Wendelin J. Stark

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (220)867.08 Total impact

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    ABSTRACT: Mechanistic and kinetic insights into the removal of soluble nanoparticles as templates for mesopores now permit scale-up of mesoporous polymer membranes. Investigations on the effect of pH and nanoparticle dissolution time showed that pH levels of 0 or 1 are necessary to enable fast pore template dissolution. Approximately 5 wt % of the originally applied nanoparticles remained in the membrane due to complete encapsulation by the polymer matrix but did not contaminate the permeate during prolonged use. We demonstrated continuous production of 17 m2 (100 m length) of membrane using a commercial roll-to-roll coating unit. These membranes were successfully applied in gravitation driven water filtration. The rates of rejection of bacteria from heavily contaminated natural pond water were higher than 99.99%.
    Industrial & Engineering Chemistry Research 01/2014; 53(22):9214–9220. · 2.24 Impact Factor
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    ABSTRACT: Objectives To investigate chemo-mechanical effects of incorporating alkaline bioactive glass nanoparticles into a light-curable dental resin matrix. Methods An unfilled Bis-GMA/TEGDMA material was infiltrated with up to 20 wt% of ultrafine SiO2–Na2O–CaO–P2O5–Bi2O3 particles. The unfilled and filled resins were investigated regarding their viscosity before setting and compared to commercially available materials. Set specimens were immersed for 21 days in phosphate buffered saline at 37 °C. Water uptake, pH, Knoop hardness, and degree of conversion of freshly polymerized and stored samples were investigated. Resin surfaces were viewed and mapped in a scanning electron microscope for the formation of calcium phosphate (Ca/P) precipitates. In addition, Raman spectroscopy was performed. Numeric values were statistically compared (p < 0.01). Results Viscosity increased with particle loading, but remained below that of a flowable dental composite material. Water uptake into and pH induction from the polymerized samples also increased with particle loading (p < 0.01). The addition of 20 wt% nanoparticles had no significant influence on microhardness, yet it slightly (p < 0.01) increased the degree of conversion after 21 days. Ca/P precipitates formed on specimens filled with 20 wt% of the particles, while they were scarce on counterparts loaded with 10 wt%, and absent on unfilled resin surfaces. Significance The results of the current study show that a Bis-GMA-based resin can be functionalized using alkaline nanoparticles. A material with bioactive properties and similar hardness as the unfilled resin was obtained by incorporating 20 wt% of ultrafine SiO2–Na2O–CaO–P2O5–Bi2O3 particles into the resin matrix.
    Dental Materials. 01/2014;
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    ABSTRACT: Wheat is the predominant crop in the world and vermin infestation remains a serious issue regarding its storage and field cultivation, especially in developing countries. In addition to physical control methods, pesticides are often applied. However, their usage includes a number of concerns regarding environmental and ecological impacts. In the present work an alternative route to protect seeds against herbivore attack is suggested. The seeds were coated with substances capable of cyanogenesis. The precursors were initially in isolated compartments separated through biodegradable polylactide layers. The HCN formation only occurred upon contact of the cyanogenic precursor (mandelonitrile) with a suitable enzyme (hydroxynitrile lyase) and thus needed to be mechanically triggered. This concept is inspired by nature and is based on the protection strategy applied by higher plants, for example apple trees. Further tests showed that the ability for germination was preserved throughout the treatment. Finally, cyanogenesis was followed and quantified in both the liquid and the gas phase and provided HCN in sufficient concentrations to serve as a pest control.
