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Michael Krämer

Publications and source records attributed to Michael Krämer.

8 recordsLinked to original sources
PubMed2006

pH-Responsive dendritic core-shell architectures as amphiphilic nanocarriers for polar drugs.

In this paper a simple general synthetic concept was used to generate dendritic core-shell architectures based on hyperbranched poly(ethylene imine) cores and different shells which contain aliphatic chains and poly(ethylene glycol) chains, respectively. Using these dendritic core-shell architectures as nanocarriers we studied the encapsulation and transport of polar drugs, such as congo red. The results showed that the acid labile nanocarriers exhibited much higher transport capacities for congo red than the bare poly(ethylene imine). This gives an opportunity for the controlled release of encapsulated dyes and drugs by a pH-triggered cleavage of the imine bond. In two cases the pH-sensitivity was studied and a relatively high stability of the imine bond was observed at pH 8, while fast cleavage occurred at pH 5-7. This pH difference correspond to the pH-shift in malignant tissues (tumor, infection) compared to normal tissue, which could trigger the release of the encapsulated drugs.

Aldehydes↗
PubMed2006

The impact of an hematocrit of 20% during normothermic cardiopulmonary bypass for elective low risk coronary artery bypass graft surgery on oxygen delivery and clinical outcome--a randomized controlled study [ISRCTN35655335].

INTRODUCTION: Cardiopulmonary bypass (CPB) induces hemodilutional anemia, which frequently requires the transfusion of blood products. The objective of this study was to evaluate oxygen delivery and consumption and clinical outcome in low risk patients who were allocated to an hematocrit (Hct) of 20% versus 25% during normothermic CPB for elective coronary artery bypass graft (CABG) surgery. METHODS: This study was a prospective, randomized and controlled trial. Patients were subjected to normothermic CPB (35 to 36 degrees C) and were observed until discharge from the intensive care unit (ICU). Outcome measures were calculated whole body oxygen delivery, oxygen consumption and clinical outcome. A nonparametric multivariate analysis of variance for repeated measurements and small sample sizes was performed. RESULTS: In a total of 54 patients (25% Hct, n = 28; 20% Hct, n = 26), calculated oxygen delivery (p = 0.11), oxygen consumption (p = 0.06) and blood lactate (p = 0.60) were not significantly different between groups. Clinical outcomes were not different between groups. CONCLUSION: These data indicate that an Hct of 20% during normothermic CPB maintained calculated whole body oxygen delivery above a critical level after elective CABG surgery in low risk patients. The question of whether a transfusion trigger in excess of 20% Hct during normothermic CPB is still supported requires a larger prospective and randomized trial.

Adult↗
PubMed2005

Monolayers of salen derivatives as catalytic planes for alkene oxidation in water.

Monolayers at the gas/water interface have been used as an adjustable catalytic system in which the molecular density may be modified. Mn(III)-salen complexes bearing perfluoroalkyl substituents have been organized as a Langmuir film on an aqueous subphase containing a urea/hydrogen peroxide adduct (UHP, the oxidant) and cinnamyl alcohol (the substrate). The catalytic activity of the monolayer for the epoxidation of the alkene dissolved in water has been demonstrated and the reaction kinetic investigated. For a constant area per molecule of catalyst, the reaction rate exhibits first-order dependence on oxidant concentration and zero-order dependence on alkene concentration, in agreement with the reaction orders reported for Mn(III)-salen-catalyzed epoxidation reactions carried out in solution. Furthermore, kinetic experiments suggest an enhanced activity of the catalysts assembled in a Langmuir film relative to that observed in bulk reaction. Finally, varying the molecular density of the catalyst at the gas/water interface highlights an important dependence of the catalytic activity of the layer with the mean molecular area. A strong increase of the catalytic properties of the monolayer was observed for a mean molecular area of 140-145 A2, an increase which was supposedly related to a modification of the Mn(III)-salen complex orientation at the interface upon compression. This hypothesis was supported by PM-IRRAS (polarization modulation infrared reflection adsorption spectroscopy) experiments performed in situ on the monolayer. Such results demonstrate that a soft and adjustable molecular system like a Langmuir film can be used to better understand the reactivity in various heterogeneous and/or pseudohomogeneous (such as those based on dendrimers) catalytic systems.

Journal Article↗
PubMed2005

Tailor-made nanoparticles via gas-phase synthesis.

Gas-phase synthesis is a well-known chemical manufacturing technique for an extensive variety of nanoscale particles. Since the potential of ultrafine and, in particular, nanoscale particles in high-performance applications has been identified, scientific and commercial interest has increased immensely, thus identifying this field as a most important technology of the future. However, nanomaterials can perform their multifunctional tasks only if they are customized in terms of chemical composition, size, and morphology to suit the application at hand. Profound knowledge of the synthesis and precise process control is crucial in meeting the stringent specifications. Although the gas-phase synthesis of ultrafine materials has been known and commercially exploited for decades, existing knowledge is based almost exclusively on empirical know-how. Process simulation is a very suitable tool for expanding the understanding of the synthesis-relevant processes, particle formation mechanisms, and operating parameters. Based on the resulting expertise some special nanoscale gas-phase products of high innovative potential have been developed.

