A rapid, instrument-free, sample-to-result nucleic acid amplification test

A portable all-in-one microfluidic device with real-time colorimetric LAMP for HPV16 and HPV18 DNA point-of-care testing

Screening for high-risk human papillomavirus (HPV) infection is one of the most important preventative measures for cervical cancer. However, fast, convenient, and low-cost HPV detection remains challenging, especially in resource-limited settings. Here, we report a portable all-in-one device (PAD) for point-of-care testing (POCT) for HPV16 and HPV18 DNA in cervical swabs. The PAD was engineered to integrate modules for extraction-free sample lysis, loop-mediated isothermal amplification (LAMP) with lyophilized reagent beads, and real-time colorimetric signal sensing into a single miniaturized device, considerably shortening the sample-to-result time to 15 min. The precision liquid handling in the completely sealed microfluidic chip is achieved by a uniquely designed pressure-balanced automatic liquid flow mechanism, thereby eliminating the need for manual manipulation of liquids and thus the risk of biohazards. The PAD employs an improved real-time colorimetric LAMP (rcLAMP) assay with a limit of detection (LOD) of 1 copy/μL, enabled by enhanced assay chemistry to maximize the reaction kinetics. To validate this device for clinical application, we tested 206 clinical cervical swab samples and obtained a sensitivity of 92.1% and a specificity of 99.0%. This custom PAD enabled by microfluidic and electronic engineering techniques can be configured for the simultaneous detection of HPV16 and HPV18 or other pathogens in point-of-care applications.

UbiNAAT: a multiplexed point-of-care nucleic acid diagnostic platform for rapid at-home pathogen detection

The COVID-19 pandemic increased demands for respiratory disease testing to facilitate treatment and limit transmission, demonstrating in the process that most existing test options were too complex and expensive to perform in point-of-care or home scenarios. Lab-based molecular techniques can detect viral RNA in respiratory illnesses but are expensive and require trained personnel, while affordable antigen-based home tests lack sensitivity for early detection in newly infected or asymptomatic individuals. The few home RNA detection tests deployed were prohibitively expensive. Here, we demonstrate a point-of-care, paper-based rapid analysis device that simultaneously detects multiple viral RNAs; it is demonstrated on two common respiratory viruses (COVID-19 and influenza A) spiked onto a commercial nasal swab. The automated device requires no sample preparation by the user after insertion of the swab, minimizing user operation steps. We incorporated lyophilized amplification reagents immobilized in a porous matrix, a novel thermally actuated valve for multiplexed fluidic control, a printed circuit board that performs on-device lysis and amplification within a cell-phone-sized disposable device. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) products are visualized via fluorescent dyes using a modified cell phone, resulting in detection of as few as 104 viral copies per swab across both pathogens within 30 minutes. This integrated platform could be commercialized in a form that would be inexpensive, portable, and sensitive; it can readily be multiplexed to detect as many as 8 different RNA or DNA sequences, and adapted to any desired RNA or DNA detection assays.

Self-Powered Microfluidics for Point-of-Care Solutions: From Sampling to Detection of Proteins and Nucleic Acids

Self-powered microfluidics presents a revolutionary approach to address the challenges of healthcare in decentralized and point-of-care settings where limited access to resources and infrastructure prevails or rapid clinical decision-making is critical. These microfluidic systems exploit physical and chemical phenomena, such as capillary forces and surface tension, to manipulate tiny volumes of fluids without the need for external power sources, making them cost-effective and highly portable. Recent technological advancements have demonstrated the ability to preprogram complex multistep liquid operations within the microfluidic circuit of these standalone systems, which enabled the integration of sensitive detection and readout principles. This chapter first addresses how the accessibility to in vitro diagnostics can be improved by shifting toward decentralized approaches like remote microsampling and point-of-care testing. Next, the crucial role of self-powered microfluidic technologies to enable this patient-centric healthcare transition is emphasized using various state-of-the-art examples, with a primary focus on applications related to biofluid collection and the detection of either proteins or nucleic acids. This chapter concludes with a summary of the main findings and our vision of the future perspectives in the field of self-powered microfluidic technologies and their use for in vitro diagnostics applications.

Low-tech vs. high-tech approaches in μPADs as a result of contrasting needs and capabilities of developed and developing countries focusing on diagnostics and point-of-care testing

Paper-based analysis has captivated scientists' attention in the field of analytical chemistry and related areas for the last two decades. Arguably no other area of modern chemical analysis is so broad and diverse in its approaches spanning from simple 'low-tech' low-cost paper-based analytical devices (PADs) requiring no or simple instrumentation, to sophisticated PADs and microfluidic paper-based analytical devices (μPADs) featuring elements of modern material science and nanomaterials affording high selectivity and sensitivity. Correspondingly diverse is the applicability, covering resource-limited scenarios on the one hand and most advanced approaches on the other. Herein we offer a view reflecting this diversity in the approaches and types of devices. The core idea of this article rests in dividing μPADs according to their type into two groups: A) instrumentation-free μPADs for resource-limited scenarios or developing countries and B) instrumentation-based μPADs as futuristic POC devices for e-diagnostics mainly aimed at developed countries. Each of those two groups is presented and discussed with the view of the main requirements in the given area, the most common targets, sample types and suitable detection approaches either implementing high-tech elements or low-tech low-cost approaches. Finally, a socioeconomic perspective is offered in discussing the fabrication and operational costs of μPADs, and, future perspectives are offered.

A novel tailed primer nucleic acid test for detection of HPV 16, 18 and 45 DNA at the point of care

Cervical cancer is a leading cause of death for women in low-resource settings despite being preventable through human papillomavirus (HPV) vaccination, early detection, and treatment of precancerous lesions. The World Health Organization recommends high-risk HPV (hrHPV) as the preferred cervical cancer screening strategy, which is difficult to implement in low-resource settings due to high costs, reliance on centralized laboratory infrastructure, and long sample-to-answer times. To help meet the need for rapid, low-cost, and decentralized cervical cancer screening, we developed tailed primer isothermal amplification and lateral flow detection assays for HPV16, HPV18, and HPV45 DNA. We translated these assays into a self-contained cartridge to achieve multiplexed detection of three hrHPV genotypes in a disposable cartridge. The developed test achieves clinically relevant limits of detection of 50-500 copies per reaction with extracted genomic DNA from HPV-positive cells. Finally, we performed sample-to-answer testing with direct lysates of HPV-negative and HPV-positive cell lines and demonstrated consistent detection of HPV16, HPV18, and HPV45 with 5000-50,000 cells/mL in < 35 min. With additional optimization to improve cartridge reliability, incorporation of additional hrHPV types, and validation with clinical samples, the assay could serve as a point-of-care HPV DNA test that improves access to cervical cancer screening in low-resource settings.