    J. Mater. Chem. A. 12/2013; 2(3).
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    ABSTRACT: Chest-wall invading malignancies usually necessitate the resection of the respective part of the thoracic wall. Gore-Tex® is the material of choice that is traditionally used to repair thoracic defects. This material is well accepted by the recipient; however, though not rejected, it is an inert material and behaves like a 'foreign body' within the thoracic wall. By contrast, there are materials that have the potential to physiologically integrate into the host, and these materials are currently under in vitro and also in vivo investigation. These materials offer a gradual but complete biodegradation over time, and severe adverse inflammatory responses can be avoided. Here, we present a novel material that is a biodegradable nanocomposite based on poly-lactic-co-glycolic acid and amorphous calcium phosphate nanoparticles in comparison to the traditionally employed Gore-Tex® being the standard for chest-wall replacement. On a mouse model of thoracic wall resection, that resembles the technique and localization applied in humans, poly-lactic-co-glycolic acid and amorphous calcium phosphate nanoparticles and Gore-Tex® were implanted subcutaneously and additionally tested in a separate series as a chest-wall graft. After 1, 2, 4 and 8 weeks cell infiltration into the respective materials, inflammatory reactions as well as neo-vascularization (endothelial cells) were determined in six different zones. While Gore-Tex® allowed for cell infiltration only at the outer surface, electrospun poly-lactic-co-glycolic acid and amorphous calcium phosphate nanoparticles were completely penetrated by infiltrating cells. These cells were composed mainly by macrophages, with only 4% of giant cells and lymphocytes. Total macrophage count increased by time while the number of IL1-β-expressing macrophages decreased, indicating a protective state towards the graft. As such, poly-lactic-co-glycolic acid and amorphous calcium phosphate nanoparticles seem to develop ideal characteristics as a material for chest-wall replacement by (a) having the advantage of full biodegradation, (b) displaying stable chest-wall structures and (c) adapting a physiological and integrating graft compared to Gore-Tex®.
    Journal of Biomaterials Applications 11/2013; · 2.76 Impact Factor
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    ABSTRACT: Movement is a key characteristic of higher organisms. During mammalian embryogenesis fetal movements have been found critical to normal tissue development. On the single cell level, however, our current understanding of stem cell differentiation concentrates on inducing factors through cytokine mediated biochemical signaling. In this study, human mesenchymal stem cells and chondrogenesis were investigated as representative examples. We show that pressureless, soft mechanical stimulation precipitated by the cyclic deformation of soft, magnetic hydrogel scaffolds with an external magnetic field, can induce chondrogenesis in mesenchymal stem cells without any additional chondrogenesis transcription factors (TGF-β1 and dexamethasone). A systematic study on the role of movement frequency revealed a classical dose-response relationship for human mesenchymal stem cells differentiation towards cartilage using mere mechanical stimulation. This effect could even be synergistically amplified when exogenous chondrogenic factors and movement were combined.
    PLoS ONE 11/2013; 8(11):e81362. · 3.53 Impact Factor
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    ABSTRACT: Palladium nanoparticles are deposited on the surface of highly magnetic carbon-coated cobalt nanoparticles. In contrast to the established synthesis of Pd nanoparticles via reduction of Pd(II) precursors, the microwave decomposition of a Pd(0) source leads to a more efficient Pd deposition, resulting in a material with considerably higher activity in the hydrogenation of alkenes. Systematic variation of the Pd loading on the carbon-coated cobalt nanoparticle surface reveals a distinct trend to higher activities with decreased loading of Pd. The activity of the catalyst is further improved by the addition of 10 vol.% diethyl-ether to iso-propanol that is found to be the solvent of choice. With respect to activity (turnover frequencies up to 11095/h), handling, recyclability through magnetic decantation, and leaching of Pd (≤6 ppm/cycle), this novel magnetic hybrid material compares favorably to conventional Pd/C or Pd@CNT catalysts.
    Advanced Functional Materials 11/2013; · 10.44 Impact Factor
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    ABSTRACT: The present study evaluates the effect of heat treatment on electrospun poly(lactide‐co‐glycolide) fibrous membranes. Both a temperature (75–150 °C) and a treatment time range (5–40 min) are tested. The effect on the fibrous structure is investigated in terms of morphology, showing that with increasing temperature or longer treatment time the fusion of fibres progresses continuously. Additionally, the tensile properties of the various scaffolds deliver results on the effect of increasing fibre‐to‐fibre linkages. Both modulus and yield increase within the heat treatment procedures. The elevated stiffness of the membranes accompanies a loss in porosity. These findings deliver insights into the tailoring of membranes that might be used in the fabrication of customised scaffolds intended for cell culture in tissue engineering.
