Fluidic Self-assembly Research Articles

Determination of the Charge Attached to Micro-scale Devices Used in Fluidic Self-Assembly Processes

ABSTRACTSelf-assembly of micron sized and smaller particles has previously been demonstrated using biologically inspired events such as DNA hybridization and interactions of ligands and receptors. In order to implement these techniques to create practical electronic devices, a quantitative measure of the amount of substance attached to the device surface just prior to the final assembly is essential. In the present investigation, this crucial quantity was investigated from the electrophoretic mobility of particles, which was ascertained by examining their motion under applied electric fields ranging from 0 to 1 V/mm. Sequential CCD camera images processed with custom software enabled calculation of particle velocities during their viscous motion in an inexpensive electrophoresis chamber filled with a low-conductivity buffer solution. A linear fit through the velocity vs. electric field data points yielded the electrophoretic mobility, which was utilized in the Stokes equation to calculate the net amount of charge present on each device. For 5.44 micron carboxyl-coated polystyrene beads, this method indicated a charge of 2.69e-15 C per particle. The manufacturer of the beads, Spherotech corporation, quoted 6.37e-11 C as the expected charge. The more than three orders of magnitude discrepancy is at least partially attributable to the electrophoretic retardation and relaxation effects of small electrolyte ions in the buffer solution. The method was also applied to silicon islands in the shape of a cone frustum with similar dimensions to the beads. A mercapto-ethane-sulfonate monolayer, attached via thiol bonds to the gold-coated surface of the islands, provided the charge. The amount of charge on an island was calculated to average 2.48e-15 C, corresponding to a density of 3.82e10 mercapto-ethane-sulfonate groups per square centimeter of Au surface.

New Technology for DNA Chips on a Microarray-Random Fluidic Self-assembly Method Using Hydrophobic Interaction

In this paper, we describe a new approach for an arrangement of many kinds of DNAs on transducers to construct a DNA microarray. A high-density array of sensor probes was prepared by randomly distributing a mixture of particles immobilized with various DNAs. A process for immobilization of the DNAs was separated from the assembly of the particles. The particles were arranged on the chip pattern by random fluidic self-assembly, using hydrophobic interaction.

Microstructure to substrate self-assembly using capillary forces

We have demonstrated the fluidic self-assembly of micromachined silicon parts onto silicon and quartz substrates in a preconfigured pattern with submicrometer positioning precision. Self-assembly is accomplished using photolithographically defined part and substrate binding sites that are complementary shapes of hydrophobic self-assembled monolayers. The patterned substrate is passed through a film of hydrophobic adhesive on water, causing the adhesive to selectively coat the binding sites. Next, the microscopic parts, fabricated from silicon-on-insulator wafers and ranging in size from 150/spl times/150/spl times/15 /spl mu/m/sup 3/ to 400/spl times/400/spl times/50 /spl mu/m/sup 3/, are directed toward the substrate surface under water using a pipette. Once the hydrophobic pattern on a part comes into contact with an adhesive-coated substrate binding site, shape matching occurs spontaneously due to interfacial free energy minimization. In water, capillary forces of the adhesive hold the parts in place with an alignment precision of less than 0.2 /spl mu/m. Permanent bonding of the parts onto quartz and silicon is accomplished by activating the adhesive with heat or ultraviolet light. The resulting rotational misalignment is within /spl sim/0.3/spl deg/. Using square sites, 98-part arrays have been assembled in less than 1 min with 100% yield. The general microassembly approach described here may be applied to parts ranging in size from the nano- to milliscale, and part and substrate materials including semiconductors, glass, plastics, and metals.

Lateral Force Microscopy Study of the Frictional Behavior of Self-Assembled Monolayers of Octadecyltrichlorosilane on Silicon/Silicon Dioxide Immersed in n-Alcohols

