A novel method of modeling multiple frequency dielectrophoresis (MFDEP) is introduced based on the concept of an effective Clausius-Mossotti factor, CM(eff), for a particle that is exposed to electrical fields of different frequencies, coming either from one or multiple pairs of electrodes. This analysis clearly illustrates how adding frequencies adds control parameters, up to two additional parameters per frequency. As a result, MFDEP can be used for a wide variety of applications, including separating particles with very similar Clausius-Mossotti spectra, trapping multiple groups of cells simultaneously, and cancelling unwanted dielectrophoretic traps. Illustrating the modeling approach, we determine the CM(eff)s for live and dead yeast cells, and then predict their equilibrium distribution on a three-electrode configuration, with two electrodes at different frequencies and the third electrode at ground. This prediction is validated experimentally, using MFDEP to selectively attract live cells to one location and dead cells to another, trapping both. These results demonstrate that the use of multiple frequencies for the manipulation of particles can enhance the performance of dielectrophoretic devices, not only for sorting, but also for such applications as patterning cells in close proximity for the formation of cell consortia.

Electrically conductive materials capable of bending and stretching are useful for a wide range of applications such as smart clothing, [1] flexible displays, [2] stretchable circuits, [3] strain gauges, [4] implants , [5] high-deformation microelectromechanical systems (MEMS), [4] and dielectric elastomer actuators (DEAs), [6] to name a few. The necessary strain varies from minimal (< 5 %) for applications such as retinal prostheses that only require conformal contact to a surface [5] to high (> 60 %) for large-strain DEAs.[7]A number of approaches have previously been used to produce stretchable electrodes. [8] Printing with carbon grease [6,9,10] or graphite powder [11,12] has been the preferred method for use with DEAs because of the low modulus of these materials, which allows large strains, but they have a relatively low conductivity and can readily be rubbed off the surface. Thin metal films deposited on elastomeric substrates, [13] such as by thermal evaporation or sputtering, can be stretched 10-20 % before failure, [14] or higher if the metal is patterned. [15] In special cases when it is possible to stretch the elastomer during deposition, so that the metal forms corrugations upon polymer relaxation, the achievable strains can be increased even further. [14,[16][17][18][19] Loading an elastomeric host polymer with a conducting material allows some degree of elasticity, but the modulus of the composite increases dramatically with loading, often before the electrical percolation threshold is reached. [20,21] Electrostatic layer-by-layer assembly has been successful in achieving highly conducting, robust, free-standing sheets capable of large strain. [22] Integrating these electrodes with other devices, however, which would include patterning the electrodes, has not yet been demonstrated. Another promising method for producing compliant electrodes is metal ion implantation, [23] which has been used in the fabrication of miniature DEAs. The advantages include patternability and an expectation of a low modulus, while the drawbacks are a rather high sheet resistance (∼ 100 kX/ square), a limited substrate size, a requirement for a low-energy ion implanter, and post-implantation bonding of the elastomer to the Si wafer. The goal of the present work was therefore to develop a new approach to achieving robust and truly elastomeric conductors that can be patterned with standard microfabrication techniques. This communication presents a method for making compliant electrodes that results in materials with a sheet resistance (R s is related to electrical conductivity r through r = 1/R s t, where t is the film thickness, so this corresponds to a conductivity of 50 S cm -1 for our 0.01 cm thick films. R s of only 2 X/& when unstretched that do not lose conductivity until they are elongated by up to 150 %. The electrodes have a Young's modulus of under 10 MPa, and they survive thousands of strain cycles. The electrodes are also photopatternable ( Fig. 1), making them compatible with microfabrication of electronic dev...

The ability to use magnets external to the body to focus therapy to deep tissue targets has remained an elusive goal in magnetic drug targeting. Researchers have hitherto been able to manipulate magnetic nanotherapeutics in vivo with nearby magnets but have remained unable to focus these therapies to targets deep within the body using magnets external to the body. One of the factors that has made focusing of therapy to central targets between magnets challenging is Samuel Earnshaw’s theorem as applied to Maxwell’s equations. These mathematical formulations imply that external static magnets cannot create a stable potential energy well between them. We posited that fast magnetic pulses could act on ferromagnetic rods before they could realign with the magnetic field. Mathematically, this is equivalent to reversing the sign of the potential energy term in Earnshaw’s theorem, thus enabling a quasi-static stable trap between magnets. With in vitro experiments, we demonstrated that quick, shaped magnetic pulses can be successfully used to create inward pointing magnetic forces that, on average, enable external magnets to concentrate ferromagnetic rods to a central location.

Loctite photopatternable adhesives 3108, 3340, and 3525 are introduced for microelectromechanical systems (MEMS) applications. These materials are patterned within minutes by exposure to ultraviolet (UV) light followed by rinsing with a solvent; no further processing is required. Because the uncured fluid is relatively insensitive to room light, this can be done on any lab bench without the requirement for a clean room. The materials can be spin-coated to obtain films, or cast between spacers for layers up to 1 cm thick, and the cured polymers range from elastomeric to rigid. These adhesives are of interest for rapid, inexpensive fabrication of relatively low-resolution features (tens to hundreds of micrometers) by photocuring. They can alternatively be molded, like polydimethylsiloxane (PDMS), to achieve high resolution, as well as irreversibly bonded after an O 2 plasma treatment. In addition, like SU8, they can be used as molds for patterning PDMS. Initial characterization of resolution, swelling, and biocompatibility were performed. One of the polymers, 3340, can be used for packaging bioMEMS-on-complementary-metal-oxide-semiconductor (CMOS) chips, exploiting its biocompatibility and its photopatternability at thicknesses of 1500 m to cover the bond wires while exposing the chip surface. As further demonstrations of the versatility of these materials, multilevel, interconnected channel structures were fabricated with a gelatin sacrificial layer, and magnetic films were prepared, since the polymers remain patternable even with additives.[1535]