227 results on '"Alexander Berg"'
Search Results
202. Ueber die Temperatur der Westseite von Südamerika
- Author
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Alexander Berg
- Subjects
General Physics and Astronomy - Abstract
n/a
- Published
- 1839
203. Two-Year Sustained Benefit of an Absorbable Implant for the Treatment of NVC
- Author
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Marion San Nicoló MD and Alexander Berghaus MD
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Otorhinolaryngology ,RF1-547 ,Surgery ,RD1-811 - Published
- 2017
- Full Text
- View/download PDF
204. NO2 and NH3 Sensing Characteristics of Inkjet Printing Graphene Gas Sensors
- Author
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Caterina Travan and Alexander Bergmann
- Subjects
graphene ,inkjet printing ,gas sensor ,Chemical technology ,TP1-1185 - Abstract
Graphene is a good candidate for filling the market requirements for cheap, high sensitivity, robust towards contamination, low noise, and low power consumption gas sensors, thanks to its unique properties, i.e., large surface, high mobility, and long-term stability. Inkjet printing is a cheap additive manufacturing method allowing fast, relatively precise and contactless deposition of a wide range of materials; it can be considered therefore the ideal technique for fast deposition of graphene films on thin substrates. In this paper, the sensitivity of graphene-based chemiresistor gas sensors, fabricated through inkjet printing, is investigated using different concentrations of graphene in the inks. Samples have been produced and characterized in terms of response towards humidity, nitrogen dioxide, and ammonia. The presented results highlight the importance of tuning the layer thickness and achieving good film homogeneity in order to maximize the sensitivity of the sensor.
- Published
- 2019
- Full Text
- View/download PDF
205. Intensity Modulated Photothermal Measurements of NO2 with a Compact Fiber-Coupled Fabry–Pérot Interferometer
- Author
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Philipp Breitegger, Benjamin Lang, and Alexander Bergmann
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nitrogen dioxide ,photothermal interferometry ,gas sensor ,optical microphone ,Chemical technology ,TP1-1185 - Abstract
Sensors for the reliable measurement of nitrogen dioxide concentrations are of high interest due the adverse health effects of this pollutant. This work employs photothermal spectroscopy to measure nitrogen dioxide concentrations at the parts per billion level. Absorption induced temperature changes are detected by means of a fiber-coupled Fabry−Pérot interferometer. The small size of the interferometer enables small detection volumes, paving the way for miniaturized sensing concepts as well as fast response times, demonstrated down to 3 s. A normalized noise equivalent absorption of 7.5 × 10 − 8 cm−1W/ Hz is achieved. Additionally, due to the rigid structure of the interferometer, the sensitivity to mechanical vibrations is shown to be minor.
- Published
- 2019
- Full Text
- View/download PDF
206. First Steps towards a Super-Compact In-Situ Laser-Induced-Incandescence Sensor System
- Author
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Martin Kupper, Jožef Pulko, Martin Kraft, and Alexander Bergmann
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Laser-Induced-Incandescence ,LII ,automotive exhaust ,photonic sensors ,soot ,in-situ sensing ,General Works - Abstract
To realize an actual in-situ Laser-Induced Incandescence (LII) sensor system for measurements in an exhaust pipe of a combustion engine, suitable components for such an application were chosen, integrated in a first prototype and tested. Key components for the proposed LII system are a super-compact high-power DPSS laser (CTR HiPoLas®) as excitation source, fast KETEK silicon photomultipliers (SiPM) as detectors and a specially designed optical measurement setup. Using a defined aerosol from a soot generator (Jing 5201 miniCAST), signals were collected at different laser energies and soot concentrations. By comparing the recorded behaviour with the literature, the incandescence effect could be reliably identified as the true source of the signal. Further long-term tests at an AVL engine testbed were performed.
