Martin Bohley

Cleaning of titanium substrates after application in a bioreactor

Plain and microstructured cp-titanium samples were studied as possible biofilm reactor substrates. The biofilms were grown by exposition of the titanium samples to bacteria in a flow cell. As bacteria the rod shaped gram negative Pseudomonas fluorescens and the spherical gram negative Paracoccus seriniphilus were chosen. Afterward, the samples were cleaned in subsequent steps: First, with a standard solvent based cleaning procedure with acetone, isopropanol, and ultrapure water and second by oxygen plasma sputtering. It will be demonstrated by means of x-ray photoelectron spectroscopy, fluorescence microscopy, and confocal laser scanning microscopy that oxygen plasma cleaning is a necessary and reliant tool to fully clean and restore titanium surfaces contaminated with a biofilm. The microstructured surfaces act beneficial to biofilm growth, while still being fully restorable after biofilm contamination. Scanning electron microscopy images additionally show, that the plasma process does not affect the microstructures. The presented data show the importance of the cleaning procedure. Just using solvents does not remove the biofilm and all its components reliably while a cleaning process by oxygen plasma regenerates the surfaces.

Micromachining of CP-Titanium on a Desktop Machine - Study on Bottom Surface Quality in Micro End Milling

With the ever-growing demand for micro products, the influence of micro-features and the functional performance of component surfaces represent a high economical potential. A competitive process for the manufacture of micro products is using micro end mills and a desktop milling machine. Since the topography of machined surfaces affects e.g. the wetting interaction or the bacterial cell adhesion, all relevant finishing steps such as the generation of a plane surface parallel to the machining table and the structuring with micro end mills have to be concerned in order to create functional surfaces. In this paper the generation of plane surfaces and chosen tool paths is described first and then the use of ultra-small micro end mills for slot milling in cp-titanium and the influence of process parameters on bottom surface quality is discussed.

Application of Ultra-Small Micro Grinding and Micro Milling Tools: Possibilities and Limitations

Current demands for flexible, individual microstructures in high quality result in high requirements for micro tools. As the tool size defines the minimum structure size, ultra-small tools are needed. To achieve tool diameters of 50 µm and lower, we investigate the complete manufacturing chain of micro machining. From the development of the machine tools and components needed to produce and apply the micro tools, the micro tools themselves, as well as the micro machining processes. Machine tools are developed with the possibility of producing the micro geometry (cutting edge design) of micro tools as well as plating processes to produce super abrasive micro grinding tools. Applying these setups, we are able to produce ultra-small micro grinding and micro milling tools with typical diameters of 50 µm and down to 4 µm. However, the application of such tools is very challenging. The article presents possibilities and limitations in manufacturing the micro tools themselves as well as microstructures made with these tools. A special emphasis will be on the influence of the tool substrate in micro milling and grain sizes in micro grinding.

Deformation behaviour of micro-milled cp-titanium specimens under tensile loading

Abstract The functionality of components can be improved by surface modifications such as micro-milling. The micro-milled structures (notches) can cause changes in the mechanical properties of the components. To be able to predict or prevent component failure, the influence of these micro-milled notches on the deformation behaviour of commercially pure titanium under tensile loading is investigated. Quasi-static tensile tests show that the tensile strength is increased by deeper notches and the fracture elongation of the specimens decreases. Metallographic analyses and nanoindentation tests of systematically interrupted tensile tests at different strain rates show the development of the plastic deformation and the inhomogeneous deformation behaviour of the material.

Untersuchung des Verschleißverhaltens von TiB2-beschichteten Mikrofräswerkzeugen

Kurzfassung Beim Einsatz von Mikrofräswerkzeugen kommt es, bedingt durch Größeneffekte, zu erhöhter Reibung und damit erhöhtem Verschleiß. Zusätzlich resultieren aus den geringen Werkzeugdurchmessern niedrige Schnittgeschwindigkeiten, sodass es zu Aufbauschneiden kommen kann. Beides wirkt sich negativ auf die Prozessergebnisgrößen aus und reduziert die Effizienz des Prozesses. Um dies zu minimieren, wurden Mikrofräswerkzeuge (Ø 50 μm) mit einer PVD-Beschichtung versehen und das Einsatzverhalten untersucht.

