Wendelin J. Stark

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (268)1072.33 Total impact

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    ABSTRACT: A novel technique for transforming cubic α-NaYF4 phosphors into the hexagonal modification is presented for the removal of O impurities that hinder upconversion luminescence.
    ChemInform 06/2014; 45(22).
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    ABSTRACT: Background Fractures with a critical size bone defect are associated with high rates of delayed- and non-union. The treatment of such complications remains a serious issue in orthopedic surgery. Adipose derived stem cells (ASCs) combined with biomimetic materials can potentially be used to increase fracture healing. Nevertheless, a number of requirements have to be fulfilled; in particular the insufficient vascularization of the bone constructs. Here, the objectives were to study the impact of ASC-derived osteoblasts on ASC-derived endothelial cells in a 3D co-culture and the effect of 40 wt % of amorphous calcium phosphate nanoparticles on the proliferation and differentiation of ASC-derived endothelial cells when present in PLGA. Materials and Methods Five primary ASC lines were differentiated towards osteoblasts (OB) and endothelial cells (EC) and two of them were chosen based on quantitative PCR results. Either a mono-culture of ASC-derived EC or a co-culture of ASC-derived EC with ASC-derived OB (1:1) was seeded on an electrospun nanocomposite of poly-(lactic-co-glycolic acid) and amorphous calcium phosphate nanoparticles (PLGA/a-CaP; reference: PLGA). The proliferation behavior was determined histomorphometrically in different zones and the expression of von Willebrand Factor (vWF) was quantified. Results Independently of the fat source (biologic variability), ASC-derived osteoblasts decelerated the proliferation behavior of ASC-derived endothelial cells in the co-culture compared to the mono-culture. However, expression of vWF was clearly stronger in the co-culture, indicating further differentiation of the ASC-derived EC into the EC lineage. Moreover, the presence of a-CaP nanoparticles in the scaffold slowed the proliferation behavior of the co-culture cells, too, going along with a further differentiation of the ASC-derived OB, when compared to pure PLGA scaffolds. Conclusions This study revealed significant findings for bone tissue-engineering. Co-cultures of ASC-derived EC and ASC-derived OB stimulate each other's further differentiation. A nanocomposite with a-CaP nanoparticles offers higher mechanical stability, bioactivity and osteoconductivity compared to mere PLGA and can easily be seeded with pre-differentiated EC and OB.
    Injury 06/2014; · 2.46 Impact Factor
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    ABSTRACT: The bombardier beetle uses attack-triggered mixing of reactants (hydrochinone, hydrogen peroxide (H2O2) and enzymes as catalysts) to defend itself against predators. Using multi-layer polymer sheets with H2O2 and catalyst (MnO2) filled compartments we developed a 2D analogous bio-inspired chemical defence mechanism for anti-vandalism applications. The reactants were separated by a brittle layer that ruptures upon mechanical attack and converts the mechanical energy trigger (usually a few Joules) into a chemical self-defence reaction involving release of steam, and optionally persistent dyes and a DNA-based marker for forensics. These surfaces effectively translate a weak mechanical trigger into an energetic chemical reaction with energy amplification of several orders of magnitude. Since the responsive materials presented here do not depend on electricity, they may provide a cost effective alternative to currently used safety systems in the public domain, automatic teller machines and protection of money transport systems. Anti-feeding protection in forestry or agriculture may similarly profit from such mechanically triggered chemical self-defending polymer surfaces.
    J. Mater. Chem. A. 05/2014; 2(22).
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    ABSTRACT: Purpose: A low direct current can be used to disinfect dental titanium implants in simulated physiologic environments. The aim of this study was to determine whether this treatment affects implant surface structure and cytocompatibility. Materials and Methods: Titanium test disks with a sandblasted, acid-etched, large-grit (SLA) surface were placed as anodes in an electrolytic bath with physiologic saline and treated with 15 mA of current for 15 minutes. Surfaces were analyzed by light and electron microscopy and contact angle measurement. Depth profile analyses of SLA disks were run at subsurface levels from 0 to 1,000 nm. The proliferation and viability of preosteoblastic cells and human foreskin fibroblasts on implant surfaces were assessed. Alkaline phosphatase (ALP) activity was determined with and without exposure to bone morphogenetic protein-2 (BMP-2). Mineralization was determined after 4 weeks. Results: A blue discoloration was observed after treating the SLA disks, but no damage was recognized microscopically. An oxidation layer formed on the surface and the wettability of the disks increased significantly. Cell proliferation and initial maturation were not affected by the treatment. Mineralization and ALP activity of BMP-exposed cells, however, were slightly but significantly reduced on test disks. Conclusions: The current study showed that the alterations in implant color after electrochemical treatment did not reflect significant surface changes, which would preclude cell adhesion and growth or have a major impact on osteoblastic differentiation or maturation.
