Conformation of ring polymers in 2D constrained environments.

The combination of ring closure and spatial constraints has a fundamental effect on the statistics of semiflexible polymers such as DNA. However, studies of the interplay between circularity and constraints are scarce and single-molecule experimental data concerning polymer conformations are missing. By means of atomic force microscopy we probe the conformation of circular DNA molecules in two dimensions and in the concentrated regime (above the overlap concentration c*). Molecules in this regime experience a collapse, and their statistical properties agree very well with those of simulated vesicles under pressure. Some circular molecules also create confining regions in which other molecules are trapped. Thus we show further that spatially confined molecules fold into specific conformations close to those found for linear chains, and strongly dependent on the size of the confining box.

Bovine serum albumin adsorption on nano-rough platinum surfaces studied by QCM-D.

The adsorption of bovine serum albumin (BSA) on platinum surfaces with a root-mean-square roughness ranging from 1.49nm to 4.62nm was investigated using quartz crystal microbalance with dissipation (QCM-D). Two different BSA concentrations, 50microg/ml and 1mg/ml, were used, and the adsorption studies were complemented by monitoring the antibody interaction with the adsorbed BSA layer. The adsorption process was significantly influenced by the surface nano-roughness, and it was observed that the surface mass density of the adsorbed BSA layer is enhanced in a non-trivial way with the surface roughness. From a close examination of the energy dissipation vs. frequency shift plot obtained by the QCM-D technique, it was additionally observed that the BSA adsorption on the roughest surface is subject to several distinct adsorption phases revealing the presence of structural changes facilitated by the nano-rough surface morphology during the adsorption process. These changes were in particular noticeable for the adsorption at the low (50microg/ml) BSA concentration. The results confirm that the nano-rough surface morphology has a significant influence on both the BSA mass uptake and the functionality of the resulting protein layer.

Influence of implantation on the electrochemical properties of smooth and porous TiN coatings for stimulation electrodes.

OBJECTIVE: To determine whether changes in electrochemical properties of porous titanium nitride (TiN) electrodes as a function of time after implantation are different from those of smooth TiN electrodes. APPROACH: Eight smooth and 8 porous TiN coated electrodes were implanted in 8 rats. Before implantation, voltage transients, cyclic voltammograms and impedance spectra were recorded in phosphate buffered saline (PBS). After implantation, these measurements were done weekly to investigate how smooth and porous electrodes were affected by implantation. MAIN RESULTS: The electrode capacitance of the porous TiN electrodes decreased more than the capacitance of the smooth electrodes due to acute implantation under fast measurement conditions (such as stimulation pulses). This indicates that protein adhesion presents a greater diffusion limitation for counter-ions for the porous than for the smooth electrodes. The changes in electrochemical properties during the implanted period were similar for smooth and porous TiN electrodes, indicating that cell adhesion poses a similar diffusion limitation for smooth and porous electrodes. SIGNIFICANCE: This knowledge can be used to optimize the porous structure of the TiN film, so that the effect of protein adhesion on the electrochemical properties is diminished. Alternatively, an additional coating could be applied on the porous TiN that would prevent or minimize protein adhesion.

Biofouling resistance of boron-doped diamond neural stimulation electrodes is superior to titanium nitride electrodes in vivo.

OBJECTIVE: The goal of this study was to assess the electrochemical properties of boron-doped diamond (BDD) electrodes in relation to conventional titanium nitride (TiN) electrodes through in vitro and in vivo measurements. APPROACH: Electrochemical impedance spectroscopy, cyclic voltammetry and voltage transient (VT) measurements were performed in vitro after immersion in a 5% albumin solution and in vivo after subcutaneous implantation in rats for 6 weeks. MAIN RESULTS: In contrast to the TiN electrodes, the capacitance of the BDD electrodes was not significantly reduced in albumin solution. Furthermore, BDD electrodes displayed a decrease in the VTs and an increase in the pulsing capacitances immediately upon implantation, which remained stable throughout the whole implantation period, whereas the opposite was the case for the TiN electrodes. SIGNIFICANCE: These results reveal that BDD electrodes possess a superior biofouling resistance, which provides significantly stable electrochemical properties both in protein solution as well as in vivo compared to TiN electrodes.

Enhancement of protein adsorption induced by surface roughness.

Using quartz crystal microbalance with dissipation and ellipsometry, we show that during adsorption of fibrinogen on evaporated tantalum films the saturation uptake increases with increasing root-mean-square roughness (from 2.0 to 32.9 nm) beyond the accompanying increase in surface area. This increase is attributed to a change in the geometrical arrangement of the fibrinogen molecules on the surface. For comparison, the adsorption of a nearly globular protein, bovine serum albumin, was studied as well. In this case, the adsorption was less influenced by the roughness. Simple Monte Carlo simulations taking into account surface roughness and the anisotropic shape of fibrinogen reproduce the experimentally observed trend.