Scientific publications

Indentation experiments on the nanometre scale have been performed by means of atomic force microscopy in ultra-high vacuum on KBr(100) surfaces. The surfaces yield in the form of discrete surface displacements with a typical length scale of 1 Å. These surface displacements are detected in both normal and lateral directions. Measurement of the lateral tip displacement requires a load-dependent calibration due to the load dependence of the effective lateral compliance. Correlation of the lateral and normal displacements for each glide event allow identification of the activated slip system. The results are discussed in terms of the resolved shear stress in indentation experiments and of typical results in atomistic simulations of nanometre-scale indentation.

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.

The influence of anion adsorption on friction forces in an electrochemical environment has been studied by means of lateral force microscopy on Au(1 1 1) surfaces. Sensitivity to atomic stick-slip motion allows to reveal sulphate adsorption in ordered layers under the sliding tip at potentials lower than expected from cyclic voltammetry for the open surface. No ordered adsorption is found in lateral force measurements for the weakly adsorbed perchlorate anions. Correspondingly, some increase in friction in the anion adsorption regime is observed for sulphate but none for perchlorate adsorption. Friction increases significantly at the onset of oxidation in both sulphuric and perchloric acid solutions.

Friction between the sliding tip of an atomic force microcope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomicscale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only abouve a load threshold at which wear Of the surface is initiated. In contrast, irregular stick slip motion and a linear increase of friction with load are. observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange Mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions.

Multiple studies demonstrate the influence of the limbic system on the processing of sensory events and attentional guidance. But the mechanisms involved therein are yet not entirely clear. The close connection of handling incoming sensory information and memory retrieval, like in the case of habituation towards insignificant stimuli, suggests a crucial impact of the hippocampus on the direction of attention. In this paper we thus present a neurofunctional forward model of a hippocampal comparator function based on the theory of theta-regulated attention. Subsequently we integrated this comparator model into a multiscale framework for the simulation of evoked responses. The results of our simulations were compared to experimental data on electroencephalographic (EEG) correlates of habituation towards familiar stimuli using time-scale analysis. In consequence we are able to present additional evidence for limbic influences on the direction of attention driven by stimulus novelty and a systems neuroscience framework for the statements given in the theta-regulated attention hypothesis.

A novel adaptive and approximate shift-invariant wavelet packet feature extraction scheme for event-related potentials (ERPs) in the electroencephalogram (EEG) is introduced in this paper. In this algorithm, the shift-invariant wavelet packed decomposition is done by integrating a cost function for decimation decision in each sub-band expansion. Additionally, a shape adaptation of the wavelet is implemented to find the best adapted wavelet shape for a given class of ERPs. This scheme is used to analyze the time course of the impact of single-pulse transcranial magnetic stimulation (TMS) to the auditory ERPs. We show that the proposed scheme is able to extract even slightest impacts of TMS, making it a promising tool for the extraction of weak ERPs components, particularly in hybrid TMS-EEG/ERP setups.

Large-scale neural correlates of auditory selective attention reflected in the electroencephalogram (EEG) have been identified by using the complex wavelet-phase stability measure (WPS). In this paper, we study the feasibility of using this amplitude independent measure, the WPS in extracting the correlates of attention by comparing its performance to the widely used linear interdependency measures, i.e., the wavelet coherence and the correlation coefficient. The outcome reveals that the WPS outperforms the other two measures in discriminating both the attended and unattended single sweep auditory late responses (ALRs). It is concluded that the proposed WPS provides a faster (in terms of less sweeps which are required) and robust objective quantification of selective attention.

Cells communicate with their external environment via focal adhesions and generate activation signals that in turn trigger the activity of the intracellular contractile machinery. These signals can be triggered by mechanical loading that gives rise to a cooperative feedback loop among signaling, focal adhesion formation, and cytoskeletal contractility, which in turn equilibrates with the applied mechanical loads. We devise a signaling model that couples stress fiber contractility and mechano-sensitive focal adhesion models to complete this above mentioned feedback loop. The signaling model is based on a biochemical pathway where IP3 molecules are generated when focal adhesions grow. These IP3 molecules diffuse through the cytosol leading to the opening of ion channels that disgorge Ca2+ from the endoplasmic reticulum leading to the activation of the actin/myosin contractile machinery. A simple numerical example is presented where a one-dimensional cell adhered to a rigid substrate is pulled at one end, and the evolution of the stress fiber activation signal, stress fiber concentrations, and focal adhesion distributions are investigated. We demonstrate that while it is sufficient to approximate the activation signal as spatially uniform due to the rapid diffusion of the IP3 through the cytosol, the level of the activation signal is sensitive to the rate of application of the mechanical loads. This suggests that ad hoc signaling models may not be able to capture the mechanical response of cells to a wide range of mechanical loading events.

Because nanoparticles are promising tools in drug delivery, quantification of their cellular binding and uptake is an emerging question. Therefore, rhodamine B isothiocyanate-labeled silica nanoparticles with different sizes and surface modifications were investigated concerning their uptake in Caco-2 cells. Flow cytometry studies exhibited a size- and time-dependent association for unmodified nanoparticles (50 and 77 nm), whereas larger particles (94 nm) and polyethylene glycol-modified nanoparticles showed no cellular interaction. A second approach dealt with particles with adsorbed propidium iodide (PI) to distinguish between internalized and adsorbed nanoparticles. These particles only give a fluorescence signal when associated with nucleic acids inside the cell, whereas particles adsorbed to the outer cell surface are not detected. PI-labeled nanoparticles (21 nm) showed a time-dependent uptake, exhibiting a signal in the cytoplasm but less in the nucleus. These novel PI-labeled nanoparticles in combination with flow cytometry are innovative tools for the quantification of nanoparticulate uptake.