    Macromolecular Materials and Engineering 11/2013; 298(11). · 2.78 Impact Factor
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    ABSTRACT: Nanomaterials are increasingly suggested for selective adsorption and extraction of complex compounds in biomedicine. Binding of the latter requires specific surface modifications of the nanostructures. However, even complicated macromolecules such as proteins can afford affinities towards basic surface characteristics such as hydrophobicity, topology and electrostatic charge. In this study we address to these more basic physical interactions. In a model system, the interaction of bovine serum albumin and amyloid β 42 fibrillar aggregates with carbon coated cobalt nanoparticles, functionalized with various polymers differing in character was studied. The possibility of rapid magnetic separation upon binding to the surface represents a valuable tool to study surface interactions and selectivities. We find that the surface interaction of Aβ 42 fibrillar aggregates is of mostly hydrophobic nature. Since bovine serum albumin (BSA) is conformationally adaptive, it is known to bind surfaces with widely differing properties (charge, topology and hydrophobicity). However, the rate of tight binding (no desorption upon washing) can vary largely depending on the extent of necessary conformational changes for a specific surface. We found that BSA can only bind slowly to polyethylenimine coated nanomagnets. Under competitive conditions (high BSA excess compared to β 42 fibrillar aggregates), this effect is beneficial for targeting the fibrillar species. These findings highlight the possibility of selective extractions from complex media when advantageous basic physical surface properties are chosen.
    Langmuir 10/2013; · 4.38 Impact Factor
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    ABSTRACT: The combination of force and flexibility enables controlled and soft movements. In sharp contrast, presently used machines are solid and mostly based on stiff driveshafts or cog wheels. Magnetic elastomers are realized through dispersion of small particles in polymer matrices and have attracted significant interest as soft actuators for controlled movement or conveying and are particularly attractive candidates for magnetic pump applications. At present, low magnetic particle loading and thus limited actuator strength have restricted the application of such materials. Here, the direct incorporation of metal microparticles into a very soft and flexible silicone and its application as an ultra-flexible, yet strong magnetic tube, is described. Because metals have a far higher saturation magnetization and higher density than oxides, the resulting increased force/volume ratio afforded significantly stronger magnetic actuators with high mechanical stability, flexibility, and shape memory. Elliptical inner diameter shape of the tubing allowed a very efficient contraction of the tube by applying an external magnetic field. The combination of magnetic silicone tubes and a magnetic field generating device results in a magnetic peristaltic pump.
    Advanced Functional Materials 08/2013; 23(31). · 10.44 Impact Factor
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    ABSTRACT: In a number of clinical conditions such as intoxication, bacteraemia or autoimmune diseases the removal of the disease-causing factor from blood would be the most direct cure. However, physicochemical characteristics of the target compounds limit the applicability of classical filtration and diffusion-based processes. In this work, we present a first in vivo magnetic blood purification rodent animal model and demonstrate its ability to rapidly clear toxins from blood circulation using two model toxins with stable plasma levels (lead (Pb(2+)) and digoxin). Ultra-strong functionalized metal nanomagnets are employed to eliminate the toxin from whole blood in an extracorporeal circuit. In the present experimental demonstration over 40% of the toxin (i.e. lead or digoxin) was removed within the first 10 minutes and over 75% within 40 minutes. After capturing the target substance, a magnetic trap prevents the toxin-loaded nanoparticles from entering the blood circulation. Elemental analysis and magnetic hysteresis measurements confirm full particle recovery by simple magnetic separation (residual particle concentration below 1 μg mL(-1) (detection limit)). We demonstrate that magnetic separation-based blood purification offers rapid blood cleaning from noxious agents, germs or other deleterious materials with relevance to a number of clinical conditions. Based on this new approach, current blood purification technologies can be extended to efficiently remove disease-causing factors, e.g. overdosed drugs, bacteria or cancer cells without being limited by filter cut-offs or column surface saturation.