    The International journal of oral & maxillofacial implants 05/2014; 29(3):735-42. · 1.49 Impact Factor
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    ABSTRACT: Intravascular application of magnetic nanocarriers is a critical step in the development of new therapeutic strategies, including magnetic drug targeting or hyperthermia. However, injection of particulate matter bears the intrinsic risk of contact activation of the blood coagulation cascade. In this work, we use point-of-care assays to study coagulation dynamics and clotting parameters in blood samples exposed to relevant concentrations of surface-functionalized carbon-coated iron carbide nanomagnets using unmodified nanomagnets and poly(ethylene)glycol-functionalized nanomagnets with different end-groups, including –OCH3, –NH2, –COOH, –IgG, and –ProteinA-protected-IgG (–IgG-ProtA). Silica nanoparticles with a comparable surface area are used as a reference material. For magnetic nanoparticles, we observe a decrease in clotting time by 25% compared to native blood at concentrations of 1 mg mL−1, independent of the surface functionalization, and only minor differences in receptor expression on platelets (GP-IIb-IIIa, CD62, and CD63) relative to control samples were observed. Interestingly, the inter-subject variance of the clotting time is similar to the nanoparticle-induced effect in a single subject with average clotting time. Whilst the present study is based on in vitro assays and a small group of healthy blood donors, the comparison to broadly used silica nanoparticles, and the fact that experimental intergroup variability is comparable to the observed effects from the carbon-coated nanomagnets suggests continuing investigations on their potential clinical use.
    Journal of Materials Chemistry 04/2014; · 6.63 Impact Factor
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    ABSTRACT: The luxury angora rabbit fiber stands out from other animal hair. Its porous structure combines exceptional insulation and comfortable wear. On page 1831, W. J. Stark and co‐workers present a method by which a structurally equivalent protein filament can be synthesized in a continuous process. Raw material slaughterhouse waste‐derived gelatin is utilized, adding value to this low‐cost biopolymer.
    Advanced Functional Materials 04/2014; 24(13). · 10.44 Impact Factor
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    ABSTRACT: In the pursuit of robust and reusable biocatalysts for industrial synthetic chemistry, nanobiotechnology is cur-rently taking a significant part. Recently, enzymes have been immobilized on different nanoscaffold supports. Carbon coated metallic nanoparticles were found to be a practically useful support for enzyme immobilization due to their large surface area, high magnetic saturation and manipulative surface chemistry. In this study carbon coated cobalt nanoparticles were chemically functionalized (diazonium chemistry), activated for bioconjugation (N,N-Disuccinimidyl carbonate) and subsequently used in enzyme immobilization. Three enzymes: β-glucosidase, α-chymotrypsin and lipase B were successfully covalently immobilized on the magnetic nonsupport. The enzyme-particle conjugates formed retained their activity and stability after immobilization and were effi-ciently recycled from milliliter to liter scales in short recycle times.
    Bioconjugate Chemistry 03/2014; · 4.82 Impact Factor
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    ABSTRACT: Applications of upconversion phosphors have grown extensively in number during the past decade. Hexagonal sodium yttrium fluoride (β-NaYF4) is known to be one of the best host lattices for upconversion materials. We developed a novel technique for transforming cubic sodium yttrium fluoride (α-NaYF4) phosphors into the hexagonal modification and remove oxygen impurities that hinder the upconversion luminescence. We transformed cubic α-NaYF4 nanoparticles from flame-spray synthesis with a particle size less than 50 nm into more efficient β-NaYF4 phosphors. The application of SnF2 and ZnF2 as oxygen scavengers allowed the formation of the pure hexagonal phase and improved the upconversion luminescence intensity. The developed process utilizes no free HF gas in the production and does not contaminate the upconversion phosphors with scavenger material. The treatment increases the particle size to between approximately 500 nm and 1 μm. Upconversion luminescence spectra revealed the characteristic blue Tm3+ and green Er3+ emissions of β-NaYF4: Yb,Tm and Yb,Er, respectively.