- Published
- 2019
- Full Text
- View/download PDF
207. Fast Optical Humidity Sensor Based on Hydrogel Thin Film Expansion for Harsh Environment
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Anton Buchberger, Sebastian Peterka, Anna Maria Coclite, and Alexander Bergmann
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humidity measurement ,polymer ,hydrogel ,thin film ,initial chemical vapor deposition ,laser interference ,spectral reflectance ,Flory–Huggins theory ,interaction parameter ,Chemical technology ,TP1-1185 - Abstract
With the application of a recently developed deposition method called initiated chemical vapor deposition (iCVD), responsive hydrogel thin films in the order of a few hundred nanometers were created. When in contact with humid air, the hydrogel layer increases its thickness considerably. The measurement of the thickness change was realized interferometrically with a laser and a broadband light source in two different implementations. The relative change in thickness with respect to humidity can be described with the Flory⁻Huggins theory. The required Flory⁻Huggins interaction parameter was determined for the actual hydrogel composition. The setup was designed without electric components in the vicinity of the active sensor layer and is therefore applicable in harsh environments such as explosive or corrosive ones. The implemented sensor prototype delivered reproducible relative humidity ( R H ) values and the achieved response time for an abrupt change of the humidity τ 63 ≤ 2.5 s was about three times faster compared to one of the fastest commercially available sensors on the market.
- Published
- 2019
- Full Text
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208. Charging-Based PN Sensing of Automotive Exhaust Particles
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Mario Anton Schriefl, Matthias Longin, and Alexander Bergmann
- Subjects
particle number ,diffusion charging ,photoelectric charging ,General Works - Abstract
Mobile measurement of particle number concentration (PN) in the exhaust of motor vehicles has recently become an integral part of emission legislation. Charge-based sensing techniques for the examination of PN, like Diffusion Charging (DC), represent a promising alternative to condensational particle counters (CPCs) as established PN sensors, because they enable to build robust, compact and energy efficient systems. However, due to the charging process, particle properties like size and morphology have a big impact on the sensor’s PN response. For particles of different size and shape we experimentally investigated those impacts using own-built charging-based sensors. The PN response of the DC sensor showed desired behavior for compact NaCl particles, but less satisfying behavior for combustion aerosol standard (CAST) particles, which is a widely used test aerosol for automotive applications. With a photoelectric charger, the PN response of CAST particles was significantly better.
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- 2019
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209. Characterization of a Capacitive Sensor for Particulate Matter
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Paul Maierhofer, Marco Carminati, Giorgio Ferrari, Georg Röhrer, Marco Sampietro, and Alexander Bergmann
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particle sensor ,capacitive sensing ,particle deposition ,General Works - Abstract
We characterize a novel micro-sensor with pairs of interdigitated combs of microelectrodes designed to detect particles in air. We evaluate the sensor’s response to 1 µm Polystyrene Latex (PSL) particles experimentally and crosscheck the results with simulations. Experiment and simulation show good consistency. Based on the promising results we propose a redesign of the capacitive particle sensor with respect to PM2.5.
- Published
- 2018
- Full Text
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210. Fast Humidity Sensors for Harsh Environment
- Author
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Anton Buchberger, Anna Maria Coclite, and Alexander Bergmann
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humidity measurement ,polymer ,hydrogel ,thin film ,initial chemical vapor deposition ,iCVD ,thin film thickness ,laser interference ,spectral reflectance ,Flory-Huggins theory ,General Works - Abstract
With the application of a recently developed deposition method called initiated chemical vapor deposition (iCVD), remarkably fast responsive hydrogel thin films in the order of a few hundred nanometers were created. When in contact with humid air, the hydrogel layer extends its thickness manifold, which can be detected. The verification of the thickness change was realized interferometrically with a laser and a white light input source in two different implementations. The setup was designed without electric components in the vicinity of the active sensor layer and is therefore applicable in harsh and explosive environment. The achieved response time for an abrupt change of the humidity τ63 ≤ 2.5 s is about three times lower compared to one of the fastest commercially available sensors on the market.
