Me-Doped Ti–Me intermetallic thin films used for dry biopotential electrodes: a comparative case study

sensors Article Me-Doped Ti–Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study Cláudia Lopes 1,* , Patrique Fiedler 2,* , Marco Sampaio Rodrigues 1, Joel Borges 1, Maurizio Bertollo 3,4 , Eduardo Alves 5, Nuno Pessoa Barradas 6, Silvia Comani 3,7 , Jens Haueisen 2,8 and Filipe Vaz 1   Citation: Lopes, C.; Fiedler, P.; Rodrigues, M.S.; Borges, J.; Bertollo, M.; Alves, E.; Barradas, N.P.; Comani, S.; Haueisen, J.; Vaz, F. Me-Doped Ti–Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study. Sensors 2021,21, 8143. https://doi.org/ 10.3390/s21238143 Academic Editor: Laszlo B. Kish Received: 4 November 2021 Accepted: 1 December 2021 Published: 6 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Centro de Física das Universidades do Minho e Porto, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal; [email protected] (M.S.R.); joelbor[email protected] (J.B.); [email protected] (F.V.) 2Institute of Biomedical Engineering and Informatics, Technische Universität Ilmenau, 98693 Ilmenau, Germany; [email protected] 3Behavioral Imaging and Neural Dynamics Center, University “G. d’Annunzio” of Chieti-Pescara, 66100 Chieti, Italy; [email protected] (M.B.); [email protected] (S.C.) 4Department of Medicine and Aging Sciences, University “G. d’Annunzio” of Chieti-Pescara, 66100 Chieti, Italy 5 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal; [email protected] 6Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa EN10, 2695-066 Bobadela, Portugal; [email protected] 7Department of Neuroscience Imaging and Clinical Sciences, University “G. d’Annunzio” of Chieti-Pescara, 66100 Chieti, Italy 8Department of Neurology, Biomagnetic Center, Jena University Hospital, 07747 Jena, Germany *Correspondence: [email protected] (C.L.); [email protected] (P.F.) Abstract: In a new era for digital health, dry electrodes for biopotential measurement enable the monitoring of essential vital functions outside of specialized healthcare centers. In this paper, a new type of nanostructured titanium-based thin film is proposed, revealing improved biopotential sensing performance and overcoming several of the limitations of conventional gel-based electrodes such as reusability, durability, biocompatibility, and comfort. The thin films were deposited on stainless steel (SS) discs and polyurethane (PU) substrates to be used as dry electrodes, for non-invasive monitoring of body surface biopotentials. Four different Ti–Me (Me = Al, Cu, Ag, or Au) metallic binary systems were prepared by magnetron sputtering. The morphology of the resulting Ti–Me systems was found to be dependent on the chemical composition of the films, specifically on the type and amount of Me. The existence of crystalline intermetallic phases or glassy amorphous structures also revealed a strong influence on the morphological features developed by the different systems. The electrodes were tested in an in-vivo study on 20 volunteers during sports activity, allowing study of the application-specific characteristics of the dry electrodes, based on Ti–Me intermetallic thin films, and evaluation of the impact of the electrode–skin impedance on biopotential sensing. The electrode–skin impedance results support the reusability and the high degree of reliability of the Ti–Me dry electrodes. The Ti–Al films revealed the least performance as biopotential electrodes, while the Ti–Au system provided excellent results very close to the Ag/AgCl reference electrodes. Keywords: thin-films; binary intermetallic systems; Ti–Me dry electrodes; polymers; electrode–skin impedance; biopotential measurement 1. Introduction Commonly used conventional electrodes for biopotential recordings, in clinical and research applications, are silver–silver chloride (Ag/AgCl) based electrodes, in combination with electrolyte gels or pastes. These electrodes present the characteristics of non-polarizable electrodes including outstanding reliability, and low, almost frequencyindependent skin contact impedance. Thus, Ag/AgCl electrodes are considered the gold Sensors 2021,21, 8143. https://doi.org/10.3390/s21238143 https://www.mdpi.com/journal/sensors Sensors 2021,21, 8143 2 of 17 standard for measurements involving low voltage signals of electrophysiological origin such as e.g., electromyography (EMG), electroencephalography (EEG), or electrocardiography (ECG). Nevertheless, and despite their popularity, the gel-based electrodes present several considerable drawbacks such as short service life, extensive preparation time, and skin reaction ranging from liquid-related discomfort to eventual allergic contact dermatitis [ 1 , 2 ]. Furthermore, the use of gel has the further drawback of dehydration, leading to increased skin contact impedance and motion artifacts, in turn reducing the signal quality and limiting the electrode application environment and lifetime [3,4]. To overcome the limitations of conventional gel-based electrodes, improved hydrogels [ 2 , 5 – 7 ] and semi-dry [ 8 – 11 ] and dry electrodes [ 3 , 12 – 14 ] have been proposed and increasingly used in recent years. Semi-dry electrodes have been suggested as a compromise between conventional gel-based and completely dry