Novel TiNx-based biosignal electrodes for electroencephalography

A composition, structural, morphological and electrochemical study of two titanium nitride (TiNx) thin films, sample A and sample B, obtained by dc reactive sputtering on titanium substrates was carried out in this paper. In order to assess their applicability to be used as dry electrodes, several EEG signal acquisition tests were performed using the TiNx electrodes and compared to signals acquired with conventional silver/silver-chloride (Ag/AgCl) electrodes. The two films displayed some compositional differences, as sample B was over-stoichiometric (x = 1.34, x = N/Ti atomic ratio) and sample A was close-stoichiometric (x = 0.94). XRD diffractograms showed that both samples developed a similar fcc crystalline structure (d-TiN phase). XRD peak fitting showed that sample B (over-stoichiometric) has a more oriented structure (highly [1 1 1] textured), with larger grains. A columnar-type structure with pyramid-like shape at column's top was common to both TiNx films. This morphology, in addition to some columnar disaggregation, gives rise to a rather rough surface and porous structure. Electrochemical impedance spectroscopy results in artificial sweat showed that the electric properties of the samples remain unchanged, even after prolonged contact with sweat. The comparison of EEG signals, simultaneously recorded using the novel TiNx electrodes and conventional Ag/AgCl electrodes, showed very similar results. The root mean square deviation, which was computed for different electrode-electrolyte combinations and signal sequences of 5 s, was in the range of 4.5-9.4 mu V and 6.7-20.4 mu V for samples A and B, respectively. An additional test with two sets of Ag/AgCl electrodes revealed similar results, thus indicating most of the signal differences are related to the spatial distance of the compared electrodes as well as environmental noise. Therefore, our results lead to the conclusion that the novel TiNx electrodes show promising signal quality for application in EEG biosignal measurements.