- Published
- 2018
- Full Text
- View/download PDF
211. An Intrinsically Pressure Insensitive Low Cost Particle Number Diluter Featuring Flow Monitoring
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Markus Bainschab and Alexander Bergmann
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engine exhaust ,particle number ,dilution ,flow monitoring ,General Works - Abstract
We present a low cost Particle Number (PN) diluter including mass flow monitoring. The device consists of a commercial hypodermic needle, a High Efficiency Particulate Air (HEPA) filter. and a custom-made flow sensor. The flow sensor is used to monitor the diluter’s performance and enable in-time replacement of the low cost elements used. Neither the sampling flow rate nor the pressure drop drastically change the dilution factor introduced by the presented device. This makes the presented device especially useful for particle number measurements at positions close to the tailpipe of internal combustion engine powered vehicles, where aggravating, fast pressure pulsations complicate correct sampling.
- Published
- 2018
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212. A Precise Gas Dilutor Based on Binary Weighted Critical Flows to Create NO2 Concentrations
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Philipp Breitegger and Alexander Bergmann
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gas diluter ,nitrogen dioxide ,metrology ,General Works - Abstract
A gas diluter based on critical orifices was built and evaluated. The gas diluter is capable of creating dilution ratios of 1:1400 at a total flow of 6.5 L/min. An extended uncertainty analysis of gas concentrations and dilution ratios according to the Guide to the Expression of Uncertainty in Measurement was conducted. A gas cylinder of 5.16 ppm NO2 with a relative uncertainty of 1.5% (k = 1) can be diluted down to a concentration of 3.69 ppb NO2 (dilution ratio of 1:1400) at an uncertainty of 1.9% (k = 1). The results are in good agreement with reference NO2 measurements, conducted with a chemiluminescence detector (CLD, European reference method EN14211; 2005).
- Published
- 2018
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213. Modelling a Holographic Particle Counter
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Georg Brunnhofer and Alexander Bergmann
- Subjects
aerosol particle model ,Condensation Particle Counter ,In-Line holography ,General Works - Abstract
In order to design an imaging unit of a novel holographic particle counter an aerosol particle model was developed to generate a virtual hologram plane of an aerosol volume of interest. The herein presented model combines the three essential components to help dimensioning a target detection unit: (i) an In-Line holography model with a reference light source and a basic transfer function of an imager to take into account imager size, pixel pitch and exposure time; (ii) an aerosol particle model with particles of variable count, size and spatial distribution; and (iii) the possibility to import fluid dynamics simulation data to simulate the particle flow in an arbitrary sampling volume.
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- 2018
- Full Text
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214. Laser Driver and Analysis Circuitry Development for Quartz-Enhanced Photoacoustic Spectroscopy of NO2 for IoT Purpose
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Alexander Kerschhofer, Philipp Breitegger, and Alexander Bergmann
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photoacoustic spectroscopy ,laser driver ,lock in amplifier ,DDS ,air pollution measurement ,internet of things ,General Works - Abstract
The rising effort to track local air pollution measurements require low-cost air quality sensors that provide good accuracy, long-term stability and possibly Internet of Things (IoT) connectivity. To provide such a solution and avoid cost-intensive equipment the development of a low-cost environmental sensor system was started. To measure the pollutant NO2, a quartz-enhanced photoacoustic spectroscopy (QEPAS) setup was established. A pulsed 450 nm laser diode excites NO2 molecules due to its strong absorption at this wavelength and causes a vibrational-translational relaxation, which results in an acoustic wave. The acoustic wave is detected by a quartz tuning fork (QTF) which generates a weak electrical signal proportional to the NO2 concentration. To realize this at low cost, a laser driver and an analysis circuit including a lock-in amplifier and analog-to-digital conversion were developed. We present first results, which proof the functionality of the circuitry compared to a more expensive laboratory setup.
- Published
- 2018
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215. Components of the plasminogen activation system promote engraftment of porous polyethylene biomaterial via common and distinct effects.
