Suzan Meijs

Electrochemical properties of titanium nitride nerve stimulation electrodes: an in vitro and in vivo study.

The in vivo electrochemical behavior of titanium nitride (TiN) nerve stimulation electrodes was compared to their in vitro behavior for a period of 90 days. Ten electrodes were implanted in two Göttingen minipigs. Four of these were used for electrical stimulation and electrochemical measurements. Five electrodes were kept in Ringers solution at 37.5°C, of which four were used for electrical stimulation and electrochemical measurements. The voltage transients measured in vivo were 13 times greater than in vitro at implantation and they continued to increase with time. The electrochemical properties in vivo and the tissue resistance (Rtissue) followed a similar trend with time. There was no consistent significant difference between the electrochemical properties of the in vivo and in vitro electrodes after the implanted period. The differences between the in vivo and in vitro electrodes during the implanted period show that the evaluation of electrochemical performance of implantable stimulation electrodes cannot be substituted with in vitro measurements. After the implanted period, however, the performance of the in vivo and in vitro electrodes in saline was similar. In addition, the changes observed over time during the post-implantation period regarding the electrochemical properties of the in vivo electrodes and Rtissue were similar, which indicates that these changes are due to the foreign body response to implantation.

Biofouling resistance of boron-doped diamond neural stimulation electrodes is superior to titanium nitride electrodes in vivo

OBJECTIVE The goal of this study was to assess the electrochemical properties of boron-doped diamond (BDD) electrodes in relation to conventional titanium nitride (TiN) electrodes through in vitro and in vivo measurements. APPROACH Electrochemical impedance spectroscopy, cyclic voltammetry and voltage transient (VT) measurements were performed in vitro after immersion in a 5% albumin solution and in vivo after subcutaneous implantation in rats for 6 weeks. MAIN RESULTS In contrast to the TiN electrodes, the capacitance of the BDD electrodes was not significantly reduced in albumin solution. Furthermore, BDD electrodes displayed a decrease in the VTs and an increase in the pulsing capacitances immediately upon implantation, which remained stable throughout the whole implantation period, whereas the opposite was the case for the TiN electrodes. SIGNIFICANCE These results reveal that BDD electrodes possess a superior biofouling resistance, which provides significantly stable electrochemical properties both in protein solution as well as in vivo compared to TiN electrodes.

Increased Charge Storage Capacity of Titanium Nitride Electrodes by Deposition of Boron-doped Nanocrystalline Diamond Films

The aim of this study was to investigate the feasibility of depositing a thin layer of boron-doped nanocrystalline diamond (B-NCD) on titanium nitride (TiN) coated electrodes and the effect this has on charge injection properties. The charge storage capacity increased by applying the B-NCD film, due to the wide potential window typical for B-NCD. The impedance magnitude was higher and the pulsing capacitance lower for B-NCD compared to TiN. Due to the wide potential window, however, a higher amount of charge can be injected without reaching unsafe potentials with the B-NCD coating. The production parameters for TiN and B-NCD are critical, as they influence the pore resistance and thereby the surface area available for pulsing.

Comparison of the electrochemical properties of smooth and porous TiN electrode coatings in rats

Eight smooth and 8 porous titanium nitride (TiN) coated electrodes were implanted in 8 rats. Before implantation, voltage transients (VTs), cyclic voltammograms (CVs) and impedance spectra were recorded in phosphate buffered saline (PBS). During the implanted period, these measurements were repeated weekly. The VTs and CVs at high sweep rates of the porous electrodes were more affected by implantation as compared to the smooth electrodes. The charge injection (Qinj) and charge storage capacity (CSC) of both electrode types decreased during the first 3 weeks after implantation. This indicates that protein adhesion directly after implantation presents a diffusion limitation for the porous electrodes only, while cell adhesion limits diffusion for both smooth and porous electrodes.

Increasing Voltage Transients Using Implanted Titanium Nitride Neural Stimulation Electrodes

The electrochemical properties of porous titanium nitride (TiN) stimulation electrode coatings were investigated in vivo in the chronic setting. Four titanium pins were coated with porous TiN and implanted in the pelvic region of two minipigs. Electrochemical impedance spectroscopy (EIS) was performed daily, while voltage transient measurements (VTM), electrical stimulation and cyclic voltammetry (CV) were performed every other week. Electrical stimulation was applied successfully during the course of the study. Voltage transients and tissue impedance increased, while charge storage decreased during the first 3-4 weeks after implantation. This is most likely related to encapsulation of the electrode.

