1000-fold sample focusing on paper-based microfluidic devices

Unveiling the state of the art: a systematic review and meta-analysis of paper-based microfluidic devices

Introduction

This systematic review and meta-analysis present a comprehensive evaluation of paper-based microfluidic devices, focusing on their applications in immunoassays. These devices are emerging as innovative solutions to democratize access to diagnostic technologies, especially in resource-limited settings. Our review consolidates findings from diverse studies to outline advancements in paper-based microfluidic technology, including design intricacies and operational efficacy. Key advantages such as low cost, portability, and ease of use are highlighted.

Materials and Methods

The review categorizes literature based on the design and operational nuances of these diagnostic tools, exploring various methodologies, fabrication techniques, detection methods, and applications, particularly in protein science. The meta-analysis extends to the diverse applications of these technologies, providing a framework for classifying and stratifying their uses in diagnostics.

Results and discussion

Notable findings include a critical analysis of performance metrics, such as sensitivity and specificity. The review addresses challenges, including the need for further validation and optimization for broader clinical applications. A critical discussion on the validation processes, including cross-validation and rigorous control testing, is provided to ensure the robustness of microfluidic devices. This study offers novel insights into the computational strategies underpinning these technologies and serves as a comprehensive roadmap for future research, potentially broadening the impact across the protein science universe.

Fast and sensitive smartphone colorimetric detection of whole blood samples on a paper-based analytical device

BACKGROUND

Methods based on paper-based analytical devices (PAD) and smartphone photographic colorimetric detection have become representative instrument-independent point-of-care testing (POCT) platforms due to their low cost and simplicity. However, the detection of target components from whole blood sample still presents challenges in terms of field preparation of small amounts of blood sample and detection sensitivity. This paper presents a rapid online processing method for whole blood samples on PAD based on plasma separation membrane (PSM), and combined with electrokinetic stacking and selective chromatic reaction. Real-time smartphone-based colorimetric detection of free hemoglobin (FHb) and human serum albumin (HSA) was successfully demonstrated.

RESULTS

With the proposed method, both detections for low and high concentration analytes could be implemented. The limits of detection of 16.6 mg L-1 for FHb and 0.67 g L-1 for HSA were obtained, respectively, with RSD below 8 %. The reliability of the method was verified by the recovery test and desktop spectrophotometric method. The detection results for real blood samples were in agreement with that by clinical methods.

SIGNIFICANCE AND NOVELTY

The PAD method is inexpensive, simple and fast, and detection of a whole blood sample of 5 μL can be finished in 5 min. This work shows that POCT of biomarkers from whole blood with PAD is possible without using any desktop facilities.

Paper-Based Microfluidic Chips for Food Hazard Factor Detection: Fabrication, Modification, and Application

Food safety and quality are paramount concerns for ensuring the preservation of human life and well-being. As the field of food processing continues to advance, there is a growing interest in the development of fast, instant, cost-effective, and convenient methods for detecting food safety issues. In this context, the utilization of paper-based microfluidic chips has emerged as a promising platform for enabling rapid detection, owing to their compact size, high throughput capabilities, affordability, and low resource consumption, among other advantages. To shed light on this topic, this review article focuses on the functionalization of paper-based microfluidic surfaces and provides an overview of the latest research and applications to colorimetric analysis, fluorescence analysis, surface-enhanced Raman spectroscopy, as well as their integration with paper-based microfluidic platforms for achieving swift and reliable food safety detection. Lastly, the article deliberates on the challenges these analytical methods and presents insights into their future development prospects in facilitating rapid food safety assessment.

Tuning the electrophoretic separations on a surface-accessible and flexible fibre-based microfluidic devices

Electrophoresis on textile fiber substrates provides a unique surface-accessible platform for the movement, separation and concentration of charged analytes. The method employs the inherently inbuilt capillary channels existing within textile structures, which can support electroosmotic and electrophoretic transport processes upon applying an electric field. Unlike confined microchannels in classical chip-based electrofluidic devices, the capillaries formed by the roughly oriented fibers within textile substrates can impact the reproducibility of the separation process. Here, we report an approach for precise experimental conditions affecting the electrophoretic separation of two tracer solutes, fluorescein (FL) and rhodamine B (Rh-B) on textile-based substrates. A Box-Behnken response surface design methodology has been used to optimise the experimental conditions and predict the separation resolution of a solute mixture using polyester braided structures. The magnitude of the electric field, sample concentration and sample volume are of primary importance to the separation performance of the electrophoretic devices. Here, we use a statistical approach to optimise these parameters to achieve rapid and efficient separation. While a higher potential was shown to be required to separate solute mixtures of increasing concentration and sample volume, this was counteracted by a reduced separation efficiency due to joule heating, which caused electrolyte evaporation on the unenclosed textile structure at electric fields above 175 V cm^-1. Using the approach presented here, optimal experimental conditions can be predicted to limit joule heating and attain effective separation resolution without compromising the analysis time on simple and low-cost textile substrates.