Electric Conductivity↗
PubMed2004

Dendritic polyamines: simple access to new materials with defined treelike structures for application in nonviral gene delivery.

Polycationic dendrimers are interesting nonviral vectors for in vitro DNA delivery. We describe a simple approach to the synthesis of dendritic polyamines with different molecular weights and adjustable flexibility (degrees of branching; DB). Both parameters influence the transfection efficiency and the cell toxicity of the polymer. Functionalization of hyperbranched polyethylenimine (PEI) by a two-step procedure generated fully branched pseudodendrimers (analogues of polypropylenimine (PPI) and polyamidoamine (PAMAM) dendrimers). The DNA transfection efficiencies observed for these polymers depended on the cell line investigated. The highest efficiencies were observed for polymers whose unfunctionalized PEI cores had molecular weights in the range M(w)=6000-25 000 g mol(-1). The cytotoxicity of the dendrimers generally rises with increasing core size. The data collected for NIH/3T3 and COS-7 cells indicate a maximum transfection efficiency at around 60 % branching for the PPI analogues, and at a PEI-core molecular weight of M(w)=25 000 g mol(-1). PAMAM functionalization of PEI (M(w)=5000 and 21 000 g mol(-1)) leads to polymers with little or no cytotoxity in the cell lines investigated.

Animals↗
PubMed2004

Syntheses and phase-transfer properties of dendritic nanocarriers that contain perfluorinated shell structures.

Perfect dendrimers that contain perfluorinated shells have recently attracted attention because they have been shown to encapsulate polar molecules in supercritical CO(2) and catalytically active metal nanoparticles in perfluorinated solvents. Moreover, they can then be easily separated after reaction from the biphasic organic/fluorous system. In this paper several dendritic architectures that contain perfluorinated shells were derived by covalent modification of glycerol dendrimers ([G0.5]-[G3.5]), hyperbranched polyglycerol, and polyethyleneimine. These core-shell architectures show interesting physicochemical properties. For example, they are soluble in fluorinated solvents, they are able to transport different guest molecules, and they display thermomorphic behavior. The transport capacity of these molecular nanocarriers increases significantly when amino groups are present in the core. Certain functionalized polyethyleneimines that contain perfluorinated shells show high transport capacities (up to 3 dye molecules per nanocarrier) in perfluorinated solvents. Moreover, these perfluoro-functionalized dendritic polyethyleneimines can act as templates that stabilize nanoparticles; for example, encapsulation and subsequent chemical reduction of Ag(I) ions. Silver nanoparticles with a narrow size distribution (3.9+/-1 nm) have been prepared and characterized by transmission electron microscopy. Furthermore, it has been demonstrated that the encapsulated guest molecules remain accessible to small molecules after transport into the fluorous phase. Therefore, dendritic nanocarriers that contain perfluorinated shells are currently being investigated as polar environments in nonpolar reaction media such as fluorous phases and supercritical CO(2), in particular, for application in homogenous catalysis.

Journal Article↗
PubMed2002

Radiotherapy for chordomas and low-grade chondrosarcomas of the skull base with carbon ions.

PURPOSE: Compared to photon irradiation, carbon ions provide physical and biologic advantages that may be exploited in chordomas and chondrosarcomas. METHODS AND MATERIALS: Between August 1998 and December 2000, 37 patients with chordomas (n = 24) and chondrosarcomas (n = 13) were treated with carbon ion radiotherapy within a Phase I/II trial. Tumor conformal application of carbon ion beams was realized by intensity-controlled raster scanning with pulse-to-pulse energy variation. Three-dimensional treatment planning included biologic plan optimization. The median tumor dose was 60 GyE (GyE = Gy x relative biologic effectiveness). RESULTS: The mean follow-up was 13 months. The local control rate after 1 and 2 years was 96% and 90%, respectively. We observed 2 recurrences outside the gross tumor volume in patients with chordomas. Progression-free survival was 100% for chondrosarcomas and 83% for chordomas at 2 years. Partial remission after carbon ion radiotherapy was observed in 6 patients. Treatment toxicity was mild. CONCLUSION: These are the first data demonstrating the clinical feasibility, safety, and effectiveness of scanning beam delivery of ion beams in patients with skull base tumors. The preliminary results in patients with skull base chordomas and low-grade chondrosarcomas are encouraging, although the follow-up was too short to draw definite conclusions concerning outcome. In the absence of major toxicity, dose escalation might be considered.

Adolescent↗