Microfluidic Paper-Based Analytical Devices: From Design to Applications.

Microfluidic paper-based analytical devices (µPADs) have garnered significant interest as a promising analytical platform in the past decade. Compared with traditional microfluidics, µPADs present unique advantages, such as easy fabrication using established patterning methods, economical cost, ability to drive and manipulate flow without equipment, and capability of storing reagents for various applications. This Review aims to provide a comprehensive review of the field, highlighting fabrication methods available to date with their respective advantages and drawbacks, device designs and modifications to accommodate different assay needs, detection strategies, and the growing applications of µPADs. Finally, we discuss how the field needs to continue moving forward to realize its full potential.

USB powered microfluidic paper-based analytical devices.

Microfluidic paper-based analytical devices (µPADs) allow user-friendly and portable chemical determinations, although they provide limited applicability due to insufficient sensitivity. Several approaches have been proposed to address poor sensitivity in µPADs, but they frequently require bulky equipment for power and/or read-outs. Universal serial buses (USB) are an attractive alternative to less portable power sources and are currently available in many common electronic devices. Here, USB-powered µPADs (USB µPADs) are proposed as a fusion of both technologies to improve performance without adding instrumental complexity. Two ITP USB µPADs were developed, both powered by a 5 V potential provided through standard USB ports. The first device was fabricated using the origami approach. Its operation was analyzed experimentally and numerically, yielding a two-order-of-magnitude sample focusing in 15 min. The second ITP USB µPAD is a novel design, which was numerically prototyped with the aim of handling larger sample volumes. The reservoirs were moved away from the ITP channel and capillary action was used to drive the sample and electrolytes to the separation zone, predicting 25-fold sample focusing in 10 min. USB µPADs are expected to be adopted by minimally-trained personnel in sensitive areas like resource-limited settings, the point-of-care and in emergencies.

Rapid Bacteria Detection at Low Concentrations Using Sequential Immunomagnetic Separation and Paper-Based Isotachophoresis.

Detecting bacteria is important in the fields of human health, environmental monitoring, and food safety. Foodborne pathogens alone are estimated to cause 420 000 deaths annually, with low-income regions affected most. Despite improvements in bacterial detection, fast, disposable, low-cost, sensitive, and user-friendly methods are still needed. Traditional methods for detecting bacteria rely primarily on cell culturing or polymerase chain reaction (PCR), which require highly trained personnel and a central laboratory and take several hours or even days to deliver results. Low-cost methods like lateral flow immunoassays exist but frequently suffer from poor sensitivity and/or lack quantitative results. Here, a rapid method for detecting bacteria at very low concentrations is presented using two sequential preconcentration steps. In the first preconcentration step, the sample is mixed with antibody-modified magnetic particles and free antibodies conjugated to ß-galactosidase (ß-gal). The target bacteria are isolated and concentrated using immunomagnetic separation. The isolated bacteria are then incubated with chlorophenol red-ß-d-galactopyranoside (CPRG), which reacts with ß-gal to produce chlorophenol red (CPR) in a bacteria concentration-dependent manner. In the second step, CPR and CPRG are separated and focused using an isotachophoretic microfluidic paper-based analytical device, significantly improving the final detection limit relative to paper-based devices lacking the focusing mechanism. Moreover, CPR and CPRG form two visible color bands that act as test and control bands, respectively, improving assay robustness. The method was tested with E. coli DH5-α and successfully detected concentrations as low as 9.2 CFU/mL in laboratory samples and 920 CFU/mL in apple juice samples in ∼90 min.

Flow-S: A Field-Deployable Device with Minimal Hands-On Effort to Concentrate and Quantify Schistosoma Circulating Anodic Antigen (CAA) from Large Urine Volumes.