Paper based microfluidic devices: a review of fabrication techniques and applications

A wide range of applications are possible with paper-based analytical devices, which are low priced, easy to fabricate and operate, and require no specialized equipment. Paper-based microfluidics offers the design of miniaturized POC devices to be applied in the health, environment, food, and energy sector employing the ASSURED (Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment free and Deliverable to end users) principle of WHO. Therefore, this field is growing very rapidly and ample research is being done. This review focuses on fabrication and detection techniques reported to date. Additionally, this review emphasises on the application of this technology in the area of medical diagnosis, energy generation, environmental monitoring, and food quality control. This review also presents the theoretical analysis of fluid flow in porous media for the efficient handling and control of fluids. The limitations of PAD have also been discussed with an emphasis to concern on the transformation of such devices from laboratory to the consumer.

Magnetic Control-Enhanced Lateral Flow Technique for Ultrasensitive Nucleic Acid Target Detection

In this work, a lateral flow (LF)-based detection strategy termed magnetic control-enhanced LFA (MCLF) was proposed to detect nucleic acid sequences at attomolar sensitivity. In the proposed MCLF method, magnetic controllers which are magnetic nanoparticles modified with antibodies against the labels on capture sequences were used to interact with the unreacted labeled capture sequence (CS-label) to improve the detection limit. By regulating the movement of magnetic probes (magnetic controllers) with a simple magnet under the lateral flow strip, the movement of magnetic probes bounded with unreacted CS-label in the sample flow could be reduced. Therefore, the target sequence-containing sandwich structures will arrive at the test zone prior, to interact with the recognition ligands, whereby the capture efficiency of the sandwich structures could be increased because the unreacted capture sequences at the test zone will be reduced. With the colorimetric signal from gold nanoparticle-based probes, the proposed MCLF technique could recognize as low as 100 aM of DNA target sequences by naked eyes, and the responding range of MCLF is from 100 aM to 10 pM.

Electrophoretic µPAD for Purification and Analysis of DNA Samples

In this work, the fabrication and characterization of a simple, inexpensive, and effective microfluidic paper analytic device (µPAD) for monitoring DNA samples is reported. The glass microfiber-based chip has been fabricated by a new wax-based transfer-printing technique and an electrode printing process. It is capable of moving DNA effectively in a time-dependent fashion. The nucleic acid sample is not damaged by this process and is accumulated in front of the anode, but not directly on the electrode. Thus, further DNA processing is feasible. The system allows the DNA to be purified by separating it from other components in sample mixtures such as proteins. Furthermore, it is demonstrated that DNA can be moved through several layers of the glass fiber material. This proof of concept will provide the basis for the development of rapid test systems, e.g., for the detection of pathogens in water samples.

Online and offline preconcentration techniques on paper-based analytical devices for ultrasensitive chemical and biochemical analysis: A review

Microfluidic paper-based analytical devices (μPADs) have attracted much attention over the past decade. They embody many advantages, such as abundance, portability, cost-effectiveness, and ease of fabrication, making them superior for clinical diagnostics, environmental monitoring, and food safety assurance. Despite these advantages, μPADs lack the high sensitivity to detect many analytes at trace levels than other commercial analytical instruments such as mass spectrometry. Therefore, a preconcentration step is required to enhance their sensitivity. This review focuses on the techniques used to separate and preconcentrate the analytes onto the μPADs, such as ion concentration polarization, isotachophoresis, and field amplification sample stacking. Other separations and preconcentration techniques, including liquid-solid and liquid-liquid extractions coupled with μPADs, are also reviewed and discussed. In addition, the fabrication methods, advantages, disadvantages, and the performance evaluation of the μPADs concerning their precision and accuracy were highlighted and critically assessed. Finally, the challenges and future perspectives have been discussed.

