Electrochemical and biological characterization of thin-film platinum-iridium alloy electrode coatings: a chronic in vivo study

Abstract

Objective. To evaluate the electrochemical properties, biological response, and surface characterization of an electrodeposited Platinum-Iridium (Pt-Ir) electrode coating on cochlear implants subjected to chronic stimulation in vivo. Approach. Electrochemical impedance spectroscopy (EIS), charge storage capacity (CSC), charge injection limit (CIL), and voltage transient (VT) impedance were measured bench-top before and after implant and in vivo. Coated Pt-Ir and uncoated Pt electrode arrays were implanted into cochlea of normal hearing rats and stimulated for ∼4 h d, 5 d week−1 for 5 weeks at levels within the normal clinical range. Neural function was monitored using electrically-evoked auditory brainstem responses. After explant, the electrode surfaces were assessed, and cochleae examined histologically. Main results. When measured on bench-top before and after stimulation, Pt-Ir coated electrodes had significantly lower VT impedance (p < 0.001) and significantly higher CSC (p < 0.001) and CIL (p < 0.001) compared to uncoated Pt electrodes. In vivo, the CSC and CIL of Pt-Ir were significantly higher than Pt throughout the implantation period (p= 0.047 and p< 0.001, respectively); however, the VT impedance (p= 0.3) was not. There was no difference in foreign body response between material cohorts, although cochleae implanted with coated electrodes contained small deposits of Pt-Ir. There was no evidence of increased neural loss or loss of neural function in either group. Surface examination revealed no Pt corrosion on any electrodes. Significance. Electrodeposited Pt-Ir electrodes demonstrated significant improvements in electrochemical performance on the bench-top and in vivo compared to uncoated Pt. Neural function and tissue response to Pt-Ir electrodes were not different from uncoated Pt, despite small deposits of Pt-Ir in the tissue capsule. Electrodeposited Pt-Ir coatings offer promise as an improved electrode coating for active neural prostheses.

Noble Metals for Modern Implant Materials: MOCVD of Film Structures and Cytotoxical, Antibacterial, and Histological Studies

“…That is why, for this group of samples, the growth of FC should be expected with longer implantation. The observed results correlated with recent work of Dalrymple et al [ 61 ], which showed that Pt–Ir-coated steel electrodes became fibrotic during cochlear implantation and the capsule thickness slightly increased after 5 weeks of implantation. This was detected by an increase in the impedance, while no histological data were obtained.…”

“…The maturation of FC by 3 months due to the thickening and compaction of connective tissue fibers synthesized by fibroblasts led to a decrease in their average thickness. Our results show that implants with noble metal coatings caused the formation of a dense fibrous capsule with a minimal level of immune response from the very beginning, which was consistent with the abovementioned data obtained for PtIr electrodes [61]. In this case, the increase in FC over time observed for samples with PtIr coating may have been due to the nonspecific toxic effect of Pt ions leaching from their surface, when the death of fibroblasts inside the capsule led to the start of the repair mechanism and the formation of a new layer of the fibrous capsule.…”

Stimulation of the Dorsal Root Ganglion using an Injectrode®

“…Pt electrodes, which are more clinically-relevant, also exhibit more capacitive behaviour at low frequencies, indicated by a phase closer to -90°. This behaviour has been reported in several in vivo studies using Pt electrodes, in which the low frequency (1 Hz) impedance phase was approximately -66° (Shepherd et al, 2021) and the 100 Hz impedance phase ranged from -56° to -72° (Dalrymple et al, 2020a(Dalrymple et al, , 2020bShepherd et al, 2021). This is similar to how the stainless steel electrode behaved here, suggesting that the Injectrode may also accumulate less charge than a Pt electrode.…”

“…Gold or platinum are more appropriate than silver for chronic implants because they are biocompatible and are less likely to elicit a strong tissue response (Wellman et al, 2018). A strong tissue response would increase the impedance at the electrode-tissue interface, thereby requiring more charge injection to effectively stimulate the neural target (Cogan, 2008;Dalrymple, 2021;Dalrymple et al, 2020a). The Injectrode design is promising for chronic use as it conforms to the tissue around it, limiting the potential for compression of nearby structures, like the DRG, as compared to stiffer electrode leads currently used for DRG stimulation.…”

Exploring polymeric biomaterials in developing neural prostheses

“…43 Moreover, unlike metal electrodes, conductive polymers are also capable of preventing skin injuries. 34 CPs are being extensively studied as a method to mitigate limitations aforementioned in miniaturization due to their beneficial characteristics such as biocompatibility, environmental 17 Cassar et al, 18 Dalrymple et al 19 Aurum Biocompatibility, good conductivity Kim et al 20 Iridium oxide Good biostability, high charge storage capacity Hasenkamp et al 21…”