Linda A. Domeier

Low-distortion, high-strength bonding of thermoplastic microfluidic devices employing case-II diffusion-mediated permeant activation

We demonstrate a new method for joining thermoplastic surfaces to produce microfluidic devices. The method takes advantage of the sharply defined permeation boundary of case-II diffusion to generate dimensionally controlled, activated bonding layers at the surfaces being joined. The technique is capable of producing bonds that exhibit cohesive failure, while preserving the fidelity of fine features in the bonding interface. This approach is uniquely suited to production of layered microfluidic structures, as it allows the bond-forming interface between plastic parts to be precisely manipulated at micrometre length scales. Distortions in microfluidic device channels are limited to the size scale of the permeant-swollen layer; 6 microm deep channels are routinely produced with no detectable cross-sectional distortions. Conventional thermal diffusion bonding of identical parts yields less strongly bonded microfluidic structures with increasingly severe dimensional compressions as bonding temperatures approach the thermoplastic glass-transition temperature: a preliminary rheological analysis is consistent with the observed compressions. The bond-enhancing procedure is easily integrated in standard process flows, uses inexpensive reagents, and requires no specialized equipment.

LIGA: metals, plastics, and ceramics

LIGA, an acronym from the German words for Lithography, Electroforming, and Molding, is being evaluated worldwide as a method to produce microparts from engineering materials. Much of the work to date in LIGA has focused on producing metal microparts, with nickel as the most common material of choice. There is a growing interest in producing plastic parts replicated from LIGA metal masters due largely to microanalytical instrumentation and medical applications. These plastic replicates are generally made by either hot embossing or injection molding. Ceramic replication, of particular interest for high temperature applications or to produce piezoelectric or magnetic microparts, is also emerging as an area of interest. In this paper, a model of the LIGA exposure and development processes is presented along with the result of numerical optimization of mask design and process cost. The baseline processes for a cost- effective method to produce metal microparts are discussed, along with replication methods and result for plastics and ceramics.

N-Cyanoimides via the Cyanation of Imides

Abstract The conversion of cyclic imides to the corresponding N-cyanoimides has been carried out using cyanogen bromide as the nitrile source. This methodology provides a convenient route for the preparation of both aromatic and aliphatic N-cyanoimides.

Microstructure and mechanical properties of nickel microparts electroformed in replicated LIGA molds

A novel process for the rapid replication of electroforming plastic micromolds has been developed and is now being used to produce plated nickel test specimens. The process combines hot embossing or injection molding with metallic microscreens to produce sacrificial electroforming molds with conducting bases and insulating sidewalls. The replicated micromolds differ from standard LIGA molds in that the holes in the microscreen act as insulating defects in the electroforming base. The effects of such defects on the materials properties of electroformed microparts will be discussed and it will be shown that when the surface irregularities corresponding to the microscreen holes are removed, mechanical properties are experimentally indistinguishable from those found in conventionally processed LIGA specimens.

Micromolding and sintering of nanoparticle preforms into microparts

A new replication technology that produces, high aspect ratio ceramic or metal microparts by micromolding and sintering nanoparticle preforms is presented. In this LIGA replication technique, an epoxy based nanoparticle slurry is cast into sacrificial plastic micromolds produced by injection molding. The epoxy is allowed to cure and, if desired, excess epoxy is polished off to produce individual micropart preforms. The micromold is then dissolved in methylene chloride and the micropart preforms are sintered in either air (oxide ceramics) or 4% hydrogen in argon (nickel). This presentation will discuss the effects of the epoxy formulation, the microcasting procedure, and the sintering schedule on the materials properties of the final sintered microparts. It will be shown that this replication technique produces ceramic or metal microparts with micron size features and mechanical properties comparable to those of macroscopic materials.

