Nanostructured Ti-based thin films. A Versatile Platform for Biopotential Sensing and Neurorehabilitation

Four distinct Ti-based thin film systems, doped with different metals (Au, Ag, Cu, Al), have been prepared by magnetron sputtering, allowing precise control over their chemical composition and microstructure. The strategic incorporation of these metals induces significant variations in phase composition, grain morphology, crystallographic orientation, and surface topography, which directly impact the electrical conductivity, mechanical flexibility, and electrochemical stability. These tunable properties are crucial for optimising their performance in biomedical applications, particularly as functional interfaces for biopotential sensing. All the systems exhibit three distinct regimes based on their chemical composition. At low metal contents, Ti-based films establish α-Ti(metal) metastable solid solutions. For intermediate metal/Ti ratios, the precipitation of intermetallic phases leads to high structural disorder, giving rise to different microstructures depending on the metal type. At higher ratios, the systems display contrasting morphologies, from well-defined domains to amorphous structures. The mechanical properties vary accordingly: Ti-Au and Ti-Cu films demonstrate superior toughness (H/E ≈ 0.1) and high elastic recovery, whereas Ti-Ag and Ti-Al, characterised by columnar and brittle intermetallic structures, exhibit lower plastic deformation resistance (H/E < 0.04). Electrical resistivity is also metal-dependent, with Ti-Au and Ti-Cu films maintaining nearly constant resistivity (~180 μΩ·cm) due to their Thin Film Metallic Glasses-like morphology, while Ag- and Al-rich films exhibit resistivity variations (130–270 μΩ·cm) linked to their crystalline structures. These Ti-based systems have been implemented as advanced dry biopotential electrodes, namely on the integration of novel neuro-rehabilitation systems combining electroencephalography (EEG), electrocardiography (ECG), electromyography (EMG), and functional electrical stimulation (FES). Ti-Au and Ti-Cu electrodes demonstrated superior electromechanical performance and in vivo signal acquisition, outperforming conventional Ag/AgCl electrodes. Their dense, disordered structures contribute to enhanced durability, while Ti-Cu electrodes exhibited prolonged reusability, maintaining high-fidelity signal recording for at least 24 hours. The integration of these biocompatible, flexible thin films onto polymeric substrates ensures mechanical adaptability and stable skin-electrode interaction, reinforcing their potential in bioelectronic and neurorehabilitation systems.