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Biomedical subjects

Klavs F Jensen

Publications and source records attributed to Klavs F Jensen.

At least 19 recordsLinked to original sources
PubMed2006

Multiphase microfluidics: from flow characteristics to chemical and materials synthesis.

We review transport characteristics of pressure-driven, multiphase flows through microchannel networks tens of nanometres to several hundred of micrometres wide with emphasis on conditions resulting in enhanced mixing and reduced axial dispersion. Dimensionless scaling parameters useful in characterizing multiphase flows are summarized along with experimental flow visualization techniques. Static and dynamic stability considerations are also included along with methods for stabilizing multiphase flows through surface modifications. Observed gas-liquid and immiscible liquid-liquid flows are summarized in terms of flow regime diagrams and the different flows are related to applications in chemistry and materials synthesis. Means to completely separate multiphase flows on the microscale and guidelines for design of scalable multiphase systems are also discussed.

Journal Article↗
PubMed2006

Density functional theory study of ligand binding on CdSe (0001), (0001), and (1120) single crystal relaxed and reconstructed surfaces: implications for nanocrystalline growth.

To gain a better understanding of the influence of ligand-surface interactions on nanocrystalline growth, periodic density functional theory calculations were employed in the study of the binding of organic ligands on the relaxed nonpolar (1120) and polar Se terminated (0001) surfaces and the relaxed and vacancy and adatom reconstructed Cd terminated (0001) surface. We examined chemisorption properties of phosphine, amine, phosphine oxide, carboxylic acid, and phosphinic acid model ligands, including preferred binding sites and geometries, vibrational frequencies, and binding energetics, and compared findings to intrinsic growth via addition of CdSe molecules or Cd and Se atoms. Our results indicate that binding of the ligands is preferred in the electron-poor 1-fold sites on all surfaces, with secondary coordination of the acidic ligands through the hydroxyl hydrogen to the electron-rich surface sites. In general ligand adsorption directly obstructs binding sites for growth species on the (1120) surface and only indirectly on the two polar surfaces. The order of binding affinities on the (1120) and (0001) surfaces is PH(3) < OPH(3) approximately HCOOH < NH(3) < OPH(2)OH and that on the (0001) surface is OPH(3) approximately HCOOH < OPH(2)OH < NH(3) < PH(3). Our findings corroborate the experimental observation that incorporation of the nonbulky phosphinic acid-type ligands with high affinity and high selectivity for both the (1120) and (0001) surfaces strongly enhances unidirectional growth on the (0001) surface, while incorporation of either bulky ligands or ligands with moderate affinity does not. Higher affinity of all traditionally used ligands for the (1120) surface compared to the (0001) surface also suggests that new ligands should be engineered to achieve the synthesis of novel shapes that require preferential growth on the (1120) surface.

Journal Article↗
PubMed2006

Cells on chips.

Microsystems create new opportunities for the spatial and temporal control of cell growth and stimuli by combining surfaces that mimic complex biochemistries and geometries of the extracellular matrix with microfluidic channels that regulate transport of fluids and soluble factors. Further integration with bioanalytic microsystems results in multifunctional platforms for basic biological insights into cells and tissues, as well as for cell-based sensors with biochemical, biomedical and environmental functions. Highly integrated microdevices show great promise for basic biomedical and pharmaceutical research, and robust and portable point-of-care devices could be used in clinical settings, in both the developed and the developing world.

Animals↗
PubMed2006

Continuous dielectrophoretic size-based particle sorting.

