Bert Müller

Strained-layer growth and islanding of germanium on Si(111)-(7 × 7) studied with STM

Abstract The growth of in situ prepared germanium layers on Si(111)-(7 × 7) has been studied as a function of substrate temperature and coverage. At room temperature, Ge grows in irregular clusters arranged in an ordered array on the substrate and the (7 × 7) reconstruction is preserved. At elevated temperature, in the submonolayer range triangular islands form with preferred growth in [ 1 1 2] direction. The islands show Si-like (7 × 7) and (5 × 5) DAS reconstruction. Ge nucleates preferentially at step edges and at (7 × 7) domain boundaries. Coverages over 2 ML result in a completely (5 × 5) reconstructed layer. On substrate with a ( 3 × 3 ) R 30° adatom arrangement after boron segregation, the Ge epilayer also exhibits DAS reconstructions of the same kind found on the pure Si substrates. Above 4 ML the formation of 3D islands is observed, which show mainly (113) and (111) facets. The islands are relaxed and show a mixture of c(2 × 8), c(2 × 4), and (2 × 2) reconstructions known for bulk Ge(111), when they are grown below 450° C. At a higher deposition temperature a (7 × 7) reappears on top of the 3D islands. Defects emerging from the bulk have been imaged.

Shear-stress sensitive lenticular vesicles for targeted drug delivery

Atherosclerosis results in the narrowing of arterial blood vessels and this causes significant changes in the endogenous shear stress between healthy and constricted arteries. Nanocontainers that can release drugs locally with such rheological changes can be very useful. Here, we show that vesicles made from an artificial 1,3-diaminophospholipid are stable under static conditions but release their contents at elevated shear stress. These vesicles have a lenticular morphology, which potentially leads to instabilities along their equator. Using a model cardiovascular system based on polymer tubes and an external pump to represent shear stress in healthy and constricted vessels of the heart, we show that drugs preferentially release from the vesicles in constricted vessels that have high shear stress.

The morphology of anisotropic 3D-printed hydroxyapatite scaffolds

Three-dimensional (3D) scaffolds with tailored pores ranging from the nanometer to millimeter scale can support the reconstruction of centimeter-sized osseous defects. Three-dimensional-printing processes permit the voxel-wise fabrication of scaffolds. The present study rests upon 3D-printing with nano-porous hydroxyapatite granulates. The cylindrical design refers to a hollow bone with higher density at the periphery. The millimeter-wide central channel follows the symmetry axis and connects the perpendicularly arranged micro-pores. Synchrotron radiation-based micro computed tomography has served for the non-destructive characterization of the scaffolds. The 3D data treatment is essential, since, for example, the two-dimensional distance maps overestimate the mean distances to the material by 33-50% with respect to the 3D analysis. The scaffolds contain 70% micrometer-wide pores that are interconnected. Using virtual spheres, which might be related to the cells migrating along the pores, the central channel remains accessible through the micro-pores for spheres with a diameter of up to (350+/-35)mum. Registering the tomograms with their 3D-printing matrices has yielded the almost isotropic shrinking of (27+/-2)% owing to the sintering process. This registration also allows comparing the design and tomographic data in a quantitative manner to extract the quality of the fabricated scaffolds. Histological analysis of the scaffolds seeded with osteogenic-stimulated progenitor cells has confirmed the suitability of the 3D-printed scaffolds for potential clinical applications.

High-resolution tomographic imaging of a human cerebellum: comparison of absorption and grating-based phase contrast

Human brain tissue belongs to the most impressive and delicate three-dimensional structures in nature. Its outstanding functional importance in the organism implies a strong need for brain imaging modalities. Although magnetic resonance imaging provides deep insights, its spatial resolution is insufficient to study the structure on the level of individual cells. Therefore, our knowledge of brain microstructure currently relies on two-dimensional techniques, optical and electron microscopy, which generally require severe preparation procedures including sectioning and staining. X-ray absorption microtomography yields the necessary spatial resolution, but since the composition of the different types of brain tissue is similar, the images show only marginal contrast. An alternative to absorption could be X-ray phase contrast, which is known for much better discrimination of soft tissues but requires more intricate machinery. In the present communication, we report an evaluation of the recently developed X-ray grating interferometry technique, applied to obtain phase-contrast as well as absorption-contrast synchrotron radiation-based microtomography of human cerebellum. The results are quantitatively compared with synchrotron radiation-based microtomography in optimized absorption-contrast mode. It is demonstrated that grating interferometry allows identifying besides the blood vessels, the stratum moleculare, the stratum granulosum and the white matter. Along the periphery of the stratum granulosum, we have detected microstructures about 40 µm in diameter, which we associate with the Purkinje cells because of their location, size, shape and density. The detection of individual Purkinje cells without the application of any stain or contrast agent is unique in the field of computed tomography and sets new standards in non-destructive three-dimensional imaging.

Impact of nanometer-scale roughness on contact-angle hysteresis and globulin adsorption

Besides surface chemistry, the surface roughness on the micrometer scale is known to dominate the wetting behavior and the biocompatiblity properties of solid-state materials. The significance of topographic features with nanometer size, however, has yet to be demonstrated. Our approach is based on well-defined Ge nanopyramids naturally grown on Si(001) using ultrahigh vacuum chemical vapor deposition, where the nanopyramid density can be precisely controlled by the growth conditions. Since the geometry of the nanopyramids, often termed dome clusters, is known, the surface roughness can be characterized by the Wenzel ratio with previously unattainable precision. Dynamic contact-angle measurements and adsorption of γ-globulin as a function of that ratio demonstrate the strong correlation between surface nanoarchitecture, on one hand, and wetting behavior and biocompatibility, on the other hand. Related x-ray photoelectron spectroscopy measurements reveal that potential changes of surface composition can be...

