A broadband tunable filter for the infrared spectral region is desired for use as a wavelength selective element in a miniature absorption spectrometer. We present the design, fabrication, packaging, and characterization of a bulk micromachined Fabry-Perot interferometer (FPI) for meeting this need. A novel approach to fabricate a MEMS-based tunable resonant cavity using two separate wafers bonded using a "lock-and-key" spacer design is outlined, with the goal of realizing electrostatically actuated membranes from films predeposited on base substrates. This ability could enable the pursuit of MEMS devices without in-house chemical vapor deposition (CVD) capability, after overcoming the shortcomings of bulk micromachining. The FPI device was designed with a planar structure comprising two face-to-face bonded chips of overall lateral dimension 10×10 mm with deflection regions of 2×2 mm. The device employs electrostatic actuation to tune the output wavelength, for which finite element modeling predicted low (<1 V) actuation voltages for movement of the membrane. Experimental results from device testing (mechanical) were found to differ from the theoretical predictions, primarily due to fabrication issues. Specifically, the device performance was found to be greatly influenced by the amount of residual silicon on the wafer chip following inductively coupled plasma (ICP) backside etching, with high voltages (~30 times higher than modeled) required for actuation of the device. Through a combination of modeling and experimental measurements, it is demonstrated that the ability to produce MEMS devices by releasing membranes from films predeposited on substrates is highly susceptible to error in etching and packaging.
A layer-by-layer nanoassembly (LbL) allows production of ultrathin films with a precision of 1-2 nm and needed composition across the multilayer. It was used in combination with traditional lithography to develop micropatterns in ordered nanoparticle multilayers. A selective nanoparticle film growth was also demonstrated for microchannel silicon chips. Microfluidic properties of nanoorganized polymer microcapsules were studied with the microchannel device. Nanoorganized microcapsules production: A LbL-assembly of 20-nm thick
poly(styrenesulfonate) / poly(allylamine) shell on microtemplates and loading such hollow polyion shells with enzymes allowed fabrication of catalytic "bioreactors," as it was demonstrated for glucose oxidase, hemoglobin, and myoglobin ensembles.
A microfluidic system was designed, fabricated and implemented to study the behavior of polyelectrolyte capsules flowing in microscale channels. The silicon component of the system contains microchannels that leads into constrictions, which were fabricated using lithography techniques. Polyelectrolyte microcapsules were also fabricated with well-known layer-by-layer assembly technique, on a spherical template. Once the template was removed, the resulting hollow capsules were introduced into the system. The behavior of the capsules at the constrictions was visualized and the properties of the capsules were investigated. Capsules recovered from the system appear to have a undergone a plastic deformation.
Miniaturization is fast gaining attention in chemical processes that are conventionally carried out on a lab-scale or larger. Major progress and landmarks has been made during the last five years. A microreactor system has been developed for fast catalyst development and process optimization. This paper focuses on the issues of microreactor system design and characterization for fast and accurate reaction analysis.
Components of a microspectrometer for operation in the IR range has been designed, fabricated, and characterized. An adjustable Fabry-Perot interferometer is used to select the resonant frequency of the system through electrostatic actuation, allowing tuning for certain optical frequencies to pass. Silicon microfabrication techniques are employed for the fabrication of the device. The intended use of the device is for spectroscopic study of liquids in biomedical and environmental applications; therefore, a sample containment chamber has been integrated into the device. The device was designed using finite element modeling to determine the stress distribution on the silicon nitride membrane due to deflection and the voltage required for the suitable displacement of the membrane to which one mirror is attached. The devices have been fabricated using a combination of processing steps to sputter gold mirrors on nitride membranes, to deposit electrodes and spacers using evaporation and photosensitive polyimide, to etch channels and sacrificial layers, and to bond chips to obtain a resonant cavity. Optical characterization was performed with an FTIR spectrometer. Initial results presented here support the feasibility of the approach in developing standalone microspectrometers for analysis of aqueous samples including biological fluids.
Micro-scale biochemical reactors have been developed from a polydimethylsiloxane/enzyme (PDMS-E)biopolymer. Micro-reactor channels 125 micrometers in depth, 500 micrometers wide by 50 centimeters long contain fixed triangular features for enhanced fluid mixing. All channel features are composed of the same PDMS-E material. Conversions of urea by urease enzyme of up to 70% have been obtained at an overall flowrate of 0.4 mL/min. Additional PDMS-E biopolymer systems containing amyloglucosidase (for converting starch to glucose) have demonstrated enzymatic activity.
This paper describes ongoing work in the development of microreactor-based systems for applications in the chemical process industry. The microreactors discussed here are formed from silicon using robust micromachining processes to produce devices with micrometer-scale fluidic structures including passageways for the introduction and removal of gases, and a reaction zone with a thin-film catalyst. We describe experiments done to characterize these reactors for use as development tools for industrial catalytic processes in terms of catalyst screening, acquisition of rate laws, and determination of optimal process conditions. The system studied here, the reaction of a cyclic olefin (cyclohexene) with hydrogen in the presence of platinum catalyst, is a model for industrially important catalytic hydrogenation and dehydrogenation reactions.
An experimental system has been designed and constructed to conduct gas- solid heterogeneous catalytic reactions in microreactors. This apparatus is inteded to be used for any exothermic or endothermic reaction, including those with multiple feeds. It can be used to test the effectiveness of a microreactor design for a particular catalyst or to test the behavior of the catalyst itself. The system uses a test block that is plumbed for multiple feeds and vacuum to hold down a standard size microreactor chip. This chip has two exit vias, which includes one for the reactor effluent and one for the exit stream from a possible reactor membrane wall. The reactors are systems of channels with a smallest cross-dimension as small as 5 micrometers. The experimental system is equipped with temperature control and automatic data acquisition. The reactors can be stacked in order to scale up to higher throughput. A simulator has been developed that accounts for the unique physical aspects of reaction and flow in very small channels. Along with design, it assist in determining operating conditions and interpreting experimental results.
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