In recent years, the invasive and implantable devices have been well applied in cochlear implant, deep brain stimulation, visual prosthesis and other neural prostheses to diagnose or treat diseases.  

As a key bridge between internal tissue and external devices, neural electrodes are developing towards miniaturization and integration, which will provide higher efficiency of electrical stimulation/recording in clinical practice.  

However, the interface impedance is extremely high with the shrinking size of electrode, which severely reduces its charge storage and injection capacity, and thus limits its practical application.  

In addition, despite of the excellent catalytic properties of platinum (Pt) and iridium (Ir), the electrolysis efficiency of water is often limited by the high overpotential of oxygen evolution reaction (OER).  

Based on the above considerations, Prof. WU Tianzhun￠s gruop from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences developed Pt and Ir nanomaterial in their previous work (Electrochimica Acta (2017), Adv. Mater. Interfaces (2019), ACS Appl. Mater. Interfaces (2020), IEEE Sens. J. (2021)) to improve the electrical performance and stimulation efficiency.  

Recently, Prof. WU’s team further developed a flower-shaped Pt nanocrystal with intensive high-surface area as an intermediate layer for accumulating IrOx (<3 wt% Ir) with enhanced adhesion, showing a multiplier effect.  

The study, which was published in Advanced Materials Interfaces, was selected as front cover.  

Compared with bare Pt electrode of the same size, the impedance of IrOx/Pt flower coated microelectrode at 1 kHz was down to ≈2 kW (reduction of 94.23%). The corresponding cathodic charge storage capacity (CSCc) and charge injection capacity (CIC) were increased up to 202.75±2.18 mC×cm^-2 and 6.53±0.16 mC×cm^-2 respectively.  

IrOx layer adhered tightly to Pt nanocrystals, demonstrating robust chronic stability under continuous electrostimulation for 1′10⁸ cycles. 

Besides, the as-prepared coatings showed good biosafety, and exhibited promising electrocatalytic activity towards OER in 0.5 M H₂SO₄, requiring a low overpotential of about 150 mV to reach 10 mA×cm^-2.  

IrOx helped reducing the Tafel slope of Pt flower from 162.9 mV×dec^-1 to 41.1 mV×dec^-1 drastically, also with excellent durability after chronoamperometry test.  

After 48 hours of culture, the surface coverage of Escherichia coli on IrOx/Pt flower electrode was much lower than that on the planar Pt electrode, which confirmed its potential antibacterial ability.  

"This method effectively solved the existing technical shortcomings with strong operability, and can be mass-produced," said Dr. ZENG Qi, the author of this study, "it can be widely used in neural interface, water oxidation, anti-biological pollution, and is expected to be used in flexible bioelectronics and energy storage such as neural prostheses, efficient stimulation/recording electrodes and biosensing, and other practical applications." 

Application schematic diagram. (Image by SIAT)

The schematic of electrode modification process, and properties of neural interface, water oxidation and anti-microbial contamination. (Image by SIAT)