Continuous-flow dielectrophoretic (DEP) particle separation based on size is demonstrated in a microfluidic device. Polystyrene microspheres suspended in a neutrally buoyant aqueous solution are used as model particles to study DEP induced by an array of slanted, planar, interdigitated electrodes inside of a soft-lithography microchannel. The E-field gradients from the slanted electrodes impart a net transverse force component on the particles that causes them to "ratchet" across the channel. Over the length of the device, larger particles are deflected more than smaller particles according to the balance of hydrodynamic drag and DEP forces. Consequently, a flow-focused particle suspension containing different-sized particles is fractionated as the beads flow and separate down the length of the device. The flow behavior of spherical particles is modeled, and the total transverse particle displacement in the microfluidic device predicts fourth-order size and voltage and second-order inverse flow rate dependences. The model is verified experimentally for a range of flow rates, particle sizes, and E-field strengths.

Journal Article↗
PubMed2006

Microchemostat-microbial continuous culture in a polymer-based, instrumented microbioreactor.

In a chemostat, microbial cells reach a steady state condition at which cell biomass production, substrates and the product concentrations remain constant. These features make continuous culture a unique and powerful tool for biological and physiological research. We present a polymer-based microbioreactor system integrated with optical density (OD), pH, and dissolved oxygen (DO) real-time measurements for continuous cultivation of microbial cells. Escherichia coli (E. coli) cells are continuously cultured in a 150 microL, membrane-aerated, well-mixed microbioreactor fed by a pressure-driven flow of fresh medium through a microchannel. Chemotaxisial back growth of bacterial cells into the medium feed channel is prevented by local heating. Using poly(ethylene glycol) (PEG)-grafted poly(acrylic acid) (PAA) copolymer films, the inner surfaces of poly(methyl methacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) of the microbioreactor are modified to generate bio-inert surfaces resistant to non-specific protein adsorption and cell adhesion. The modified surfaces of microbioreactor effectively reduce wall growth of E. coli for a prolonged period of cultivation. Steady state conditions at different dilution rates are demonstrated and characterized by steady OD, pH, and DO levels.

Acrylic Resins↗
PubMed2006

Flow-induced deformation of shallow microfluidic channels.

We study the elastic deformation of poly(dimethylsiloxane) (PDMS) microchannels under imposed flow rates and the effect of this deformation on the laminar flow profile and pressure distribution within the channels. Deformation is demonstrated to be an important consideration in low aspect ratio (height to width) channels and the effect becomes increasingly pronounced for very shallow channels. Bulging channels are imaged under varying flow conditions by confocal microscopy. The deformation is related to the pressure and is thus non-uniform throughout the channel, with tapering occurring along the stream-wise axis. The measured pressure drop is monitored as a function of the imposed flow rate. For a given pressure drop, the corresponding flow rate in a deforming channel is found to be several times higher than expected in a non-deforming channel. The experimental results are supported by scaling analysis and computational fluid dynamics simulations coupled to materials deformation models.

Computer Simulation↗
PubMed2006

A well-mixed, polymer-based microbioreactor with integrated optical measurements.

We describe a 150 microL microbioreactor fabricated in poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) to cultivate microbial cell cultures. Mixing is achieved by a small magnetic stir bar and fluorescent sensors are integrated for on-line measurement of pH and dissolved oxygen. Optical transmission measurements are used for cell density. The body of the reactor is poly(methylmethacrylate) with a thin layer of poly (dimethylsiloxane) for aeration, oxygen diffuses through this gas-permeable membrane into the microbioreactor to support metabolism of bacterial cells. Mixing in the reactor is characterized by observation of mixing of dyes and computational fluid dynamics simulations. The oxygenation is described in terms of measured K(L)a values for microbioreactor, 20-75/h corresponding to increasing stirring speed 200-800 rpm. Escherichia coli cell growth in the microbioreactor is demonstrated and the growth behavior is benchmarked with conventional bench-scale bioreactors, flasks and tubes. Batch culture experiments with Saccharomyces cerevisiae further demonstrate the reproducibility and flexibility of the microbioreactor system.

Bioreactors↗
PubMed2006

In situ measurement of bioluminescence and fluorescence in an integrated microbioreactor.

