Paper as a novel material platform for devices

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oduction Device platforms can be radically transformed by introducing new materials at a fundamental level. Microﬂuidic devices, for example, were initially constructed from glass and silicon.1 With advances in fabrication techniques, devices constructed on thin polymer substrates emerged and expanded the landscape of potential applications by decreasing the cost and simplifying the materials and equipment required to fabricate microﬂuidic devices.1 Similarly, in the last decade, microelectronics has experienced a fundamental materials expansion from expensive rigid substrates to now low-cost ﬂexible materials.2–4 Paper is the latest material to transform the device landscape. Despite the fact that paper was developed nearly 2,000 years ago and has been broadly used in scientiﬁc applications, including chromatography, for over 100 years,5 new and important uses for this intriguing material are still emerging. From a materials perspective, paper offers a number of useful attributes: (1) It is low in cost and ubiquitously available in a variety of forms; (2) it has the ability to wick ﬂuids via capillary action; (3) it has a very high surface area due to its microﬁber composition; (4) it has the capacity for ﬁltration and separation of microscopic components, such as cells; (5) it is amenable to a wide range of printing technologies; (6) it is an ideal medium for dry chemical storage; (7) it demonstrates very strong adhesion to a variety of materials; (8) it can have high reﬂectivity and can be easily dyed; (9) it can be cut

using a number of low-cost tools and standard techniques; and (10) it can be stacked/folded to generate 3D structures. Cost is a particularly important consideration, as devices are being developed for new market sectors (e.g., diagnostics, packaging), including 80% of the world that lives on less than $10 per day. This population, approximately 5 billion people, comprises an underserved (and largely untapped) market with great potential.6 By re-engineering complex devices, particularly from a materials point of view, advances in health care, energy, communications, and electronics are poised to have a global impact. The ability to provide reliable medical information on the spot at very low cost may shift the paradigm away from centralized labs, which can delay results and subsequent treatment by weeks in developing nations. Advances in telecommunications—an industry that has successfully accessed developing world markets—integrated with novel low-cost diagnostic platforms, offer a completely new approach to practicing health care across the globe.7 Diagnostic devices based on paper in the form of lateral ﬂow tests (e.g., home pregnancy tests) have existed for decades.8 Lateral ﬂow has proven to be a useful platform for a large number of immunoassays. The ﬁeld of paper microﬂuidics stands to build substantially upon the form and function of lateral ﬂow devices. The ability to pattern paper allows for a degree of ﬂ uid control not previously demonstrated. Fluidic operations such as splitting,9 mix

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