Protein-Based Anchoring Methods for Nucleic Acid Detection in Lateral Flow Format Assays

The use of lateral flow assays to detect nucleic acid targets has many applications including point-of-care diagnostics, environmental monitoring, and food safety. A sandwich format, similar to that in protein immunoassays, is often used to capture the target nucleic acid sequence with an immobilized complementary strand anchored to a substrate, and then to visualize this event using a complementary label nucleic acid bound to a nanoparticle label. A critical component of high-sensitivity nucleic acid detection is to utilize high-density capture surfaces for the effective capture of target nucleic acid. Multiple methods have been reported, including the use of streptavidin-based protein anchors that can be adsorbed to the lateral flow substrate and that can utilize the high-affinity streptavidin-biotin linkage to bind biotinylated nucleic acid capture sequences for subsequent target nucleic acid binding. However, these protein anchors have not been systematically characterized for use in the context of nucleic acid detection. In this work, we characterize several protein-based anchors on nitrocellulose for (i) capturing the robustness of the attachment of the protein anchor, (ii) capturing nucleic acid density, and (iii) targeting nucleic acid capture. Further, we demonstrate the signal gains in target nucleic acid hybridization made by increasing the density of capture nucleic acid on a nitrocellulose substrate using multiple applications of protein loading onto nitrocellulose. Finally, we use our high-density capture surfaces to demonstrate high-sensitivity nucleic acid detection in a lateral flow assay (in the context of a SARS-CoV-2 sequence), achieving a LOD of approximately 0.2 nM.

Sample-to-answer sensing technologies for nucleic acid preparation and detection in the field

Efficient sample preparation and accurate disease diagnosis under field conditions are of great importance for the early intervention of diseases in humans, animals, and plants. However, in-field preparation of high-quality nucleic acids from various specimens for downstream analyses, such as amplification and sequencing, is challenging. Thus, developing and adapting sample lysis and nucleic acid extraction protocols suitable for portable formats have drawn significant attention. Similarly, various nucleic acid amplification techniques and detection methods have also been explored. Combining these functions in an integrated platform has resulted in emergent sample-to-answer sensing systems that allow effective disease detection and analyses outside a laboratory. Such devices have a vast potential to improve healthcare in resource-limited settings, low-cost and distributed surveillance of diseases in food and agriculture industries, environmental monitoring, and defense against biological warfare and terrorism. This paper reviews recent advances in portable sample preparation technologies and facile detection methods that have been / or could be adopted into novel sample-to-answer devices. In addition, recent developments and challenges of commercial kits and devices targeting on-site diagnosis of various plant diseases are discussed.

An integrated isothermal nucleic acid amplification test to detect HPV16 and HPV18 DNA in resource-limited settings

High-risk human papillomavirus (HPV) DNA testing is widely acknowledged as the most sensitive cervical cancer screening method but has limited availability in resource-limited settings, where the burden of cervical cancer is highest. Recently, HPV DNA tests have been developed for use in resource-limited settings, but they remain too costly for widespread use and require instruments that are often limited to centralized laboratories. To help meet the global need for low-cost cervical cancer screening, we developed a prototype, sample-to-answer, point-of-care test for HPV16 and HPV18 DNA. Our test relies on isothermal DNA amplification and lateral flow detection, two technologies that reduce the need for complex instrumentation. We integrated all test components into a low-cost, manufacturable platform, and performance of the integrated test was evaluated with synthetic samples, provider-collected clinical samples in a high-resource setting in the United States, and self-collected clinical samples in a low-resource setting in Mozambique. We demonstrated a clinically relevant limit of detection of 1000 HPV16 or HPV18 DNA copies per test. The test requires six user steps, yields results in 45 min, and can be performed using a benchtop instrument and minicentrifuge by minimally trained personnel. The projected per-test cost is <$5, and the projected instrumentation cost is <$1000. These results show the feasibility of a sample-to-answer, point-of-care HPV DNA test. With the inclusion of other HPV types, this test has the potential to fill a critical gap for decentralized and globally accessible cervical cancer screening.

Toward Rapid and Accurate Molecular Diagnostics at Home

The global outbreaks of infectious diseases have significantly driven an imperative demand for rapid and accurate molecular diagnostics. Nucleic acid amplification tests (NAATs) feature high sensitivity and high specificity; however, the labor-intensive sample preparation and nucleic acid amplification steps remain challenging in order to carry out rapid and precision molecular diagnostics at home. This review discusses the advances and challenges of automatic solutions of sample preparation integrated with on-chip nucleic acid amplification for effective and accurate molecular diagnostics at home. The sample preparation methods of whole blood, urine, saliva/nasal swab, and stool on chip are examined. Then, the repurposable integrated sample preparation on a chip using various biological samples is investigated. Finally, the on-chip NAATs that can be integrated with automated sample preparation are evaluated. The user-friendly approaches with combined sample preparation and NAATs can be the game changers for next-generation rapid and precision home diagnostics.

Portable Paper-Based Nucleic Acid Enrichment for Field Testing

Point-of-care testing (POCT) can be the method of choice for detecting infectious pathogens; these pathogens are responsible for not only infectious diseases such as COVID-19, but also for certain types of cancers. For example, infections by human papillomavirus (HPV) or Helicobacter pylori (H. pylori) are the main cause of cervical and stomach cancers, respectively. COVID-19 and many cancers are treatable with early diagnoses using POCT. A variety of nucleic acid testing have been developed for use in resource-limited environments. However, questions like unintegrated nucleic acid extraction, open detection systems increase the risk of cross-contamination, and dependence on expensive equipment and alternating current (AC) power supply, significantly limit the application of POCT, especially for on-site testing. In this paper, a simple portable platform is reported capable of rapid sample-to-answer testing within 30 min based on recombinase polymerase amplification (RPA) at a lower temperature, to detect SARS-CoV-2 virus and H. pylori bacteria with a limit of detection as low as 4 × 102 copies mL-1 . The platform used a battery-powered portable reader for on-chip one-pot amplification and fluorescence detection, and can test for multiple (up to four) infectious pathogens simultaneously. This platform can provide an alternative method for fast and reliable on-site diagnostic testing.

