Mechanical Stability of Nano-Coatings on Clinically Applicable Electrodes, Generated by Electrophoretic Deposition

The mechanical stability of implant coatings is crucial for medical approval and transfer to clinical applications. Here, electrophoretic deposition (EPD) is a versatile coating technique, previously shown to cause significant post-surgery impedance reduction of brain stimulation platinum electrodes. However, the mechanical stability of the resulting coating has been rarely systematically investigated. In this work, pulsed-DC EPD of laser-generated platinum nanoparticles (PtNPs) on Pt-based, 3D neural electrodes is performed and the in vitro mechanical stability is examined using agarose gel, adhesive tape, and ultrasonication-based stress tests. EPD-generated coatings are highly stable inside simulated brain environments represented by agarose gel tests as well as after in vivo stimulation experiments. Electrochemical stability of the NP-modified surfaces is tested via cyclic voltammetry and that multiple scans may improve coating stability could be verified, indicated by higher signal stability following highly invasive adhesive tape stress tests. The brain sections post neural stimulation in rats are analyzed via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Measurements reveal higher levels of Pt near the region stimulated with coated electrodes, in comparison to uncoated controls. Even though local concentrations in the vicinity of the implanted electrode are elevated, the total Pt mass found is below systemic toxicologically relevant concentrations.

Comparing Direct and Pulsed-Direct Current Electrophoretic Deposition on Neural Electrodes: Deposition Mechanism and Functional Influence

Electrophoretic deposition (EPD) of platinum nanoparticles (PtNPs) on platinum-iridium (Pt-Ir) neural electrode surfaces is a promising strategy to tune the impedance of electrodes implanted for deep brain stimulation in various neurological disorders such as advanced Parkinson's disease and dystonia. However, previous results are contradicting as impedance reduction was observed on flat samples while in three-dimensional (3D) structures, an increase in impedance was observed. Hence, defined correlations between coating properties and impedance are to date not fully understood. In this work, the influence of direct current (DC) and pulsed-DC electric fields on NP deposition is systematically compared and clear correlations between surface coating homogeneity and in vitro impedance are established. The ligand-free NPs were synthesized via pulsed laser processing in liquid, yielding monomodal particle size distributions, verified by analytical disk centrifugation (ADC). Deposits formed were quantified by UV-vis supernatant analysis and further characterized by scanning electron microscopy (SEM) with semiautomated interparticle distance analyses. Our findings reveal that pulsed-DC electric fields yield more ordered surface coatings with a lower abundance of particle assemblates, while DC fields produce coatings with more pronounced aggregation. Impedance measurements further highlight that impedance of the corresponding electrodes is significantly reduced in the case of more ordered coatings realized by pulsed-DC depositions. We attribute this phenomenon to the higher active surface area of the adsorbed NPs in homogeneous coatings and the reduced particle-electrode electrical contact in NP assemblates. These results provide insight for the efficient EPD of bare metal NPs on micron-sized surfaces for biomedical applications in neuroscience and correlate coating homogeneity with in vitro functionality.

Optimizing in Vitro Impedance and Physico-Chemical Properties of Neural Electrodes by Electrophoretic Deposition of Pt Nanoparticles

Neural electrodes suffer from an undesired incline in impedance when in permanent contact with human tissue. Nanostructures, induced by electrophoretic deposition (EPD) of ligand-free laser-generated nanoparticles (NPs) on the electrodes are known to stabilize impedance in vivo. Hence, Pt surfaces were systematically EPD-coated with Pt NPs and evaluated for impedance as well as surface coverage, contact angle, electrochemically active surface area (ECSA) and surface oxidation. The aim was to establish a systematic correlation between EPD process parameters and physical surface properties. The findings clearly reveal a linear decrease in impedance with increasing surface coverage, which goes along with a proportional reduction of the contact angle and an increase in ECSA and surface oxidation. EPD process parameters, prone to yield surface coatings with low impedance, are long deposition times (40-60 min), while high colloid concentrations (>250 μg mL^-1 ) and electric field strengths (>25 V cm^-1 ) should be avoided due to detrimental NP assemblage effects.

Electrophoretic deposition of ligand-free platinum nanoparticles on neural electrodes affects their impedance in vitro and in vivo with no negative effect on reactive gliosis

BACKGROUND

Electrodes for neural stimulation and recording are used for the treatment of neurological disorders. Their features critically depend on impedance and interaction with brain tissue. The effect of surface modification on electrode impedance was examined in vitro and in vivo after intracranial implantation in rats. Electrodes coated by electrophoretic deposition with platinum nanoparticles (NP; <10 and 50 nm) as well as uncoated references were implanted into the rat's subthalamic nucleus. After postoperative recovery, rats were electrostimulated for 3 weeks. Impedance was measured before implantation, after recovery and then weekly during stimulation. Finally, local field potential was recorded and tissue-to-implant reaction was immunohistochemically studied.

RESULTS

Coating with NP significantly increased electrode's impedance in vitro. Postoperatively, the impedance of all electrodes was temporarily further increased. This effect was lowest for the electrodes coated with particles <10 nm, which also showed the most stable impedance dynamics during stimulation for 3 weeks and the lowest total power of local field potential during neuronal activity recording. Histological analysis revealed that NP-coating did not affect glial reactions or neural cell-count.

CONCLUSIONS

Coating with NP <10 nm may improve electrode's impedance stability without affecting biocompatibility. Increased impedance after NP-coating may improve neural recording due to better signal-to-noise ratio.