Adhesion forces of the sea-water bacterium Paracoccus seriniphilus on titanium: Influence of microstructures and environmental conditions

The bacterial attachment to surfaces is the first step of biofilm formation. This attachment is governed by adhesion forces which act between the bacterium and the substrate. Such forces can be measured by single cell force spectroscopy, where a single bacterium is attached to a cantilever of a scanning force microscope, and force-distance curves are measured. For the productive sea-water bacterium Paracoccus seriniphilus, pH dependent measurements reveal the highest adhesion forces at pH 4. Adhesion forces measured at salinities between 0% and 4.5% NaCl are in general higher for higher salinity. However, there is an exception for 0.9% where a higher adhesion force was measured than expected. These results are in line with zeta potential measurements of the bacterium, which also show an exceptionally low zeta potential at 0.9% NaCl. In the absence of macromolecular interactions, the adhesion forces are thus governed by (unspecific) electrostatic interactions, which can be adjusted by pH and ionic strength. It is further shown that microstructures on the titanium surface increase the adhesion force. Growth medium reduces the interaction forces dramatically, most probably through macromolecular bridging.

Paracoccus seriniphilus adhered on surfaces: Resistance of a seawater bacterium against shear forces under the influence of roughness, surface energy, and zeta potential of the surfaces

Bacteria in flowing media are exposed to shear forces exerted by the fluid. Before a biofilm can be formed, the bacteria have to attach to a solid surface and have to resist these shear forces. Here, the authors determined dislodgement forces of single Paracoccus seriniphilus bacteria by means of lateral force microscopy. The first measurement set was performed on very flat glass and titanium (both as very hydrophilic samples with water contact angles below 20°) as well as highly oriented pyrolytic graphite (HOPG) and steel surfaces (both as more hydrophobic surfaces in the context of biological interaction with water contact angles above 50°). The different surfaces also show different zeta potentials in the range between -18 and -108 mV at the measurement pH of 7. The second set comprised titanium with different RMS (root mean square) roughness values from a few nanometers up to 22 nm. Lateral forces between 0.5 and 3 nN were applied. For Paracoccus seriniphilus, the authors found as a general trend that the surface energy of the substrate at comparable roughness determines the detachment process. The surface energy is inversely proportional to the initial adhesion forces of the bacterium with the surface. The higher the surface energy (and the lower the initial adhesion force) is, the easier the dislodgement of the bacteria happens. In contrast, electrostatics play only a secondary role in the lateral dislodgement of the bacteria and may come only into play if surface energies are the same. Furthermore, the surface chemistry (glass, titanium, and steel as oxidic surfaces and HOPG as a nonoxidic surface) seems to play an important role because HOPG does not completely follow the above mentioned general trend found for the oxide covered surfaces. In addition, the roughness of the substrates (made of the same material) is limiting the lateral dislodgement of the bacteria. All examined structures with RMS roughness of about 8-22 nm on titanium prevent the bacteria from the lateral dislodgement compared to polished titanium with an RMS roughness of about 3 nm.

Recent Developments in Desktop-Sized Machine Tools

The miniaturization of components and the functionalization via micro structures demands for flexible and economic manufacturing processes. Micro machining, i.e. micro milling and micro grinding can meet these requirements. In this paper, desktop-sized machine tools and their components that were developed at our institute are presented. With those machine tools, micro tools can be machined and used in one clamping, allowing for increased machining quality. Grooves milled with such machine tools achieve a bottom surface roughness below 10 nanometer.

Analyzing the influence of microstructured surfaces on the lactic acid production of Lactobacillus delbrueckii lactis in a flow-through cell system

Microorganisms growing in biofilms might be possible biocatalysts for future biotechnological production processes. Attached to a surface and embedded in an extracellular polymeric matrix, they create their preferred environment and form robust cultures for continuous systems. With the objective of implementing highly efficient processes, productive biofilms need to be understood comprehensively. In this study, the influence of microstructured metallic surfaces on biofilm productivity was researched. To conduct this study, titanium and stainless steel sheets were polished, micromilled, as well as coated with particles. Subsequently, the metal sheets were exposed to the lactic acid producing Lactobacillus delbrueckii subsp. lactis under laminar and homogeneous flow conditions in a custom‐built flow cell. A proof‐of‐concept showed that biofilm formation in the system only occurred on the designated substratum. Following a 24‐h batch cultivation for primary biofilm development, the culture was continuously provided with glucose containing medium. As different experimental series have indicated, the process resulted to be stable for up to eleven days. Primary metabolite productivity averaged around 6–7 g/(L h). Interestingly, the productivity was shown to be affected neither by the type of metal, nor by the applied microstructures. Nevertheless, a higher dry biomass weight determined on micro‐milled substratum indicates a complementary differentiation of biofilm components in future experiments.