Promise and perils of paper-based point-of-care nucleic acid detection for endemic and pandemic pathogens

From HIV and influenza to emerging pathogens like COVID-19, each new infectious disease outbreak has highlighted the need for massively-scalable testing that can be performed outside centralized laboratory settings at the point-of-care (POC) in order to prevent, track, and monitor endemic and pandemic threats. Nucleic acid amplification tests (NAATs) are highly sensitive and can be developed and scaled within weeks while protein-based rapid tests require months for production. Combining NAATs with paper-based detection platforms are promising due to the manufacturability, scalability, and simplicity of each of these components. Typically, paper-based NAATs consist of three sequential steps: sample collection and preparation, amplification of DNA or RNA from pathogens of interest, and detection. However, these exist within a larger ecosystem of sample collection and interpretation workflow, usability, and manufacturability which can be vastly perturbed during a pandemic emergence. This review aims to explore the challenges of paper-based NAATs covering sample-to-answer procedures along with three main types of clinical samples; blood, urine, and saliva, as well as broader operational, scale up, and regulatory aspects of device development and implementation. To fill the technological gaps in paper-based NAATs, a sample-in-result-out system that incorporates the integrated sample collection, sample preparation, and integrated internal amplification control while also balancing needs of users and manufacturability upfront in the early design process is required.

A low-cost, paper-based hybrid capture assay to detect high-risk HPV DNA for cervical cancer screening in low-resource settings

Cervical cancer is a leading cause of cancer death for women in low-resource settings. The World Health Organization recommends that cervical cancer screening programs incorporate HPV DNA testing, but available tests are expensive, require laboratory infrastructure, and cannot be performed at the point-of-care. We developed a two-dimensional paper network (2DPN), hybrid-capture, signal amplification assay and a point-of-care sample preparation protocol to detect high-risk HPV DNA from exfoliated cervical cells within an hour. The test does not require expensive equipment and has an estimated cost of <$3 per test without the need for batching. We evaluated performance of the paper HPV DNA assay with short synthetic and genomic HPV DNA targets, HPV positive and negative cellular samples, and two sets of clinical samples. The first set of clinical samples consisted of 16 biobanked, provider-collected cervical samples from a study in El Salvador previously tested with careHPV and subsequently tested in a controlled laboratory environment. The paper HPV DNA test correctly identified eight of eight HPV-negative clinical samples and seven of eight HPV-positive clinical samples. We then performed a field evaluation of the paper HPV DNA test in a hospital laboratory in Mozambique. Cellular controls generated expected results throughout field testing with fully lyophilized sample preparation and 2DPN reagents. When evaluated with 16 residual self-collected cervicovaginal samples previously tested by the GeneXpert HPV assay ("Xpert"), the accuracy of the HPV DNA paper test in the field was reduced compared to testing in the controlled laboratory environment, with positive results obtained for all eight HPV-positive samples as well as seven of eight HPV-negative samples. Further evaluation showed reduction in performance was likely due in part to increased concentration of exfoliated cells in the self-collected clinical samples from Mozambique compared with provider-collected samples from El Salvador. Finally, a formal usability assessment was conducted with users in El Salvador and Mozambique; the assay was rated as acceptable to perform after minimal training. With additional optimization for higher cell concentrations and inclusion of an internal cellular control, the paper HPV DNA assay offers promise as a low-cost, point-of-care cervical cancer screening test in low-resource settings.

REverSe TRanscrIptase chain termination (RESTRICT) for selective measurement of nucleotide analogs used in HIV care and prevention

Sufficient drug concentrations are required for efficacy of antiretroviral drugs used in HIV care and prevention. Measurement of nucleotide analogs, included in most HIV medication regimens, enables monitoring of short- and long-term adherence and the risk of treatment failure. The REverSe TRanscrIptase Chain Termination (RESTRICT) assay rapidly infers the concentration of intracellular nucleotide analogs based on the inhibition of DNA synthesis by HIV reverse transcriptase enzyme. Here, we introduce a probabilistic model for RESTRICT and demonstrate selective measurement of multiple nucleotide analogs using DNA templates designed according to the chemical structure of each drug. We measure clinically relevant concentrations of tenofovir diphosphate, emtricitabine triphosphate, lamivudine triphosphate, and azidothymidine triphosphate with agreement between experiment and theory. RESTRICT represents a new class of activity-based assays for therapeutic drug monitoring in HIV care and could be extended to other diseases treated with nucleotide analogs.

Paper-based sensors for bacteria detection

The detection of pathogenic bacteria is essential to prevent and treat infections and to provide food security. Current gold-standard detection techniques, such as culture-based assays and polymerase chain reaction, are time-consuming and require centralized laboratories. Therefore, efforts have focused on developing point-of-care devices that are fast, cheap, portable and do not require specialized training. Paper-based analytical devices meet these criteria and are particularly suitable to deployment in low-resource settings. In this Review, we highlight paper-based analytical devices with substantial point-of-care applicability for bacteria detection and discuss challenges and opportunities for future development.

Rapid detection of hepatitis C virus using recombinase polymerase amplification

Over 71 million people are infected with hepatitis C virus (HCV) worldwide, and approximately 400,000 global deaths result from complications of untreated chronic HCV. Pan-genomic direct-acting antivirals (DAAs) have recently become widely available and feature high cure rates in less than 12 weeks of treatment. The rollout of DAAs is reliant on diagnostic tests for HCV RNA to identify eligible patients with viremic HCV infections. Current PCR-based HCV RNA assays are restricted to well-resourced central laboratories, and there remains a prevailing clinical need for expanded access to decentralized HCV RNA testing to provide rapid chronic HCV diagnosis and linkage to DAAs in outpatient clinics. This paper reports a rapid, highly accurate, and minimally instrumented assay for HCV RNA detection using reverse transcription recombinase polymerase amplification (RT-RPA). The assay detects all HCV genotypes with a limit of detection of 25 copies per reaction for genotype 1, the most prevalent in the United States and worldwide. The clinical sensitivity and specificity of the RT-RPA assay were both 100% when evaluated using 78 diverse clinical serum specimens. The accuracy, short runtime, and low heating demands of RT-RPA may enable implementation in a point-of-care HCV test to expand global access to effective treatment via rapid chronic HCV diagnosis.

Translating diagnostics and drug delivery technologies to low-resource settings

Diagnostics and drug delivery technologies engineered for low-resource settings aim to meet their technical design specifications using strategies that are compatible with limited equipment, infrastructure, and operator training. Despite many preclinical successes, very few of these devices have been translated to the clinic. Here, we identify factors that contribute to the clinical success of diagnostics and drug delivery systems for low-resource settings, including the need to engage key stakeholders at an early stage, and provide recommendations for the clinical translation of future medical technologies.