    Nanoscale 07/2013; 5(18). · 6.74 Impact Factor
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    ABSTRACT: Biomaterials made of biodegradable poly(α-hydroxyesters) like poly(lactide-co-glycolide) (PLGA) are known to decrease the pH in the vicinity of the implants. Bioactive glass (BG) is being investigated as a counteracting agent buffering the acidic degradation products. However, in dentistry the question arises whether an antibacterial effect is rather obtained from pure PLGA or from BG/PLGA composites, as BG has been proved antimicrobial. In the present study the antimicrobial properties of electrospun PLGA and BG45S5/PLGA fibres were investigated using human oral bacteria (specified with mass spectrometry) incubated for up to 24 h. BG45S5 nanoparticles were prepared by flame spray synthesis. The change in colony-forming units (CFU) of the bacteria was put into correlation with the pH of the medium during incubation. Morphology and structure of the scaffolds as well as the appearance of the bacteria were followed by microscopy. Additionally, we studied if the presence of BG45S5 had an influence on the degradation speed of the polymer. Finally, it turned out that the pH increase induced by BG45S5 presence in the scaffold did not last long enough to show a reduction in CFU. On the contrary, pure PLGA demonstrated antibacterial properties that should be taken into consideration when designing biomaterials for dental applications.
    Acta biomaterialia 06/2013; · 5.68 Impact Factor
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    ABSTRACT: Unprecedented magnetic borohydride exchange (mBER), magnetic Wang aldehyde (mWang) and magnetic amine resins were prepared from highly magnetic polymer-coated cobalt or iron nanoparticles. Microwave irradiation was used to obtain excellent degrees of functionalization (>95 %) and loadings (up to 3.0 mmol g(-1) ) in short reaction times of 15 min or less. A small library of ureas and thioureas was synthesized by the exclusive application of these magnetic resins. As a first step, a reductive amination of aromatic and aliphatic aldehydes was carried out with mBER. The excess of primary amine needed to complete the reaction was subsequently scavenged selectively by mWang. Simple magnetic decantation from the resins resulted in secondary amines in good to excellent yields and purities. The used magnetic resins were efficiently regenerated and reused for the next run. In a second step, the secondary amines were converted to trisubstituted (thio)ureas in excellent yields and purities by stirring with an excess of iso(thio)cyanate, which was scavenged by addition of the magnetic amine resin after completion of the reaction. The whole reaction sequence is carried out without any purification apart from magnetic decantation; moreover, conventional magnetic stirring can be used as opposed to the vortexing required for polystyrene resins.
    Chemistry - A European Journal 06/2013; · 5.93 Impact Factor
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    ABSTRACT: Magnetic separation-based blood purification enables rapid and selective removal of endotoxin and bacteria from human whole blood. Functional assays assessing key steps in host defense against bacteria show an attenuated inflammatory response in purified samples and less chemotactic activity. Beatrice Beck-Schimmer and co-workers introduce on page 829 this promising new approach for future sepsis treatment.
    Advanced healthcare materials. 06/2013; 2(6):828.
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    ABSTRACT: Magnetic nanomaterials find increasing application as separation agents to rapidly isolate target compounds from complex biological media (i.e., blood purification). The responsiveness of the used materials to external magnetic fields (i.e., their saturation magnetization) is one of the most critical parameters for a fast and thorough separation. In the present study, magnetite (Fe3O4) and non-oxidic cementite (Fe3C) based carbon-coated nanomagnets are characterized in detail and compared regarding their separation behavior from human whole blood. A quantification approach for iron-based nanomaterials in biological samples with strong matrix effects (here, salts in blood) based on platinum spiking is shown. Both materials are functionalized with polyethyleneglycol (PEG) to improve cytocompatibility (confirmed by cell toxicity tests) and dispersability. The separation performance is tested in two setups, namely under stationary and different flow-conditions using fresh human blood. The results reveal a superior separation behavior of the cementite based nanomagnets and strongly suggest the use of nanomaterials with high saturation magnetizations for magnetic retention under common blood flow conditions such as in veins.
