Paper, broadly defined as thin, porous sheets, is currently being used to create novel devices for diagnostics, microfluidics, and electronics that ideally combine low cost and high performance. A “device,” in this context, can be defined as an object that serves to provide information or function to a user in response to input. This issue will highlight some of these novel devices and provide examples of potential applications. We begin with an overview of paper’s unique properties and how these properties lead to a potential for changing the integrated microfluidic and flexible electronics landscape. We then discuss methods for patterning paper as well as specific fluidic operations that are possible on paper. Finally, we conclude with an overview of electronic devices on paper and a brief outlook on the future of this emerging field.

Microfluidic paper-based analytical devices (μPADs) use the passive capillary-driven flow of aqueous solutions through patterned paper channels to transport a sample fluid into distinct detection zones that contain the reagents for a chemical assay. These devices are simple, affordable, portable, and disposable; they are, thus, well suited for diagnostic applications in resource-limited environments. Adding screen-printed electrodes to the detection zones of a μPAD yields a device capable of performing electrochemical assays (an EμPAD). Electrochemical detection has the advantage over colorimetric detection that it is not affected by interference from the color of the sample and can be quantified with simple electronics. The accessibility of EμPADs, however, is limited by the requirement for an external potentiostat to power and interpret the electrochemical measurement. New developments in paper-based electronics may help loosen this requirement. This review discusses the current capabilities and limitations of EμPADs and paper-based electronics, and sketches the ways in which these technologies can be combined to provide new devices for diagnostic testing.

The traditional paradigm for obtaining a quantitative measurement in point-of-care (POC) assays may not be adequate for extremely resource-limited environments, such as remote villages in the developing world. In standard quantitative POC assays, sample volume and assay time must be controlled. Furthermore, thermally stable assay reagents, a power supply, and an electronic reader must be available. Arranging all of these variables in a single assay results in systems that are too complicated, expensive, and user-intensive for extremely resource-limited environments. This overview describes new approaches in various areas of materials science that are beginning to redefine how quantitative POC assays are achieved, with a focus on approaches that use paper as the platform for the assays. Such approaches should have an immediate impact in the developing world, but also may transform quantitative POC assays in a variety of other settings, where quantitative information about the health of people, plants, animals, and the environment would help individuals better assess and manage their lives.

Cellulose is one of the most abundant organic materials on earth, and cellulose paper is ubiquitous in our daily life. Re-engineering cellulose fibers at the nanoscale will allow this renewable material to be applied to advanced energy storage systems and optoelectronic devices. In this article, we examine the recent development of nanofibrillated cellulose and discuss how the integration of other nanomaterials leads to a wide range of applications. The unique properties of nanofibrillated cellulose enable multi-scale structuring of the functional composites, which can be tailored to develop new concepts of energy and electronic devices. Tapping into the nanostructured materials offered by nature can offer many opportunities that will take nanotechnology research to a new level.

The need to improve health outcomes in the developing world and to moderate healthcare costs in developed countries has resulted in an increased interest in sophisticated, inexpensive, and instrument-free point-of-care diagnostics using porous materials. One major segment of the paper-based diagnostics effort is focused on developing high-performance point-of-care tests using porous nitrocellulose membranes. This review provides a perspective on the nature, history, and future of nitrocellulose-based assays. Beginning as a protein blotting substrate, porous nitrocellulose membranes have grown to be the most commonly used lateral flow substrate and are the primary membranes used in two-dimensional paper networks for user-friendly multistep assays. In addition to the historical context, we examine assay development considerations, such as the physics of flow in porous media, reagent deposition and storage, and detection methods.

Bioactive paper, or paper that exhibits biorecognition properties, is a promising tool for on-site environmental monitoring. A critical parameter for production of paper-based sensors is the immobilization of biomolecules onto paper with retention of long-term activity. High loading, control over biomolecule orientation and accessibility, and protection of biomolecules from external denaturation need to be provided by the immobilization technique. We present here a summary of the current strategies for enzyme, antibody, bacteriophage, and aptamer immobilization on cellulose and provide examples of the application of bioactive paper in environmental monitoring.

The ability to control the shape of metal nanocrystals is central to advances in many areas of modern science and technology, including catalysis, plasmonics, electronics, and biomedicine. This article provides a brief overview of our recent efforts toward the development of solution-phase methods for shape-controlled synthesis of metal nanocrystals. While the synthetic methods only involve simple redox reactions, we have been working diligently to understand the complex nucleation and growth mechanisms leading to the formation of metal nanocrystals with desired shapes and related properties. We hope this review will inspire new ideas and concepts in the general area of nanomaterial synthesis, expand our ability to engineer the properties of metals for various applications, and contribute to the realization of sustainable use for some of the scarcest materials.