Near-digital amplification in paper improves sensitivity and speed in biplexed reactions

The simplest point-of-care assays are usually paper and plastic devices that detect proteins or nucleic acids at low cost and minimal user steps, albeit with poor limits of detection. Digital assays improve limits of detection and analyte quantification by splitting a sample across many wells (or droplets), preventing diffusion, and performing analyte amplification and detection in multiple small wells. However, truly digital nucleic acid amplification tests (NAATs) require costly consumable cartridges that are precisely manufactured, aligned, and operated to enable low detection limits. In this study, we demonstrate how to implement near-digital NAATs in low-cost porous media while approaching the low limits of detection of digital assays. The near-digital NAAT was enabled by a paper membrane containing lyophilized amplification reagents that automatically, passively meters and distributes a sample over a wide area. Performing a NAAT in the paper membrane while allowing diffusion captures many of the benefits of digital NAATs if the pad is imaged at a high spatial resolution during amplification. We show that the near-digital NAAT is compatible with a low-cost paper and plastic disposable cartridge coupled to a 2-layer rigid printed circuit board heater (the MD NAAT platform). We also demonstrate compatibility with biplexing and imaging with mobile phones with different camera sensors. We show that the near-digital NAAT increased signal-to-noise ratios by ~ 10×, improved limits of detection from above 10³ copies of methicillin-resistant Staphylococcus aureus genomic DNA to between 100 and 316 copies in a biplexed reaction containing 10⁵ copies of co-amplifying internal amplification control DNA, and reduced time-to-result from 45 min of amplification to 15-20 min for the positive samples.

A carbon-black-embedded poly(dimethylsiloxane)-paper hybrid device for energy-efficient nucleic-acid amplification in point-of-care testing

A paper-based device patterned with a carbon-black-poly(dimethylsiloxane) (PDMS) mixture is developed as a heating platform for nucleic-acid amplification tests. The photothermal effect of carbon black under 808 nm laser irradiation is used to conduct loop-mediated isothermal amplification (LAMP) to detect Escherichia coli (E. coli) O157:H7, a foodborne pathogen. We characterize the heat generation of carbon black by changing its concentration and the hardness of PDMS. Then, we optimize the minimum laser power required to perform LAMP. The proposed paper-based device requires less than 15 min to perform LAMP, and the result can be confirmed based on the color change observed by the naked eye. The rfbE gene of E. coli O157:H7 is specifically amplified, with a detection limit of 10² CFU mL^-1. Amplification is also performed by using a laboratory-made laser-diode device, which consumes only 2 W h during its operation. The low cost, disposability, and easy fabrication of the paper-based device make it a powerful tool for point-of-care testing.

Quantitative isothermal amplification on paper membranes using amplification nucleation site analysis

Quantitative nucleic acid amplification tests (qNAATs) are critical in treating infectious diseases, such as in HIV viral load monitoring or SARS-CoV-2 testing, in which viral load indicates viral suppression or infectivity. Quantitative PCR is the gold standard tool for qNAATs; however, there is a need to develop point-of-care (POC) qNAATs to manage infectious diseases in outpatient clinics, low- and middle-income countries, and the home. Isothermal amplification methods are an emerging tool for POC NAATs as an alternative to traditional PCR-based workflows. Previous works have focused on relating isothermal amplification bulk fluorescence signals to input copies of target nucleic acids for sample quantification with limited success. In this work, we show that recombinase polymerase amplification (RPA) reactions on paper membranes exhibit discrete fluorescent amplification nucleation sites. We demonstrate that the number of nucleation sites can be used to quantify HIV-1 DNA and viral RNA in less than 20 minutes. An image-analysis algorithm quantifies nucleation sites and determines the input nucleic acid copies in the range of 67-3000 copies per reaction. We demonstrate a mobile phone-based system for image capture and onboard processing, illustrating that this method may be used at the point-of-care for qNAATs with minimal instrumentation.

Reversible Thermo-Responsive Valve for Microfluidic Paper-Based Analytical Devices

Fluid control on a paper channel is necessary for analysis with multiple reagents, such as enzyme-linked immunosorbent assay (ELISA) in microfluidic paper-based analytical devices (µPADs). In this study, a thermo-responsive valve was fabricated by polymerizing N-isopropylacrylamide on a PVDF porous membrane by plasma-induced graft polymerization. The polymerized membrane was observed by scanning electron microscopy (SEM), and it was confirmed that more pores were closed at temperatures below 32 °C and more pores were opened at temperatures above 32 °C. Valve permeability tests confirmed that the proposed polymerized membrane was impermeable to water and proteins at temperatures below 32 °C and permeable to water at temperatures above 32 °C. The valve could also be reversibly and repeatedly opened and closed by changing the temperature near 32 °C. These results suggest that plasma-induced graft polymerization may be used to produce thermo-responsive valves that can be opened and closed without subsequent loss of performance. These results indicate that the thermo-responsive valve fabricated by plasma-induced graft polymerization could potentially be applied to ELISA with µPADs.

Application of Nanomaterials in the Prevention, Detection, and Treatment of Methicillin-Resistant Staphylococcus aureus (MRSA)

Due to differences in geographic surveillance systems, chemical sanitization practices, and antibiotic stewardship (AS) implementation employed during the COVID-19 pandemic, many experts have expressed concerns regarding a future surge in global antimicrobial resistance (AMR). A potential beneficiary of these differences is the Gram-positive bacteria MRSA. MRSA is a bacterial pathogen with a high potential for mutational resistance, allowing it to engage various AMR mechanisms circumventing conventional antibiotic therapies and the host’s immune response. Coupled with a lack of novel FDA-approved antibiotics reaching the clinic, the onus is on researchers to develop alternative treatment tools to mitigate against an increase in pathogenic resistance. Mitigation strategies can take the form of synthetic or biomimetic nanomaterials/vesicles employed in vaccines, rapid diagnostics, antibiotic delivery, and nanotherapeutics. This review seeks to discuss the current potential of the aforementioned nanomaterials in detecting and treating MRSA.