A laboratory-based lateral flow (LF) test that utilizes up-converting reporter particles (UCP) for ultrasensitive quantification of Schistosoma circulating anodic antigen (CAA) in urine is a well-accepted test to identify active infection. However, this UCP-LF CAA test requires sample pre-treatment steps not compatible with field applications. Flow, a new low-cost disposable, allows integration of large-volume pre-concentration of urine analytes and LF detection into a single field-deployable device. We assessed a prototype Flow-Schistosoma (Flow-S) device with an integrated UCP-LF CAA test strip, omitting all laboratory-based steps, to enable diagnosis of active Schistosoma infection in the field using urine. Flow-S is designed for large-volume (5-20 mL) urine, applying passive paper-based filtration and antibody-based CAA concentration. Samples tested for schistosome infection were collected from women of reproductive age living in a Tanzania region where S. haematobium infection is endemic. Fifteen negative and fifteen positive urine samples, selected based on CAA levels quantified in paired serum, were analyzed with the prototype Flow-S. The current Flow-S prototype, with an analytical lower detection limit of 1 pg CAA/mL, produced results correlated with the laboratory-based UCP-LF CAA test. Urine precipitates occurred in frozen banked samples and affected accurate quantification; however, this should not occur in fresh urine. Based on the findings of this study, Flow-S appears suitable to replace the urine pre-treatment required for the laboratory-based UCP-LF CAA test, thus allowing true field-based applications with fresh urine samples. The urine precipitates observed with frozen samples, though less important given the goal of testing fresh urines, warrant additional investigation to evaluate methods for mitigation. Flow-S devices permit testing of pooled urine samples with applications for population stratified testing. A field test with fresh urine samples, a further optimized Flow-S device, and larger statistical power has been scheduled.

Capillary driven microfluidic sequential flow device for point-of-need ELISA: COVID-19 serology testing.

A capillary-driven microfluidic sequential flow device, designed for eventual at-home or doctor's office use, was developed to perform an enzyme-linked immunosorbent assay (ELISA) for serology assays. Serology assays that detect SARS-CoV-2 antibodies can be used to determine prior infection, immunity status, and/or individual vaccination status and are typically run using well-plate ELISAs in centralized laboratories, but in this format SARs-CoV-2 serology tests are too expensive and/or slow for most situations. Instead, a point-of-need device that can be used at home or in doctor's offices for COVID-19 serology testing would provide critical information for managing infections and determining immune status. Lateral flow assays are common and easy to use, but lack the sensitivity needed to reliably detect SARS-CoV-2 antibodies in clinical samples. This work describes a microfluidic sequential flow device that is as simple to use as a lateral flow assay, but as sensitive as a well-plate ELISA through sequential delivery of reagents to the detection area using only capillary flow. The device utilizes a network of microfluidic channels made of transparency film and double-sided adhesive combined with paper pumps to drive flow. The geometry of the channels and storage pads enables automated sequential washing and reagent addition steps with two simple end-user steps. An enzyme label and colorimetric substrate produce an amplified, visible signal for increased sensitivity, while the integrated washing steps decrease false positives and increase reproducibility. Naked-eye detection can be used for qualitative results or a smartphone camera for quantitative analysis. The device detected antibodies at 2.8 ng mL-1 from whole blood, while a well-plate ELISA using the same capture and detection antibodies could detect 1.2 ng mL-1. The performance of the capillary-driven immunoassay (CaDI) system developed here was confirmed by demonstrating SARS-CoV-2 antibody detection, and we believe that the device represents a fundamental step forward in equipment-free point-of-care technology.

Capillary flow-driven immunoassay platform for COVID-19 antigen diagnostics.

