Developments in the in vitro diagnostics (IVD) industry have been driven by global trends such as the prevalence of chronic diseases, an aging population, the increase in the occurrence of contagious diseases, and the influence of technology innovators. These trends, plus the desire for ease of use and a general acceptance of personalized care by consumers in developed and developing countries, have influenced IVD developments.
The market segment of point-of-care testing (POCT), in particular, has expanded significantly in response to these trends. POCT enables the rapid detection of disease in both the field and in settings with the patient nearby. Speed of detection aids in faster and more effective disease diagnosis, monitoring, and management.
Technological innovations have made a strong positive impact on this market segment. Biosensor technology is one of the key innovations for point-of-care testing because it has dramatically increased test accuracy and helped with the management of vast amounts of data. Other technological innovations developed for POCT have also improved its performance. These include nanotechnologies that have boosted optical, electrical, magnetic, and chemical properties that have further tuned test sensitivity and specificity.
Clinicians have relied upon lateral flow assays (LFAs) since the 1970s for diagnostic testing. The efficacy of dipstick assays goes back even earlier, to the 1940s. Such paper-based devices incorporate materials such as cellulose or nitrocellulose. These materials are inexpensive, disposable, widely available, and lend themselves to the mass production of POCT devices.
Additional advantages include their ease of safe handling, hydrophilic quality, and biocompatibility with most samples.
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In the last decade, the interest and use of microfluidic paper-based analytical devices (μPADs) have expanded considerably. μPADs have evolved from the same principle as LFAs: they employ layers of hydrophilic cellulose fibers to move a liquid sample from an inlet to an outlet where the chemical or biochemical reaction occurs.
The route in which the sample travels from the inlet to the outlet is known as the flow channel. It is at this location where the essential difference between LFAs and μPADs lies. LFAs rely on hydrostatic pressure or electro-osmotic flow to move the sample from inlet to outlet. However, μPADs create directed flow due to the patterning made on the substrate. While this design adjustment seems almost too simple, it has facilitated the ability to perform complicated sample preparation steps and opened up the potential for detecting multiple diseases on a single device. Moreover, as is the case with LFAs, paper-based microfluidic devices used for POCT are proving to work extremely well for certain disease diagnoses and clinical management in both developed and developing countries.
Recently, μPADs have been developed to detect Ebola virus RNA, Salmonella, and Hepatitis C antibodies, among others. Their portability, ease of use, and low cost have encouraged researchers to continue their development in detecting even more biomarkers and pathogens.
μPADs are well-suited for initial screenings in which a simple "yes or no" is needed for quick disease diagnosis, especially in resource-limited POCT settings. However, if quantitative analysis is required, paper-based microfluidic devices are limited by poor sensitivity and loss of reproducibility.
Generally speaking, the instability of reagents in the field is a problem that limits the commercialization of μPADs at this time. At the root of these devices' lack of precision is the difficulty of controlling the variability of dry reagents instilled during dissolution and rehydration in the paper matrix. Another issue is the inability to rely on two or more moving fluids uniformly mixing within the paper matrix.
An additional challenge for μPADs is balancing specificity to prevent false positives against keeping the device simple and easy to use. This is a lesser issue than the prevention of false negatives, but fine-tuning the devices for better efficiency is still needed.
A related challenge to specificity is the need to increase the multiplexing capability of μPADs. The architecture of these devices lends itself to testing for multiple vital markers, and doing so would be cost-effective. As with specificity, however, researchers are challenged by trying to develop effective devices that are user-friendly and straightforward.
Research groups are addressing the aforementioned challenges by incorporating novel biosensors into μPADs. For example, one group recently developed a μPAD capable of multiplexed detection of cancer cells by coating the paper device with nanocomposites that contained DNA aptamers.
The development of hollow-channel μPADs with integrated microwires has proven that higher sensitivity, lower detection limits, and faster analysis times through electrochemistry are possible. Another set of researchers has used self-assembled monolayers (SAMs) or organic molecular assemblies in conjunction with electrochemical impedance spectroscopy (EIS) to produce μPADs that have met the detection limits of West Nile Virus required for clinical diagnostics.
While the mass commercialization of paper-based microfluidics is still at least several years away, μPADs show great promise in delivering rapid, robust, accurate, inexpensive, and simple forms of point-of-care testing. Right now, some of these devices are excellent for sensitivity or specificity, and others have strong multiplexing capabilities. But researchers will need to continue their efforts to develop a μPAD that overcomes all of the current challenges, meets criteria for data interpretation and tracking, and continue to provide the benefits that make them so ideal for point-of-care testing.