Reporter strains of bacteria that emit light or a fluorescent marker in response to specific conditions in their environment are having a significant impact in many areas of biology, including toxicity assays for environmental pollutants, chemical detection, and gene expression profiling. We have demonstrated methods for in situ measurements of bioluminescence and fluorescence from bacterial cultures grown in 50 microL instrumented microbioreactors. Results from microbioreactors were compared to results obtained from conventional 500 mL batch bioreactors and shake flasks. Experiments were conducted with reporter strains of Escherichia coli in which luxCDABE or gfp was fused to a promoter that was either expressed constitutively, or that responded to oxygen limitation. With these reporter strains, we have demonstrated the ability to obtain information on growth conditions within the microbioreactor. We have also shown that the large aspect ratio of the microbioreactor provides a unique advantage over measurements in larger bioreactors by reducing the inner filter effect in on-line measurements and eliminating the need for error-prone off-line dilutions. In addition, continuous on-line monitoring of genes in real-time, when expanded to include entire reporter libraries, could potentially provide a true dynamic picture of cellular gene expression from which the kinetics of gene expression can be untangled and elucidated.

Bioreactors↗
PubMed2006

Micro free-flow IEF enhanced by active cooling and functionalized gels.

Rapid free-flow IEF is achieved in a microfluidic device by separating the electrodes from the focusing region with porous buffer regions. Moving the electrodes outside enables the use of large electric fields without the detrimental effects of bubble formation in the active region. The anode and cathode porous buffer regions, which are formed by acrylamide functionalized with immobilized pH groups, allow ion transport while providing buffering capacity. Thermoelectric cooling mitigates the effects of Joule heating on sample focusing at high field strengths (approximately 500 V/cm). This localized cooling was observed to increase device performance. Rapid focusing of low-molecular-weight p/ markers and Protein G-mouse IgG complexes demonstrate the versatility of the technique. Simulations provide insight into and predict device performance based on a well-defined sample composition.

Acrylic Resins↗
PubMed2005

Gene expression analysis of Escherichia coli grown in miniaturized bioreactor platforms for high-throughput analysis of growth and genomic data.

Combining high-throughput growth physiology and global gene expression data analysis is of significant value for integrating metabolism and genomics. We compared global gene expression using 500 ng of total RNA from Escherichia coli cultures grown in rich or defined minimal media in a miniaturized 50-microl bioreactor. The microbioreactor was fabricated out of poly(dimethylsiloxane) (PDMS) and glass and equipped to provide on-line, optical measurements. cDNA labeling for microarray hybridizations was performed with the GeniconRLS system. From these experiments, we found that the expression of 232 genes increased significantly in cells grown in minimum medium, including genes involved in amino acid biosynthesis and central metabolism. The expression of 275 genes was significantly elevated in cells grown in rich medium, including genes involved in the translational and motility apparatuses. In general, these changes in gene expression levels were similar to those observed in 1,000-fold larger cultures. The increasing rate at which complete genomic sequences of microorganisms are becoming available offers an unprecedented opportunity for investigating these organisms. Our results from microscale cultures using just 500 ng of total RNA indicate that high-throughput integration of growth physiology and genomics will be possible with novel biochemical platforms and improved detection technologies.

Bioreactors↗
PubMed2005

Properties of the CdSe(0001), (0001), and (1120) single crystal surfaces: Relaxation, reconstruction, and adatom and admolecule adsorption.

Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.

Journal Article↗
PubMed2005

Development of a multiplexed microbioreactor system for high-throughput bioprocessing.

A multiplexed microbioreactor system for parallel operation of multiple microbial fermentation is described. The system includes miniature motors for magnetic stirring of the microbioreactors and optics to monitor the fermentation parameters optical density (OD), dissolved oxygen (DO), and pH, in-situ and in real time. The microbioreactors are fabricated out of poly(methylmethacrylate)(PMMA) and poly(dimethylsiloxane)(PDMS), and have a working volume of 150 microl. Oxygenation of the cells occurs through a thin PDMS membrane at the top of the reactor chamber. Stirring is achieved with a magnetic spin bar in the reactor chamber. Parallel microbial fermentations with Escherichia coli are carried out in four stirred microbioreactors and demonstrate the reproducible performance of the multiplexed system. The profiles for OD, DO, and pH compare favourably to fermentations performed in bioreactor systems with multiple bench-scale reactors. Finally, the multiplexed system is used to compare two different reactor designs, demonstrating that the reproducibility of the system permits the quantification of microbioreactor performance.