Paper-Based Test for Rapid On-Site Screening of SARS-CoV-2 in Clinical Samples

Detection methods for monitoring infectious pathogens has never been more important given the need to contain the spread of the COVID-19 pandemic. Herein we propose a highly sensitive magnetic-focus-enhanced lateral flow assay (mLFA) for the detection of SARS-CoV-2. The proposed mLFA is simple and requires only lateral flow strips and a reusable magnet to detect very low concentrations of the virus particles. The magnetic focus enhancement is achieved by focusing the SARS-CoV-2 conjugated magnetic probes in the sample placed in the lateral flow (LF) strips for improved capture efficiency, while horseradish peroxidase (HRP) was used to catalyze the colorimetric reaction for the amplification of the colorimetric signal. With the magnetic focus enhancement and HRP-based amplification, the mLFA could yield a highly sensitive technology for the recognition of SARS-CoV-2. The developed methods could detect as low as 400 PFU/mL of SARS-CoV-2 in PBS buffer based on the visible blue dots on the LF strips. The mLFA could recognize 1200 PFU/mL of SARS-CoV-2 in saliva samples. With clinical nasal swab samples, the proposed mLFA could achieve 66.7% sensitivity and 100% specificity.

Assessing citric acid-derived luminescent probes for pH and ammonia sensing: A comprehensive experimental and theoretical study

The present work reports on the assessment of luminescent probes derived from citric acid (CA) and β-aminothiols (namely, l-cysteine (Cys) and cysteamine) for instrumental and smartphone-based fluorimetric sensing purposes. Remarkably, the evaluated luminescent probes derived from natural compounds showed pH-dependent dual excitation/dual emission features. Both fluorophores hold promise for the ratiometric fluorimetric sensing of pH, being especially convenient for the smartphone-based sensing of pH via ratiometric analysis by proper selection of B and G color channels. Time dependent density functional theory (TDDFT) calculations allowed to substantiate the pH dependent structure-property relationship and to unveil the critical role of the CA derived carboxyl group, these findings contributing to the fundamental knowledge on these systems for the rational design of new fluorophores and in establishing fluorescence sensing mechanisms of CA-derived systems. Besides, paper-based devices modified with CA-Cys were implemented in a three-phase separation approach for sensitive and selective ammonia sensing, yielding a remarkable enrichment factor of 389 and a limit of detection of 37 μM under optimal conditions. The proposed approach was successfully applied to the determination of ammonia nitrogen and extractable ammonium in water samples and marine sediments, respectively.

Electrochemical Microfluidic Paper-Based Aptasensor Platform Based on a Biotin-Streptavidin System for Label-Free Detection of Biomarkers

Timely and rapid detection of biomarkers is extremely important for the diagnosis and treatment of diseases. However, going to the hospital to test biomarkers is the most common way. People need to spend a lot of money and time on various tests for potential disease detection. To make the detection more convenient and affordable, we propose a paper-based aptasensor platform in this work. This device is based on a cellulose paper, on which a three-electrode system and microfluidic channels are fabricated. Meanwhile, novel nanomaterials consisting of amino redox graphene/thionine/streptavidin-modified gold nanoparticles/chitosan are synthesized and modified on the working electrode of the device. Through the biotin-streptavidin system, the aptamer whose 5'end is modified with biotin can be firmly immobilized on the electrode. The detection principle is that the current generated by the nanomaterials decreases proportionally to the concentration of targets owing to the combination of the biomarker and its aptamer. 17β-Estradiol (17β-E2), as one of the widely used diagnostic biomarkers of various clinical conditions, is adopted for verifying the performance of the platform. The experimental results demonstrated that this device enables the determination of 17β-E2 in a wide linear range of concentrations of 10 pg mL^-1 to 100 ng mL^-1 and the limit of detection is 10 pg mL^-1 (S/N = 3). Moreover, it enables the detection of targets in clinical serum samples, demonstrating its potential to be a disposable and convenient integrated platform for detecting various biomarkers.

Thread-based isotachophoresis for DNA extraction and purification from biological samples

A rapid, low-cost, and disposable microfluidic thread-based isotachophoresis method was developed for the purification and preconcentration of nucleic acids from biological samples, prior to their extraction and successful analysis using quantitative polymerase chain reaction (qPCR). This approach extracts and concentrates protein-free DNA from the terminating electrolyte buffer, via a continuous sampling approach, resulting in significant focussing of the extracted DNA upon a 6 cm length nylon thread. The platform was optimised using the preconcentration of a fluorescent dye, showing a 600-fold concentration capacity within <5 min. The system was then applied to the one-step extraction of lambda DNA - an E. coli bacteriophage - spiked into whole blood, exhibiting the exclusion of PCR inhibitors. The extraction efficiency from the thread material following concentration was consistent, between 94.4-113.9%. The determination of lambda DNA in whole blood was achieved within a linear range of 1.0-1 × 105 fg μL-1 (20-2 × 106 copies per μL). This technique demonstrates great potential for the development of thread-based affordable analytical and diagnostic devices based upon DNA and RNA isolation.

