This paper describes the use of a modified x,y-plotter to generate hydrophilic channels by printing a solution of hydrophobic polymer (poly(dimethyl siloxane; PDMS) dissolved in hexanes onto filter paper. The PDMS penetrates the depth of the paper, and forms a hydrophobic wall that aqueous solutions cannot cross. The minimum size of printed features is approximately 1 mm; this resolution is adequate for the rapid prototyping of hand-held, visually read, diagnostic assays (and other microfluidic systems) based on paper. After curing the printed PDMS, the paper-based devices can be bent or folded to generate three-dimensional (3D) systems of channels. Capillary action pulls aqueous samples into the paper channels. Colorimetric assays for the presence of glucose and protein are demonstrated in the printed devices; spots of Bromothymol Blue distinguished samples with slightly basic pH (6.5) from samples with slightly acidic pH (8.0). The work also describes using printed devices that can be loaded using multipipettes, and printed flexible, foldable channels in paper over areas larger than 100 cm 2 . This paper describes the use of a modified desktop plotter to fabricate simple patterns of hydrophilic microchannels in paper. We defined the boundaries of the microchannels by printing a solution of poly(dimethyl siloxane) (PDMS) in hexanes onto filter paper, using an x,y-plotter as a print-engine. Because PDMS is an elastomer, the paper could be bent and folded, without destroying the integrity of the channels. The channels have a minimum width of 1 mm, and the minimum spacing between two channels can be as small as 1 mm. These dimensions are large relative to those normally encountered in microfluidic systems, but they are the right size for the basic analytical and diagnostic devices that are our primary objectives, since readout of these devices will often involve visual observations of unmagnified spots of analytes. 1 We believe that this low-cost system for printing hydrophobic polymers and other materials on paper will be useful for prototyping simple paper-based diagnostics assays.A variety of simple diagnostic tests (for example, "dip-stick" tests) rely on paper-based assays. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Some of these tests analyze environmental conditions, 16,17 or detect illness (often in the developing world). [18][19][20] We have recently patterned paper into hydrophilic regions demarcated by hydrophobic walls using photolithography and conventional photoresists (SU-8; and poly(methyl methacrylate), or PMMA). 1 The hydrophilic regions act as microfluidic channels, in which capillary action wicks aqueous samples into the device. This design provides the basis for diagnostic systems in paper that are more complex than the simplest dip-stick assays (in the sense that multiple assays can be performed on a small array with ~5-20 μL of blood, urine, or other fluid), but simpler and more affordable than highertechnology microfluidic assays. 20 An advantage of using PDMS to pattern the p...

This report shows that the direction of polarization of attached mammalian cells determines the direction in which they move. Surfaces micropatterned with appropriately functionalized selfassembled monolayers constrain individual cells to asymmetric geometries (for example, a teardrop); these geometries polarize the morphology of the cell. After electrochemical desorption of the self-assembled monolayers removes these constraints and allows the cells to move across the surface, they move toward their blunt ends.motility ͉ polarity ͉ self-assembled monolayers T his report demonstrates that imposed polarity of an adherent mammalian cell, that is, its morphology as characterized by a wide front (typically the blunt end) and a narrow rear (typically the sharp end), determines its direction of motility (1, 2). We patterned self-assembled monolayers (SAMs) on gold to confine single cells initially to polarized shapes (3, 4). A brief pulse of voltage applied to the gold released the cells from their constraints and allowed them to move freely across the surface (4). The initial direction of motility of cells correlated with the polarity of their original shape; we conclude that polarization of the shape of cells is sufficient to determine their directions of motion.The migration of mammalian cells typically includes the following processes: (i) morphological polarization (characterized by a wide front and a narrow rear); (ii) extension of membranes toward the direction of motility; (iii) formation of attachments between these leading membranes and the substrate; (iv) movement of the bulk of the cell body; and (v) release of attachments from the substrate at the sharp end (2). These processes together result in net translocation of the cell body (Fig. 1A). Many types of motile mammalian cells adopt a ''teardrop'' shape with a wide leading edge (dominated by structures termed the lamellipodia) that extends in the front and a narrow tail that releases and retracts. Most types of cells can polarize and move without stimuli (2, 5). Under the influence of a stimulus (chemical or mechanical), cells can polarize and move directionally (toward or away from the stimulus). It is not clear, however, whether morphological polarity of the cell itself can determine the direction of motility. We addressed this uncertainty by defining the polarity of adherent cells using an asymmetrically patterned substrate without a gradient of stimulant. We then released the constraint on the shape and location of the cells and assessed the direction of motility for individual cells. This approach is, to our knowledge, the first test of the hypothesis that the shape of a cell determines the direction of its motion. Other parameters that characterize the motion of cells, such as their speed and their tendency to make turns, are not affected by the initial constraints.Recently, Parker et al. (6) showed that cells, when confined to a square shape, in the absence of gradients of stimulant, preferentially extended their lamellipodia from the corners (Fi...

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...

This paper describes the use of micropatterned agarose stamps prepared by molding against PDMS masters to print patterns of bacteria on agar plates. Topographically patterned agarose stamps were inked with suspensions of bacteria; these stamps generated patterns of bacteria with features as small as 200 microm over areas as large as 50 cm2. Stamps with many small features (>200 microm) were used to study patterns of bacteria growing on media containing gradients of small molecules; stamps with larger features (>750 microm) were used to print different strains of bacteria simultaneously. The stamp transfers only a small percentage of cells that are on its surface to the agar at a time; it is thus possible to replica-pattern hundreds of times with a single inking. The use of soft stamps provides other useful functions. Stamps are easily customized to provide a range of patterns. When culture media is included in the agarose stamp, cells divide and thrive on the surface. The resulting "living stamp" regenerates its "ink" and can be used to pattern surfaces repetitively for a month. This method is rapid, reproducible, convenient, and can be used to control the pattern, spacing, and orientation between colonies of different bacteria.

Metal in microfluidic channels: The fabrication of electromagnets (solder) with micron‐scale dimensions in poly(dimethylsiloxane) in close proximity (ca. 10‐μm separation) to microfluidic channels is described (see pictures, top view and cross section). The method only has four steps and can be completed in 10 min. By passing an electric current through the magnets, magnetic field gradients are generated inside the microfluidic channels, which can be used to capture and release superparamagnetic beads in the microfluidic channel.