Paper based microfluidics: A forecast toward the most affordable and rapid point-of-care devices

The microfluidic industry has evolved through years with acquired scientific knowledge from different, and already developed industries. Consequently, a wide range of materials like silicon from the electronic industry to all the way, silicone, from the chemical engineering industry, has been spotted to solve similar challenges. Although a typical microfluidic chip, fabricated from glass or polymer substrates offers definite benefits, however, paper-based microfluidic analytical devices (μPADs) possess numerous special benefits for practical implementation at a lower price. Owing to these features, in recent years, paper microfluidics has drawn immense interest from researchers in industry and academia alike. These devices have wider applications with advantages like lower cost, speedy detection, user-easiness, biocompatibility, sensitivity, and specificity etc. when compared to other microfluidic devices. Therefore, these sensitive but affordable devices fit themselves into point-of-care (POC) testing with features in demand like natural disposability, situational flexibility, and the capability to store and analyze the target at the point of requirement. Gradually, advancements in fabrication technologies, assay development techniques, and improved packaging capabilities, have contributed significantly to the real-time identification and health investigation through paper microfluidics; however, the growth has not been limited to the biomedical field; industries like electronics, energy storage and many more have expanded substantially. Here, we represent an overall state of the paper-based microfluidic technology by covering the fundamentals, working principles, different fabrication procedures, applications for various needs and then to make things more practical, the real-life scenario and practical challenges involved in launching a device into the market have been revealed. To conclude, recent contribution of μPADs in the 2020 pandemic and potential future possibilities have been reviewed.

Quantitative Isothermal Amplification on Paper Membranes using Amplification Nucleation Site Analysis

Quantitative nucleic acid amplification tests (qNAATs) are critical in treating infectious diseases, such as in HIV viral load monitoring or SARS-CoV-2 testing, in which viral load indicates viral suppression or infectivity. Quantitative PCR is the gold standard tool for qNAATs; however, there is a need to develop point-of-care (POC) qNAATs to manage infectious diseases in outpatient clinics, low- and middle-income countries, and the home. Isothermal amplification methods are an emerging tool for POC NAATs as an alternative to traditional PCR-based workflows. Previous works have focused on relating isothermal amplification bulk fluorescence signals to input copies of target nucleic acids for sample quantification with limited success. In this work, we show that recombinase polymerase amplification (RPA) reactions on paper membranes exhibit discrete fluorescent amplification nucleation sites. We demonstrate that the number of nucleation sites can be used to quantify HIV-1 DNA and RNA in less than 20 minutes. An image-analysis algorithm quantifies nucleation sites and determines the input nucleic acid copies in the range of 67-3,000 copies per reaction. We demonstrate a mobile phone-based system for image capture and onboard processing, illustrating that this method may be used at the point-of-care for qNAATs with minimal instrumentation.

A survey of 3D printing technology applied to paper microfluidics

Paper microfluidics is a rapidly growing subfield of microfluidics in which paper-like porous materials are used to create analytical devices that are well-suited for use in field applications. 3D printing technology has the potential to positively affect paper microfluidic device development by enabling tools and methods for the creation of devices with well-defined and tunable fluidic networks of porous matrices for high performance signal generation. This critical review focuses on the progress that has been made in using 3D printing technologies to advance the development of paper microfluidic devices. We describe printing work in three general categories: (i) solid support structures for paper microfluidic device components; (ii) channel barrier definition in existing porous materials; and (iii) porous channels for capillary flow, and discuss their value in advancing paper microfluidic device development. Finally, we discuss major areas of focus for highest impact on the next generation of paper microfluidics devices.

Point-of-care diagnostics: recent developments in a pandemic age

In this review, we provide an overview of developments in point-of-care (POC) diagnostics during the COVID-19 pandemic. We review these advances within the framework of a holistic POC ecosystem, focusing on points of interest - both technological and non-technological - to POC researchers and test developers. Technologically, we review design choices in assay chemistry, microfluidics, and instrumentation towards nucleic acid and protein detection for severe acute respiratory coronavirus 2 (SARS-CoV-2), and away from the lab bench, developments that supported the unprecedented rapid development, scale up, and deployment of POC devices. We describe common features in the POC technologies that obtained Emergency Use Authorization (EUA) for nucleic acid, antigen, and antibody tests, and how these tests fit into four distinct POC use cases. We conclude with implications for future pandemics, infectious disease monitoring, and digital health.

Frugal Science Powered by Curiosity

More than 60 years ago, Richard Feynman gave a lecture titled "There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics", where he called on others to join the then-nascent field of nanotechnology. In a similar spirit, we wish to invite chemists, biologists, physicists, bioengineers, educators, high school students, and inventors of all backgrounds to join us in the emerging field of frugal science. In this Review, we define frugal science and use six case studies to describe the broad applications of frugal science, from synthetic biology to disease diagnostics. We conclude by establishing an argument for curiosity-driven research through frugal science to enable broader access in chemical and bioengineering research and drive innovation.

Using airflow-driven, evaporative gradients to improve sensitivity and fluid control in colorimetric paper-based assays

Microfluidic paper-based analytical devices (μPADs) are foundational devices for point-of-care testing, yet suffer from limitations in regards to their sensitivity and capability in handling complex assays. Here, we demonstrate an airflow-based, evaporative method that is capable of manipulating fluid flows within paper membranes to offer new functionalities for multistep delivery of reagents and improve the sensitivity of μPADs by 100-1000 times. This method applies an air-jet to a pre-wetted membrane, generating an evaporative gradient such that any solutes become enriched underneath the air-jet spot. By controlling the lateral position of this spot, the solutes in the paper strip are enriched and follow the air jet trajectory, driving the reactions and enhancing visualization for colorimetric readout in multistep assays. The technique has been successfully applied to drive the sequential delivery in multistep immunoassays as well as improve sensitivity for colorimetric detection assays for nucleic acids and proteins via loop-mediated isothermal amplification (LAMP) and ELISA. For colorimetric LAMP detection of the COVID-19 genome, enrichment of the solution on paper could enhance the contrast of the dye in order to more clearly distinguish between the positive and negative results to achieve a sensitivity of 3 copies of SARS-Cov-2 RNAs. For ELISA, enrichment of the oxidized TMB substrate yielded a sensitivity increase of two-to-three orders of magnitude when compared to non-enriched samples - having a limit of detection of around 200 fM for IgG. Therefore, this enrichment method represents a simple process that can be easily integrated into existing detection assays for controlling fluid flows and improving detection of biomarkers on paper.

Piecewise Isothermal Nucleic Acid Testing (PINAT) for Infectious Disease Detection with Sample-to-Result Integration at the Point-of-Care

We developed a piecewise isothermal nucleic acid test (PINAT) as a platform technology for diagnosing pathogen-associated infections, empowered by an illustrative novel methodology that embeds an exclusive DNA-mediated specific probing reaction with the backbone of an isothermal reverse transcription cum amplification protocol for detecting viral RNA. In a point-of-care format, this test is executable in a unified single-step, single-chamber procedure, leading to seamless sample-to-result integration in an inexpensive, scalable, pre-programmable, and customizable portable device, with mobile-app-integrated interpretation and analytics involving minimal manually operative procedures. The test exhibited a high sensitivity and specificity of detection when assessed using 200 double-blind patient samples for detecting SARS-CoV-2 infection by the Indian Council of Medical Research (ICMR), and subsequently using 170 double-blind patient samples in a point-of-care format outside controlled laboratory settings as performed by unskilled technicians in an organized clinical trial. We also established its efficacy in detecting Influenza A infection by performing the diagnosis at the point of collection with uncompromised detection rigor. The envisaged trade-off between advanced laboratory-based molecular diagnostic procedures and the elegance of common rapid tests renders the method ideal for deployment in resource-limited settings towards catering the needs of the underserved.

