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At least 109 records · Page 6Linked to original sources

PubMed2004

Mechanistic studies on enzymatic reactions by electrospray ionization MS using a capillary mixer with adjustable reaction chamber volume for time-resolved measurements.

Mass spectrometry (MS)-based techniques have enormous potential for kinetic studies on enzyme-catalyzed processes. In particular, the use of electrospray ionization (ESI) MS for steady-state measurements is well established. However, there are very few reports of MS-based studies in the pre-steady-state regime, because it is difficult to achieve the time resolution required for this type of experiment. We have recently developed a capillary mixer with adjustable reaction chamber volume for kinetic studies by ESI-MS with millisecond time resolution (Wilson, D. J.; Konermann, L. Anal. Chem. 2003, 75, 6408-6414). Data can be acquired in kinetic mode, where the concentrations of selected reactive species are monitored as a function of time, or in spectral mode, where entire mass spectra are obtained for selected reaction times. Here, we describe the application of this technique to study the kinetics of enzyme reactions. The hydrolysis of p-nitrophenyl acetate by chymotrypsin was chosen as a simple chromophoric model system. On-line addition of a "makeup solvent" immediately prior to ionization allowed the pre-steady-state accumulation of acetylated chymotrypsin to be monitored. The rate constant for acetylation, as well as the dissociation constant of the enzyme-substrate complex obtained from these data, is in excellent agreement with results obtained by conventional stopped-flow methods. Bradykinin was chosen to illustrate the performance of the ESI-MS-based method with a nonchromophoric substrate. In this case, the unfavorable rate constant ratio for acylation and deacylation of the enzyme precluded measurements in the pre-steady-state regime. Steady-state experiments were carried out to determine the turnover number and the Michaelis constant for bradykinin. The methodologies used in this work open a wide range of possibilities for future ESI-MS-based kinetic assays in enzymology.

Acetylation↗

PubMed2004

Microfluidic T-form mixer utilizing switching electroosmotic flow.

This paper presents a microfluidic T-form mixer utilizing alternatively switching electroosmotic flow. The microfluidic device is fabricated on low-cost glass slides using a simple and reliable fabrication process. A switching DC field is used to generate an electroosmotic force which simultaneously drives and mixes the fluid samples. The proposed design eliminates the requirements for moving parts within the microfluidic device and delicate external control systems. Two operation modes, namely, a conventional switching mode and a novel pinched switching mode, are presented. Computer simulation is employed to predict the mixing performance attainable in both operation modes. The simulation results are then compared to those obtained experimentally. It is shown that a mixing performance as high as 97% can be achieved within a mixing distance of 1 mm downstream from the T-junction when a 60 V/cm driving voltage and a 2-Hz switching frequency are applied in the pinched switching operation mode. This study demonstrates how the driving voltage and switching frequency can be optimized to yield an enhanced mixing performance. The novel methods presented in this study provide a simple solution to mixing problems in the micro-total-analysis-systems field.

Electrochemistry↗

PubMed1992

Instrumentation of a high-shear mixer: evaluation and comparison of a new capacitive sensor, a watt meter, and a strain-gage torque sensor for wet granulation monitoring.

A high-shear mixer was instrumented with a new capacitive sensor, a watt meter, and a strain-gage torque sensor. The output from the capacitive sensor was split into two channels, which monitor both resistive and capacitive changes during the granulation process. The outputs of the devices were related to properties of the wet granulation. The percentage moisture content related linearly to the amplitude channel response. Yield values provided a rheological property to relate with power consumption and torque measurement. Power consumption and torque furnished a similar map of the granulation process. The amplitude channel of the capacitive sensor appears to be more clearly differentiated between binder levels in hydrous lactose-HPMC granulations than either power consumption or torque measurement, based on particle size distributions.

Chemistry, Pharmaceutical↗

PubMed2003

Time-resolved Fourier transform infrared spectrometry using a microfabricated continuous flow mixer: application to protein conformation study using the example of ubiquitin.