Bioreactors↗
PubMed2005

Cell stimulus and lysis in a microfluidic device with segmented gas-liquid flow.

We describe a microfluidic device with rapid stimulus and lysis of mammalian cells for resolving fast transient responses in cell signaling networks. The device uses segmented gas-liquid flow to enhance mixing and has integrated thermoelectric heaters and coolers to control the temperature during cell stimulus and lysis. Potential negative effects of segmented flow on cell responses are investigated in three different cell types, with no morphological changes and no activation of the cell stress-sensitive mitogen activated protein kinases observed. Jurkat E6-1 cells are stimulated in the device using alpha-CD3, and the resulting activations of ERK and JNK are presented for different time points. Stimulation of cells performed on chip results in pathway activation identical to that of conventionally treated cells under the same conditions.

Antibodies, Monoclonal↗
PubMed2005

Surfactant-enhanced liquid-liquid extraction in microfluidic channels with inline electric-field enhanced coalescence.

Continuous microfluidic liquid-liquid extraction is realized in a microfluidic device by generating emulsions with large interfacial areas for mass transfer, and subsequently breaking these emulsions using electric fields into easily separated segments of immiscible liquids (plugs). The microfluidic device employs insulated electrodes in a potassium hydroxide-etched channel to create large electric fields (100 kV m(-1)) that drive coalescence of the emulsion phase. The result is a transition from disperse to slug flow that can then readily be separated by gravity. Extractions of phenol and p-nitrophenol from an aqueous to hexane-surfactant solution serve as model systems. In addition to the increased surface area in the emulsion, extraction efficiency is enhanced by reverse micelles resulting from the presence of surfactants. The surfactant concentration is varied approximately 1-10 wt% and a general two-parameter model is developed to quantify the extraction behavior and demonstrate the effectiveness of reverse micelle enhanced extraction.

Micelles↗
PubMed2005

Micromixing of miscible liquids in segmented gas-liquid flow.

We present an integrated microfluidic system that achieves efficient mixing between two miscible liquid streams by introducing a gas phase, forming a segmented gas-liquid (slug) flow, and completely separating the mixed liquid and gas streams in a planar capillary separator. The recirculation motion associated with segmented flow enhances advection in straight microchannels without requiring additional fabrication steps. Instantaneous velocity fields are quantified by microscopic particle image velocimetry (muPIV). Velocities in the direction normal to the channel amount to approximately 30% of the bulk liquid velocity inside a liquid segment. This value depends only weakly on the length of a liquid segment. Spatial concentration fields and the extent of mixing (EOM) are obtained from pulsed-laser fluorescence microscopy and confocal scanning microscopy measurements. The mixing length is reduced 2-3-fold in comparison with previously reported chaotic micromixers that use three-dimensional microchannel networks or patterned walls. Segmented gas-liquid microflows allow mixing times to be varied over several orders of magnitude between milliseconds and second time scales.

Journal Article↗
PubMed2004

A microfluidic electroporation device for cell lysis.

We demonstrate a micro-electroporation device for cell lysis prior to subcellular analysis. Simple circuit models show that electrical lysis method is advantageous because it is selective towards plasma membrane while leaving organelle membrane undamaged. In addition, miniaturization of this concept leads to negligible heat generation and bubble formation. The designed microdevices were fabricated using a combination of photolithography, metal-film deposition, and electroplating. We demonstrate the electro-lysis of human carcinoma cells in these devices to release the subcellular materials.

Cell Fractionation↗