Long-term dry storage of enzyme-based reagents for isothermal nucleic acid amplification in a porous matrix for use in point-of-care diagnostic devices

Nucleic acid amplification test (NAAT)-based point-of-care (POC) devices are rapidly growing for use in low-resource settings. However, key challenges are the ability to store the enzyme-based reagents in dry form in the device and the long-term stability of those reagents at elevated temperatures, especially where ambient temperatures could be as high as 45 °C. Here, we describe a set of excipients including a combination of trehalose, polyethylene glycol and dextran, and a method for using them that allows long-term dry storage of enzyme-based reagents for an isothermal strand displacement amplification (iSDA) reaction in a porous matrix. Various porous materials, including nitrocellulose, cellulose, and glass fiber, were tested. Co-dried reagents for iSDA always included those that amplified the ldh1 gene in Staphylococcus aureus (a polymerase and a nicking enzyme, 4 primers, dNTPs and a buffer). Reagents also either included a capture probe and a streptavidin-Au label required for lateral flow (LF) detection after amplification, or a fluorescent probe used for real-time detection. The reagents showed the best stability in a glass fiber matrix when stored in the presence of 10% trehalose and 2.5% dextran. The reagents were stable for over a year at ∼22 °C as determined by lateral flow detection and gel electrophoresis. The reagents also exhibited excellent stability after 360 h at 45 °C; the assay still detected as few as 10 copies of ldh1 gene target by lateral flow detection, and 50 copies with real-time fluorescence detection. These results demonstrate the potential for incorporation of amplification reagents in dry form in point-of-care devices for use in a wide range of settings.

Nucleic acid analysis on paper substrates (NAAPs): an innovative tool for Point of Care (POC) infectious disease diagnosis

The cost-effective rapid diagnosis of infectious diseases is an essential and important factor for curing such diseases in the global public health care picture. Owing to poor infrastructure and lack of sanitation, these diseases have an extreme impact on remote and rural areas, especially in developing countries, and there are unresolved challenges. Molecular diagnosis, such as nucleic acid analysis, plays a key role in the significant treatment of numerous infectious diseases. Current molecular diagnostic assays require a sophisticated laboratory setup with expensive components. Molecular diagnosis on a microfluidic point-of-care (POC) platform is attractive to researchers for disease detection with proper prevention. Compared to various microfluidic substrate materials, paper-based POC technologies offer significant cost-effective solutions over high-cost clinical instruments to fill the gap between the needs of users and affordability. Low-cost paper-based microfluidic POC technologies provide portable and disposable diagnostic systems for multiple disease detection that may be extremely useful in remote areas. This article presents a critical review of paper-based microfluidic device technology which has become an imminent platform to adjust the current health scenario for the detection of diseases using different stages of nucleic acid analysis, such as extraction, amplification and detection of nucleic acid, with future perspectives for paper substrates.

Fluorometric Paper-Based, Loop-Mediated Isothermal Amplification Devices for Quantitative Point-of-Care Detection of Methicillin-Resistant Staphylococcus aureus (MRSA)

Loop-mediated isothermal amplification (LAMP) has been widely used to detect many infectious diseases. However, minor inconveniences during the steps of adding reaction ingredients and lack of simple color results hinder point-of-care detection. We therefore invented a fluorometric paper-based LAMP by incorporating LAMP reagents, including a biotinylated primer, onto a cellulose membrane paper, with a simple DNA fluorescent dye incubation that demonstrated rapid and accurate results parallel to quantitative polymerase chain reaction (qPCR) methods. This technology allows for instant paper strip detection of methicillin-resistant Staphylococcus aureus (MRSA) in the laboratory and clinical samples. MRSA represents a major public health problem as it can cause infections in different parts of the human body and yet is resistant to commonly used antibiotics. In this study, we optimized LAMP reaction ingredients and incubation conditions following a central composite design (CCD) that yielded the shortest reaction time with high sensitivity. These CCD components and conditions were used to construct the paper-based LAMP reaction by immobilizing the biotinylated primer and the rest of the LAMP reagents to produce the ready-to-use MRSA diagnostic device. Our paper-based LAMP device could detect as low as 10 ag (equivalent to 1 copy) of the MRSA gene mecA within 36-43 min, was evaluated using both laboratory (individual cultures of MRSA and non-MRSA bacteria) and clinical blood samples to be 100% specific and sensitive compared to qPCR results, and had 35 day stability under 25 °C storage. Furthermore, the color readout allows for quantitation of MRSA copies. Hence, this device is applicable for point-of-care MRSA detection.

Nitrocellulose-bound achromopeptidase for point-of-care nucleic acid tests

Enzymes are the cornerstone of modern biotechnology. Achromopeptidase (ACP) is a well-known enzyme that hydrolyzes a number of proteins, notably proteins on the surface of Gram-positive bacteria. It is therefore used for sample preparation in nucleic acid tests. However, ACP inhibits DNA amplification which makes its integration difficult. Heat is commonly used to inactivate ACP, but it can be challenging to integrate heating into point-of-care devices. Here, we use recombinase polymerase amplification (RPA) together with ACP, and show that when ACP is immobilized on nitrocellulose paper, it retains its enzymatic function and can easily and rapidly be activated using agitation. The nitrocellulose-bound ACP does, however, not leak into the solution, preventing the need for deactivation through heat or by other means. Nitrocellulose-bound ACP thus opens new possibilities for paper-based Point-of-Care (POC) devices.

Reverse transcription lesion-induced DNA amplification: An instrument-free isothermal method to detect RNA

One challenge in point-of-care (POC) diagnostics is the lack of room-temperature methods for RNA detection based on enzymatic amplification and visualization steps. Here we perform reverse transcription lesion-induced DNA amplification (RT-LIDA), an isothermal amplification method that only requires T4 DNA ligase. RT-LIDA involves the RNA-templated ligation of DNA primers to form complementary DNA (cDNA) followed by toehold-mediated strand displacement of the cDNA and its exponential amplification via our isothermal ligase chain reaction LIDA. Each step is tuned to proceed at 28 °C, which falls within the range of global room temperatures. Using RT-LIDA, we can detect as little as ∼100 amol target RNA and can distinguish RNA target from total cellular RNA. Finally, we demonstrate that the resulting DNA amplicons can be detected colorimetrically, also at room temperature, by rapid, target-triggered disassembly of DNA-modified gold nanoparticles. This integrated amplification/detection platform requires no heating or visualization instrumentation, which is an important step towards realizing instrument-free POC testing.

