Viscoelastic properties of low-viscosity liquids studied with thickness-shear mode resonators.

The network analysis method was applied to AT cut quartz blanks (f(0) = 10 MHz), which were loaded with liquids of low and medium viscosity (water, methanol, ethanol, 1-propanol, 1-butanol, glycerol solutions). The shift of the resonance frequency Δf could be separated into a term due to rigidly coupled mass Δf(rig) and a term due to viscous damping Δf - Δf(rig). From the difference Δf - Δf(rig) and the broadening of the resonance curve, the complex shear modulus G = G' + iωη(L) was calculated. The viscosity coefficients η(L) are in good agreement with literature data. As G' > 0, it can be concluded that the examined fluids also reveal elasticity at shear frequencies in the MHz range. For the low-viscosity liquids, elastic contributions resulting from collective interactions of molecules are measurable but small and neglectable in most applications. The medium viscous liquid glycerol (98%) begins to exhibit considerable elasticity, resulting from the relaxation of separate molecules.

Characterization and optimization of microelectrode arrays for in vivo nerve signal recording and stimulation.

Revealing the complex signal-processing mechanisms and interconnection patterns of the nervous system has long been an intriguing puzzle. As a contribution to its understanding the optimization of the impedance behavior of implantable electrode arrays with via holes is discussed here. Peripheral axons will regenerate through these holes allowing for simultaneous nerve stimulation and signal recording. This approach is part of the ESPRIT project INTER and may eventually lead to devices driving sensory motor prosthesis with closed loop control. In the first set of experiments, micromachined platinum electrode arrays were prepared, characterized and optimized for nerve signal recording. The results of these studies are based on impedance spectroscopy and microscopic techniques. Equivalent circuits were modeled describing formally the electrical response behavior with ohmic resistances between 500 omega and 10 k omega. To attain low impedances for all electrodes on the INTER device, platinum from H2PtCl6 was electrodeposited, and sputter technology as well as electrochemical deposition from H2IrCl6 solution were used to produce thin iridium films. For the former, a lift-off process was established at one of the institutes to generate electrode structures with a line width of 5 microns. As a result in all three cases the electrodes showed almost constant impedances over the entire frequency range (10 Hz-1 kHz), which is relevant for nerve signal recording. In the second set of experiments, electrodes were optimized to allow for nerve stimulation. For this purpose, the charge delivery capacity (CDC) had to be increased and the impedance had to be decreased. Iridium oxide is the material of choice, because its CDC is much higher than the CDC of platinum at 75 microC/cm2 (Ziaie et al., 1991, IEEE Sensors & Actuators Transducers, 6, 124-127). A significant increase of the electrochemically active surface of the electrode structures could be observed by measuring the surface roughness. In first experiments, an activated iridium oxide film was formed with cyclic voltammetry and was evaluated using scanning force microscopy and impedance spectroscopy. The evaluation of the cyclic voltammograms showed a CDC up to 400 mC/cm2 for sputter deposited and oxidatively treated iridium films. Further investigations are directed towards increasing the stability of the iridium oxide electrodes with regard to long-term implants. Parallel experiments aim at the controlled axon adhesion without changing the impedance behavior of the described electrodes.

A simple and versatile PSA system for heavy metal determinations.

An easy to build potentiometric stripping analysis system for heavy metal determinations (Zn, Cd, Pb, Cu) is described which reaches good sensitivity and precision. Main components are an IBM compatible computer, an ADDA-conversion card and two selfmade electrodes, so it can be realized for one tenth of the price of complete commercial systems. A graphite pencil lead covered with a Hg-film is used as working electrode and a Ag/AgCl electrode as a combined reference/counter electrode. Reproducibility and calibration measurements are reported, heavy metal determinations on soil samples are compared with AAS results.