- Author
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Christoph A Reichel, Maximilian E T Hessenauer, Kerstin Pflieger, Markus Rehberg, Sandip M Kanse, Stefan Zahler, Fritz Krombach, Alexander Berghaus, and Sebastian Strieth
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Medicine ,Science - Abstract
Rapid fibrovascularization is a prerequisite for successful biomaterial engraftment. In addition to their well-known roles in fibrinolysis, urokinase-type plasminogen activator (uPA) and tissue plasminogen activator (tPA) or their inhibitor plasminogen activator inhibitor-1 (PAI-1) have recently been implicated as individual mediators in non-fibrinolytic processes, including cell adhesion, migration, and proliferation. Since these events are critical for fibrovascularization of biomaterial, we hypothesized that the components of the plasminogen activation system contribute to biomaterial engraftment. Employing in vivo and ex vivo microscopy techniques, vessel and collagen network formation within porous polyethylene (PPE) implants engrafted into dorsal skinfold chambers were found to be significantly impaired in uPA-, tPA-, or PAI-1-deficient mice. Consequently, the force required for mechanical disintegration of the implants out of the host tissue was significantly lower in the mutant mice than in wild-type controls. Conversely, surface coating with recombinant uPA, tPA, non-catalytic uPA, or PAI-1, but not with non-catalytic tPA, accelerated implant vascularization in wild-type mice. Thus, uPA, tPA, and PAI-1 contribute to the fibrovascularization of PPE implants through common and distinct effects. As clinical perspective, surface coating with recombinant uPA, tPA, or PAI-1 might provide a novel strategy for accelerating the vascularization of this biomaterial.
- Published
- 2015
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216. Intramolecular electron and energy transfer in an axial ZnP-pyridylfullerene complex as studied by X- and W-band time-resolved EPR spectroscopy
- Author
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Tamar Galili, Ayelet Regev, Klaus Möbius, David I. Schuster, Alexander Berg, Anton Savitsky, and Haim Levanon
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Porphyrins ,Time Factors ,Fullerene ,Photochemistry ,Analytical chemistry ,Electrons ,Sensitivity and Specificity ,law.invention ,Electron transfer ,law ,Liquid crystal ,Organometallic Compounds ,Physical and Theoretical Chemistry ,Spectroscopy ,Electron paramagnetic resonance ,Chemistry ,Electron Spin Resonance Spectroscopy ,Temperature ,Acceptor ,Photoexcitation ,Zinc ,Crystallography ,Energy Transfer ,Intramolecular force ,Anisotropy ,Fullerenes - Abstract
Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.
217. A co-amorphous form of a substance and a protein
- Author
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Jaya Mishra, Adam Bohr, Thilo Heinz Alexander Berg, Löbmann, K., Thomas Rades, Holger Grohganz, and Jorrit Jeroen Water
218. Defect-induced infrared electroluminescence from radial GaInP/AlGaInP quantum well nanowire array light- emitting diodes.
- Author
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Laiq Hussain, Mohammad Karimi, Alexander Berg, Vishal Jain, Magnus T Borgström, Anders Gustafsson, Lars Samuelson, and Håkan Pettersson
- Subjects
LIGHT emitting diodes ,ELECTROLUMINESCENCE ,NANOWIRES - Abstract
Radial GaInP/AlGaInP nanowire array light-emitting diodes (LEDs) are promising candidates for novel high-efficiency solid state lighting due to their potentially large strain-free active emission volumes compared to planar LEDs. Moreover, by proper tuning of the diameter of the nanowires, the fraction of emitted light extracted can be significantly enhanced compared to that of planar LEDs. Reports so far on radial growth of nanowire LED structures, however, still point to significant challenges related to obtaining defect-free radial heterostructures. In this work, we present evidence of optically active growth-induced defects in a fairly broad energy range in vertically processed radial GaInP/AlGaInP quantum well nanowire array LEDs using a variety of complementary experimental techniques. In particular, we demonstrate strong infrared electroluminescence in a spectral range centred around 1 eV (1.2 μm) in addition to the expected red light emission from the quantum well. Spatially resolved cathodoluminescence studies reveal a patchy red light emission with clear spectral features along the NWs, most likely induced by variations in QW thickness, composition and barriers. Dark areas are attributed to infrared emission generated by competing defect-assisted radiative transitions, or to trapping mechanisms involving non-radiative recombination processes. Possible origins of the defects are discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
219. Resveratrol as a pan-HDAC inhibitor alters the acetylation status of histone [corrected] proteins in human-derived hepatoblastoma cells.