Numerical simulations of paper-based electrophoretic separations with open-source tools

A new tool for the solution of electromigrative separations in paper-based microfluidics devices is presented. The implementation is based on a recently published complete mathematical model for describing these types of separations, and was developed on top of the open-source toolbox electroMicroTransport, based on OpenFOAM^® , inheriting all its features as native 3D problem handling, support for parallel computation, and a GNU GPL license. The presented tool includes full support for paper-based electromigrative separations (including EOF and the novel mechanical and electrical dispersion effects), compatibility with a well-recognized electrolyte database, and a novel algorithm for computing and controlling the electric current in arbitrary geometries. Additionally, the installation on any operating system is available due to its novel installation option in the form of a Docker image. A validation example with data from literature is included, and two extra application examples are provided, including a 2D free-flow IEF problem, which demonstrates the capabilities of the toolbox for dealing with computational and physicochemical modeling challenges simultaneously. This tool will enable efficient and reliable numerical prototypes of paper-based electrophoretic devices to accompany the contemporary fast growth in paper-based microfluidics.

HIV detection from human serum with paper-based isotachophoretic RNA extraction and reverse transcription recombinase polymerase amplification

The number of people living with HIV continues to increase with the current total near 38 million, of which about 26 million are receiving antiretroviral therapy (ART). These treatment regimens are highly effective when properly managed, requiring routine viral load monitoring to assess successful viral suppression. Efforts to expand access by decentralizing HIV nucleic acid testing in low- and middle-income countries (LMICs) has been hampered by the cost and complexity of current tests. Sample preparation of blood samples has traditionally relied on cumbersome RNA extraction methods, and it continues to be a key bottleneck for developing low-cost POC nucleic acid tests. We present a microfluidic paper-based analytical device (μPAD) for extracting RNA and detecting HIV in serum, leveraging low-cost materials, simple buffers, and an electric field. We detect HIV virions and MS2 bacteriophage internal control in human serum using a novel lysis and RNase inactivation method, paper-based isotachophoresis (ITP) for RNA extraction, and duplexed reverse transcription recombinase polymerase amplification (RT-RPA) for nucleic acid amplification. We design a specialized ITP system to extract and concentrate RNA, while excluding harsh reagents used for lysis and RNase inactivation. We found the ITP μPAD can extract and purify 5000 HIV RNA copies per mL of serum. We then demonstrate detection of HIV virions and MS2 bacteriophage in human serum within 45-minutes.

16S rRNA Monitoring Point-of-Care Magnetic Focus Lateral Flow Sensor

The detection and profiling of pathogenic bacteria is critical for human health, environmental, and food safety monitoring. Herein, we propose a highly sensitive colorimetric strategy for naked eye screening of 16S ribosomal RNA (16S rRNA) from pathogenic agents relevant to infections, human health, and food safety monitoring with a magnetic focus lateral flow sensor (mLFS) platform. The method developed was demonstrated in model 16S rRNA sequences of the pathogen Escherichia coli O157:H7 to detect as low as 1 fM of targets, exhibiting a sensitivity improved by ∼5 × 10⁵ times compared to the conventional GNP-based colorimetric lateral flow assay used for oligonucleotide testing. Based on the grayscale values, semi-quantitation of up to 1 pM of target sequences was possible in ∼45 min. The methodology could detect the target 16S rRNA from as low as 32 pg/mL of total RNA extracted from pathogens. Specificity was demonstrated with total RNA extracted from E. coli K-12 MG1655, Bacillus subtilis (B. subtilis), and Pseudomonas aeruginosa (P. aeruginosa). No signal was observed from as high as 320 pg/mL of total RNA from the nontarget bacteria. The recognition of target 16S rRNA from 32 pg/mL of total RNA in complex matrices was also demonstrated. The proposed mLFS method was then extended to monitoring B. subtilis and P. aeruginosa. Our approach highlights the possibility of extending this concept to screen specific nucleic acid sequences for the monitoring of infectious pathogens or microbiome implicated in a range of diseases including cancer.

