Mixing chemical or biological samples with reagents for chemical analysis is one of the most time consuming operations on microfluidic platforms. This is primarily due to the low rate of diffusive transport in liquid systems. Additionally, much research has focused on detection, rather than sample preparation. In response, we describe a mixer for microfluidic sample preparation based on the electrokinetic phenomenon of induced-charge-electroosmosis (ICEO). ICEO creates microvortices within a fluidic channel by application of alternating current (AC) electric fields. The microvortices are driven by electrostatic forces acting on the ionic charge induced by the field near polarizable materials. By enabling mixing to be turned on or off within a channel of fixed volume, these electronically controlled mixers prevent sample dilution-a common problem with other strategies. A three-dimensional model based on the finite volume method was developed to calculate the electric field, fluid flow, and mass transport in a multi-species liquid. After preliminary experiments, the model was used to rapidly prototype a wide range of designs. A new microfabrication process was developed for devices with vertical sidewalls having conductive metal coatings and embedded electrodes. Mixing experiments were carried out in the devices and the results were compared to the model.

Patterned amine-functionalized self-assembled monolayers have potential as a template for the deposition and patterning of a wide variety of materials on silicon surfaces, including biomolecules. Results are presented here for low-energy electron-beam patterning of 2-aminopropyltriethoxysilane and (aminoethylaminomethyl)phenethyltrimethoxysilane self-assembled monolayers on silicon substrates. On these ultrathin (1–2 nm) monolayers, lower electron beam energies (<5 keV) produce higher resolution patterns than high-energy beams. Auger electron spectroscopy indicates that low-energy electron exposure primarily damages the amine groups. At 1 keV, a dose of 40 μC/cm2 is required to make the patterns observable by lateral force microscopy. Features as small as 80 nm were exposed at 2 keV on these monolayers. After exposure, palladium colloids and aldehyde- and protein-coated polystyrene fluorescent spheres adhered only to unexposed areas of the monolayers.

Adhesive templates for biomolecule patterning were fabricated on silicon and gold by low-voltage (1 kV) electron beam lithography of an inert self-assembled monolayer, followed by backfilling the exposed regions with an amine-terminated monolayer. Amine-terminated monolayers selectively attached either the desired materials or linker molecules that subsequently bound other materials including antibodies. Lines (300 nm wide) of 20-nm polystyrene beads were formed on gold by exposing a mercaptohexadecanoic acid (MHDA) monolayer, then backfilling with cysteamine, and selectively attaching aldehyde-coated beads to the amines. Attachment density was found to vary sharply around a critical dose, making the technique useful for patterns such as gradients which require varying density. An optimal dose of 200 µC/cm 2 was found for attaching fluorescent polystyrene spheres to MHDA-cysteamine templates. A cycling process was developed for aligning patterns of two or more kinds of polystyrene spheres. Biotin was tethered to the amine templates, making the technique applicable to high-resolution patterning of biomaterials with the widely used avidin-biotin binding system.

We have fabricated undoped GaAs/AlxGa1−xAs heterojunctions in which an electric field produced by a top gate confines carriers to the interface, and where contact is made to carriers at the interface using a novel self-aligned contacting process. Densities for both electrons and holes ranging from n2D < 1010/cm2 to n2D ≳ 5 × 1011/cm2 are obtainable with mobilities comparable to those measured in high quality modulation-doped heterojunctions.

Articles you may be interested inPolythiophene-based charge dissipation layer for electron beam lithography of zinc oxide and gallium nitride J. Vac. Sci. Technol. B 28, 817 (2010); 10.1116/1.3460903 High density patterned quantum dot arrays fabricated by electron beam lithography and wet chemical etching Appl. Phys. Lett. 93, 111117 (2008); 10.1063/1.2981207Sub-50 nm stencil mask for low-energy electron-beam projection lithography Electron projection lithography mask format layer stress measurement and simulation of pattern transfer distortion J.

Soft, stretchable, sewable, and strain‐sensitive optical fibers are reported. Intrinsically stretchable optical waveguides capable of >100% strain are formed by coating a commercially available elastic urethane fiber (“Stretch Magic”) with a silicone cladding having a lower refractive index. These 1–2 mm diameter fibers can be attached to textiles using a sewing machine. Because the attaching threads create microbends in the fiber, the sewn fibers are prestrained for monotonic intensity‐versus‐strain behavior. Installed in a stretchable piece of athletic tape, they detect strains originating from changing muscle shapes during weight‐bearing activity.

Electrical impedance-based particle detection or Coulter counting, offers a lab-on-chip compatible method for flow cytometry. Developments in this area will produce devices with greater portability, lower cost, and lower power requirements than fluorescence-based flow cytometry. Because conventional Coulter apertures are prone to clogging, hydrodynamic focusing improves the device by creating fluid-walled channels with variable width to increase sensitivity without the associated risk of blocking the channel. We describe a device that focuses the sample in three dimensions, creating a narrow sample stream on the floor of the channel for close interaction with sensing electrodes. The key to this design is a stepped outlet channel fabricated in a single layer with soft lithography. In contrast to previous impedance-based designs, the new design requires minimal alignment with the substrate. Three-dimensional focusing maximizes the sensitivity of the device to cell-size particles within much larger channels. Impedance-based particle sensing experiments within this device show an increase in percentage conductivity change by a factor of 2.5 over devices that only focus the sample in the horizontal direction.

Fabrication and applications are discussed for a visible-wavelength micropolarizer array consisting of a linear polarizer and a micropatterned liquid-crystal (LC) cell. LC alignment direction is controlled by means of depositing an optically transparent gold film at an oblique angle and coating the surface with an alkanethiol self-assembled monolayer. Microdomains of two perpendicular LC alignment directions are created by photolithography and etching of the gold layer, rotating the substrate 90 deg, and depositing a second oblique gold layer in the etched areas. The resulting array is used for polarization-difference imaging (PDI), a technique that enhances image contrast in the presence of scattering. Images obtained with the array require more processing than do conventional PDI images, but this method eliminates the need for an electronically activated LC filter and is especially suited to systems whose filters are closely integrated with optical sensor arrays.