    Chemistry of Materials 03/2014; 26(6):2015–2020. · 8.54 Impact Factor
  • Journal of Non-Crystalline Solids 03/2014; 387:41-46. · 1.72 Impact Factor
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    ABSTRACT: A method to encapsulate DNA in heat-resistant and inert magnetic particles was developed. An inexpensive synthesis technique based on co-precipitation was utilized to produce Fe2O3 nanoparticles, which were further functionalized with ammonium groups. DNA was adsorbed on this magnetic support and the DNA/magnet nanocluster was surface coated with a dense silica layer by sol-gel chemistry. The materials were further surface modified with hexyltrimethoxysilane to achieve particle dispersibility in hydrophobic liquids. The hydrodynamic particle sizes were evaluated by analytical disc-centrifugation and the magnetic properties were investigated by vibrating sample magnetometry. The obtained nanoengineered encapsulates showed good dispersion abilities in various non-aqueous fluids and did not affect the optical properties of the hydrophobic dispersant when present at concentrations lower than 1000 µg/L. Upon magnetic separation and particle dissolution, the DNA could be recovered unharmed and was analyzed by quantitative real-time PCR and Sanger sequencing. DNA encapsulated within the magnetic particles was stable for 2 years in decalin at room temperature and the stability was further tested at elevated temperatures. The new magnetic DNA/silica encapsulates were utilized to developed a low-cost platform for the tracing/tagging of oils and oil derived products, requiring 1 µg/L = 1 ppb levels of the taggant and allowing quantification of taggant concentration on a logarithmic scale. The procedure was tested for the barcoding of a fuel (gasoline), a cosmetic oil (bergamot oil), and a food grade oil (extra virgin olive oil), being able to verify the authenticity of the products.
    ACS Nano 02/2014; · 12.03 Impact Factor
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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: In this article, we describe the application of surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with the use of amine functionalized graphene-coated cobalt nanoparticles (CoC–NH2 nanoparticles) to analyse aromatic hydrophobic compounds that are known environmental contaminants, including polycyclic aromatic hydrocarbons (PAHs) and pentachlorophenol (PCP). Our results demonstrated that SALDI-MS can detect PCP, anthracene, and pyrene in water. In particular, the CoC–NH2 nanoparticles proved to be an e cient means of capturing PCP in water because of the high adsorption capacity of the nanoparticles for PCP, which resulted in a detectability of 100 ppt. Furthermore, the CoC– NH2 nanoparticles also functioned as an adsorbent for solid-phase extraction of per uorooctane sulfonate (PFOS) from human serum, displaying good performance with a detectability of 10 ppb by SALDI-MS.
    Mass Spectrometry. 01/2014; 3:A0028.
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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: Due to their porous structure, angora rabbit fibers make for some of the highest quality wool. The application of these fibers on a technical scale is not feasible due to their limited availability and high price. Here, a robust fiber preparation method is reported based on an unusual spinning process, where a non-equilibrated, ternary system of protein, solvent, and non-solvent is continuously processed into strong fibers with minimal energy input and harmless solvents. Gelatin - the degradation product of collagen - is chosen as the protein component because of its immense availability from slaughterhouse waste. Due to the sponge-like structure of the ternary gelatin/water/2-propanol spinning mixture, fibers with internal cavities are produced. The porous nature of these fibers resembles the morphology of angora rabbit fibers. Despite their high porosity, the here-obtained gelatin fibers show clear re-orientation of the fibrous protein and attain a mechanical performance similar to other bio- (e.g., wool, tendon collagen) and synthetic polymers (e.g., polytetrafluoroethylene, polyamide 6). These promising results motivate for broader investigations on the spinning of non-equilibrium protein mixtures and suggest the use of porous gelatin fibers in textiles.
    Advanced Functional Materials 11/2013; 24(13). · 10.44 Impact Factor
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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: On page 6057, W. J. Stark and co-workers present a chemically active filtration membrane that removes bacteria and the essential growth nutrient phosphate in a single filtration step. This allows the suppression of the microbial regrowth processes and thus, provides a simple device for the preparation of drinking water and subsequent safe storage.
    Advanced Materials 11/2013; 25(42):5989. · 15.41 Impact Factor

Publication Stats

6k Citations
1,072.33 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
  • 2008
    • Ijuí Charity Hospital
      Ijuhy, Rio Grande do Sul, Brazil