Precise electroosmotic flow measurements on paper substrates

A novel method for electroosmotic flow (EOF) measurement on paper substrates is presented; it is based on dynamic mass measurements by simply using an analytical balance. This technique provides a more reliable alternative to other EOF measurement methods on porous media. The proposed method is used to increase the amount and quality of the available information about physical parameters that characterize fluid flow on microfluidic paper-based analytical devices (μPADs). Measurements were performed on some of the most frequently used materials for μPADs, i.e., Whatman #1 , S&S, and Muntktell 00A filter paper. Obtained experimental results are consistent with the few previously reported data, either experimental or numerical, characterizing EOF in paper substrates. Moreover, a thorough analysis is presented for the quantification of the different effects that affect the measurements such as Joule effect and evaporation. Experimental results enabled, for the first time, to establish well-defined electroosmotic characteristics for the three substrates in terms of the magnitude of EOF as funtion of pH, enabling researchers to make a rational choice of the substrate depending on the electrophoretic technique to be implemented. The measurement method can be described as robust, reliable, and affordable enough for being adopted by researchers and companies devoted to electrophoretic μPADs and related technologies.

Recent Advances in Microfluidic Paper-Based Analytical Devices toward High-Throughput Screening

Microfluidic paper-based analytical devices (µPADs) have become promising tools offering various analytical applications for chemical and biological assays at the point-of-care (POC). Compared to traditional microfluidic devices, µPADs offer notable advantages; they are cost-effective, easily fabricated, disposable, and portable. Because of our better understanding and advanced engineering of µPADs, multistep assays, high detection sensitivity, and rapid result readout have become possible, and recently developed µPADs have gained extensive interest in parallel analyses to detect biomarkers of interest. In this review, we focus on recent developments in order to achieve µPADs with high-throughput capability. We discuss existing fabrication techniques and designs, and we introduce and discuss current detection methods and their applications to multiplexed detection assays in relation to clinical diagnosis, drug analysis and screening, environmental monitoring, and food and beverage quality control. A summary with future perspectives for µPADs is also presented.

Enhanced sample pre-concentration by ion concentration polarization on a paraffin coated converging microfluidic paper based analytical platform

Microfluidic paper-based analytical devices (μPADs) represent a modest and feasible alternative for conventional analytical methods. However, the inadequate sensitivity of these devices limits the possible applications of μPADs. In this scenario, inducing ion concentration polarization (ICP) on μPADs has shown promise to overcome this limitation by preconcentrating the analytes of interest. Here, we report a μPAD implementing ICP using an off-shelf Nafion® membrane as the perm selective membrane. Two types of devices with a geometrical configuration of a straight channel converging at the middle connecting to circular reservoirs at the end of channels were fabricated. The devices are comprised of a single input channel and an absorption channel. The Nafion membrane is attached to the absorption channel of the device, which is encased by heating with paraffin films at both sides to lower the electro-osmotic flow generated by an applied DC electric field that is needed for ICP. The field induced ICP enables obtaining a maximum concentration factor of more than 2000 folds for fluorescein sodium salt solution on the μPAD. Also, since evaporation of the sample solution was reported to be of great influence on the concentration factor, we analyze the effect of sample solution evaporation on sample preconcentration. Furthermore, our reported fabrication method for μPAD can lower the fabrication cost down to 0.3 USD. This device shows the potential to be developed for serving as a diagnostic and environmental monitoring platform.

Simultaneous electrokinetic stacking and separation of anionic and cationic species on a paper fluidic channel

On-line enrichment is effective for improving the sensitivity of paper-based analytical devices (PADs). Electrokinetic stacking of ionic species - anionic or cationic species, respectively, on a paper-based fluidic channel has been well demonstrated in the literature. In this work, we further demonstrated that both anionic and cationic species can be electrokinetically stacked and separated simultaneously on the same paper fluidic channel. The feasibility of the proposed method was visually demonstrated by using a colored cationic probe of Rhodamine 6G and an anionic probe of Brilliant Blue. With the introduction of a background electrolyte (BGE) consisting of weak acid and weak base salt, two electric field gradients can be developed on the same paper fluidic channel when a DC voltage was applied. Both of the anionic and cationic species from the reservoirs can be simultaneously stacked as separate bands on the two field gradients, respectively. Under optimized conditions, two orders of magnitude enrichment factors can be achieved for the anionic and cationic probes as characterized by colorimetric analysis by smartphone imaging. The applicability of this method was further demonstrated by stacking and separation of copper ions/nitrite and even amphoteric ions-proteins of phycocyanin (blue, pI 4.4)/cytochrome C (brown, pI 10.2). Potential applications can be found not only for a PAD based point of care test (POCT), but also for sample pretreatment in protein analysis considering the friendliness of the BGE to the mass spectrometer.

