Gerhard Heusel

In vitro study of titanium nitride electrodes for neural stimulation

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 0.2mC/cm2 charge density for square shaped electrodes with 50μm × 50μm dimensions in safe operation, which is less compared to previous reports. The safe operation window for TiN was confirmed to be ±1V 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.

In vitro study of iridium electrodes for neural stimulation

Iridium is one of the main electrode materials for applications like neural stimulation. Iridium has a higher charge injection capacity when activated and transformed into AIROF (activated iridium oxide film) using specific electrical signals [1]. Activation is not possible in stimulating devices, if they do not include the necessary circuitry for activation. We introduce a method for iridium electrode activation requiring minimum additional on-chip hardware. In the main part, the lifetime behavior of iridium electrodes is investigated. These results may be interesting for applications not including on-chip activation hardware, and also because activation has drawbacks such as worse mechanical properties and reproducibility of AIROF.