Paper-Based Electrical Respiration Sensor.

Current methods of monitoring breathing require cumbersome, inconvenient, and often expensive devices; this requirement sets practical limitations on the frequency and duration of measurements. This article describes a paper-based moisture sensor that uses the hygroscopic character of paper (i.e. the ability of paper to adsorb water reversibly from the surrounding environment) to measure patterns and rate of respiration by converting the changes in humidity caused by cycles of inhalation and exhalation to electrical signals. The changing level of humidity that occurs in a cycle causes a corresponding change in the ionic conductivity of the sensor, which can be measured electrically. By combining the paper sensor with conventional electronics, data concerning respiration can be transmitted to a nearby smartphone or tablet computer for post-processing, and subsequently to a cloud server. This means of sensing provides a new, practical method of recording and analyzing patterns of breathing.

Broadly available imaging devices enable high-quality low-cost photometry.

This paper demonstrates that, for applications in resource-limited environments, expensive microplate spectrophotometers that are used in many central laboratories for parallel measurement of absorbance of samples can be replaced by photometers based on inexpensive and ubiquitous, consumer electronic devices (e.g., scanners and cell-phone cameras). Two devices, (i) a flatbed scanner operating in transmittance mode and (ii) a camera-based photometer (constructed from a cell phone camera, a planar light source, and a cardboard box), demonstrate the concept. These devices illuminate samples in microtiter plates from one side and use the RGB-based imaging sensors of the scanner/camera to measure the light transmitted to the other side. The broadband absorbance of samples (RGB-resolved absorbance) can be calculated using the RGB color values of only three pixels per microwell. Rigorous theoretical analysis establishes a well-defined relationship between the absorbance spectrum of a sample and its corresponding RGB-resolved absorbance. The linearity and precision of measurements performed with these low-cost photometers on different dyes, which absorb across the range of the visible spectrum, and chromogenic products of assays (e.g., enzymatic, ELISA) demonstrate that these low-cost photometers can be used reliably in a broad range of chemical and biochemical analyses. The ability to perform accurate measurements of absorbance on liquid samples, in parallel and at low cost, would enable testing, typically reserved for well-equipped clinics and laboratories, to be performed in circumstances where resources and expertise are limited.

"Paper Machine" for Molecular Diagnostics.

Clinical tests based on primer-initiated amplification of specific nucleic acid sequences achieve high levels of sensitivity and specificity. Despite these desirable characteristics, these tests have not reached their full potential because their complexity and expense limit their usefulness to centralized laboratories. This paper describes a device that integrates sample preparation and loop-mediated isothermal amplification (LAMP) with end point detection using a hand-held UV source and camera phone. The prototype device integrates paper microfluidics (to enable fluid handling) and a multilayer structure, or a "paper machine", that allows a central patterned paper strip to slide in and out of fluidic path and thus allows introduction of sample, wash buffers, amplification master mix, and detection reagents with minimal pipetting, in a hand-held, disposable device intended for point-of-care use in resource-limited environments. This device creates a dynamic seal that prevents evaporation during incubation at 65 °C for 1 h. This interval is sufficient to allow a LAMP reaction for the Escherichia coli malB gene to proceed with an analytical sensitivity of 1 double-stranded DNA target copy. Starting with human plasma spiked with whole, live E. coli cells, this paper demonstrates full integration of sample preparation with LAMP amplification and end point detection with a limit of detection of 5 cells. Further, it shows that the method used to prepare sample enables concentration of DNA from sample volumes commonly available from fingerstick blood draw.

From the bench to the field in low-cost diagnostics: two case studies.

Despite the growth of research in universities on point-of-care (POC) diagnostics for global health, most devices never leave the laboratory. The processes that move diagnostic technology from the laboratory to the field--the processes intended to evaluate operation and performance under realistic conditions--are more complicated than they might seem. Two case studies illustrate this process: the development of a paper-based device to measure liver function, and the development of a device to identify sickle cell disease based on aqueous multiphase systems (AMPS) and differences in the densities of normal and sickled cells. Details of developing these devices provide strategies for forming partnerships, prototyping devices, designing studies, and evaluating POC diagnostics. Technical and procedural lessons drawn from these experiences may be useful to those designing diagnostic tests for developing countries, and more generally, technologies for use in resource-limited environments.

Evaluation of a density-based rapid diagnostic test for sickle cell disease in a clinical setting in Zambia.