    Advanced Functional Materials 04/2013; 23(39):4888. · 10.44 Impact Factor
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    ABSTRACT: Deinking is a fundamental part of paper recycling. As the global paper consumption rises and exceeds even the annual paper production, recycling of this raw material is of high importance. Magnetic ink based on carbon coated magnetic nanoparticles enables an alternative approach to state of the art paper deinking. Magnetic deinking comprises three steps (pre- selection, washing and magnetic separation of fibers). Pre-separation of printed from non-printed scraps of paper is feasible and reduces the paper mass which has to be fed into a deinking process. A consecutively washing process removes surficial magnetic ink that can be collected by application of a permanent magnet. Still printed parts are subjected to a further continuous magnetic deinking step, where magnetic and non- magnetic paper fibers can be separated. Magnetic deinking of a model print allows recovery of more than 80% of bright fibers without any harsh chemical treatment and the re- collection of more than 82% of magnetic ink.
    Langmuir 03/2013; · 4.38 Impact Factor
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    ABSTRACT: An important aspect in risk assessment of nanoparticles (NPs) is to understand their environmental interactions. We used hydroponic plant cultures to study nanoparticle-plant-root interaction and translocation and exposed wheat and pumpkin to suspensions of uncoated CeO(2)-NP for 8d (primary particle size 17-100nm, 100mgL(-1)) in the absence and presence of fulvic acid (FA) and gum arabic (GA) as representatives of different types of natural organic matter. The behavior of CeO(2)-NPs in the hydroponic solution was monitored regarding agglomeration, sedimentation, particle size distribution, surface charge, amounts of root association, and translocation into shoots. NP-dispersions were stable over 8d in the presence of FA or GA, but with growing plants, changes in pH, particle agglomeration rate, and hydrodynamic diameter were observed. None of the plants exhibited reduced growth or any toxic response during the experiment. We found that CeO(2)-NPs translocated into pumpkin shoots, whereas this did not occur in wheat plants. The presence of FA and GA affected the amount of CeO(2) associated with roots (pure>FA>GA) but did not affect the translocation factor. Additionally, we could confirm via TEM and SEM that CeO(2)-NPs adhered strongly to root surfaces of both plant species.
    Chemosphere 01/2013; · 3.14 Impact Factor
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    ABSTRACT: This work describes a magnetic separation-based approach using polymyxin B-functionalized metal alloy nanomagnets for the rapid elimination of endotoxins from human blood in vitro and functional assays to evaluate the biological relevance of the blood purification process. Playing a central role in gram-negative sepsis, bacteria-derived endotoxins are attractive therapeutic targets. However, both direct endotoxin detection in and removal from protein-rich fluids remains challenging. We present the synthesis and functionalization of ultra-magnetic cobalt/iron alloy nanoparticles and a magnetic separation-based approach using polymyxin B-functionalized nanomagnets to remove endotoxin from human blood in vitro. Conventional chromogenic Limulus Amebocyte Lysate assays confirm decreased endotoxin activity in purified compared to untreated samples. Functional assays assessing key steps in host defense against bacteria show an attenuated inflammatory mediator expression from human primary endothelial cells in response to purified blood samples compared to untreated blood and less chemotactic activity. Exposing Escherichia coli-positive blood samples to polymyxin B-functionalized nanomagnets even impairs the ability of gram-negative bacteria to form colony forming units, thus making magnetic separation based blood purification a promising new approach for future sepsis treatment.
    Advanced healthcare materials. 12/2012; 2(6).