The interplay of surface morphology and strain relief in surfactant mediated growth of Ge on Si(111)

Abstract The growth of Ge on Si is strongly modified by surface active species called surfactants. While the effectiveness of Sb as a surfactant in forcing layer-by-layer growth of Ge on Si(111) and in creating a misfit adjusting dislocation network confined to the Si Ge-interface has been demonstrated in previous studies, the dynamic growth process on an atomic scale leading to this result is still unknown. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by spot profile analysing low energy electron diffraction during the deposition. The change from islanding to layer-by-layer growth is seen in the oscillatory intensity variation of the (00)-spot. To relieve the strain the Ge-film forms a microscopically rough surface of small triangular and defect free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. After the overgrowth of the dislocations the surface smoothes again showing a much larger terrace length. The periodic dislocation network at the interface gives rise to an elastic deformation of the surface, which results in a spot splitting in LEED. Thus, for the first time the dynamics of the formation of a dislocation network has been observed in situ during the growth process. Surprisingly, the dislocation network is already completed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro-rough surface morphology, providing innumerous nucleation sites for dislocation.

High density resolution in synchrotron-radiation-based attenuation-contrast microtomography

During the last few years microtomography using synchrotron radiation (SR) has become a standard technique to characterize samples 3-dimensionally in the fields of biology, medicine and materials science. The GKSS Research Center Geesthacht, Germany, is responsible for developing and running the microtomography experiments at the SR-facility DESY, Hamburg, Germany. The application of SRμCT using attenuation-contrast at the beamlines W2/HARWI-II and BW2 of the storage ring DORIS III results in high throughput investigations. For achieving tomograms showing not only high spatial resolution but also high density resolution special emphasis was given to the stability of the used monochromators and the calibration of the total system. The influence of the photon statistic from the measurement to the tomograms is simulated and the achieved high density resolution is demonstrated showing selected results.

Non-destructive three-dimensional evaluation of a polymer sponge by micro-tomography using synchrotron radiation

X-ray micro-tomography, a non-destructive technique is used to uncover the complex 3-D micro-architecture of a degradable polymer sponge designed for bone augmentation. The measurements performed at HASYLAB at DESY are based on a synchrotron radiation source resulting in a spatial resolution of about 5.4 microm. In the present communication we report the quantitative analysis of the porosity and of the pore architecture. First, we elucidate that synchrotron radiation at the photon energy of 9 keV has an appropriate cross section for this low-weight material. Modifications in sponge micro-architecture during measurement are not detected. Second, the treatment of the data, an amount of 2.5 Gbyte to generate binary data is described. We compare the 3-D with the 2-D analysis in a quantitative manner. The obtained values for the mean distance to material within the sponge calculated from 2-D and 3-D data of the whole tomogram differ significantly: 12.5 microm for 3-D and 17.6 microm for 2-D analysis. If the pores exhibit a spherical shape as frequently found, the derived mean pore diameter, however, is overestimated only by 6% in the 2-D image analysis with respect to the 3-D evaluation. This approach can be applied to different porous biomaterials and composites even in a hydrated state close to physiological conditions, where any surface preparation artifact is avoided.

Tailoring Selective Laser Melting Process Parameters for NiTi Implants

Complex-shaped NiTi constructions become more and more essential for biomedical applications especially for dental or cranio-maxillofacial implants. The additive manufacturing method of selective laser melting allows realizing complex-shaped elements with predefined porosity and three-dimensional micro-architecture directly out of the design data. We demonstrate that the intentional modification of the applied energy during the SLM-process allows tailoring the transformation temperatures of NiTi entities within the entire construction. Differential scanning calorimetry, x-ray diffraction, and metallographic analysis were employed for the thermal and structural characterizations. In particular, the phase transformation temperatures, the related crystallographic phases, and the formed microstructures of SLM constructions were determined for a series of SLM-processing parameters. The SLM-NiTi exhibits pseudoelastic behavior. In this manner, the properties of NiTi implants can be tailored to build smart implants with pre-defined micro-architecture and advanced performance.

Protein adsorption and monocyte activation on germanium nanopyramids

Germanium can form defect-free pyramidal islands on Si(1 0 0)-2 x 1 with a height of 15 nm and a width of 60 nm. Using chemical vapor deposition we have prepared substrates with different nanopyramid densities to study the impact on contact angles, protein adsorption and cell behavior. The advancing contact angle of a water droplet of millimeter size significantly raises with nanopyramid density. The dynamic contact angle measurements reveal that the substrate surface is highly hydrophilic. On such a surface the adsorption of hydrophilic proteins, i.e. albumin and globulin, is drastically increased by the presence of nanopyramids. More important, however, the globulin is inactive after adsorption on nanopyramid edges. This observation is supported by the cytokine release of IL-1beta and TNF-alpha of monocyte-like cell line U937. Consequently, the presence of nanopyramidal structures gives rise to less inflammatory reactions.