We report on the use of time-resolved Fourier transform infrared spectroscopy (FT-IR) to study chemically induced conformational changes of proteins using the example of ubiquitin. For this purpose a micromachined mixer is coupled to a conventional IR transmission cell with a pathlength of 25 microm and operated in both the continuous and the stopped-flow mode. This experimental set-up allows the elucidation of reaction pathways in the time frame of about 500 milliseconds to seconds with little reagent consumption and low pressure. For continuous flow measurements employed in the time frame from 0.5 to 1.4 s the reaction time is determined by the flow rate used as the connection between the point of confluence in the micromixer and the flow cell was kept constant in all experiments. For stopped-flow experiments (>1.4 s) the time is determined by data acquisition of the rapid scanning infrared spectrometer. Ubiquitin, a small well-known protein with 76 amino acid residues, changes its conformation from native to A-state with the addition of methanol under low pH conditions. We investigated the conformational change in the time frame from 0.5 to 10 s by mixing ubiquitin (20% methanol-d(4)) with an 80% methanol-d(4) solution at pD 2 by evaluating the time dependent changes in the amide I band of the protein.

Amides↗

PubMed2003

Electro-hydrodynamic micro-fluidic mixer.

Fluid mixing in microchannels is needed for many applications ranging from bio-arrays to micro-reactors, but is typically difficult to achieve. A simple geometry micro-mixer is proposed based on the electro-hydrodynamic (EHD) force present when the fluids to be mixed have different electrical properties and are subjected to an electric field. The electrodes are arranged so that the electric field is perpendicular to the interface between the two fluids, creating a transversal secondary flow. The technique is demonstrated experimentally using the flow of two liquids with identical viscosity and density, but different electrical properties. The volume flow rate and average velocity are 0.26 microl s(-1) and 4.2 mm s(-1), respectively, corresponding to a Reynolds number Re= 0.0174. The effect of a continuous (DC) electric field and two alternating (AC)- sinusoidal and square - electric fields is explored. At the appropriate parameter values, very good mixing takes place in less than 0.1 s, over a very short distance (within a fraction of the width 250 microm of the electrodes).

Journal Article↗

PubMed2004

Micro magnetic stir-bar mixer integrated with parylene microfluidic channels.

Previously, we reported a micro magnetic stir-bar mixer driven by an external rotating magnetic field and its rapid mixing performance in polydimethyl-siloxane (PDMS) channels. The PDMS piece with embedded fluid channels were manually aligned to a glass substrate and assembled. In this paper, we report the fabrication and testing results of a micro magnetic stir-bar monolithically integrated in parylene surface-micromachined channels with improved design features, including small tolerance of the stir-bar to channel wall (10 microm). Using of parylene based microchannels with improved design not only provides improved mixing, but also eliminates certain problems associated with PDMS-based channels. For example, porosity of PDMS causes evaporation and absorption of chemicals and thus channels made of PDMS are prone to cross-contamination. We have also demonstrated that the magnetic stir-bar can be used to pump liquid in micro channels.

Dimethylpolysiloxanes↗

PubMed2006

Microbioreactor arrays with integrated mixers and fluid injectors for high-throughput experimentation with pH and dissolved oxygen control.

We have developed an integrated array of microbioreactors, with 100 microL working volume, comprising a peristaltic oxygenating mixer and microfluidic injectors. These integrated devices were fabricated in a single chip and can provide a high oxygen transfer rate (k(L)a approximately 0.1 s(-1)) without introducing bubbles, and closed loop control over dissolved oxygen and pH (+/-0.1). The system was capable of supporting eight simultaneous Escherichia coli fermentations to cell densities greater than 13 g-dcw L(-1) (1 cm OD(650 nm) > 40). This cell density was comparable to that achieved in a 4 litre reference fermentation, conducted with the same strain, in a bench scale stirred tank bioreactor and is more than four times higher than cell densities previously achieved in microbioreactors. Bubble free oxygenation permitted near real time optical density measurements which could be used to observe subtle changes in the growth rate and infer changes in the state of microbial genetic networks. Our system provides a platform for the study of the interaction of microbial populations with different environmental conditions, which has applications in basic science and industrial bioprocess development. We leverage the advantages of microfluidic integration to deliver a disposable, parallel bioreactor in a single chip, rather than robotically multiplexing independent bioreactors, which opens a new avenue for scaling small scale bioreactor arrays with the capabilities of bench scale stirred tank reactors.

Ammonium Hydroxide↗

PubMed2001

Lifetimes of intermediates in the beta -sheet to alpha -helix transition of beta -lactoglobulin by using a diffusional IR mixer.