Although simple and low-cost interventions for sickle cell disease (SCD) exist in many developing countries, child mortality associated with SCD remains high, in part, because of the lack of access to diagnostic tests for SCD. A density-based test using aqueous multiphase systems (SCD-AMPS) is a candidate for a low-cost, point-of-care diagnostic for SCD. In this paper, the field evaluation of SCD-AMPS in a large (n = 505) case-control study in Zambia is described. Of the two variations of the SCD-AMPS used, the best system (SCD-AMPS-2) demonstrated a sensitivity of 86% (82-90%) and a specificity of 60% (53-67%). Subsequent analysis identified potential sources of false positives that include clotting, variation between batches of SCD-AMPS, and shipping conditions. Importantly, SCD-AMPS-2 was 84% (62-94%) sensitive in detecting SCD in children between 6 months and 1 year old. In addition to an evaluation of performance, an assessment of end-user operability was done with health workers in rural clinics in Zambia. These health workers rated the SCD-AMPS tests to be as simple to use as lateral flow tests for malaria and HIV.

Density-based separation in multiphase systems provides a simple method to identify sickle cell disease.

Although effective low-cost interventions exist, child mortality attributable to sickle cell disease (SCD) remains high in low-resource areas due, in large part, to the lack of accessible diagnostic methods. The presence of dense (ρ > 1.120 g/cm(3)) cells is characteristic of SCD. The fluid, self-assembling step-gradients in density created by aqueous multiphase systems (AMPSs) identifies SCD by detecting dense cells. AMPSs separate different forms of red blood cells by density in a microhematocrit centrifuge and provide a visual means to distinguish individuals with SCD from those with normal hemoglobin or with nondisease, sickle-cell trait in under 12 min. Visual evaluation of a simple two-phase system identified the two main subclasses of SCD [homozygous (Hb SS) and heterozygous (Hb SC)] with a sensitivity of 90% (73-98%) and a specificity of 97% (86-100%). A three-phase system identified these two types of SCD with a sensitivity of 91% (78-98%) and a specificity of 88% (74-98%). This system could also distinguish between Hb SS and Hb SC. To the authors' knowledge, this test demonstrates the first separation of cells by density with AMPSs, and the usefulness of AMPSs in point-of-care diagnostic hematology.

Is the focus on "molecules" obsolete?

The technologies developed in analytical chemistry have defined in spectacular detail the properties of molecules. The field now faces enormously important and interesting problems of which molecules are only a part: for example, understanding the nature of life; helping to manage megacities, oceans, and atmospheres; and making health care (especially diagnostics) affordable and relevant. The emergence of these problems involving molecular systems raises the issue of how (and what) analytical chemistry should teach. Historically, it has been essential to chemistry in teaching the science of measurement. As complicated analytical techniques proliferate, it must consider how to balance teaching the uses of sophisticated devices and the fundamentals of analysis and measurement. This review (by an admiring but nonanalytical chemist) sketches the essential role of analytical methods--especially simple ones made up on the spot--in guiding research in new fields, with examples from self-assembled monolayers, soft lithography, paper diagnostics, and self-assembly; and suggests issues in teaching.

A paper-based multiplexed transaminase test for low-cost, point-of-care liver function testing.

In developed nations, monitoring for drug-induced liver injury through serial measurements of serum transaminases [aspartate aminotransferase (AST) and alanine aminotransferase (ALT)] in at-risk individuals is the standard of care. Despite the need, monitoring for drug-related hepatotoxicity in resource-limited settings is often limited by expense and logistics, even for patients at highest risk. This article describes the development and clinical testing of a paper-based, multiplexed microfluidic assay designed for rapid, semiquantitative measurement of AST and ALT in a fingerstick specimen. Using 223 clinical specimens obtained by venipuncture and 10 fingerstick specimens from healthy volunteers, we have shown that our assay can, in 15 min, provide visual measurements of AST and ALT in whole blood or serum, which allow the user to place those values into one of three readout "bins" [<3× upper limit of normal (ULN), 3 to 5× ULN, and >5× ULN, corresponding to tuberculosis/HIV treatment guidelines] with >90% accuracy. These data suggest that the ultimate point-of-care fingerstick device will have high impact on patient care in low-resource settings.

Analog modeling of Worm-Like Chain molecules using macroscopic beads-on-a-string.

This paper describes an empirical model of polymer dynamics, based on the agitation of millimeter-sized polymeric beads. Although the interactions between the particles in the macroscopic model and those between the monomers of molecular-scale polymers are fundamentally different, both systems follow the Worm-Like Chain theory.

Paper-based analytical device for electrochemical flow-injection analysis of glucose in urine.

This article describes a new design for a paper-based electrochemical system for flow-injection analysis. Capillary wicking facilitates a gravity-driven flow of buffer solution continuously through paper and nitrocellulose, from a buffer reservoir at one end of the device to a sink at the other. A difference in height between the reservoir and the sink leads to a continuous and constant flow. The nitrocellulose lies horizontally on a working electrode, which consists of a thin platinum layer deposited on a solid support. The counter and reference electrodes are strategically positioned upstream in the buffer reservoir. A simple pipetting device was developed for reliable application of (sub)microliter volumes of sample without the need of commercial micropipets; this device did not damage the nitrocellulose membrane. Demonstration of the system for the determination of the concentration of glucose in urine resulted in a noninvasive, quantitative assay that could be used for diagnosis and monitoring of diabetes. This method does not require disposable test strips, with enzyme and electrodes, that are thrown away after each measurement. Because of its low cost, this system could be used in medical environments that are resource-limited.