electrodes. By applying a small amount of electrolyte to the electrode–skin point of contact, a stable, low-impedance contact can be established, without individual electrode preparation and without major gel residues in the hair. However, semi-dry electrodes are primarily used for EEG applications and are subject to an inherent limitation of the electrolyte in the respective reservoir. Consequently, similar to conventional gel-based electrodes they are not suitable for long-term applications. Furthermore, the advantages of using semi-dry electrodes for applications on bare skin and skin with low hair density in ECG and EMG are limited and do not outweigh the increased requirements for preparation, cleaning, and disinfection. On bare skin, dry electrodes normally exhibit higher contact impedances than the conventional gel-based wet electrodes, but when combined with state-of-the-art electronics, following a short settling time, the electrodes can record bioelectric signals with signal-to-noise ratios similar to the gel electrodes. Moreover, the use of soft, flexible substrate materials fosters the adaptivity of the electrode to the body shape, increasing the long-term wearing comfort and improving the electrode–skin contact, especially during body motion [ 3 , 12 – 14 ]. Dry electrodes are thus user-friendly solutions offering improved applicability without the need for hydrogels or other types of wet electrolytes [3,12–17]. Different dry electrode shapes, grades of flexibility, and material compositions have been suggested, depending on the intended application (e.g., EMG, ECG, or EEG) and respectively required properties. The current state-of-the-art in dry and semi-dry electrodes is the use of flexible materials, either coated or doped with electrically conductive components. The range of doped materials primarily includes elastomers doped with graphite [ 18 , 19 ], and silver (Ag) [ 20 , 21 ] components, as well as carbon nanotubes and nanofibers [ 19 , 22 , 23 ]. Production cost, complexity, and reproducibility as well as constraints in electrode shape, mechanical characteristics, and durability have limited these materials to prototypes thus far. Coated elastomers have been suggested primarily with Ag [ 24 ], Ag/AgCl [ 11 , 13 , 16 , 25 ], gold (Au) [ 26 ], and titanium (Ti) [ 17 , 27 , 28 ]. While silverbased coatings are known to provide excellent electrochemical and thus bioelectric signal characteristics, their biocompatibility in long-term repetitive applications remains an often discussed and questioned limitation [ 29 ]. The costs of gold coatings have also limited these electrode types from widespread use. Ti-based coatings have been proposed as pure metal coatings and in various compositions. A major advantage of these coatings is their well-known biocompatibility and a wide range of established medical applications [ 30 , 31 ]. Moreover, thin-film technology applied to common flexible polymer substrates presents important alternatives aimed at cost-efficient, flexible, and reliable biopotential dry electrodes solutions. The possibility to customize the films’ nanostructures profiting from the low weight and flexibility of the polymer substrates paves the way for the development of a new generation of biomedical sensors. The use of biocompatible thin films enables the activation and functionalization of non-conductive surfaces. When deposited on mechanically flexible polymers, the thin film provides highly conductive pathways for biosignals without compromising the polymer’s elastic modulus, while improving the surface mechanical resistance [ 17 , 32 ]. However, a systematic comparison of Ti-based thin film compositions Sensors 2021,21, 8143 3 of 17 used for bioelectric signal acquisition is missing to date, since existing literature focuses on individual metal combinations only and lacks assessment of cross-system differences. Our investigations focused on a multi-parameter study and comparison of the most promising Ti-based coatings, dedicated to the development of dry electrodes for biopotential measurement. We functionalize flexible polyurethane substrates with different Ti–Me binary intermetallic thin films doped with increasing amounts of Me metals. The Ti–Me intermetallic thin films were prepared by magnetron sputtering, an efficient, economic, and environmentally friendly technology, chosen for its versatility and high adaptability to industrial processes. Ti is a biocompatible material and Ti-based alloys are being applied in an increasing number of medical applications due to their excellent mechanical, physical, and biological performance. By adding a metal “Me” to the Ti matrix, we aimed to improve the well-known properties of pure Ti due to the formation of intermetallic-like compounds. Without considerable differences in biocompatibility and considering their applicability in biomedical sensing, the functional properties of intermetallic compounds proved to be far more advantageous than those of pure metals [ 27 , 30 , 32 – 35 ]. In the binary Ti–Me intermetallic thin films systems the electrical, chemical, and mechanical characteristics of the individual elements prevail [ 27 , 32 , 33 ]. The Ti–Me dry electrodes were assessed in terms of electrode–skin impedances, which are