The extremely slow alpha-helix/beta-sheet transition of proteins is a crucial step in amylogenic diseases and represents an internal rearrangement of local contacts in an already folded protein. These internal structural rearrangements within an already folded protein are a critical aspect of biological action and are a product of conformational flow along unknown metastable local minima of the energy landscape of the compact protein. We use a diffusional IR mixer with time-resolved Fourier transform IR spectroscopy capable of 400-micros time resolution to show that the trifluoroethanol driven beta-sheet to alpha-helix transition of beta-lactoglobulin proceeds via a compact beta-sheet intermediate with a lifetime of 7 ms, small compared with the overall folding time of beta-lactoglobulin.

Diffusion↗

PubMed2004

Drug-beta-cyclodextrin containing pellets prepared with a high-shear mixer.

This work was aimed at investigating the preparation of beta-cyclodextrin-microcrystalline cellulose pellets by means of a high-shear mixer, both in the absence or in the presence of ibuprofen as model drug. Drug loading of pellets was accomplished by means of two alternative techniques: 1) solution layering or 2) powder layering. The prepared pellets were characterised in terms of size distribution, shape factor, friability and dissolution rate. The interaction between ibuprofen and beta-cyclodextrin was monitored by Differential Scanning Calorimetry (DSC). Micro Fourier Transform Infrared spectroscopy (MicroFTIR) was applied to determine the distribution of components within each pellet on a micro scale. Pellets with narrow size distribution and containing up to about 90% of BCD were prepared using water as binder. The process yield resulted around 84 and 63% for drug-free and medicate pellets respectively. Drug loaded pellets with favourable technological and biopharmaceutical characteristics can be obtained both by powder or solution layering techniques. The latter proved to be more suitable for producing pellets with high drug contents, reduced friability and high drug dissolution rates.

Cellulose↗

PubMed1976

Performance and application of the Quantiflex air/oxygen mixer.

The Quantiflex air/oxygen mixer is designed to dispense mixtures of air and oxygen with separate controls for total gas flow rate and oxygen concentration of the mixture within the range 21-100%. A monitoring flowmeter is provided for the mixture and also, as a safety measure, for the oxygen component. This serves as an indicator that oxygen is flowing and also permits independent calculation of the oxygen concentration of the mixture. Delivered oxygen concentrations were found to be within +/- 2% of the indicated value at flow rates between 4 and 12 litre/min with the input pressures of either or both gases at 208-415 kPa (30-60 lbf/in2.) gauge, and with or without an output pressure of 20 kPa. At total flow rates of 1.5-2 litre/min there was a maximum discrepancy of 4% below and 8% above the indicated concentration in some delivered concentrations. Acceptability, ease, accuracy and quickness of use by nurses were compared with current methods using separate flowmeters for air and oxygen and calculating the required flow rates by means of arithmetic, graph and special-purpose slide-rule (Blease). The Quantiflex prototype was the most acceptable, the easiest, the most accurate and the fastest of the techniques investigated.

Evaluation Studies as Topic↗

PubMed2004

Microfluidic mixers: from microfabricated to self-assembling devices.

This paper begins with a survey of both passive and active microfluidic mixers that have been implemented in recent years. It then describes a micromixing device based on dynamic self-assembly. This device is easy to fabricate and has excellent working characteristics in the continuous-flow mode. The paper concludes with a brief discussion of possible applications of self-assembly in microfluidics.

Complex Mixtures↗

PubMed1989

A computer-controlled anaesthetic gas mixer.

The theory and construction of a prototype computer-controlled gas mixer and flow regulator are described. The device is capable of controlling and mixing flows of oxygen, nitrous oxide and air. The accuracy and repeatability of the device is very high under given conditions, and its performance is quantified. The flow rate and oxygen concentration are selected from a microcomputer and the control program is written in BASIC. Possible improvements to the system are discussed as are uses in both clinical and research work.

Air↗

PubMed1966

Multicapillary mixer of solutions.

A mixer made from a bundle of glass tubules can mix two solutions within 30 microseconds, with a total-solution flow rate of 1.33 milliliters per second. One solution passes through the interstices of the bundle; the other moves through the lumens of the tubes.

Medical Laboratory Science↗

PubMed2001

The change in characteristics of microcrystalline cellulose during wet granulation using a high-shear mixer.