Immunobinding-induced alteration in the electrophoretic mobility of proteins: An approach to studying the preconcentration of an acidic protein under cationic isotachophoresis

The objective of this study is to explore an approach for analyzing negatively charged proteins using paper-based cationic ITP. The rationale of electrophoretic focusing the target protein with negative charges under unfavorable cationic ITP condition is to modify the electrophoretic mobility of the target protein through antigen-antibody immunobinding. Cationic ITP was performed on a paper-based analytical device that was fabricated using fiberglass paper. The paper matrix was modified with (3-aminopropyl)trimethoxysilane to minimize sample attraction to the surface for cationic ITP. Negatively charged BSA was used as the model target protein for the cationic ITP experiments. No electrophoretic mobility was observed for BSA-only samples during cationic ITP experimental condition. However, the presence of a primary antibody to BSA significantly improved the electrokinetic behavior of the target protein. Adding a secondary antibody conjugated with amine-rich quantum dots to the sample further facilitated the concentrating effect of ITP, reduced experiment time, and elevated the stacking ratio. Under our optimized experimental conditions, the cationic ITP-based paper device electrophoretically stacked 94% of loaded BSA in less than 7 min. Our results demonstrate that the technique has a broad potential for rapid and cost-effective isotachphoretic analysis of multiplex protein biomarkers in serum samples at the point of care.

Magnetic Focus Lateral Flow Sensor for Detection of Cervical Cancer Biomarkers

We report on a magnetic focus lateral flow biosensor (mLFS) for ultrasensitive detection of protein biomarkers in a practical format. With valosin-containing protein as a target protein, we show that the developed mLFS concept could detect as low as 25 fg/mL with magnetic focus to enhance target capture efficiency to deliver a 10⁶-fold improvement in sensitivity compared to that of conventional lateral flow (LF) systems. The conceptualized strategy utilizes a simple magnet placed beneath the three-dimensional printed LF device to concentrate the targets at the signal zone without any additional instrumentation. In addition, protein mixtures extracted from the tissue of cervical cancer patients was also utilized to validate the sensor. To investigate the effect of magnetic focus on sensitivity, surface-enhanced Raman spectroscopy and dark-field imaging was utilized to characterize the distribution and movement of Fe₃O₄ core-Au shell nanoprobes in a model LF strip. Our experiments show that the magnetic focus results in an increased interaction time between the magnetic probe-labeled targets and the capture antibody, yielding a higher capture efficiency, allowing for ultrasensitive detection of the target not possible before with LF. The proposed mLFS can be utilized to detect a range of trace protein biomarkers for early diagnosis and can be combined with diverse pretreatment and signal amplification steps to query complex samples.

Emerging paper microfluidic devices

Paper has unique advantages over other materials, including low cost, flexibility, porosity, and self-driven liquid pumping, thus making it widely used in various fields in biology, chemistry, physics and materials science.

Controlling Dispersion during Single-Cell Polyacrylamide-Gel Electrophoresis in Open Microfluidic Devices

New tools for measuring protein expression in individual cells complement single-cell genomics and transcriptomics. To characterize a population of individual mammalian cells, hundreds to thousands of microwells are arrayed on a polyacrylamide-gel-coated glass microscope slide. In this "open" fluidic device format, we explore the feasibility of mitigating diffusional losses during lysis and polyacrylamide-gel electrophoresis (PAGE) through spatial control of the pore-size of the gel layer. To reduce in-plane diffusion-driven dilution of each single-cell lysate during in-microwell chemical lysis, we photopattern and characterize microwells with small-pore-size sidewalls ringing the microwell except at the injection region. To reduce out-of-plane-diffusion-driven-dilution-caused signal loss during both lysis and single-cell PAGE, we scrutinize a selectively permeable agarose lid layer. To reduce injection dispersion, we photopattern and study a stacking-gel feature at the head of each <1 mm separation axis. Lastly, we explore a semienclosed device design that reduces the cross-sectional area of the chip, thus reducing Joule-heating-induced dispersion during single-cell PAGE. As a result, we observed a 3-fold increase in separation resolution during a 30 s separation and a >2-fold enhancement of the signal-to-noise ratio. We present well-integrated strategies for enhancing overall single-cell-PAGE performance.