an important parameter of electrode–skin contact. Electrode–skin interfacial impedance spectroscopy was performed in vivo on 20 young volunteers during sports activity, comparing Ti–Me thin films deposited on both PU and SS substrates with conventional hydrogel-based adhesive electrodes. Furthermore, the same pair of Ti–Me dry electrodes was used over all the in-vivo tests (20 measurements including cleaning and disinfection over the course of 14 days) providing a qualitative measure of the electrodes’ stability and wearing effects on the thin film. 2. Materials and Methods 2.1. Ti–Me Dry Electrode Preparation The dry electrodes were prepared by deposition of Ti–Me thin films on flexible polyurethane, PU, substrates and stainless steel, SS, discs, using a custom-made sputtering deposition system. For reference and comparison purposes, commercial hydrogel-based Ag/AgCl electrodes (SNAP Euro ECG electrodes, Foam-Solid gel, FIAB, Firenze, Italy) with a base diameter of 15 mm, were also used. The PU substrates (Biresin U1419, Sika, Germany) adopt a multiwave design previously validated for EEG applications [ 15 , 36 ]. The multiwave electrode concept, as well as the wave height and arrangement (see Figure 1for details), was implemented to maximize the ability of the electrodes to pass through an eventual minor hair layer on the forehead during EEG measurements [ 15 , 37 ]. The same design was adopted for this study. The selective deformation of the PU waves allows them to easily pass through hair on the skin, while the adaptivity to irregular body contours is provided by the PU flexibility, avoiding the unpleasant procedure of shaving the skin before a surface EMG (sEMG) recording when using flat (disc) electrodes. Sensors 2021, 21, x FOR PEER REVIEW 4 of 18 Figure 1. Top and side-view of the multiwave PU substrate containing 30 wave pins in a honeycomb configuration. The relatively short waves provide an improved feeling of comfort and ensure proper interfacial electrode–skin contact [15,37]. The use of SS enabled the investigation of the substrate influence on the overall behavior and stability of the Ti–Me electrodes. The SS discs (reference AISI 316 L, Roqlaser, Portugal) were produced with a thickness of 0.5 mm, while the base diameter (15 mm) was kept equal to the polymer substrates. In addition, silicon substrates (~400 mm2) with (100) orientation (boron-doped p-type) were added to the sputtering chamber during the Ti–Me thin film depositions for subsequent chemical and microstructural characterization analysis. Prior to the depositions, the substrates were cleaned with ethanol and activated by plasma treatment in pure Argon (Ar) atmosphere (working pressure of 80 Pa) to ensure strong adhesion of the Ti–Me films to the substrates. The plasma treatments were promoted during 900 s by a plasma cleaner (Plasma System Zepto, Diener electronic GmbH & Co. KG, Ebhausen, Germany), equipped with a 13.56 MHz generator, at a power of 50 W and a rotary pump [16,38]. This procedure was especially important to increase the surface energy and wettability of the PU substrates without modifying the structural properties of the bulk polymer [39–41]. After the activation, the substrates were placed on a 3-dimensional grounded sample holder positioned in the center of the sputtering chamber at 70 mm from the magnetron. For the depositions, a Ti target (purity: 99.99 at.%, dimensions: 200 mm × 100 mm × 6 mm) was modified with metallic pellets of Al, Cu, Ag or Au (area: 16 mm2, thickness: 0.5 mm) homogeneously glued with conductive Ag glue over the racetrack of the target. The type and number of Me pellets varied according to the Ti–Me system under preparation in order to achieve a wide range of chemical compositions for each system. The depositions were carried out at room temperature, keeping the Ar flow constant while the substrate holder was in a rotation mode (speed of 5 rpm) to ensure the homogeneity of the deposited samples. The direct current (DC) density was set to 75 A/m2 and the maximum deposition temperature reached was 60 °C, avoiding thermal degradation of the PU substrates [39]. The base pressure was always below 2.0 × 10−4 Pa, and the Ar pressure was kept constant at 3.0 × 10−1 Pa. 2.2. Chemical and (Micro)Structural Analysis The quantitative chemical composition of the Ti–Me thin films was evaluated by Rutherford backscattering spectrometry (RBS) analysis. The analysis was carried out in a small RBS chamber, using monoenergetic and collimated beams of 4 He+ (1.5–2 MeV) and/or 1 H+ (2.3 MeV) ions accelerated ~2.5 MeV by a Van der Graaf accelerator, to perpendicularly collide on the sample. Inside the chamber, three detectors were positioned to record the energies of the backscattered ions: one Si surface barrier detector located at a 140° scattering angle, and two pin-diode detectors located symmetrical to each other, both at 165° (detector 3 on the same side as detector 2). The angle of incidence was 0° (normal incidence). The in-depth composition profiles were simulated using the NDF software, after three different measurements in each sample [42,43]. Figure 1. Top and side-view of the multiwave PU substrate containing 30 wave pins in a honeycomb configuration. The relatively short waves provide an