The objective of this study was to investigate the mechanism of hard granule formation and to demonstrate the applicability of X-ray diffraction methods for studying the polymeric pharmaceutical excipients. Using a high-shear mixer, microcrystalline cellulose (MCC) was granulated with water as the granulating liquid. The hardness of the MCC granules increased with granulation time and the amount of water added. The specific surface area measured by the N2 adsorption method was reduced during the process. Crystallite size of cellulose, calculated by Scherrer's equation adapted for wide angle X-ray diffraction method, decreased with granulation time and with increasing amounts of water added. Debye plots for X-ray small scattering patterns suggested that the average magnitude of the continuous solid region in MCC granules became significantly greater, whereas the specific surface area of the MCC granules, calculated from Debye plots, became smaller in comparison with that of intact MCC. These findings suggested that the long-chain structures in MCC were disrupted, resulting in smaller units with shorter chain lengths due to the strong shear force of the impeller. These smaller units then form a network within the granules. Thus, MCC granules are strengthened with longer granulation time and greater amounts of water, resulting in a more intricate network. The change in MCC chain length and physical structure can be experimentally detected using the small-angle X-ray scattering and wide-angle powder X-ray diffraction methods.

Cellulose↗

PubMed2001

Interaction of microcrystalline cellulose and water in granules prepared by a high-shear mixer.

Microcrystalline cellulose (MCC) granules were prepared by wet granulation using a high-shear mixer. Physical characteristics of the granules were investigated using near IR spectrometry, thermogravimetry and isothermal water vapor adsorption. Near IR spectra of dried MCC granules prepared for various granulation times exhibited different peak intensities at 1428, 1772, and 1920 nm, which were assigned to functional groups of cellulose or water. On isothermogravimetric analysis, the rate of dehydration of water was shown to decrease with granulation time. These results suggest that the physical structure of MCC could change during the granulation process, and the interaction between MCC and water was gradually strengthened. The isothermal water vapor adsorption curves suggested that the amorphous region of MCC would be divided by the strong shear force of the impeller, because the high adsorption ability of intact MCC in the low humidity region was diminished in granules collected following 5 and 10 min of granulation. It was suggested that MCC formed a network which caught water within its structure during the wet granulation process.

Cellulose↗

PubMed2002

Optimization of operating conditions in a high-shear mixer using dem model: determination of optimal fill level.

For the purpose of evaluating optimal fill level of starting materials in a high-shear mixer, discrete element method (DEM) simulation was conducted to visualize kinetic status between particles. The simulation results obtained by changing fill levels were used to determine solid fraction of particles, particle velocity, particle velocity vector, and kinetic energy and discuss the flow pattern. Optimal fill level was obtained from the information on these matters. It was pointed out that understanding the kinetic energy between particles in an agitating vessel was effective in determining the optimal fill level. Granulation experiment was conducted to validate the optimal fill level obtained by the simulation, confirming the good agreement between these two results. It was pointed out that determination of kinetic energy between particles through the simulation was effective in obtaining an index of the kinetic status of particles. Further, it was confirmed that the simulation could provide more information than conventional granulation experiments could provide and also helpful in optimizing the operating conditions.

Drug Compounding↗

PubMed2004

Mode multiplexing in optical frequency mixers.

Asymmetric Y junctions allow the development of a new class of optical frequency mixers that utilize higher-order waveguide modes for signal processing. We measure high-contrast (>30 dB) mode sorting in asymmetric Y junctions by use of a novel technique: efficient TM00, TM10, and TM20 mode mixing in a periodically poled lithium niobate waveguide. We also demonstrate an odd-to-even mode wavelength converter capable of spectral inversion without offset or bidirectional wavelength conversion.

Journal Article↗

PubMed2006

Timing noise measurement of 320 GHz optical pulses using an improved optoelectronic harmonic mixer.

An improved optoelectronic harmonic mixer (OEHM) has been employed for measuring the timing noise of 320 GHz optical pulses that are generated from a 160 GHz mode-locked laser diode by the temporal Talbot effect. The OEHM makes use of a low-drive voltage LiNbO3 modulator and a W-band unitraveling carrier photodiode for converting the 320 GHz pulse intensity into a low-frequency electrical signal. The time domain demodulation technique has been used for the precise evaluation of phase noise power spectral density. The rms timing jitter has been estimated to be 311 fs for the 10 Hz-18.6 MHz bandwidth.

Journal Article↗