Nanocone-Array-Based Platinum-Iridium Oxide Neural Microelectrodes: Structure, Electrochemistry, Durability and Biocompatibility Study

Abstract

Neural interfaces provide a window for bio-signal modulation and recording with the assistance of neural microelectrodes. However, shrinking the size of electrodes results in high electrochemical impedance and low capacitance, thus limiting the stimulation/recording efficiency. In order to achieve critical stability and low power consumption, here, nanocone-shaped platinum (Pt) with an extensive surface area is proposed as an adhesive layer on a bare Pt substrate, followed by the deposition of a thin layer of iridium oxide (IrO_x) to fabricate high-performance nanocone-array-based Pt-IrO_x neural microelectrodes (200 μm in diameter). A uniform nanocone-shaped Pt with significant roughness is created via controlling the ratio of NH₄⁺ and Pt⁴⁺ ions in the electrolyte, which can be widely applicable for batch production on multichannel flexible microelectrode arrays (fMEAs) and various substrates with different dimensions. The Pt-IrO_x nanocomposite-coated microelectrode presents a significantly low impedance down to 0.72 ± 0.04 Ω cm² at 1 kHz (reduction of ~92.95%). The cathodic charge storage capacity (CSC_c) and charge injection capacity (CIC) reaches up to 52.44 ± 2.53 mC cm^-2 and 4.39 ± 0.36 mC cm^-2, respectively. Moreover, superior chronic stability and biocompatibility are also observed. The modified microelectrodes significantly enhance the adhesion of microglia, the major immune cells in the central nervous system. Therefore, such a coating strategy presents great potential for biomedical and other practical applications.

Keywords: biocompatibility; iridium oxide; nanostructure; neural microelectrodes; platinum.

Figure 1

Flexible microelectrode array (fMEA). (a) Schematic of the microfabrication process, including spin-coating, photo-lithography, reactive ion etching and peeling off (i–vi). (b) Photographic image of fMEA (200 μm in diameter of electrode site), welded onto a printed circuit board (PCB). (c) Magnified view of coated (Pt, IrO_x, or IrO_x/Pt nanostructure) and uncoated (bare Pt) electrode sites.

Figure 2

SEM images of coated microelectrodes with different deposition processes. (a) Bare Pt electrode. Morphology of nanostructured Pt deposited in electrolytes containing (b) PtCl₄, (c) PtCl₄ + (NH₂CH₂)₂·2HCl, (d) PtCl₄ + NH₄Cl, (e) (NH₄)₂PtCl₆ and (f) PtCl₄ + (NH₄)₂PtCl₆ respectively. The insets in (e,f) show the enlarged images of Pt nanocluster and nanocone respectively.

Figure 3

Electrochemical performance of microelectrodes before and after modification under different deposition process conditions. (a) EIS and (b) CV measurements on microelectrodes carrying different Pt nanodendrites in comparison with bare Pt (unmodified). The insets in (b) show the overall view of cluster−shaped and nanocone−shaped Pt deposited on single−channel electrodes, respectively.

Figure 4

Batch production of Pt nanocones. Pt nanocone array electrodeposited on (a) electrophysiological fMEA, (b) platinized silicon wafers, and (c) Pt wire and cochlear implant. The numbers marked on panels (a–c) were chosen to observe the morphology, with point 1 as the unmodified electrode for reference. (d) The initial status and (e) after 1 kg load for 72 h of a Pt−nanocone−array−based platinized silicon wafer, the inset in (e) shows the enlarged image of Pt nanocone with excellent stability. (f) The variation rate of impedance and CSC_c after ultrasonic tests for 1 h in PBS.

Figure 5

Morphology and electrochemical properties of different coated microelectrodes. SEM images of (a) IrO_x and (b,c) IrO_x/Pt nanocone composites. (d–f) The comparison of impedance at 1 kHz, CSC_c, and CIC of bare Pt, Pt nanocone, IrO_x, and IrO_x/Pt nanocone−coated microelectrodes. *** denotes p < 0.001, **** denotes p < 0.0001 by Student’s t test. (g) Chronic stability of IrO_x/Pt nanocone−coated microelectrodes with different electrostimulation duration times at 100 Hz; the inset implies the stable structure of Pt nanocones.

Figure 6

Biocompatibility evaluation of different microelectrodes on primary mouse microglia. Three microglia cultures were assayed for each experimental group. (a) Immunostaining of microglia-specific marker Iba1 (green) after culturing with extract from the indicated microelectrodes, and DAPI (blue)-stained nuclei. Scale bar is 100 µm. (b) Quantification of Iba1-positive microglia adhered to substrates following culture with different microelectrode extracts from (a). (c) Quantification of microglia viability following culture with the indicated microelectrode extracts using CCK8 viability assay. * denotes p < 0.05 by Student’s t test, NS denotes not significant.