Lateral force microscopy (LFM) was used to evaluate the frictional behavior of surfaces modified with self-assembled monolayers (SAMs) and immersed in n-alcohols (CH3(CH2)xOH where x = 0−8,11) as a function of applied normal load, sliding velocity, and solvent chain length. SAMs were formed from octadecyltrichlorosilane (OTS) on silicon/silicon dioxide substrates. The objective was to investigate how the solvent environment affected the frictional behavior of OTS and to characterize the effectiveness of OTS as a boundary lubricant in liquid environments for applications such as fluidic self-assembly and microelectromechanical devices. Three characteristic frictional regimes were observed at low, intermediate, and high loads. Maxima as a function of the sliding velocity appeared in the frictional forces for intermediate applied normal loads of ∼20−40 nN for x = 1−8. These maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length of the solvent. The frictional maxima were interpreted by adapting concepts of viscoelasticity for bulk polymer systems to the two-dimensional systems of SAMs. Maxima were interpreted to result from localized relaxation processes in the SAMs that depend on the extent of solvent partitioning in the compressed region under the tip. The characteristic relaxation times of the alkyl chains increased with increased applied normal load and with increased solvent chain length. The behavior as a function of x was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion into defects. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation based on the adsorption and desorption of the chain tails from the surface of the atomic force microscopy tip. The total frictional forces was consistent with superposition of relaxation processes and viscous drag on the tip and plowing effects that become dominant at high applied normal loads.

Application of micromachine techniques to biotechnological research

Novel methods to construct a multianalyte biosensing chip are described. A method is two-step immobilization in which materials are not immobilized directly but indirectly via small support materials. Biomaterials are immobilized on a certain support particle in the first step. The particles are placed or stuck on a chip in the second step. The other method is random fluidic self-assembly of the microsupports for reducing the complication in the second step on the chip. The combination of the methods enables us to immobilize various kinds of biomaterials densely on a chip. The ways to avoid possible problems due to random distribution are discussed. Several examples are also described.

Immobilization of Biomaterial by Random Fluidic Self-Assembly Method

We propose a novel method for individual immobilization of biomaterial for biosensor application. Various kinds of biomaterial were first immobilized on certain supports such as glass beads or microfabricated metal particles of which size is almost the same as the size of transducer element. Then the suspension mixture of the various support was arranged on the transducer array by the fluidic self-assembly method at random. Glass beads immobilized with glucose oxidase and/or peroxidase were successfully arranged by this method using gravity, as a short-range force required in self-assembly. The beads gave chemiluminescence with the addition of luminol and its substrate. The metal particles consisted of nickel and/or gold layers formed by electroplating and evaporation on a photolithographically patterned chip. The coin-shaped particles were arranged on a nickel dot array by magnetic force interaction as the short-range force. The direction of binding of the particles were also controllable using the multilayer structure.

TRACTION SINGULARITIES ON SHARP CORNERS AND EDGES IN STOKES FLOWS

An integral identity developed by Brenner (1964) is used in the “reversed” context to derive a fixed point iterative scheme for the surface tractions on a rigid particle of arbitrary shape submerged in Stokes flows. The iterative approach facilitates the solution of very large systems of equations and thus the employment of high resolution discretization schemes. The utility of the approach is demonstrated by illustrative computations of the tractions on regular polyhedra, with special emphasis on the results near the edges and corners where (integrable) singular behavior is expected to provide a stringent test for the numerical method. Excellent agreement between our numerical estimates for the exponent of these singularities with the analytical result (local 2-D analysis) were obtained. The emphasis in this work is on validation of the approach, but references to applications, such as fluidic self-assembly of semiconductor microstructures, are provided.

Integration of fluidically self-assembled optoelectronic devices using a silicon-based process

Fluidic self-assembly (FSA) is a technique for efficient wafer-scale placement of large numbers of small or microscopic devices into receptor sites etched in a substrate. This paper reports the first complete planar process for integrating optoelectronic devices on a silicon wafer using fluidic self-assembly. Methods for bonding the devices into the receptor sites and subsequently contacting, planarizing, and interconnecting them are outlined. >

Fluidic self-assembly of InGaAs vertical cavitysurface emitting lasers onto silicon

Vertical cavity surface emitting lasers (VCSELs) have been successfully integrated by fluidic self-assembly (FSA) into 10 /spl mu/m deep holes etched into an Si substrate. These 40/spl times/40/spl times/10 /spl mu/m/sup 3/ devices were fully planarised and metallised after integration. The VCSELs operated pulsed at O.98 /spl mu/m with a threshold current as low as 75 mA and a turn-on voltage of 2.9 V.

Fluidic self-assembly for the integration of GaAs light-emitting diodes on Si substrates

The integration of GaAs optoelectronic devices on Si VLSI is important for many high-bandwidth communication applications. In this paper we describe a novel technique for the quasi-monolithic integration of GaAs light-emitting diodes on Si substrates that utilizes fluid transport and shape differentiation for placement and orientation. GaAs light-emitting diodes fabricated into trapezoidal blocks are suspended in a carrier fluid and deposited over holes etched in Si for integration. Top-side ring contact and bottom electrical contact are fabricated on the blocks prior to integration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>