteins and nucleic acids showing the versatility of this technology.

Towards nano-diagnostics for bacterial infections

Sensitive, specific and rapid diagnosis of infectious diseases is essential for effective and economic medical care. Focused medical treatment of the patient enabled by pathogen-specific diagnosis may benefit the patient, may reduce cost, and may minimize the risk of drug resistance development. The rapid progress in micro and nanotechnologies contributes to the development of novel diagnostic methods. This critical review assesses emerging nanotechnologies for diagnosis of bacterial infection in developed and developing countries on the background of the current state of the art and includes particular challenges and pitfalls posed by a number of specific pathogens.

Diagnostic performance study of an antigen microarray for the detection of antiphospholipid antibodies in human serum

The parallelization of clinically relevant antigens in a microarray format is of growing importance due to the ability to measure multiple antigen-antibody interactions. With the development of a microarray for the detection of antiphospholipid antibodies we focussed on one important autoimmune disease that is still diagnostically challenging. Reasons are the heterogeneity of the autoantibodies and the unspecific clinical symptoms.

For the covalent immobilization of antigenic structures, glass transducers were coated with 11-aminoundecyltrimethoxysilane (11-AUTMS). In total 35 antiphospholipid syndrome (APS) patients, six patients with lupus erythematosus and 24 healthy controls were investigated on a microarray format using polarized imaging reflectometric interference spectroscopy.

The novel surface modification based on the short derivative 11-AUTMS resulted in a selective biosensor allowing a clear differentiation of patient and control samples. It combined proteinogenic as well as phospholipid-derived antigens, namely β

Multiplexed determination of serological parameters has a great potential. We have shown that our biosensor is capable of detecting four different APS relevant antibodies in parallel exhibiting a sensitivity and specificity comparable to existing ELISA methods.

Advances in nanowire transistors for biological analysis and cellular investigation

Electrical biosensors based on silicon nanowire field-effect transistors (SiNW-FETs) have attracted enormous interest in the biosensing field. SiNW-FETs have proven to be significant and efficient in detecting diverse biomolecular species with the advantages of high probing sensitivity, target selectivity, real-time recording and label-free detection. In recent years, significant advances in biosensors have been achieved, particularly for cellular investigation and biomedical diagnosis. In this critical review, we will report on the latest developments in biosensing with SiNW-FETs and discuss recent advancements in the innovative designs of SiNW-FET devices. This critical review introduces the basic instrumental setup and working principle of SiNW-FETs. Technical approaches that attempted to enhance the detection sensitivity and target selectivity of SiNW-FET sensors are discussed. In terms of applications, we review the recent achievements with SiNW-FET biosensors for the investigations of protein-protein interaction, DNA/RNA/PNA hybridization, virus detection, cellular recording, biological kinetics, and clinical diagnosis. In addition, the novel architecture designs of the SiNW-FET devices are highlighted in studies of live neuron cells, electrophysiological measurements and other signal transduction pathways. Despite these remarkable achievements, certain improvements remain necessary in the device performance and clinical applications of FET-based biosensors; thus, several prospects about the future development of nanowire transistor-based instruments for biosensing employments are discussed at the end of this review.

Rapid microfluidic solid-phase extraction system for hyper-methylated DNA enrichment and epigenetic analysis

Genetic sequence and hyper-methylation profile information from the promoter regions of tumor suppressor genes are important for cancer disease investigation. Since hyper-methylated DNA (hm-DNA) is typically present in ultra-low concentrations in biological samples, such as stool, urine, and saliva, sample enrichment and amplification is typically required before detection. We present a rapid microfluidic solid phase extraction (μSPE) system for the capture and elution of low concentrations of hm-DNA (≤1 ng ml(-1)), based on a protein-DNA capture surface, into small volumes using a passive microfluidic lab-on-a-chip platform. All assay steps have been qualitatively characterized using a real-time surface plasmon resonance (SPR) biosensor, and quantitatively characterized using fluorescence spectroscopy. The hm-DNA capture/elution process requires less than 5 min with an efficiency of 71% using a 25 μl elution volume and 92% efficiency using a 100 μl elution volume.

Integrated label-free silicon nanowire sensor arrays for (bio)chemical analysis

We present a label-free (bio)chemical analysis platform that uses all-electrical silicon nanowire sensor arrays integrated with a small volume microfluidic flow-cell for real-time (bio)chemical analysis and detection. The integrated sensing platform contains an automated multi-sample injection system that eliminates erroneous sensor responses from sample switching due to flow rate fluctuations and provides precise sample volumes down to 10 nl. Biochemical sensing is demonstrated with real-time 15-mer DNA-PNA (peptide nucleic acid) duplex hybridization measurements from different sample concentrations in a low ionic strength, and the equilibrium dissociation constant KD ≈ 140 nM has been extracted from the experimental data using the first order Langmuir binding model. Chemical sensing is demonstrated with pH measurements from different injected samples in flow that have sensitivities consistent with the gate-oxide materials. A differential sensor measurement configuration results in a 30× reduction in sensor drift. The integrated label-free analysis platform is suitable for a wide range of small volume chemical and biochemical analyses.

A microfluidic device for immuno-affinity-based separation of mitochondria from cell culture

In this work, we present a method to isolate mitochondria of mammalian cells after cell disruption on microscale. The device is composed of linear microchannels cast in PDMS (polydimethylsiloxane). Specific antibodies against the translocase outer membrane protein of the mitochondria are immobilized on the surface of the substrate using an avidin-biotin sandwich construct. The mitochondria can be captured in the channel, whereas the remains of the cell lysate flow out the chip unhindered. The captured mitochondria can be observed directly on chip. A successful immobilization of pre-isolated mitochondria was shown at a flow rate between 0 and 5 μl min(-1) (≈0-2.5 mm s(-1)). After fluorescence staining, we demonstrated that the mitochondria covered around 3% of the channel surface. The mitochondria appeared in a distinct spherical shape with a diameter of around 0.8-1.2 μm. Further validation of the microfluidic device using non-treated cell lysate was done at 2 μl min(-1). The immobilized mitochondria were smaller with a diameter of around ≈490 nm. We observed a surface coverage of around 4%. The immobilized mitochondria were active and stable for over 2 h without cooling and were shown to be able to produce ATP under stage 3 respiration on chip.