This paper describes several low‐cost methods for fabricating flexible electronic circuits on paper. The circuits comprise i) metallic wires (e.g., tin or zinc) that are deposited on the substrate by evaporation, sputtering, or airbrushing, and ii) discrete surface‐mountable electronic components that are fastened with conductive adhesive directly to the wires. These electronic circuits—like conventional printed circuit boards—can be produced with electronic components that connect on both sides of the substrate. Unlike printed circuit boards made from fiberglass, ceramics, or polyimides, however, paper can be folded and creased (repeatedly), shaped to form three‐dimensional structures, trimmed using scissors, used to wick fluids (e.g., for microfluidic applications) and disposed of by incineration. Paper‐based electronic circuits are thin and lightweight; they should be useful for applications in consumer electronics and packaging, for disposable systems for uses in the military and homeland security, for applications in medical sensing or low‐cost portable diagnostics, for paper‐based microelectromechanical systems, and for applications involving textiles.

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.

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This paper describes a model of the motion of superparamagnetic beads in a microfluidic channel under the influence of a weak magnetic field produced by an electric current passing through a coplanar metal wire. The model based on the conventional expression for the magnetic force experienced by a superparamagnetic bead (suspended in a biologically relevant medium) and the parameters provided by the manufacturer failed to match the experimental data. To fit the data to the model, it was necessary to modify the conventional expression for the force to account for the non-zero initial magnetization of the beads, and to use the initial magnetization and the magnetic susceptibility of the beads as adjustable parameters. The best-fit value of susceptibility deviated significantly from the value provided by the manufacturer, but was in good agreement with the value computed using the magnetization curves measured independently for the beads from the same vial as those used in the experiment. The results of this study will be useful to researchers who need an accurate prediction of the behavior of superparamagnetic beads in aqueous suspensions under the influence of weak magnetic fields. The derivation of the force on a magnetic bead due to a magnetic field also identifies the correct treatment to use for this interaction, and resolves discrepancies present throughout the literature.

This Communication describes a method of fabricating complex metallic microstructures in 3D by injecting liquid solder into microfluidic channels, and allowing the solder to cool and solidify; after fabrication, the metallic structures can be flexed, bent, or twisted. This method of fabrication-which we call microsolidics-takes advantage of the techniques that were developed for fabricating microfluidic channels in poly(dimethylsiloxane) (PDMS) in 2D and 3D, uses surface chemistry to control the interfacial free energy of the metal-PDMS interface, and uses techniques based on microfluidics, but ultimately generates solid metal structures. This approach makes it possible to build flexible electronic circuits or connections between circuits, complex embedded or freestanding 3D metal microstructures, 3D electronic components, and hybrid electronic-microfluidic devices.There are several techniques for making metal microstructures in 3D. Electroplating and electroless deposition are routinely used to construct microstructures with metallic layers several nanometers to several microns thick in 2D or 3D. [1][2][3][4][5][6][7][8][9][10][11] To generate solid replicas of 3D objects, several groups have developed a technique, referred to as "microcasting", to form metals in order to fabricate microparts (e.g., posts and gears) with features as small as 10 lm and aspect ratios as high as 10 from steel, zirconia, and alumina. [12,13] Techniques based on LIGA (Lithographie, Galvanoformung, und Abformung) produce even more complicated metallic objects by depositing a metal onto a molded polymer template that is subsequently removed to yield an open structure (such as a honeycomb arrangement of cells). [14,15] In principle, these approaches can be used to pattern metals of any thickness to produce features with an aspect ratio that is larger than that produced using electroplating. Solder reflow is a standard technique in electronic packaging, [16,17] and has recently been combined with micromolding in channels to form custom-made solder pieces for batches of chips.[18] The technique has also been used to form 3D connections (e.g., bridging opposite sides of an electronic circuit board or substrate): Lauffer and co-workers and Ference and co-workers describe similar approaches to bridge electrical "islands" of metal by heating solid rods of solder "on chip"-the solder flows along trenches, holes, or metal strips (formed lithographically) and produces electrically conductive wires between the top and bottom surfaces of the substrate. [19,20] A growing interest in flexible sensors and displays has fueled the development of polymer-metal composites. Research in this field includes composites of metal in PDMS with optical functions, [21] conductive PDMS-carbon nanotube composites, [22,23] substrates for surface-enhanced Raman spectroscopy, [24,25] spherically curved metal oxide semiconductor field-effect transistors, [26] and flexible gold-polymer nanocomposites as passive components. [27,28] In addition to materials with electrica...