improved feeling of comfort and ensure proper interfacial electrode–skin contact [15,37]. Sensors 2021,21, 8143 4 of 17 The use of SS enabled the investigation of the substrate influence on the overall behavior and stability of the Ti–Me electrodes. The SS discs (reference AISI 316 L, Roqlaser, Portugal) were produced with a thickness of 0.5 mm, while the base diameter (15 mm) was kept equal to the polymer substrates. In addition, silicon substrates (~400 mm 2 ) with (100) orientation (boron-doped p-type) were added to the sputtering chamber during the Ti–Me thin film depositions for subsequent chemical and microstructural characterization analysis. Prior to the depositions, the substrates were cleaned with ethanol and activated by plasma treatment in pure Argon (Ar) atmosphere (working pressure of 80 Pa) to ensure strong adhesion of the Ti–Me films to the substrates. The plasma treatments were promoted during 900 s by a plasma cleaner (Plasma System Zepto, Diener electronic GmbH & Co. KG, Ebhausen, Germany), equipped with a 13.56 MHz generator, at a power of 50 W and a rotary pump [ 16 , 38 ]. This procedure was especially important to increase the surface energy and wettability of the PU substrates without modifying the structural properties of the bulk polymer [39–41]. After the activation, the substrates were placed on a 3-dimensional grounded sample holder positioned in the center of the sputtering chamber at 70 mm from the magnetron. For the depositions, a Ti target (purity: 99.99 at.%, dimensions: 200 mm ×100 mm ×6 mm ) was modified with metallic pellets of Al, Cu, Ag or Au (area: 16 mm 2 , thickness: 0.5 mm) homogeneously glued with conductive Ag glue over the racetrack of the target. The type and number of Me pellets varied according to the Ti–Me system under preparation in order to achieve a wide range of chemical compositions for each system. The depositions were carried out at room temperature, keeping the Ar flow constant while the substrate holder was in a rotation mode (speed of 5 rpm) to ensure the homogeneity of the deposited samples. The direct current (DC) density was set to 75 A/m 2 and the maximum deposition temperature reached was 60 ◦ C, avoiding thermal degradation of the PU substrates [ 39 ]. The base pressure was always below 2.0 × 10 −4 Pa, and the Ar pressure was kept constant at 3.0 ×10−1Pa. 2.2. Chemical and (Micro)Structural Analysis The quantitative chemical composition of the Ti–Me thin films was evaluated by Rutherford backscattering spectrometry (RBS) analysis. The analysis was carried out in a small RBS chamber, using monoenergetic and collimated beams of 4 He + (1.5–2 MeV) and/or 1 H + (2.3 MeV) ions accelerated ~2.5 MeV by a Van der Graaf accelerator, to perpendicularly collide on the sample. Inside the chamber, three detectors were positioned to record the energies of the backscattered ions: one Si surface barrier detector located at a 140 ◦ scattering angle, and two pin-diode detectors located symmetrical to each other, both at 165 ◦ (detector 3 on the same side as detector 2). The angle of incidence was 0 ◦ (normal incidence). The in-depth composition profiles were simulated using the NDF software, after three different measurements in each sample [42,43]. The crystal structure and phase distribution of the Ti–Me thin films were carried out employing X-ray diffraction (XRD), using a Bruker D8 Discover diffractometer, operating with Cu–K α radiation ( λ = 1.5406 Å). The XRD patterns were deconvoluted, assuming the Rietveld method with a Pearson VII function. The corresponding structures were analyzed by means of the Inorganic Crystal Structure Database (ICSD) using the TOPAS software (Bruker Corp., Billerica, MA, USA). The plan-view and cross-section micrographs used to evaluate the morphological features and to determine the thickness of the Ti–Me thin films were performed using a high-resolution scanning electron microscope (SEM; FEI Nova NanoSEM 200), with X-ray microanalysis and electron backscattered diffraction analysis, operating at 15 keV. 2.3. In-Vivo Electrode–Skin Interfacial Impedance Measurements All electrode types were tested on a group of 20 healthy male volunteers, age 23.7 ±3.3 years , regular practitioners of cycling at least twice a week. The participants Sensors 2021,21, 8143 5 of 17 reported no recent history of physical injuries, chronic diseases (e.g., neurological or dermatological diseases) or use of drugs. The study was approved by the Ethics Committee of the University “G. d’Annunzio” of Chieti-Pescara (Italy) and all volunteers provided written informed consent. Each volunteer wore two pairs of electrodes of the same type on the (i) rectus femoris and (ii) vastus medialis of the dominant leg. The electrodes were placed on the skin after cleaning with 70% ethyl alcohol and with a distance of 20 mm center to center in the direction of the muscle fibers, following the Surface ElectroMyoGraphy for the NonInvasive Assessment of Muscles (SENIAM) guidelines [ 44 ]. Rigid zinc oxide strapping tape (Strappal, BSN Medical, Hamburg, Germany) was used to fix and keep the same pressure applied to all the electrodes. The electrode–skin interfacial impedance was then measured with a custom-made and portable device constituted by a USB-6216 