Claus Burkhardt

Formation of cell-iron-mineral aggregates by phototrophic and nitrate-reducing anaerobic Fe(II)-oxidizing bacteria.

Microbial anaerobic Fe(II) oxidation at neutral pH produces poorly soluble Fe(III) which is expected to bind to cell surfaces causing cell encrustation and potentially impeding cell metabolism. The challenge for Fe(II)-oxidizing prokaryotes therefore is to avoid encrustation with Fe(III). Using different microscopic techniques we tracked Fe(III) minerals at the cell surface and within cells of phylogenetically distinct phototrophic and nitrate-reducing Fe(II)-oxidizing bacteria. While some strains successfully prevented encrustation others precipitated Fe(III) minerals at the cell surface and in the periplasm. Our results indicate differences in the cellular mechanisms of Fe(II) oxidation, transport of Fe(II)/Fe(III) ions, and Fe(III) mineral precipitation.

CYTOCENTERING: a novel technique enabling automated cell-by-cell patch clamping with the CYTOPATCH chip.

Automats for patch clamping suspended cells in whole-cell configuration must (1) bring isolated cells in contact with patch contacts, (2) form gigaseals, and (3) establish stable intracellular access that allows for high quality recording of ionic currents. Single openings in planar substrates seem to be intriguing simple solutions for these problems, but due to the low rate of formation of whole-cell configurations we discarded this approach. Single openings are not suited for both attracting cells to the opening by suction and forming gigaseals with subsequent membrane rupture. To settle the three tasks with a mechanical microstructure we developed the socalled CYTOCENTERING technique to apply to suspended cells the same operation sequence as in conventional patch clamping. With this method we immobilized selected cells from a flowing suspension on the tip of a patch pipette by suction with a success rate of 97% and formed gigaseals with a success rate of 68%. Subsequent whole-cell recordings and intracellular staining with Lucifer yellow proved the stable access to the cytoplasm. Currently, a chip with an embedded suction opening in glass surrounding the microstructured contact pipette is under development. The processing of this CYTOPATCH chip is compatible to large-volume production. The CYTOPATCH automat will allow for fully automated, parallel, and asynchronous whole-cell recordings.

Helium Ion Microscopy (HIM) for the imaging of biological samples at sub-nanometer resolution

Scanning Electron Microscopy (SEM) has long been the standard in imaging the sub-micrometer surface ultrastructure of both hard and soft materials. In the case of biological samples, it has provided great insights into their physical architecture. However, three of the fundamental challenges in the SEM imaging of soft materials are that of limited imaging resolution at high magnification, charging caused by the insulating properties of most biological samples and the loss of subtle surface features by heavy metal coating. These challenges have recently been overcome with the development of the Helium Ion Microscope (HIM), which boasts advances in charge reduction, minimized sample damage, high surface contrast without the need for metal coating, increased depth of field, and 5 angstrom imaging resolution. We demonstrate the advantages of HIM for imaging biological surfaces as well as compare and contrast the effects of sample preparation techniques and their consequences on sub-nanometer ultrastructure.

Patch-clamping of primary cardiac cells with micro-openings in polyimide films

Patch-clamping is a powerful method for investigating the function and regulation of ionic channels. Currently, great efforts are being made to automate this method. As a step towards this goal, the feasibility of patch-clamping primary cells with a microscopic opening in a planar substrate was tested. Using standard microfabrication and ion beam technology, small-diameter openings (2 and 4 μm) were formed in polyimide films (thickness 6.5 μm). Single cells (sheep Purkinje heart cells, Chinese hamster ovary cells) in a suspension were positioned on top of the opening and sucked towards the opening to improve adhesion of the cell to the planar substrate, hence increasing the seal resistance. Voltage/current measurements yielded a median seal resistance of 1.3MΩ with 4 μm openings (n=24) and 26.0 MΩ with 2 μm openings (n=75), respectively. With 2 μm openings, successful loose-patch recordings of TTX-sensitive inward currents and action potentials in sheep Purkinje heart cells (n=18) were made. In rare cases, gigaseals (n=4) were also measured, and a whole-cell configuration (n=1) could be established. It was concluded that the simple planar patch approach is suitable for automated loosepatch recordings from cells in suspension but will hardly be suitable for highthroughput whole-cell patch-clamping with high-resistance seals.

Evaluation of Electron Microscopic Sample Preparation Methods and Imaging Techniques for Characterization of Cell-Mineral Aggregates

Scanning Electron Microscopy (SEM) is used to image geomicrobiological samples, typically containing interfaces between “hard and soft materials” such as minerals and cells, which represent challenges for artifact-free preparation for high-resolution imaging. We used cell-mineral aggregates produced during microbial Fe(II) oxidation and Fe(III) reduction to evaluate different sample preparation and imaging techniques. Both rapid freezing and standard critical point drying (CPD) preserve structures of geomicrobiological samples, at least the ones obtained for Fe(II)-oxidizing and Fe(III)-reducing bacteria, without artifacts. We recommend a SEM sample preparation scheme for geomicrobiological specimens and discuss critical parameters like fixation, dehydration, coating, and acceleration voltages.