- Author
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Sascha Venturelli, Alexander Berger, Alexander Böcker, Christian Busch, Timo Weiland, Seema Noor, Christian Leischner, Sabine Schleicher, Mascha Mayer, Thomas S Weiss, Stephan C Bischoff, Ulrich M Lauer, and Michael Bitzer
- Subjects
Medicine ,Science - Abstract
The polyphenolic alcohol resveratrol has demonstrated promising activities for the prevention and treatment of cancer. Different modes of action have been described for resveratrol including the activation of sirtuins, which represent the class III histone deacetylases (HDACs). However, little is known about the activity of resveratrol on the classical HDACs of class I, II and IV, although these classes are involved in cancer development or progression and inhibitors of HDACs (HDACi) are currently under investigation as promising novel anticancer drugs. We could show by in silico docking studies that resveratrol has the chemical structure to inhibit the activity of different human HDAC enzymes. In vitro analyses of overall HDAC inhibition and a detailed HDAC profiling showed that resveratrol inhibited all eleven human HDACs of class I, II and IV in a dose-dependent manner. Transferring this molecular mechanism into cancer therapy strategies, resveratrol treatment was analyzed on solid tumor cell lines. Despite the fact that hepatocellular carcinoma (HCC) is known to be particularly resistant against conventional chemotherapeutics, treatment of HCC with established HDACi already has shown promising results. Testing of resveratrol on hepatoma cell lines HepG2, Hep3B and HuH7 revealed a dose-dependent antiproliferative effect on all cell lines. Interestingly, only for HepG2 cells a specific inhibition of HDACs and in turn a histone hyperacetylation caused by resveratrol was detected. Additional testing of human blood samples demonstrated a HDACi activity by resveratrol ex vivo. Concluding toxicity studies showed that primary human hepatocytes tolerated resveratrol, whereas in vivo chicken embryotoxicity assays demonstrated severe toxicity at high concentrations. Taken together, this novel pan-HDACi activity opens up a new perspective of resveratrol for cancer therapy alone or in combination with other chemotherapeutics. Moreover, resveratrol may serve as a lead structure for chemical optimization of bioavailability, pharmacology or HDAC inhibition.
- Published
- 2013
- Full Text
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220. Correction: Resveratrol as a Pan-HDAC Inhibitor Alters the Acetylation Status of Jistone Proteins in Human-Derived Hepatoblastoma Cells.
- Author
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Sascha Venturelli, Alexander Berger, Alexander Böcker, Christian Busch, Timo Weiland, Seema Noor, Christian Leischner, Sabine Schleicher, Mascha Mayer, Thomas S. Weiss, Stephan C. Bischoff, Ulrich M. Lauer, and Michael Bitzer
- Subjects
Medicine ,Science - Published
- 2013
- Full Text
- View/download PDF
221. Selective protection of human liver tissue in TNF-targeting of cancers of the liver by transient depletion of adenosine triphosphate.