Injection molded microfluidic devices for biological sample separation and detection

We are developing a variety of microsystems for the separation and detection of biological samples. At the heart of these systems, inexpensive polymer microfluidic chips carry out sample preparation and analysis. Fabrication of polymer microfluidic chips involves the creation of a master in etched silicon or glass; plating of the master to produce a nickel stamp; large lot chip replication by injection molding; precision chip sealing; and chemical modification of channel surfaces. Separation chips rely on insulator-based dielectrophoresis for the separation of biological particles. Detection chips carry out capillary electrophoresis to detect fluorescent tags that identify specific biological samples. Since the performance and reliability of these microfluidic chips are very sensitive to fluidic impedance, electromagnetic flux, and zeta potential, the microchannel dimensions, shape, and surface chemistry have to be tightly controlled during chip fabrication and use. This paper will present an overview of chip design, fabrication, and testing. Dimensional metrology data, surface chemistry characterization, and chip performance data will be discussed in detail.

Fabrication and characterization of polymer microfluidic devices for bio-agent detection

Sandia and Lawrence Livermore National Laboratories are developing a briefcase-sized, broad-spectrum bioagent detection system. This autonomous instrument, the BioBriefcase, will monitor the environment and warn against bacterium, virus, and toxin based biological attacks. At the heart of this device, inexpensive polymer microfluidic chips will carry out sample preparation and analysis. Fabrication of polymer microfluidic chips involves the creation of a master in etched glass; plating of the master to produce a nickel stamp; large lot chip replication by injection molding; and thermal chip sealing. Since the performance and reliability of microfluidic chips are very sensitive to fluidic impedance and to electromagnetic fluxes, the microchannel dimensions and shape have to be tightly controlled during chip fabrication. In this talk, we will present an overview of chip design and fabrication. Metrology data collected at different fabrication steps and the dimensional deviations of the polymer chip from the original design will be discussed.

Analysis of a process for replication of electroforming molds with integral microscreens

A process for the rapid replication of electroforming plastic micromolds has been developed at Sandia National Laboratories, Livermore, CA. The process is based on injection molding of plastic replicates with integral metallic screens to produce sacrificial electroforming molds in which the metallic screen acts as the conducting base and the plastic features provide insulating sidewalls. The process consists of injecting molten PMMA via a center-gate into a disk-shaped mold cavity in which a sandwich of a flow channel plate, porous Nickel foam, and metallic microscreen are placed on top of the LIGA-fabricated tooling. A numerical model for the coupled heat transfer and fluid flow phenomena is used to investigate the effects of various process parameters on the mold-filling behavior. The results from the parametric studies are presented and discussed.

Sacrificial layer for the fabrication of electroformed cantilevered LIGA microparts

The use of silver filled PMMA as a sacrificial layer for the fabrication of multilevel LIGA microparts is presented. In this technique, a bottom level of standard electroformed LIGA parts is first produced on a metallized substrate such as a silicon wafer. A methyl methacrylate formulation mixed with silver particles is then cast and polymerized around the bottom level of metal parts to produce a conducting sacrificial layer. A second level of PMMA x-ray resist is adhered to the bottom level of metal parts and conducting PMMA and patterned to form another level of electroformed features. This presentation will discuss some the requirements for the successful fabrication of multilevel, cantilevered LIGA microparts. It will be shown that by using a silver filled PMMA, a sacrificial layer can be quickly applied around LIGA components; cantilevered microparts can be electroformed; and the final parts can be quickly released by dissolving the sacrificial layer in acetone.

Epoxy Foam Encapsulants: Processing and Dielectric Characterization

The dielectric performance of epoxy foams was investigated to determine if such materials might provide advantages over more standard polyurethane foams in the encapsulation of electronic assemblies. Comparisons of the dielectric characteristics of epoxy and urethane encapsulant foams found no significant differences between the two resin types and no significant difference between as-molded and machined foams. This study specifically evaluated the formulation and processing of epoxy foams using simple methylhydrosiloxanes as the flowing agent and compared the dielectric performance of those to urethane foams of similar density.