A fully automated microfluidic PCR-array system for rapid detection of multiple respiratory tract infection pathogens

Rapid and accurate identification of respiratory tract infection pathogens is of utmost importance for clinical diagnosis and treatment, as well as prevention of pathogen transmission. To meet this demand, a microfluidic chip-based PCR-array system, Onestart, was developed. The Onestart system uses a microfluidic chip packaged with all the reagents required, and the waste liquid is also collected and stored on the chip. This ready-to-use system can complete the detection of 21 pathogens in a fully integrated manner, with sample lysis, nucleic acid extraction/purification, and real-time PCR sequentially implemented on the same chip. The entire analysis process is completed within 1.5 h, and the system automatically generates a test report. The lower limit-of-detection (LOD) of the Onestart assay was determined to be 1.0 × 10³ copies·mL⁻¹. The inter-batch variation of cycle threshold (Ct) values ranged from 0.08% to 0.69%, and the intra-batch variation ranged from 0.9% to 2.66%. Analytical results of the reference sample mix showed a 100% specificity of the Onestart assay. The analysis of batched clinical samples showed consistency of the Onestart assay with real-time PCR. With its ability to provide rapid, sensitive, and specific detection of respiratory tract infection pathogens, application of the Onestart system will facilitate timely clinical management of respiratory tract infections and effective prevention of pathogen transmission.

Fully integrated microfluidic devices for qualitative, quantitative and digital nucleic acids testing at point of care

Benefiting from emerging miniaturized and equipment-free nucleic acid testing (NAT) technologies, fully integrated NAT devices at point of care (POC) with the capability of "sample-in-answer-out" are proceeding at a break-neck speed to eliminate complex operations and reduce the risk of contamination. Like the development of polymerase chain reaction (PCR) technology (the standard technique for NAT), the detection signal of fully integrated NAT devices has evolved from qualitative to quantitative and recently to digital readout, aiming at expanding their extensive applications through gradually improving detection sensitivity and accuracy. This review firstly introduces the existing commercial products, and then illustrates recent fully integrated microfluidic devices for NAT at POC from the aspect of detection signals (i.e., qualitative, quantitative and digital). Importantly, the key issues of existing commercial products and the main challenges between scientific research and product development are discussed. On this basis, we envision that the MARCHED (miniaturized, automatic, reagent-preloaded, commercializable, high-throughput, environment-independent and disposable) NAT devices are expected to be realized in the near future.

Advances in point-of-care nucleic acid extraction technologies for rapid diagnosis of human and plant diseases

Global health and food security constantly face the challenge of emerging human and plant diseases caused by bacteria, viruses, fungi, and other pathogens. Disease outbreaks such as SARS, MERS, Swine Flu, Ebola, and COVID-19 (on-going) have caused suffering, death, and economic losses worldwide. To prevent the spread of disease and protect human populations, rapid point-of-care (POC) molecular diagnosis of human and plant diseases play an increasingly crucial role. Nucleic acid-based molecular diagnosis reveals valuable information at the genomic level about the identity of the disease-causing pathogens and their pathogenesis, which help researchers, healthcare professionals, and patients to detect the presence of pathogens, track the spread of disease, and guide treatment more efficiently. A typical nucleic acid-based diagnostic test consists of three major steps: nucleic acid extraction, amplification, and amplicon detection. Among these steps, nucleic acid extraction is the first step of sample preparation, which remains one of the main challenges when converting laboratory molecular assays into POC tests. Sample preparation from human and plant specimens is a time-consuming and multi-step process, which requires well-equipped laboratories and skilled lab personnel. To perform rapid molecular diagnosis in resource-limited settings, simpler and instrument-free nucleic acid extraction techniques are required to improve the speed of field detection with minimal human intervention. This review summarizes the recent advances in POC nucleic acid extraction technologies. In particular, this review focuses on novel devices or methods that have demonstrated applicability and robustness for the isolation of high-quality nucleic acid from complex raw samples, such as human blood, saliva, sputum, nasal swabs, urine, and plant tissues. The integration of these rapid nucleic acid preparation methods with miniaturized assay and sensor technologies would pave the road for the "sample-in-result-out" diagnosis of human and plant diseases, especially in remote or resource-limited settings.

Integrated nucleic acid testing system to enable TB diagnosis in peripheral settings

To facilitate treatment and limit transmission of tuberculosis (TB), new methods are needed to enable rapid and affordable diagnosis of the disease in high-burden low-resource settings. We have developed a prototype integrated nucleic acid testing device to detect Mycobacterium tuberculosis (M.tb) in sputum. The device consists of a disposable cartridge and compact, inexpensive instrument that automates pathogen lysis, nucleic acid extraction, isothermal DNA amplification and lateral flow detection. A liquefied and disinfected sputum sample is manually injected into the cartridge, and all other steps are automated, with a result provided in <1.5 h. Cell disruption and DNA extraction is executed within a four-port active valve containing a miniature bead blender (based on PureLyse® technology, Claremont BioSolutions LLC). The DNA-containing eluate is combined with dry master-mix reagents and target DNA is isothermally amplified. Amplified master-mix is then pumped into a lateral flow strip chamber for detection. The entire process is performed in a single-use closed-system cartridge to prevent amplicon carryover. For testing of M.tb-spiked sputum the system provided a limit of detection of 5 × 103 colony forming units (CFU) per mL. None of the negative sputum-only controls yielded a false-positive result. Testing of 45 clinical sputum specimens from TB cases and controls relative to a validated manual qPCR-based comparator method revealed a preliminary sensitivity of 90% and specificity of 96%. With further development, the herein described integrated nucleic acid testing device can enable TB diagnosis and treatment initiation in the same clinical encounter in near-patient low-resource settings of high TB burden countries.