- Author
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Timo Weiland, Kathrin Klein, Martina Zimmermann, Tobias Speicher, Sascha Venturelli, Alexander Berger, Heike Bantel, Alfred Königsrainer, Martin Schenk, Thomas S Weiss, Albrecht Wendel, Matthias Schwab, Michael Bitzer, and Ulrich M Lauer
- Subjects
Medicine ,Science - Abstract
BackgroundTumor necrosis factor alpha (TNF) is able to kill cancer cells via receptor-mediated cell death requiring adenosine triphosphate (ATP). Clinical usage of TNF so far is largely limited by its profound hepatotoxicity. Recently, it was found in the murine system that specific protection of hepatocytes against TNF's detrimental effects can be achieved by fructose-mediated ATP depletion therein. Before employing this quite attractive selection principle in a first clinical trial, we here comprehensively investigated the interdependence between ATP depletion and TNF hepatotoxicity in both in vitro and ex vivo experiments based on usage of primary patient tissue materials.MethodsPrimary human hepatocytes, and both non-tumorous and tumorous patient-derived primary liver tissue slices were used to elucidate fructose-induced ATP depletion and TNF-induced cytotoxicity.ResultsPHH as well as tissue slices prepared from non-malignant human liver specimen undergoing a fructose-mediated ATP depletion were both demonstrated to be protected against TNF-induced cell death. In contrast, due to tumor-specific overexpression of hexokinase II, which imposes a profound bypass on hepatocytic-specific fructose catabolism, this was not the case for human tumorous liver tissues.ConclusionNormal human liver tissues can be protected transiently against TNF-induced cell death by systemic pretreatment with fructose used in non-toxic/physiologic concentrations. Selective TNF-targeting of primary and secondary tumors of the liver by transient and specific depletion of hepatocytic ATP opens up a new clinical avenue for the TNF-based treatment of liver cancers.
- Published
- 2012
- Full Text
- View/download PDF
222. Delayed contrast-enhanced MRI of the coronary artery wall in takayasu arteritis.
- Author
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Christopher Schneeweis, Bernhard Schnackenburg, Matthias Stuber, Alexander Berger, Udo Schneider, Jing Yu, Rolf Gebker, Robert G Weiss, Eckart Fleck, and Sebastian Kelle
- Subjects
Medicine ,Science - Abstract
Takayasu arteritis (TA) is a rare form of chronic inflammatory granulomatous arteritis of the aorta and its major branches. Late gadolinium enhancement (LGE) with magnetic resonance imaging (MRI) has demonstrated its value for the detection of vessel wall alterations in TA. The aim of this study was to assess LGE of the coronary artery wall in patients with TA compared to patients with stable CAD.We enrolled 9 patients (8 female, average age 46±13 years) with proven TA. In the CAD group 9 patients participated (8 male, average age 65±10 years). Studies were performed on a commercial 3T whole-body MR imaging system (Achieva; Philips, Best, The Netherlands) using a 3D inversion prepared navigator gated spoiled gradient-echo sequence, which was repeated 34-45 minutes after low-dose gadolinium administration.No coronary vessel wall enhancement was observed prior to contrast in either group. Post contrast, coronary LGE on IR scans was detected in 28 of 50 segments (56%) seen on T2-Prep scans in TA and in 25 of 57 segments (44%) in CAD patients. LGE quantitative assessment of coronary artery vessel wall CNR post contrast revealed no significant differences between the two groups (CNR in TA: 6.0±2.4 and 7.3±2.5 in CAD; p = 0.474).Our findings suggest that LGE of the coronary artery wall seems to be common in patients with TA and similarly pronounced as in CAD patients. The observed coronary LGE seems to be rather unspecific, and differentiation between coronary vessel wall fibrosis and inflammation still remains unclear.