Reinventing (Bio)chemical Analysis with Paper

This paper focuses on one of the most commonly encountered materials in our society, namely paper. Paper is an inherently complex material, yet its use provides for chemical analysis approaches that are elegant in their simplicity of execution. In the first half of the previous century, paper in scientific research was used mainly for filtration and chromatographic separation. While its use decreased with the rise of modern elution chromatography, paper remains a versatile substrate for low-cost analytical tests. Recently, we have seen renewed interest to work with paper in (bio)analytical science, a result of the growing demand for inexpensive, portable analysis. Dried blood spotting, paper microfluidics, and paper spray ionization are areas in which paper is (re)establishing itself as an important material. These research areas all exploit several properties of paper, including stable sample storage, passive fluid movement and manipulation, chromatographic separation/extraction, modifiable surface and/or volume, easily altered shape, easy transport, and low cost. We propose that the real, and to date underexploited, potential of paper lies in utilizing its combined characteristics to add new dimensions to paper-based (bio)chemical analysis, expanding its applicability. This article provides the reader with a short historical perspective on the scientific use of paper and the developments that led to the establishment of the aforementioned research areas. We review important characteristics of paper and place them in a scientific context in this descriptive, yet critical, assessment of the achieved and the achievable in paper-based analysis. The ultimate goal is the exploration of integrative approaches at the interface between the different fields in which paper is or can be used.

Double-sided 3D printing on paper towards mass production of three-dimensional paper-based microfluidic analytical devices (3D-μPADs)

Recently, much effort has been focused on developing three-dimensional, paper-based microfluidic analytical devices (3D-μPADs) targeting in vitro diagnostics. However, 3D-μPAD fabrication typically requires tedious assembly that hinders mass production. Here, we report on a fabrication method for 3D-μPADs made of plastics without the need for additional assembly. Both sides of the paper were printed via liquid resin photopolymerization using a digital light processing (DLP) printer. The sample reservoir and detection zones are located on the top of the 3D-μPADs, and three microchannels are located on the bottom. The detection limits for glucose, cholesterol, and triglyceride (TG) in phosphate-buffered saline (PBS) were 0.3 mM, 0.2 mM, and 0.3 mM, respectively. The detectable ranges of glucose, cholesterol, and TG in human serum were 5-11 mM, 2.6-6.7 mM, and 1-2.3 mM. These results suggest that our fabrication method is suitable to mass produce 3D-μPADs with relative ease using simple fabrication processes.

Semiquantitative Nucleic Acid Test with Simultaneous Isotachophoretic Extraction and Amplification

Nucleic acid amplification tests (NAATs) provide high diagnostic accuracy for infectious diseases and quantitative results for monitoring viral infections. The majority of NAATs require complex equipment, cold chain dependent reagents, and skilled technicians to perform the tests. This largely confines NAATs to centralized laboratories and can significantly delay appropriate patient care. Low-cost, point-of-care (POC) NAATs are especially needed in low-resource settings to provide patients with diagnosis and treatment planning in a single visit to improve patient care. In this work, we present a rapid POC NAAT with integrated sample preparation and amplification using electrokinetics and paper substrates. We use simultaneous isotachophoresis (ITP) and recombinase polymerase amplification (RPA) to rapidly extract, amplify, and detect target nucleic acids from serum and whole blood in a paper-based format. We demonstrate simultaneous ITP and RPA can consistently detect 5 copies per reaction in buffer and 10 000 copies per milliliter of human serum with no intermediate user steps. We also show preliminary extraction and amplification of DNA from whole blood samples. Our test is rapid (results in less than 20 min) and made from low-cost materials, indicating its potential for detecting infectious diseases and monitoring viral infections at the POC in low resource settings.

Amplification-free detection of DNA in a paper-based microfluidic device using electroosmotically balanced isotachophoresis

We present a novel microfluidic paper-based analytical device (μPAD) which utilizes the native high electroosmotic flow (EOF) in nitrocellulose to achieve stationary isotachophoresis (ITP) focusing. This approach decouples sample accumulation from the length of the channel, resulting in significant focusing over short channel lengths. We provide a brief theory for EOF-balanced ITP focusing under continuous injection from a depleting reservoir and present the design of a short (7 mm) paper-based microfluidic channel, which allows a 200 μL sample to be processed in approximately 6 min, resulting in a 20 000-fold increase in concentration - a full order of magnitude improvement compared to previous paper-based ITP devices. We show the stability of the assay over longer (40 min) durations of time, and using Morpholino probes, we present the applicability of the device for amplification-free detection of nucleic acids, with a limit-of-detection (LoD) of 5 pM in 10 min. Finally, we utilize the small footprint of the channel and show a multiplexed platform in which 12 assays operate in parallel in a 24-well plate format.

