New dry electrodes based on iridium oxide (IrO) for non-invasive biopotential recordings and stimulation

This paper presents a new type of dry electrodes for acquisition of biopotentials and stimulation. These dry electrodes are composed by 16 microtip structures (by forming an array of 4 × 4 microtips), which were fabricated through bulk micromachining of a � 100 � -type silicon substrate in a potassium hydroxide (KOH) solution. The fabrication process was trimmed in a way that each microtip presents solid angles of 54.7 ◦ , a width in the range 150–200m, a height of 100–200m, and an inter-microtip spacing of 2 mm. The electrodes have a thin layer (obtained by reactive DC-sputtering) of iridium oxide (IrO) to improve the contact with the skin. These dry electrodes penetrate the outer skin layer (i.e. stratum corneum that is 10m thick) to allow a direct contact with the electrolyte fluids of the inner skin layers. The new electrode avoid the use of conductive gels and reduce the skin preparation time in EEG experiments, which may take about 45 min for a set of 32 standard silver/silver chloride (Ag/AgCl) electrodes. The electrode–electrolyte impedance spectrometry (IS) of the IrO thin-films was performed in a saline solution, 0.9% concentration by weight, to mimic the electrode–tissue interface. The IS measured results for the IrO coatings were comparable to the results observed for the standard Ag/AgCl electrodes. The new dry microtips array constitutes an inexpensive, low resistance and mechanically robust alternative electrode for non-invasive biopotential recording/stimulation with fast application on skin.