Substrate-Integrated Microelectrodes with Improved Charge Transfer Capacity by 3-Dimensional Micro-Fabrication

Arrays of microelectrodes with high charge transfer capacity have been fabricated and characterized. Nano-porous titanium nitride thin films were deposited onto a 3-dimensional micro-fabricated structure etched into a biocompatible polyimide insulation layer. The polymer layer was micro-patterned by electron beam lithography and etched in an oxygen-plasma. Measurements at 1 kHz/100 mV in electrolytic solution (PBS) show a significant reduction of impedance by a factor of 0.3 to 0.2 and are in good agreement with the surface gain expected from the 3-dimensional micro-pattern. The influence of the series resistance of the buffer solution on the electrode impedance is also examined by impedance spectroscopy and finite element simulation. This kind of microelectrodes may finally be used for stimulation and recording of cells and tissue.

Dynamics of Middle Ear Prostheses – Simulations and Measurements

The efficient and systematic development of a middle ear prosthesis necessitates the use of computer models for the prosthesis itself and the reconstructed middle ear. The structure and parameters of the computer model have to be verified by specific measurements of the implant and the reconstructed ear. To obtain a realistic model of a reconstructed ear, three steps of modeling and measurements have been carried out. To get a first approach of the coupling elements a mechanical test rig representing a simplified reconstructed middle ear was built. The velocity of the stapedial footplate was measured with a laser Doppler vibrometer. The corresponding computer model was formulated, and the respective parameters were determined using the measured dynamical transfer functions. In the second step, a prosthesis was implanted into a human temporal bone without inner ear. Exciting this system with noise, the velocity of the stapes footplate was measured with the laser Doppler vibrometer. Based on the multibody system approach, a mechanical computer model was generated to describe the spatial motions of the reconstructed ossicular chain. Varying some significant parameters, simulations have been carried out. To describe the dynamical behavior of the system consisting of middle and inner ear, the computer model used in the second step has been enlarged by adding a simplified structure of the inner ear. The results were compared with in situ measurements taken from living humans.

Visualization and in situ contacting of carbon nanotubes in a scanning electron microscope

A method for contacting carbon nanotubes on an insulating surface is presented. Tubes which are already connected to prefabricated electrodes can be visualized in the scanning electron microscope exploiting voltage contrast at low beam energies. Additional connections to tubes identified in this way are fabricated by in situ electron beam induced deposition from metalorganic precursors such as tungsten hexacarbonyl. A resistivity of 0.15Ωcm before annealing has been achieved so far with this material.

Concept and evaluation of an endaurally insertable middle-ear implant

A concept for a partially implantable hearing device, for which the power supply and signal transmission are provided by an optical transmission path, is evaluated. The actuator is designed to fit into the round-window niche and to couple directly to the round-window membrane. Implantable hearing aids can be a suitable solution in the case of severe hearing loss, where conventional hearing aids often fail. However, the surgical effort for an implantation is comparatively high. Therefore, the objective of our work was to provide a hearing system which combines reliable coupling to the auditory system with an easy implantation technique. The actuator was designed as a piezoelectric thin-film cantilever. The optical transmission path was realised using an infrared light-emitting diode combined with an active receiver circuit. For a voltage of 1V, the unloaded actuator presents displacement amplitudes of 1μm up to a stimulus frequency of 25kHz and forces up to 0.2mN. Proportionally larger forces can be achieved by stacking single actuators. The overall transmission loss from the electrical input of the light-emitting diode driver to the mechanical output of the unloaded actuator was less than 25dB at 1kHz and maximum output.

Addressable field emitter array: A tool for designing field emitters and a multibeam electron source

We have developed an array of 25 electrically addressable field emitters. This device is a very suitable tool for experimentally designing miniaturized electron sources with improved emission characteristics. The field emission tip itself is fabricated by a combination of electron beam induced deposition and physical vapor deposition. Thus it is possible to vary the geometrical setup of tip and extraction electrode easily and also to define the emitter material exactly. The influence of most important parameters on the tip shape has been examined and deposition of three-dimensional structures is shown. Analysis of Fowler–Nordheim-plots indicates that during first operation and increased exposure to argon, nitrogen, and oxygen a change in tip radius is the predominant effect. Two arrays of field emitters with different tip heights have been examined to show the variation in emission pattern and current distribution.