Geometry-induced injection dispersion in single-cell protein electrophoresis

Arrays of microwells are widely used to isolate individual cells, facilitate high throughput cytometry assays, and ensure compatibility of those assays with whole-cell imaging. Microwell geometries have recently been utilized for handling and preparation of single-cell lysate, prior to single-cell protein electrophoresis. It is in the context of single-cell electrophoresis that we investigate the interplay of microwell geometry (circular, rectangular, triangular) and transport (diffusion, electromigration) on the subsequent performance of single-cell polyacrylamide gel electrophoresis (PAGE) for protein targets. We define and measure injector-induced dispersion during PAGE, and develop a numerical model of band broadening sources, experimentally validate the numerical model, and then identify operating conditions (characterized through the Peclet number, Pe) that lead to microwell-geometry induced losses in separation performance. With analysis of mammalian cells as a case study, we sought to understand at what Pe is the PAGE separation performance adversely sensitized to the microwell geometry. In developing design rules, we find that for the microwell geometries that are the most suitable for isolation of mammalian cells and moderate mass protein targets, the Pe is usually small enough (Pe < ∼20) to mitigate the effect of the microwell geometry on protein PAGE of single-cell lysate. In extreme cases where the largest mammalian cells are analyzed (Pe > ∼20), consideration of Pe suggests using a rectangular - and not the widely used circular - microwell geometry to maximize protein PAGE separation performance.

Joule Heating-Induced Dispersion in Open Microfluidic Electrophoretic Cytometry

While protein electrophoresis conducted in capillaries and microchannels offers high-resolution separations, such formats can be cumbersome to parallelize for single-cell analysis. One approach for realizing large numbers of concurrent separations is open microfluidics (i.e., no microchannels). In an open microfluidic device adapted for single-cell electrophoresis, we perform 100s to 1000s of simultaneous separations of endogenous proteins. The microscope slide-sized device contains cells isolated in microwells located in a ∼40 μm polyacrylamide gel. The gel supports protein electrophoresis after concurrent in situ chemical lysis of each isolated cell. During electrophoresis, Joule (or resistive) heating degrades separation performance. Joule heating effects are expected to be acute in open microfluidic devices, where a single, high-conductivity buffer expedites the transition from cell lysis to protein electrophoresis. Here, we test three key assertions. First, Joule heating substantially impacts analytical sensitivity due to diffusive losses of protein out of the open microfluidic electrophoretic (EP) cytometry device. Second, increased analyte diffusivity due to autothermal runaway Joule heating is a dominant mechanism that reduces separation resolution in EP cytometry. Finally, buffer exchange reduces diffusive losses and band broadening, even when handling single-cell lysate protein concentrations in an open device. We develop numerical simulations of Joule heating-enhanced diffusion during electrophoresis and observe ∼50% protein loss out of the gel, which is reduced using the buffer exchange. Informed by analytical model predictions of separation resolution (with Joule heating), we empirically demonstrate nearly fully resolved separations of proteins with molecular mass differences of just 4 kDa or 12% (GAPDH, 36 kDa; PS6, 32 kDa) in each of 129 single cells. The attained separation performance with buffer exchange is relevant to detection of currently unmeasurable protein isoforms responsible for cancer progression.

Battery operated preconcentration-assisted lateral flow assay

Paper-based analytical devices (e.g. lateral flow assays) are highly advantageous as portable diagnostic systems owing to their low costs and ease of use. Because of their low sensitivity and detection limits for biomolecules, these devices have several limitations in applications for real-field diagnosis. Here, we demonstrate a paper-based preconcentration enhanced lateral flow assay using a commercial β-hCG-based test. Utilizing a simple 9 V battery operation with a low power consumption of approximately 81 μW, we acquire a 25-fold preconcentration factor, demonstrating a clear sensitivity enhancement in the colorimetric lateral flow assay; consequently, clear colors are observed in a rapid kit test line, which cannot be monitored without preconcentration. This device can also facilitate a semi-quantitative platform using the saturation value and/or color intensity in both paper-based colorimetric assays and smartphone-based diagnostics.

Rapid evaporation-driven chemical pre-concentration and separation on paper

Airflow-enhanced evaporation is investigated as a method for rapid chemical preconcentration on a thin porous substrate. The mechanism is described by combining 1D models of capillary rise, chromatography, and pervaporation concentration. It is shown that the effective length of the column can be shorter than its actual length, allowing concentrate to be held at a stagnation point and then released for separation, and that the Péclet number, which determines the concentration performance, is determined only by the substrate properties. The differential equations are solved dynamically, and it is shown that faster concentration can be achieved during capillary filling. Experiments are carried out using chromatography paper in a ducted airflow, and concentration is quantified by optical imaging of water-soluble food dyes. Good agreement with the model is obtained, and concentration factors of ≈100 are achieved in 10 min using Brilliant Blue FCF. Partial separation of Brilliant Blue from Tartrazine is demonstrated immediately following concentration, on a single unpatterned substrate. The mechanism may provide a method for improving the sensitivity of lab-on-paper devices.