Paper-based electroanalytical devices for accessible diagnostic testing
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need for accessible diagnostic devices Diagnosis is a critical part of the management of disease. Although diagnostic tests by themselves contribute a relatively small amount to the total costs of health care, the results of diagnoses determine the subsequent treatment and its costs. In the developed world, diagnostic testing is commonly carried out using dedicated instruments in well-equipped central laboratories. The procedure-based reimbursement system common to the United States and other developed countries encourages the development of sophisticated, and expensive, tests. This type of laboratory-based technology is inappropriate, however, for many applications, especially in resource-limited settings. Affordability, portability, ease-of-use, and the ability to operate with little or no supporting equipment are particularly important for tests used in the developing world, in public health, at the point of care, in drug testing and monitoring, and in agriculture (veterinary, food, environmental testing). As described in the introductory article of this issue, the potential of paper as a substrate for colorimetric diagnostic testing has already been demonstrated,1–4 but there remains a substantial opportunity for paper-based devices that can replicate other commonly used modes of detection—particularly electrochemical detection—while retaining low cost and simplicity. The realization of this opportunity depends on our
ability to build electrochemical and electronic capabilities on paper substrates.
Paper-based approaches to bioanalysis As a matrix for chemical and biochemical analysis relevant to diagnosis, paper offers five properties that contribute to meeting these requirements: (1) the capillary wicking of aqueous solutions along the cellulose fibers of the paper matrix provides a means of transporting fluids that does not require external energy; (2) the porosity and biocompatibility of paper allow reagents to be stored or immobilized in the paper device; (3) the light weight and flexibility of paper simplify the logistics of transport and storage; (4) the patterning and processing of paper can use established methods to form microfluidic channels in paper; and (5) the low cost and combustibility of paper make it suitable for disposable and safe single-use tests. Paper-based systems are, of course, already extensively used. Test strips for urinalysis use an array of paper test pads, supported on a plastic strip, to perform up to 11 colorimetric (color changing) assays in urine (glucose, protein, ketones, pH, nitrite, bilirubin, urobilinogen, leukocyte esterase, specific gravity, ascorbic acid, and blood).5 Immunochromatographic, “lateral-flow” assays move a sample (and analyte) between different zones on a linear nitrocellulose strip by passive wicking;
E. Jane Maxwell, Department of Chemistry and Chemical Biology, Harvard University; [email protected] Aaron D. Mazzeo, Department of Mechanical and Aerospace Engineering, Rutgers University; [email protected] George M. Whitesides, Wyss Institu
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