- Published
- 2012
- Full Text
- View/download PDF
223. Bias-dependent spectral tuning in InP nanowire-based photodetectors.
- Author
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Vishal Jain, Magnus Heurlin, Mohammad Karimi, Laiq Hussain, Mahtab Aghaeipour, Ali Nowzari, Alexander Berg, Gustav Nylund, Federico Capasso, Lars Samuelson, Magnus T Borgström, and Håkan Pettersson
- Subjects
NANOWIRE optical sensors ,PHOTODETECTORS ,INDIUM phosphide - Abstract
Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire–ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiO
x wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases. [ABSTRACT FROM AUTHOR]- Published
- 2017
- Full Text
- View/download PDF
224. Growth of wurtzite Al x Ga1−x P nanowire shells and characterization by Raman spectroscopy.
- Author
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Alexander Berg, Magnus Heurlin, Stylianos Tsopanidis, Mats-Erik Pistol, and Magnus T Borgström
- Subjects
- *
WURTZITE , *PHONON emissions , *RAMAN spectroscopy - Abstract
The phonon energies of AlGaP in wurtzite crystal structure are generally not known, as opposed to their zincblende counterparts, because AlGaP crystallizes in zincblende phase in bulk and thin films structures. However, in nanowires AlGaP can be grown in wurtzite crystal structure. In this work we have grown wurtzite GaP/AlGaP/GaP core–shell nanowires by use of MOVPE. After developing suitable growth conditions, the Al composition was determined by STEM-EDX measurements and the wurtzite AlGaP phonon energies by Raman spectroscopy. Raman measurements show a peak shift with increasing Al composition in the AlGaP shell. We find that the phonon energies for wurtzite AlGaP are slightly lower than for zincblende AlGaP. Our results can be used to determine the Al composition in wurtzite AlGaP by Raman scattering. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
225. La 'crisis' y como (no) reaccionar frente a ella
- Author
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Alexander Bergmann
- Subjects
Social Sciences - Published
- 1995
- Full Text
- View/download PDF
226. Growth parameter design for homogeneous material composition in ternary GaxIn1−xP nanowires.
- Author
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Alexander Berg, Filip Lenrick, Neimantas Vainorius, Jason P Beech, L Reine Wallenberg, and Magnus T Borgström
- Subjects
- *
NANOWIRES , *ELECTRIC wire , *OPTOELECTRONIC devices , *PYROLYSIS , *SCANNING transmission electron microscopy - Abstract
Ternary nanowires (NWs) often exhibit varying material composition along the NW growth axis because of different diffusion properties of the precursor molecules. This constitutes a problem for optoelectronic devices for which a homogeneous material composition is most often of importance. Especially, ternary GaInP NWs grown under a constant Ga–In precursor ratio typically show inhomogeneous material composition along the length of the NW due to the complexity of low temperature precursor pyrolysis and relative rates of growth species from gas phase diffusion and surface diffusion that contribute to synthesis of particle-assisted growth. Here, we present the results of a method to overcome this challenge by in situ tuning of the trimethylindium molar fraction during growth of ternary Zn-doped GaInP NWs. The NW material compositions were determined by use of x-ray diffraction, scanning transmission electron microscopy and energy dispersive x-ray spectroscopy and the optical properties by photoluminescence spectroscopy. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
227. In situ etching for control over axial and radial III-V nanowire growth rates using HBr.
- Author
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Alexander Berg, Kilian Mergenthaler, Mats-Erik Pistol, Magnus T Borgström, Martin Ek, and L Reine Wallenberg
- Subjects
- *
NANOWIRES , *HYDROGEN bromide , *TRANSMISSION electron microscopy , *WURTZITE , *PHOTOLUMINESCENCE - Abstract
We report on the influence of hydrogen bromide (HBr) in situ etching on the growth of InP, GaP and GaAs nanowires. We find that HBr can be used to impede undesired radial growth during axial growth for all three material systems. The use of HBr opens a window for optimizing the growth parameters with respect to the materials’ quality rather than only their morphology. Transmission electron microscopy (TEM) characterization reveals a partial transition from a wurtzite crystal structure to a zincblende upon the use of HBr during growth. For InP, defect-related luminescence due to parasitic radial growth is removed by use of HBr. For GaP, the etching with HBr reduced the defect-related luminescence, but no change in peak emission energy was observed. For GaAs, the HBr etching resulted in a shift to lower photon emission energies due to a shift in the crystal structure, which reduced the wurtzite segments. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
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