Over the last few years, the SARS-CoV-2 pandemic has made the need for rapid, affordable diagnostics more compelling than ever. While traditional laboratory diagnostics like PCR and well-plate ELISA are sensitive and specific, they can be costly and take hours to complete. Diagnostic tests that can be used at the point-of-care or at home, like lateral flow assays (LFAs) are a simple, rapid alternative, but many commercially available LFAs have been criticized for their lack of sensitivity compared to laboratory methods like well-plate ELISAs. The Capillary-Driven Immunoassay (CaDI) device described in this work uses microfluidic channels and capillary action to passively automate the steps of a traditional well-plate ELISA for visual read out. This work builds on prior capillary-flow devices by further simplifying operation and use of colorimetric detection. Upon adding sample, an enzyme-conjugated secondary antibody, wash steps, and substrate are sequentially delivered to test and control lines on a nitrocellulose strip generating a colorimetric response. The end user can visually detect SARS-CoV-2 antigen in 15-20 min by naked eye, or results can be quantified using a smartphone and software such as ImageJ. An analytical detection limit of 83 PFU/mL for SARS-CoV-2 was determined for virus in buffer, and 222 PFU/mL for virus spiked into nasal swabs using image analysis, similar to the LODs determined by traditional well-plate ELISA. Additionally, a visual detection limit of 100 PFU/mL was determined in contrived nasal swab samples by polling 20 untrained end-users. While the CaDI device was used for detecting clinically relevant levels of SARS-CoV-2 in this study, the CaDI device can be easily adapted to other immunoassay applications by changing the reagents and antibodies.

Simple manipulation of enzyme-linked immunosorbent assay (ELISA) using an automated microfluidic interface.

Among lateral flow immunoassay (LFIA) platforms, enzyme-based LFIAs provide signal amplification to improve sensitivity. However, most enzyme-based LFIAs require multiple timed steps, complicating their utility in point-of-care testing (POCT). Here, we report a microfluidic interface for LFIAs that automates sample, buffer, and reagent addition, greatly simplifying operation while achieving the high analytical stringency associated with more complex assays. The microfluidic interface also maintains the low cost and small footprint of standard LFIAs. The platform is fabricated from a combination of polyester film, double-sided adhesive tape, and nitrocellulose, and fits in the palm of your hand. All reagents are dried on the nitrocellulose to facilitate sequential reagent delivery, and the sample is used as the wash buffer to minimize steps. After the sample addition, a user simply waits 15 min for a colorimetric result. This manuscript discusses the development and optimization of the channel geometry to achieve a simple step enzyme amplified immunoassay. As a proof-of-concept target, we selected lipoarabinomannan (LAM), a WHO identified urinary biomarker of active tuberculosis, to demonstrate the device feasibility and reliability. The results revealed that the device successfully detected LAM in phosphate buffer (PBS) as well as spiked urine samples within 15 min after sample loading. The minimum concentration of color change was achieved at 25 ng mL-1.

Sealing 3D-printed parts to poly(dimethylsiloxane) for simple fabrication of Microfluidic devices.

Microfluidics has revolutionized the fields of bioanalytical chemistry, cellular biology, and molecular biology. Advancements in microfluidic technologies, however, are often limited by labor, time, and resource-intensive fabrication methods, most commonly a form of photolithography. The advent of 3D printing has helped researchers fabricate proof-of-concept microfluidics more rapidly and at lower costs but suffers from poor resolution and tedious post-processing to remove uncured resin from enclosed channels. Additionally, custom resins and printers are often needed to create entirely enclosed channels, which increases cost and complexity of fabrication. In this work we demonstrate the ability to create microfluidic devices by covalently sealing 3D-printed parts with open-faced channels to polydimethylsiloxane (PDMS). Open-faced channels are easier to print than fully enclosed channels and can be printed using an inexpensive and commercially available stereolithography 3D printer and resin. The 3D-printed parts are sealed to PDMS, a common substrate used in traditional microfluidic fabrication, using two different techniques. The first involves coating the part with a commercially available silicone spray before sealing to PDMS via plasma treatment. In the second technique, the cured methacrylate resin is silanized with (3-Aminopropyl)triethoxysilane (APTES) before binding to PDMS with plasma treatment. Both methods create a strong seal between the two substrates, which is demonstrated with several types of microfluidic devices including droplet and gradient generators.

Paper-based pump-free magnetophoresis.

