For neural stimulation, reliable high density charge transfer into tissue is required. One electrode material for these applications is titanium nitride (TiN). In this paper, a method for lifetime analysis of TiN electrodes is discussed. Our method significantly differs from open literature. The tests were run for much longer durations. Special attention was paid to the optical appearance and electrode voltage response to different input current pulses. According to our investigations, TiN electrodes are able to deliver at most 0.2 mC/cm(2) charge density for square shaped electrodes with 50 μm × 50 μm dimensions in safe operation, which is less compared to previous reports. The safe operation window for TiN was confirmed to be ± 1 V in terms of electrode potential with the counter electrode considered as reference. We found that the shape of the waveform does not affect electrode lifetime. Our measurements show that rectangular voltage waveforms inject the most amount of charge into the electrodes compared to other shapes. This makes rectangular electrode voltage signals optimal for highest charge injection at a given lifetime. In our case with square electrodes, the absolute electrode potential is found to be the more important parameter in electrode lifetime, compared to Helmholtz capacitor voltage drop.
Laser-based ion mobility (IM) spectrometry was used for the detection of neuroleptics and PAH. A gas chromatograph was connected to the IM spectrometer in order to investigate compounds with low vapour pressure. The substances were ionized by resonant two-photon ionization at the wavelengths λ = 213 and 266 nm and pulse energies between 50 and 300 μJ. Ion mobilities, linear ranges, limits of detection and response factors are reported. Limits of detection for the substances are in the range of 1-50 fmol. Additionally, the mechanism of laser ionization at atmospheric pressure was investigated. First, the primary product ions were determined by a laser-based time-of-flight mass spectrometer with effusive sample introduction. Then, a combination of a laser-based IM spectrometer and an ion trap mass spectrometer was developed and characterized to elucidate secondary ion-molecule reactions that can occur at atmospheric pressure. Some substances, namely naphthalene, anthracene, promazine and thioridazine, could be detected as primary ions (radical cations), while other substances, in particular acridine, phenothiazine and chlorprothixene, are detected as secondary ions (protonated molecules). The results are interpreted on the basis of quantum chemical calculations, and an ionization mechanism is proposed.
Electrodes are among the critical components of neural stimulation devices. Investigating electrode properties like electrode impedance, charge injection capacity, and electrode corrosion limits plays an important role in electrode development. There are many commercial devices available for this purpose. Although useful, these devices are usually expensive and often offer more functions than required. We propose a versatile setup, composed of a LabVIEW program, a National Instruments multifunctional board, and a circuit built of discrete commercial elements. The system offers basic functions used in electrochemical investigation like current and voltage injection, cyclic voltammetry, and impedance spectroscopy. It offers the functionalities of both a potentiostat and an arbitrary waveform generator. It has already been applied elsewhere.
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