Isolated Multifunction I/O data logger (National Instruments, Austin, TX, USA) in combination with a custom-made LabView software. Participants were asked to stay relaxed and quiet during the measurements. The safety of the volunteers was ensured by the design of the measurement setup and circuitry, cutting off eventual electric currents exceeding the maximum allowed value of 50 µA [17,45] . The absolute electrode–skin interfacial impedance value (Z E-S ) was determined by using a voltage divider circuit. A known voltage V1 was applied to measure V2, using a known impedance value (Zref), as schematically depicted in Figure 2. Sensors 2021, 21, x FOR PEER REVIEW 5 of 18 The crystal structure and phase distribution of the Ti–Me thin films were carried out employing X-ray diffraction (XRD), using a Bruker D8 Discover diffractometer, operating with Cu–Kα radiation (λ = 1.5406 Å). The XRD patterns were deconvoluted, assuming the Rietveld method with a Pearson VII function. The corresponding structures were analyzed by means of the Inorganic Crystal Structure Database (ICSD) using the TOPAS software (Bruker Corp., Billerica, MA, USA). The plan-view and cross-section micrographs used to evaluate the morphological features and to determine the thickness of the Ti–Me thin films were performed using a high-resolution scanning electron microscope (SEM; FEI Nova NanoSEM 200), with X-ray microanalysis and electron backscattered diffraction analysis, operating at 15 keV. 2.3. In-Vivo Electrode–Skin Interfacial Impedance Measurements All electrode types were tested on a group of 20 healthy male volunteers, age 23.7 ± 3.3 years, regular practitioners of cycling at least twice a week. The participants reported no recent history of physical injuries, chronic diseases (e.g., neurological or dermatological diseases) or use of drugs. The study was approved by the Ethics Committee of the University “G. d’Annunzio” of Chieti-Pescara (Italy) and all volunteers provided written informed consent. Each volunteer wore two pairs of electrodes of the same type on the (i) rectus femoris and (ii) vastus medialis of the dominant leg. The electrodes were placed on the skin after cleaning with 70% ethyl alcohol and with a distance of 20 mm center to center in the direction of the muscle fibers, following the Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) guidelines [44]. Rigid zinc oxide strapping tape (Strappal, BSN Medical, Hamburg, Germany) was used to fix and keep the same pressure applied to all the electrodes. The electrode–skin interfacial impedance was then measured with a custom-made and portable device constituted by a USB-6216 Isolated Multifunction I/O data logger (National Instruments, Austin, TX, USA) in combination with a custom-made LabView software. Participants were asked to stay relaxed and quiet during the measurements. The safety of the volunteers was ensured by the design of the measurement setup and circuitry, cutting off eventual electric currents exceeding the maximum allowed value of 50 μA [17,45]. The absolute electrode–skin interfacial impedance value (ZE-S) was determined by using a voltage divider circuit. A known voltage V1 was applied to measure V2, using a known impedance value (Zref), as schematically depicted in Figure 2. Figure 2. Measurement setup for the electrode–skin interfacial impedance using a custom-made impedance measurement device in a two-pole setup on the muscle of the subject’s dominant leg. Figure 2. Measurement setup for the electrode–skin interfacial impedance using a custom-made impedance measurement device in a two-pole setup on the muscle of the subject’s dominant leg. This two-pole setup provided a measure of the overall impedance of both theelectrode– skin interface and the biological tissue between the electrodes. Nevertheless, due to the methodical and reproducible experimental procedure on the electrodes’ placement, the tissue impedance was considered as not relevant for the quantitative comparison of the Ti–Me electrodes. Prior to the measurements, the custom-made portable impedance measurement device was calibrated using a commercial impedance analyzer (4192A LF, Hewlett Packard Company, Palo Alto, CA, USA) in a standard two-port setup. For the impedance range of interest, we received an error of 0.15% [17,37]. The electrode–skin interfacial impedance was determined for five distinct frequencies: 10 Hz, 50 Hz, 100 Hz, 500 Hz, and 1000 Hz, after a 60 s stabilization period, in potentiostatic measurement mode. The overall duration of each impedance spectroscopy per volunteer was about 5 min. Given the foreseen use of the Ti–Me thin films in the biomedical field as dry biopotential electrodes, the selected frequency values covered the frequency range of Sensors 2021,21, 8143 6 of 17 the standard biopotential signals of EEG (1–40 Hz [ 4 ]), sEMG (6–500 Hz [ 46 ]), and ECG (0.05 Hz–150 Hz up to 700 Hz [47]) [48]. The sequence of tests for all Ti–Me electrodes (PU and SS substrates) and the Ag/AgCl reference electrodes was randomized to avoid eventual systematic influences on the results. The same pair of Ti–Me dry electrodes was used for the 20 volunteers. After each application, the dry electrodes were removed, cleaned, and disinfected with 70% ethyl alcohol. Contrarily, the self-adhesive hydrogel-based Ag/AgCl reference electrodes were disposed of after each test. 3. Results 3.1. Ti–Me Systems—Chemical Composition The chemical composition of the four different Ti–Me intermetallic systems obtained by RBS spectra analysis, as a function of the Me exposed area on the target, is shown in Figure 3. The error in the atomic concentration determination was about 3 at.