Tutorial: design and fabrication of nanoparticle-based lateral-flow immunoassays

Lateral-flow assays (LFAs) are quick, simple and cheap assays to analyze various samples at the point of care or in the field, making them one of the most widespread biosensors currently available. They have been successfully employed for the detection of a myriad of different targets (ranging from atoms up to whole cells) in all type of samples (including water, blood, foodstuff and environmental samples). Their operation relies on the capillary flow of the sample throughout a series of sequential pads, each with different functionalities aiming to generate a signal to indicate the absence/presence (and, in some cases, the concentration) of the analyte of interest. To have a user-friendly operation, their development requires the optimization of multiple, interconnected parameters that may overwhelm new developers. In this tutorial, we provide the readers with: (i) the basic knowledge to understand the principles governing an LFA and to take informed decisions during lateral flow strip design and fabrication, (ii) a roadmap for optimal LFA development independent of the specific application, (iii) a step-by-step example procedure for the assembly and operation of an LF strip for the detection of human IgG and (iv) an extensive troubleshooting section addressing the most frequent issues in designing, assembling and using LFAs. By changing only the receptors, the provided example procedure can easily be adapted for cost-efficient detection of a broad variety of targets.

Enzyme-Assisted Nucleic Acid Detection for Infectious Disease Diagnostics: Moving toward the Point-of-Care

Driven by complex and interconnected factors, including population growth, climate change, and geopolitics, infectious diseases represent one of the greatest healthcare challenges of the 21st century. Diagnostic technologies are the first line of defense in the fight against infectious disease, providing critical information to inform epidemiological models, track diseases, decide treatment choices, and ultimately prevent epidemics. The diagnosis of infectious disease at the genomic level using nucleic acid disease biomarkers has proven to be the most effective approach to date. Such methods rely heavily on enzymes to specifically amplify or detect nucleic acids in complex samples, and significant effort has been exerted to harness the power of enzymes for in vitro nucleic acid diagnostics. Unfortunately, significant challenges limit the potential of enzyme-assisted nucleic acid diagnostics, particularly when translating diagnostic technologies from the lab toward the point-of-use or point-of-care. Herein, we discuss the current state of the field and highlight cross-disciplinary efforts to solve the challenges associated with the successful deployment of this important class of diagnostics at or near the point-of-care.

An IoT-enabled paper sensor platform for real-time analysis of isothermal nucleic acid amplification tests

Paper-based sensors can be exploited to develop low-cost, disposable, and rapid assays for the detection of a large variety of analytes. We report a paper-based sensor system for a point-of-care (POC) nucleic acid amplification test that can quantitatively detect multiple genes from different pathogens. The POC system combines a paper sensor chip and a portable instrument, which is built on an Internet of Things (IoT) platform. The paper-based sensor provides the functions of reagent storage, sample transportation, and nucleic acid amplification. The IoT instrument uses an Arduino microcontroller to control temperature, collect fluorescence images, and store the data in cloud storage via a WiFi network. A compact fluorescence reader was designed to measure fluorescence images of the amplicons during a loop-mediated isothermal amplification reaction in real-time. The real-time detection capability enables the quantitative analysis of target genes. The results show that the paper-based sensor cam distinguish multiple genes of the genomic DNA extracted from Escherichia coli and Campylobacter jejuni, with the concentration as low as 2 × 10³ copies/μL. The affordable instrument, in conjunction with the disposable paper sensor chip, would have a great potential for POC detections of pathogens.

Smartphone technology facilitates point-of-care nucleic acid diagnosis: a beginner's guide

The reliable detection of nucleic acids at low concentrations in clinical samples like blood, urine and saliva, and in food can be achieved by nucleic acid amplification methods. Several portable and hand-held devices have been developed to translate these laboratory-based methods to point-of-care (POC) settings. POC diagnostic devices could potentially play an important role in environmental monitoring, health, and food safety. Use of a smartphone for nucleic acid testing has shown promising progress in endpoint as well as real-time analysis of various disease conditions. The emergence of smartphone-based POC devices together with paper-based sensors, microfluidic chips and digital droplet assays are used currently in many situations to provide quantitative detection of nucleic acid targets. State-of-the-art portable devices are commercially available and rapidly emerging smartphone-based POC devices that allow the performance of laboratory-quality colorimetric, fluorescent and electrochemical detection are described in this review. We present a comprehensive review of smartphone-based POC sensing applications, specifically on microbial diagnostics, assess their performance and propose recommendations for the future.

Lab-on-paper for all-in-one molecular diagnostics (LAMDA) of zika, dengue, and chikungunya virus from human serum

Several tropical fever viruses transmitted by mosquitoes including zika, dengue, and chikungunya, are becoming a serious problem in global public health. Simple diagnostic tools in early stages are strongly required to monitor and prevent these diseases. Paper diagnostic platforms can provide a solution for these needs, with integration of fluidic control techniques and isothermal amplification methods. Here, we demonstrate a Lab-on-paper for all-in-one molecular diagnostics of zika, dengue, and chikungunya virus from human serum. The entire process of nucleic acid testing that involves sampling, extraction, amplification, and detection is simply operated on a single paper chip. Based on the engineered structure of paper materials and dried chemicals on the all-in-one chip, serum samples containing the target virus RNA were simply added by automatic flow from distilled water injection. Target RNA molecules were concentrated on the binding pad with chitosan and then transported to reaction pads following a pH increase for specific reverse transcription loop-mediated isothermal amplification with fluorescence signal generation. Three targets, zika virus, dengue virus, and chikungunya virus, in human serum were simultaneously detected on the all-in-one paper chip within 60 min at 65 °C. The all-in-one paper chip can be used as a real-time quantitative assay for 5-5000 copies of zika virus RNA. This all-in-one device was successfully used with 5 clinical specimens of zika and dengue virus from real patients. We believe that the proposed all-in-one paper chip can provide a portable, low-cost, user-friendly, sensitive, and specific NAT platform with great potential in point-of-care diagnostics.

Passive micropumping in microfluidics for point-of-care testing

Suitable micropumping methods for flow control represent a major technical hurdle in the development of microfluidic systems for point-of-care testing (POCT). Passive micropumping for point-of-care microfluidic systems provides a promising solution to such challenges, in particular, passive micropumping based on capillary force and air transfer based on the air solubility and air permeability of specific materials. There have been numerous developments and applications of micropumping techniques that are relevant to the use in POCT. Compared with active pumping methods such as syringe pumps or pressure pumps, where the flow rate can be well-tuned independent of the design of the microfluidic devices or the property of the liquids, most passive micropumping methods still suffer flow-control problems. For example, the flow rate may be set once the device has been made, and the properties of liquids may affect the flow rate. However, the advantages of passive micropumping, which include simplicity, ease of use, and low cost, make it the best choice for POCT. Here, we present a systematic review of different types of passive micropumping that are suitable for POCT, alongside existing applications based on passive micropumping. Future trends in passive micropumping are also discussed.