Electrochemical sensing in paper-based microfluidic devices.

This paper describes the fabrication and the performance of microfluidic paper-based electrochemical sensing devices (we call the microfluidic paper-based electrochemical devices, microPEDs). The microPEDs comprise paper-based microfluidic channels patterned by photolithography or wax printing, and electrodes screen-printed from conducting inks (e.g., carbon or Ag/AgCl). We demonstrated that the microPEDs are capable of quantifying the concentrations of various analytes (e.g., heavy-metal ions and glucose) in aqueous solutions. This low-cost analytical device should be useful for applications in public health, environmental monitoring, and the developing world.

Diagnostics for the developing world: microfluidic paper-based analytical devices.

Microfluidic paper-based analytical devices (microPADs) are a new class of point-of-care diagnostic devices that are inexpensive, easy to use, and designed specifically for use in developing countries. (To listen to a podcast about this feature, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.).

Quantifying colorimetric assays in paper-based microfluidic devices by measuring the transmission of light through paper.

This article describes a point-of-care (POC) system--comprising a microfluidic, paper-based analytical device (micro-PAD) and a hand-held optical colorimeter--for quantifying the concentration of analytes in biological fluids. The micro-PAD runs colorimetric assays, and consists of paper that has been (i) patterned to expose isolated regions of hydrophilic zones and (ii) wet with an index-matching fluid (e.g., vegetable oil) that is applied using a disposable, plastic sleeve encasement. Measuring transmittance through paper represents a new method of quantitative detection that expands the potential functionality of micro-PADs. This prototype transmittance colorimeter is inexpensive, rugged, and fully self-contained, and thus potentially attractive for use in resource-limited environments and developing countries.

Egg beater as centrifuge: isolating human blood plasma from whole blood in resource-poor settings.

This paper demonstrates that a hand-powered egg beater can be modified to serve as a centrifuge for separating plasma from human whole blood. Immunoassays used to diagnose infectious diseases often require plasma from whole blood, and obtaining plasma typically requires electrically-powered centrifuges, which are not widely available in resource-limited settings. Human whole blood was loaded into polyethylene (PE) tubing, and the tubing was attached to the paddle of an egg beater. Spinning the paddle pelleted the blood cells to the distal end of the PE tubing; the plasma remained as the supernatant. A cholesterol assay (run on patterned paper) demonstrated the suitability of this plasma for use in diagnostic assays. The physics of the system was also analyzed as a guide for the selection of other rotating systems for use in centrifugation. Egg beaters, polyethylene tubing, and paper are readily available devices and supplies that can facilitate the use of point-of-care diagnostics at sites far from centralized laboratory facilities.

Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper.

This paper describes the use of a modified x,y-plotter to generate hydrophilic channels by printing a solution of hydrophobic polymer (pol(dimethylsiloxane; PDMS) dissolved in hexanes onto filter paper. The PDMS penetrates the depth of the paper and forms a hydrophobic wall that aqueous solutions cannot cross. The minimum size of printed features is approximately 1 mm; this resolution is adequate for the rapid prototyping of hand-held, visually read, diagnostic assays (and other microfluidic systems) based on paper. After curing the printed PDMS, the paper-based devices can be bent or folded to generate three-dimensional systems of channels. Capillary action pulls aqueous samples into the paper channels. Colorimetric assays for the presence of glucose and protein are demonstrated in the printed devices; spots of Bromothymol Blue distinguished samples with slightly basic pH (8.0) from samples with slightly acidic pH (6.5). The work also describes using printed devices that can be loaded using multipipets and printed flexible, foldable channels in paper over areas larger than 100 cm2.

The origins and the future of microfluidics.

The manipulation of fluids in channels with dimensions of tens of micrometres--microfluidics--has emerged as a distinct new field. Microfluidics has the potential to influence subject areas from chemical synthesis and biological analysis to optics and information technology. But the field is still at an early stage of development. Even as the basic science and technological demonstrations develop, other problems must be addressed: choosing and focusing on initial applications, and developing strategies to complete the cycle of development, including commercialization. The solutions to these problems will require imagination and ingenuity.

The 'right' size in nanobiotechnology.

The biological and physical sciences share a common interest in small structures (the definition of 'small' depends on the application, but can range from 1 nm to 1 mm). A vigorous trade across the borders of these areas of science is developing around new materials and tools (largely from the physical sciences) and new phenomena (largely from the biological sciences). The physical sciences offer tools for synthesis and fabrication of devices for measuring the characteristics of cells and sub-cellular components, and of materials useful in cell and molecular biology; biology offers a window into the most sophisticated collection of functional nanostructures that exists.