Abiotic-biotic characterization of Pt/Ir microelectrode arrays in chronic implants

Abstract

Pt/Ir electrodes have been extensively used in neurophysiology research in recent years as they provide a more inert recording surface as compared to tungsten or stainless steel. While floating microelectrode arrays (FMA) consisting of Pt/Ir electrodes are an option for neuroprosthetic applications, long-term in vivo functional performance characterization of these FMAs is lacking. In this study, we have performed comprehensive abiotic-biotic characterization of Pt/Ir arrays in 12 rats with implant periods ranging from 1 week up to 6 months. Each of the FMAs consisted of 16-channel, 1.5 mm long, and 75 μm diameter microwires with tapered tips that were implanted into the somatosensory cortex. Abiotic characterization included (1) pre-implant and post-explant scanning electron microscopy (SEM) to study recording site changes, insulation delamination and cracking, and (2) chronic in vivo electrode impedance spectroscopy. Biotic characterization included study of microglial responses using a panel of antibodies, such as Iba1, ED1, and anti-ferritin, the latter being indicative of blood-brain barrier (BBB) disruption. Significant structural variation was observed pre-implantation among the arrays in the form of irregular insulation, cracks in insulation/recording surface, and insulation delamination. We observed delamination and cracking of insulation in almost all electrodes post-implantation. These changes altered the electrochemical surface area of the electrodes and resulted in declining impedance over the long-term due to formation of electrical leakage pathways. In general, the decline in impedance corresponded with poor electrode functional performance, which was quantified via electrode yield. Our abiotic results suggest that manufacturing variability and insulation material as an important factor contributing to electrode failure. Biotic results show that electrode performance was not correlated with microglial activation (neuroinflammation) as we were able to observe poor performance in the absence of neuroinflammation, as well as good performance in the presence of neuroinflammation. One biotic change that correlated well with poor electrode performance was intraparenchymal bleeding, which was evident macroscopically in some rats and presented microscopically by intense ferritin immunoreactivity in microglia/macrophages. Thus, we currently consider intraparenchymal bleeding, suboptimal electrode fabrication, and insulation delamination as the major factors contributing toward electrode failure.

Elastomeric and soft conducting microwires for implantable neural interfaces

“…This loss of viable neurons at the electrode-tissue interface may contribute to signal deterioration and the eventual failure of penetrating electrodes. Therefore, to improve the long-term reliability of neural probe over time, the next generation of high-performance microelectrodes must cause a lower tissue reactivity, BBB disruption, glial response and neuronal degeneration/damage [27, 35, 37]. It is also critical to consider electrode design factors including size, shape and tethering [35, 38, 39].…”

Implications of chronic daily anti-oxidant administration on the inflammatory response to intracortical microelectrodes

“…Unfortunately, following implantation of intracortical microelectrodes, multiple failure modes can occur, ultimately resulting in loss of recorded signals weeks to months after surgery (Prasad et al, 2012; Barrese et al, 2013; Prasad et al, 2014). One emerging hypothesis concerning microelectrode failure suggests a leading role for oxidative stress in altering neuronal cell viability and blood brain barrier stability at the device-tissue interface (McConnell et al, 2009; Potter et al, 2013).…”

“…One emerging hypothesis concerning microelectrode failure suggests a leading role for oxidative stress in altering neuronal cell viability and blood brain barrier stability at the device-tissue interface (McConnell et al, 2009; Potter et al, 2013). In addition, it has also been proposed that the same oxidative environment can result in breakdown/corrosion of both the insulator and the metals of the electrode itself (Schmitt et al, 1999; Barrese et al, 2013; Kozai et al, 2014; Prasad et al, 2014; Sankar et al, 2014). Therefore, given this possible role for oxidative stress events, the biological mechanisms that might create and propagate a local oxidative environment around implanted microelectrodes are being investigated.…”

Intracranial neuronal ensemble recordings and analysis in epilepsy

“…The impedance of the electrode depends on the surface area that comes into contact with the biological tissue, but for the typical 12.5-200µm diameter the impedance of Pt/Ir electrodes are in the 0.1-5MΩ range (Prasad, 2014) and tungsten electrodes in the 30-400kΩ range (Prasad and Sanchez, 2012).…”