Microfluidic magnetophoresis is a powerful technique that is used to separate and/or isolate cells of interest from complex matrices for analysis. However, mechanical pumps are required to drive flow, limiting portability and making translation to point-of-care (POC) settings difficult. Microfluidic paper-based analytical devices (µPADs) offer an alternative to traditional microfluidic devices that do not require external pumps to generate flow. However, µPADs are not typically used for particle analysis because most particles become trapped in the porous fiber network. Here we report the ability of newly developed fast-flow microfluidic paper-based analytical devices (ffPADs) to perform magnetophoresis. ffPADs use capillary action in a gap between stacked layers of paper and transparency sheets to drive flow at higher velocities than traditional µPADs. The multi-layer ffPADs allow particles and cells to move through the gap without being trapped in the paper layers. We first demonstrate that ffPADs enable magnetic particle separations in a µPAD with a neodymium permanent magnet and study key factors that affect performance. To demonstrate utility, E. coli was used as a model analyte and was isolated from human urine before detection with a fluorescently labeled antibody. A capture efficiency of 61.5% was then obtained of E. coli labeled magnetic beads in human urine. Future studies will look at the improvement of the capture efficiency and to make this assay completely off-chip without the need of a fluorescent label. The assay and device described here demonstrate the first example of magnetophoresis in a paper based, pump free microfluidic device.

Rotary manifold for automating a paper-based Salmonella immunoassay.

Foodborne pathogens are responsible for hundreds of thousands of deaths around the world each year. Rapid screening of agricultural products for these pathogens is essential to reduce and/or prevent outbreaks and pinpoint contamination sources. Unfortunately, current detection methods are laborious, expensive, time-consuming and require a central laboratory. Therefore, a rapid, sensitive, and field-deployable pathogen-detection assay is needed. We previously developed a colorimetric sandwich immunoassay utilizing immuno-magnetic separation (IMS) and chlorophenol red-ß-d-galactopyranoside for Salmonella detection on a paper-based analytical device (µPAD); however, the assay required many sample preparation steps prior to the µPAD as well as laboratory equipment, which decreased user-friendliness for future end-users. As a step towards overcoming these limitations in resource-limited settings, we demonstrate a reusable 3D-printed rotational manifold that couples with disposable µPAD layers for semi-automated reagent delivery, washing, and detection in 65 minutes. After IMS to clean the sample, the manifold performs pipette-free reagent delivery and washing steps in a sequential order with controlled volumes, followed by enzymatic amplification and colorimetric detection using automated image processing to quantify color change. Salmonella was used as the target pathogen in this project and was detected with the manifold in growth media and milk with detection limits of 4.4 × 102 and 6.4 × 102 CFU mL-1 respectively. The manifold increases user friendliness and simplifies immunoassays resulting in a practical product for in-field use and commercialization.

Paper-Based Enzyme Competition Assay for Detecting Falsified ß-Lactam Antibiotics.

Falsified and substandard antibiotics are a growing worldwide problem that leads to increased patient mortality and decreased trust in healthcare, and contributes to antimicrobial resistance. Monitoring falsified antibiotics is difficult because most falsified pharmaceuticals are most commonly found in developing countries, where detecting the active ingredient is difficult due to lack of access to complex instrumentation. Herein, we describe the development and optimization of a microfluidic paper-based analytical device (µPAD) to detect the active ingredient in the most falsified class of antibiotics, ß-lactams. The assay is based on enzyme competition, making it the first demonstrated competitive enzyme assay reported in paper-based devices. The assay uses nitrocefin, a chromogenic substrate, to compete with ß-lactam antibiotics in a reaction with ß-lactamase. A yellow color indicates legitimate drugs, while a color change from yellow to red indicates falsified drugs. In addition to testing for the active ingredient, another section of the device was added to test the sample pH to further verify results and identify common falsified ingredients like aspirin or baking soda. Calibration curves for four different antibiotics, including cefazolin, have been generated making it the first paper-based device capable of detecting both cephalosporin and penicillin antibiotics. The µPAD has also been tested with common falsified ingredients and four antibiotics in tablet or injectable form, demonstrating its potential for in-field falsified antibiotic testing.