  • Fabian M Koehler, Wendelin J Stark
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    ABSTRACT: Graphene is a two-dimensional crystalline carbon allotrope that has fascinated researchers worldwide and has extended the interest in carbon structures such as fullerenes and nanotubes. In this Account, we use electrical characterization tools to study chemistry on supported graphene. These experiments elucidate the way covalently bound phenyl units can change graphene's physical properties. Can we use chemistry to control electronic properties of graphene? What can we learn from well-known carbon allotropes like fullerenes? The surfaces of fullerenes and graphene show distinct differences in reactivity because of the high strain of sp(2) carbon in fullerenes compared with the complete lack of strain in graphene. Diazonium chemistry provides a versatile tool for attaching phenyl units covalently to carbon to produce advanced materials and electronic components, but diazonium-based carbon chemistry is strongly influenced by strain. Although fullerenes are highly reactive, graphite (stacks of graphene) remains relatively inert. We chemically introduce n- and p-like doping patterns in two-dimensional graphene using photolithography and extend the ability to chemically control doping to the chemical design of conducting and insulating areas. Thereby we can shape graphene surfaces into functional electronic devices. This Account also describes multistep synthesis on graphene-coated nanoparticles and the introduction of various functional groups on graphene surfaces. Only few functional groups can be produced directly via diazonium chemistry. To overcome this issue, we used these functional groups as starting points for more demanding organic reactions. We covalently attached chelating agents, catalysts, or polymers on the carbon surface. These more complex reactions facilitate the design of electronic modifications, intergraphene connections, and anchors for polymer incorporation. Diazonium chemistry forms strong covalent bridges between graphene and other areas of chemistry.
    Accounts of Chemical Research 12/2012; · 24.35 Impact Factor
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    ABSTRACT: Living materials: Artificial biological niches are loaded with the penicillin-producing mold Penicillium chrysogenum. This living material consumes food through a nanoporous top layer and releases the antibiotic on-site. No reloading of the active compound is needed. Gram-positive bacteria were efficiently killed if nearby, whereas Gram-negative bacteria (control experiment, not sensitive to penicillin) were not affected.
    Angewandte Chemie International Edition 10/2012; · 11.34 Impact Factor
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    ABSTRACT: Graphene-coated cobalt nanoparticles surface-functionalized with benzylamine groups (CoC-NH(2) nanomagnets) were shown to effectively enrich analytes for surface-assisted laser desorption/ionization mass spectrometry (affinity SALDI-MS) analysis. These CoC-NH(2) nanomagnets are highly suited for use with affinity SALDI-MS because their mean diameter of 30 nm, high specific surface area of 15 m(2) g(-1), and high-strength saturation magnetization of 158 emu g(-1) led to efficient extraction of analytes by magnetic separation, which in turn enabled excellent SALDI-MS performance. Surface modification of CoC nanomagnets with benzylamine groups increased the yield of peptide ions and decreased fragmentation of benzylpyridinium ions, so-called "thermometer ions" formed through soft ionization. The CoC-NH(2) nanomagnets were used to extract perfluorooctanesulfonate from large volumes of aqueous solutions by magnetic separation, which was identified directly by SALDI-MS analysis with high sensitivity even at the sub-part-per-trillion level (∼0.1 ng/L). The applicability of CoC-NH(2) nanomagnets in conjunction with SALDI-MS for the enrichment and detection of pentachlorophenol, bisphenol A, and polyfluorinated compounds (PFCs) with varying chain length, which are environmentally significant compounds, as well as small drugs, was also evaluated.
    Analytical Chemistry 09/2012; · 5.83 Impact Factor

Publication Stats

4k Citations
867.08 Total Impact Points


  • 2010–2014
    • Universität Regensburg
      • Institute of Organic Chemistry
      Ratisbon, Bavaria, Germany
    • Imperial College London
      • Department of Materials
      London, ENG, United Kingdom
  • 2011–2013
    • University of Zurich
      • Institut für Anästhesiologie
      Zürich, ZH, Switzerland
  • 2002–2012
    • ETH Zurich
      • • Department of Chemistry and Applied Biosciences
      • • Institute for Chemical and Bioengineering
      • • Institute of Process Engineering
      Zürich, ZH, Switzerland
    • Eawag: Das Wasserforschungs-Institut des ETH-Bereichs
      Duebendorf, Zurich, Switzerland
  • 2009
    • Universität Bern
      • Institut für Anatomie
      Bern, BE, Switzerland