% for Al and around 0.5 at.%. for the other chemical elements (Ti, Cu, Ag, and Au). Sensors 2021, 21, x FOR PEER REVIEW 7 of 18 Figure 3. Evolution of the atomic composition (at.%) of the four Ti–Me intermetallic systems obtained by RBS spectra analysis: (a) Ti–Al, (b) Ti–Cu, (c) Ti–Ag, and (d) Ti–Au. The connecting dashed lines represent the composition evolution trend of each system. 3.2. Ti–Me Systems—Structural Composition The addition of aluminum, copper, silver, or gold to an initial matrix of Ti opened a wide range of structural arrangements. Each Ti–Me system had developed unique structural features exclusively depending on the Me dopant and its concentration in the film. Three main structural zones were identified to be common to all the Ti–Me systems prepared: (i) a Ti-rich zone, representing all the films dominantly composed by Ti; (ii) an intermetallic zone, for films where the precipitation of Ti–Me intermetallic phases became evident; and (iii) a Me-rich zone when the metal, Me, plays the major role in the films’ composition. To simplify the structural analysis, representative XRD diffraction patterns of each Ti–Me system were selected and are depicted in Figure 4. For every system, five XRD diffraction patterns are presented, including the reference samples of pure Ti and Me thin films except for the pure Au (for the previously stated economic reasons). In the Ti-rich zone, all the Ti–Me films showed diffraction patterns very similar to the polycrystalline Ti thin film that crystallizes into a high close-packed hexagonal (hcp) structure (ICSD collection code #44872). Different crystallite orientations can be observed, with (002) being the preferred, as foreseen by Petrov et al. for hcp structures [49]. However, the incorporation of the Me atoms into the Ti crystal lattice (in substitutional or interstitial sites), even in low contents (Me < 15 at.%) leads to less intense and broader diffraction peaks. Thus, and despite being representative of the Ti hcp structure, the addition of an alloying element (Me) into the Ti matrix promotes considerable changes in the cell parameters, which were evidenced by the suppression of some crystallographic orientations (e.g., (100) or (110)). The results suggest that all the Ti-rich films showed solid Figure 3. Evolution of the atomic composition (at.%) of the four Ti–Me intermetallic systems obtained by RBS spectra analysis: ( a ) Ti–Al, ( b ) Ti–Cu, ( c ) Ti–Ag, and ( d ) Ti–Au. The connecting dashed lines represent the composition evolution trend of each system. All the Ti–Me systems were deposited with Me areas ranging between 0.16 cm 2 and 22 cm2 , with exception of the Ti–Au system, where the maximum Au exposed area was about 12 cm 2 , for purely economic reasons. As expected, the results evidence that the Ti and Me content in the films varied in inverse proportion with the Me exposed area on the target. However, a closer look at the results (Figure 3) reveals the great influence of the sputtered metal Me on the chemical composition of the films. For instance, the Ti content in the Ti–Me thin films decreased to 50 at.% for exposed areas of Cu and Ag (~12 cm 2 ) Sensors 2021,21, 8143 7 of 17 (Figure 3b,c) , whereas in the Ti–Al system it was necessary to expose about 21 cm 2 of Al pellets to have the same Ti content value (Figure 3a). By contrast, with only 6 cm 2 of Au pellets in the racetrack of the Ti target, it was possible to obtain a 50:50 concentration thin film of Ti–Au (Figure 3d). All prepared Ti–Me films proved to be chemically homogeneous in depth, with no evidence of selective diffusion, regardless the number of Me pellets placed onto the Ti target [ 32 ]. Since the atomic concentrations of Ti and Me showed an opposite trend, the Me/Ti atomic ratio was be adopted from then on to correctly identify the samples, providing a simple way to establish a comparative analysis between the systems. The wide range of chemical compositions in which the four different Ti–Me systems were prepared gave rise to the development of a unique set of structural and morphological characteristics. In the same way, the impedance of the electrode on the skin would have been a direct result of its physical response, which is a consequence of the exclusive (micro)structural features that each thin film developed. 