Induction of osteogenic differentiation by nanostructured alumina surfaces.

Permanent orthopedic implants are becoming increasingly important due to the demographic development. Their optimal osseointegration is key in obtaining good secondary stability. For anchorage dependent cells, topographic features of a surface play an essential role for cell adhesion, proliferation, differentiation and biomineralization. We studied the topographical effect of nanostructured alumina surfaces prepared by chemical vapor deposition on osteogenic differentiation and growth of human osteoblasts. Chemical vapor deposition of the single source precursor (tBuOAIH2)2 led to synthesis of one dimensional alumina nanostructures of high purity with a controlled stoichiometry. We fabricated different topographic features by altering the distribution density of deposited one dimensional nanostructures. Although the topography differed, all surfaces exhibited identical surface chemistry, which is the key requirement for systematically studying the effect of the topography on cells. Forty-eight hours after seeding, cell density and cell area were not affected by the nanotopography, whereas metabolic activity was reduced and formation of actin-fibres and focal adhesions was impaired compared to the uncoated control. Induction of osteogenic differentiation was demonstrated via up-regulation of alkaline phosphatase, bone sialoprotein, osteopontin and Runx2 at the mRNA level, demonstrating the potential of nanostructured surfaces to improve the osseointegration of permanent implants.

Development of multi scale structured Al/AI2O3 nanowires for controlled cell guidance.

Cell responses to surface and contact cell guidance are of great interest in bio-applications especially on nano- and micro scale features. Recently we showed selective cell responses on Al/Al2O3, bi-phasic nanowires (NWs). In this context, Al/Al2O3 NWs were synthesized by the chemical vapor deposition of (tBuOAIH2)2. Afterwards, linear periodic nano- and micro structured NWs were formed using laser interference lithography (LIL) technique to study the contact guidance of neurons from rat dorsal root ganglion (DRG), human umbilical vein smooth muscle cells (HUVSMC), human umbilical vein endothelial cells (HUVEC) and human osteoblast (HOB). LIL treatment did not alter surface chemistry of NWs. From our preliminary research LIL patterned NWs lead to alignment of axons contrary to non-patterned NWs. Morphology of HUVSMC changed from poly- to linear shapes and strong alignment was observed while HUVEC and HOB were not affected.

Huge increase of therapeutic window at a bioactive silver/titania nanocomposite coating surface compared to solution.

A silver containing coating used in the human body, e.g., on an implant should be both effectively antimicrobial and non-cytotoxic to human cells. It is generally believed that the biologic effect originates from silver ions released from the coating. Nanocomposites with well controlled Ag filling factor were prepared by co-sputtering, and the silver surface concentration and the silver release were determined by XPS and ICP-MS, respectively. Here we show that only a small therapeutic window exists for dissolved silver but the therapeutic window is largely increased at the surface. While the toxicity observed for mammalian cells in contact with the bioactive Ag/TiO2 nanocomposite surface and for silver ions in solution is rather similar the antimicrobial activity is drastically enhanced at the surface. A model is proposed to explain the strong increase of the antimicrobial activity at the surface. The present results not only question well-established tests for antimicrobial activity but they are also important for the design of antimicrobial coatings, e.g., for biomedical devices.

Surface segregation in TiO2-based nanocomposite thin films.

The morphology of nanocomposites plays a pivotal role in understanding their functionality and determines their capabilities for applications. The use of nanocomposite coatings requires a study of the size effects on their functional properties. Noble metal nanoparticles are promising candidates for nanocomposite thin film applications due to their antibacterial, plasmonic and photocatalytic properties. In this contribution, the morphology of Ag-TiO(2) and Au-TiO(2) nanocomposite thin films has been investigated experimentally using electron tomography in transmission electron microscopy in combination with UV/vis spectroscopy. Based on the additional 3D information obtained from tomography, we propose a two-step model towards the observed bimodal particle size in these nanocomposite thin films prepared by co-sputtering from two different sources. Furthermore, we show that the optical properties exhibit a well-defined relation with the morphology of the nanocomposite thin films. The present investigations demonstrate the potential of electron tomography for revealing the complex structure and formation processes of functional nanocomposites.

Design of a perfect black absorber at visible frequencies using plasmonic metamaterials.

The design and fabrication of a plasmonic black absorber with almost 100% absorbance spanning a broad range of frequencies from ultraviolet (UV) to the near infrared (NIR) is demonstrated. The perfect plasmonic absorber is achieved by a combination of a metal film with suitable metal/dielectric nanocomposites. Our fabrication technique is simple, versatile, cost-effective, and compatible with current industrial methods for solar absorber production.

Azobenzene-containing triazatriangulenium adlayers on Au(111): structural and spectroscopic characterization.

Adlayers of different azobenzene-functionalized derivatives of the triazatriangulenium (TATA) platform on Au(111) surfaces were studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS), and cyclic voltammetry (CV). The chemical composition of the adlayers is in good agreement with the molecular structure, i.e., different chemical groups attached to the azobenzene functionality were identified. Furthermore, the presence of the azobenzene moieties in the adlayers was verified by the vibration spectra and electrochemical data. These results indicate that the molecules remain intact upon adsorption with the freestanding functional groups oriented perpendicularly to the TATA platform and thus also to the substrate surface.

Reversible light-controlled conductance switching of azobenzene-based metal/polymer nanocomposites.

We present a new concept of light-controlled conductance switching based on metal/polymer nanocomposites with dissolved chromophores that do not have intrinsic current switching ability. Photoswitchable metal/PMMA nanocomposites were prepared by physical vapor deposition of Au and Pt clusters, respectively, onto spin-coated thin poly(methylmethacrylate) films doped with azo-dye molecules. High dye concentrations were achieved by functionalizing the azo groups with tails and branches, thus enhancing solubility. The composites show completely reversible optical switching of the absorption bands upon alternating irradiation with UV and blue light. We also demonstrate reversible light-controlled conductance switching. This is attributed to changes in the metal cluster separation upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress.

Model systems with extreme aspect ratio, tunable geometry, and surface functionality for a quantitative investigation of the Lotus effect.

Superhydrophobicity and superhydrophilicity of surfaces are key properties for fabrication of self-cleaning surfaces (Lotus effect). It is well known that the mechanism behind this is based on the surface roughness and surface functionalization. To obtain an understanding of the details of the underlying mechanism, a metal system based on a eutectic is suggested. In this study, a wide range tunability of its needlelike narrow size distributed nanostructure is demonstrated. The length of the needles as well as their density can be varied independently. In addition, an important parameter for the wettability, the roughness, is related directly to the growth parameters, which lead to excellent controllable and reproducible eutectic structures. Simply by varying etching time very high aspect ratios can be achieved, allowing studying the interaction of the very long needles with liquids. Moreover, the surface functionality can be tuned by RF-magnetron sputtering of PTFE onto the metal needles. As those layers can be very thin, our system allows, in principle, studying the transition from a metal to a polymer surface using submonolayers. Furthermore, the first contact angle measurements on the nanostructured and functionalized eutectic structures are presented and discussed.