Chronic Electrochemical Investigation of Titanium Nitride Stimulation Electrodes in vivo

For neural prostheses to be successful, good performance of neurostimulation electrodes is important. An important aspect of this is electrochemical stability and corrosion resistance. The electrochemical performance of 4 thin film titanium nitride (TiN) electrodes is investigated in a porcine animal model. In vivo impedance measurements have been made for 3 months to investigate the electrode-tissue interface and to monitor tissue resistance in vivo. The tissue resistance was low in the first week, after which it increased and stabilized. After 50 days, a dramatic change is observed in the electrode-tissue interface; charge is transferred via faradic instead of capacitive pathways. This is likely due to anodic oxidation of the TiN surface. This was not reversed, but no signs of further oxidation were observed. Neither the tissue nor the electrode appeared damaged after explantation.

Influence of fibrous encapsulation on electro-chemical properties of TiN electrodes

The aim of this study was to investigate how the electrochemical properties of porous titanium nitride stimulation electrode are affected by fibrous encapsulation in vivo. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry and voltage transient (VT) measurements were performed in vivo and in phosphate buffered saline, where the encapsulation process is absent. EIS was used as a non-invasive measurement to follow the inflammation, healing and encapsulation process. EIS showed that the healing and encapsulation process lasted 3-4 weeks. The VTs increased during the first 3-4 weeks, after which they stabilized. The charge storage capacity (CSC) decreased most during the first 3-4 weeks. The increasing VTs and decreasing CSC during the first 3-4 weeks after implantation of the in vivo electrodes seem related to healing and fibrous encapsulation. It is suggested that the charge injection pathway during the encapsulation process changes, which implies that charge injection limits are underestimated with conventional methods.

Influence of implantation on the electrochemical properties of smooth and porous TiN coatings for stimulation electrodes

OBJECTIVE To determine whether changes in electrochemical properties of porous titanium nitride (TiN) electrodes as a function of time after implantation are different from those of smooth TiN electrodes. APPROACH Eight smooth and 8 porous TiN coated electrodes were implanted in 8 rats. Before implantation, voltage transients, cyclic voltammograms and impedance spectra were recorded in phosphate buffered saline (PBS). After implantation, these measurements were done weekly to investigate how smooth and porous electrodes were affected by implantation. MAIN RESULTS The electrode capacitance of the porous TiN electrodes decreased more than the capacitance of the smooth electrodes due to acute implantation under fast measurement conditions (such as stimulation pulses). This indicates that protein adhesion presents a greater diffusion limitation for counter-ions for the porous than for the smooth electrodes. The changes in electrochemical properties during the implanted period were similar for smooth and porous TiN electrodes, indicating that cell adhesion poses a similar diffusion limitation for smooth and porous electrodes. SIGNIFICANCE This knowledge can be used to optimize the porous structure of the TiN film, so that the effect of protein adhesion on the electrochemical properties is diminished. Alternatively, an additional coating could be applied on the porous TiN that would prevent or minimize protein adhesion.

In vivo charge injection limits increased after 'unsafe' stimulation

The effect of unsafe stimulation on charge injection limits (Qinj) and pulsing capacitance (Cpulse) was investigated. Four stimulation protocols were applied: 20 mA – 200 and 400 Hz, 50 mA – 200 and 400 Hz. Increasing Qinj and Cpulse were observed for all stimulation protocols. Corrosion was not observed with any of the stimulation protocols and no tissue damage was observed for the 20 mA – 200 Hz stimulation group. This indicates that the ‘safe potential window’ may not be applicable in vivo, as no damage was done stimulating with 20 mA at 200 Hz, while damage was done using the same current at 400 Hz.

Electrochemical investigation of peripheral nerve stimulation electrodes in vivo and in vitro during 53 days

The objective of the study was to investigate the electrochemical properties of porous titanium nitride electrodes in vivo and in vitro for a period of 53 days. Four electrodes were implanted in two pigs and four electrodes were kept in phosphate buffered saline at 37.5 °C. Electrochemical impedance spectroscopy, voltage transient measurements, cyclic voltammetry and electrical stimulation were applied in vivo and in vitro. In vivo, electrical stimulation was successful, using stable currents below the safe limits. However, increasing voltage transients and decreasing charge storage capacity (CSC) were observed, which was likely related to wound healing. In vitro, decreasing CSC and increasing electrode impedance were observed, which was likely related to electrode deterioration.