3.2. Ti–Me Systems—Structural Composition The addition of aluminum, copper, silver, or gold to an initial matrix of Ti opened a wide range of structural arrangements. Each Ti–Me system had developed unique structural features exclusively depending on the Me dopant and its concentration in the film. Three main structural zones were identified to be common to all the Ti–Me systems prepared: (i) a Ti-rich zone, representing all the films dominantly composed by Ti; (ii) an intermetallic zone, for films where the precipitation of Ti–Me intermetallic phases became evident; and (iii) a Me-rich zone when the metal, Me, plays the major role in the films’ composition. To simplify the structural analysis, representative XRD diffraction patterns of each Ti–Me system were selected and are depicted in Figure 4. For every system, five XRD diffraction patterns are presented, including the reference samples of pure Ti and Me thin films except for the pure Au (for the previously stated economic reasons). In the Ti-rich zone, all the Ti–Me films showed diffraction patterns very similar to the polycrystalline Ti thin film that crystallizes into a high close-packed hexagonal (hcp) structure (ICSD collection code #44872). Different crystallite orientations can be observed, with (002) being the preferred, as foreseen by Petrov et al. for hcp structures [ 49 ]. However, the incorporation of the Me atoms into the Ti crystal lattice (in substitutional or interstitial sites), even in low contents (Me < 15 at.%) leads to less intense and broader diffraction peaks. Thus, and despite being representative of the Ti hcp structure, the addition of an alloying element (Me) into the Ti matrix promotes considerable changes in the cell parameters, which were evidenced by the suppression of some crystallographic orientations (e.g., (100) or (110)). The results suggest that all the Ti-rich films showed solid solutions characteristics (Ti = solute, Me = solvent), developing α -Ti (Me) metastable phases with cumulative local disorder and loss of crystallinity. Indeed, the low temperatures employed during the Ti–Me depositions strongly limited the atomic mobility, hindering the diffusion of species favoring the formation of metastable phases with reduced structural order [ 49 – 51 ]. The broadened and less intense Brag’s peaks/smaller crystallite sizes were evident for the Ti–Cu and Ti–Au films (Figure 4b,d) prepared in this zone. Nevertheless, and although in minor traces, evidence of the (002), (101), and (103) diffraction planes of the pure Ti film could still be noticed for the 0.12 Al/Ti and 0.13 Ag/Ti films, suggesting higher solubility of Al and Ag into the Ti matrix [32]. In the intermetallic zone, i.e., for the Ti–Me films prepared with Me/Ti ratios above 0.20, the most evident structural feature was the precipitation of the intermetallic phases in the α -Ti (Me) solid solution. However, a double trend of structural evolution for the prepared Ti–Me systems was evident. Sensors 2021,21, 8143 8 of 17 Sensors 2021, 21, x FOR PEER REVIEW 8 of 18 solutions characteristics (Ti = solute, Me = solvent), developing α-Ti (Me) metastable phases with cumulative local disorder and loss of crystallinity. Indeed, the low temperatures employed during the Ti–Me depositions strongly limited the atomic mobility, hindering the diffusion of species favoring the formation of metastable phases with reduced structural order [49–51]. The broadened and less intense Brag’s peaks/smaller crystallite sizes were evident for the Ti–Cu and Ti–Au films (Figure 4b,d) prepared in this zone. Nevertheless, and although in minor traces, evidence of the (002), (101), and (103) diffraction planes of the pure Ti film could still be noticed for the 0.12 Al/Ti and 0.13 Ag/Ti films, suggesting higher solubility of Al and Ag into the Ti matrix [32]. Figure 4. X-ray diffraction patterns of the Ti–Me representative films attending to the Me/Ti ratio for every system prepared: (a) Ti–Al system, (b) Ti–Cu system, (c) Ti–Ag system, and (d) Ti–Au system. For Ti–Me systems prepared with Al (a) and Ag (d), a break in the scale of the diffraction angle (x-axis) was introduced, to highlight the main crystallographic orientation growths of both systems. In the intermetallic zone, i.e., for the Ti–Me films prepared with Me/Ti ratios above 0.20, the most evident structural feature was the precipitation of the intermetallic phases in the α-Ti (Me) solid solution. However, a double trend of structural evolution for the prepared Ti–Me systems was evident. On the one hand, the Ti binary systems of Al and Ag (Figure 4a,c) evidenced polycrystalline thin films with the emergence of a new set of diffraction peaks attributed to intermetallic phases: Ti3Al (ICSD collection code # 191189), and Ti2Ag (ICSD collection code # 605931), respectively [32]. However, the increasing addition of Me (Al or Ag) led Figure 4. X-ray diffraction patterns of the Ti–Me representative films attending to the Me/Ti ratio for every system prepared: ( a ) Ti–Al system, ( b ) Ti–Cu system, ( c ) Ti–Ag system, and ( d ) Ti–Au system. For Ti–Me systems prepared with Al (a) and Ag (d), a break in the scale of the diffraction angle (x-axis) was introduced, to highlight the main crystallographic orientation growths of both systems. On the one hand, the Ti binary systems of Al and Ag (Figure 4a,c) evidenced polycrystalline thin films with the emergence of a new set of diffraction peaks attributed to intermetallic phases: Ti 3 Al (ICSD collection code #191189), and Ti 2 Ag (ICSD collection code #605931), respectively [ 32 ]. However, the increasing addition of Me (Al or Ag) led to the cumulative local lattice disorder resulting in the progressive amorphization of the α -Ti structure that became less evident in the diffraction patterns. On the other hand, the appearance of noticeable broad diffraction humps for the films prepared with Au or Cu left no doubt about the formation of non-crystalline amorphous structures typically featured on thin film metallic glasses (TFMGs), as discussed in previous studies [ 32 ]. The location of those broad diffraction patterns (Figure 4b,d) suggests the formation of different poorly-crystallized Ti–Cu (ICSD collection codes: #629388 for Ti 2 Cu and #103130 for Ti 3 Cu), and Ti–Au (ICSD collection code #58605 for Ti 3 Au or #197280 for TiAu) intermetallic metastable phases [ 32 ]. The TFMG behavior observed for these systems can be explained by the low solubility of Cu and Au into the Ti matrix [ 32 ] but also by the kinetic limitations employed within the sputtering process parameters (low temperatures and low-energy ion bombardment without biased substrates) [49,52]. In the transition to the Me-rich zone, the structural evolution trend for the systems prepared with Al and Au presented a turning point. The Al-rich films started to exhibit a Sensors 2021,21, 8143 9 of 17 quasi-amorphous structure evidenced by a broad diffraction domain (Figure 4a), where several Ti–Al intermetallic phases could be indexed (ICSD collection codes: #290974 for TiAl, #107009 for TiAl 2 and #190891 for TiAl 3 ), albeit it was impossible to distinguish them due to their reduced structural order [ 32 ]. Contrarily, the Au-rich film evolved to Au-rich crystalline domains (ICSD collection codes: #58607 for TiAu 2 , #109132 for TiAu 4 ) reflected in sharp diffraction peaks with reduced full width at half maximum (FWHM), Figure 4d. No considerable structural changes were observed in the intermetallic zone with growing Me addition for the Ti–Ag and Ti–Cu systems. In the Ag-rich films, the quasicrystalline Ti–Ag intermetallic phase (ICSD collection code #605934) prevailed, following the behavior observed for the Au-rich films. Additionally, for Agand Au-rich films, the deviation towards the same angular position of the fcc–Ag and Au structures together with the unfavorable thermodynamic conditions during the films’ growth, suggested the development of silverand golddoped Ti, Ag (Ti), and Au (Ti), metastable phases in crystalline nanophases coexisting with the intermetallic ones [ 32 , 49 ]. For the Ti–Cu system, the glassy amorphous structures—typical of TFMG—remained despite the shift of the diffraction hump to higher values of 2 θ observed for the film with 2.54 Cu/Ti ratio film. In accordance with the Ti–Cu equilibrium phase diagram [ 53 ], several poorly crystallized Cu-rich metastable phases (ICSD collection codes: #103128 for TiCu, #103133 for Ti 2 Cu 3 and #103134 for Ti 3 Cu 4 ) with a great tendency to amorphization could be associated with the diffraction hump located within the range (38◦to 48◦). The mutual solubility of the elements in the Ti–Me binary systems played a prominent role in the structural characteristics observed for the films prepared within the Me-rich zone. Results show that the high solubility of Ti in Agand Au-rich films (crystalline structures) and the low solubility in the films prepared with the highest contents of Cu and Al (amorphous structures) were most certainly playing a decisive role in the overall behavior [53–56]. 3.3. Ti–Me Systems—Morphological Evolution The morphological features of the Ti–Me thin films were assessed in the light of the different structural arrangements already identified. For each system, three representative films were selected, one from each structural zone (Tior Me-rich zone and intermetallic zone). To simplify the comparison of the morphological evolution, additional micrographs of the pure Ti and Me thin films were included. The plan-view and cross-section micrographs of each representative film, deposited on silicon substrates, can be seen in Figure 5. Regardless of the Ti–Me system, all the thin films prepared in the Ti-rich zone presented a columnar-like growth typical of the Ti structures, which is very common on sputter deposition processes at low temperatures and low-energy ion bombardments [ 57 , 58 ]. The non-thermodynamic equilibrium conditions were also responsible for the rough surface morphologies observed in this zone, which were especially evident in the Ti–Al and Ti–Au systems and reminded us of the three-dimensional hexagonal grain features of the Ti surface. Due to the sputtering process’ kinetic limitations, the atoms of Ti and of the respective metal do not arrive uniformly at the substrate (with the same energy), therefore contributing to the surface growth roughness phenomenon [59,60]. The morphology of the Ti–Me films prepared in the intermetallic zone reflected the distinct trends of the structural evolution previously discussed. The films prepared with Al and Ag continued to reveal the columnar growth typical of Ti (visible on the reference film), although developing denser and less porous microstructures (Figure 5a,c), which may have been related to the precipitation of crystallized intermetallic nanostructures. The formation of the Ti–Ag and Ti–Al intermetallic phases was also responsible for the coarsening of crystalline grains observed on the surface morphology, giving rise to rougher surfaces. Sensors 2021,21, 8143 16 of 17 10. 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