Nanostructured Coatings for Improved Charge Delivery to Neurons

Abstract

This chapter explores the variability and limitations of traditional stimulation electrodes by first appreciating how electrical potential differences lead to efficacious activation of nearby neurons and examining the basic electrochemical mechanisms of charge transfer at an electrode/electrolyte interface. It then covers the advantages and current challenges of emerging micro-/nanostructured electrode materials for next-generation neural stimulation microelectrodes. Introduction to Electrical Stimulation of NeuronsStimulation electrodes have many clinical applications. The most common clinically approved stimulator is the artificial cardiac pacemaker, which is implanted into patients with a slow or arrhythmic heartbeat [1]. Artificial pacemakers use electrodes placed directly in contact with the heart muscles to regulate heart rate by delivering a timed series of electrical pulses. Another common stimulation device implanted into many adults and children is the cochlear implant [2]. Cochlear duct electrodes are designed as a series of stimulation contacts arranged along a flexible silicone carrier (Fig. 4.1a). These electrodes are placed into the cochlea where they directly electrically stimulate nerve cells, bypassing damaged hair cells that transduce acoustic vibrations into electrical impulses in the underlying nerve cells. (Fig. 4.1b). Electrical stimulation in these deep brain structures, such as the basal ganglia, has been used to treat symptoms of Parkinson's disease including tremor and bradykinesia, as well as other movement disorders such as dystonia [3][4][5]. In addition, DBS is being studied as a treatment for stroke, chronic pain, major depression, and chronic obesity [6-10]. For epilepsy and major depression, vagal nerve stimulation offers an alternative that does not require brain surgery [11,12]. In the extremities, functional electrical stimulation (FES) is used to deliver electrical current to activate nerves or muscles in disabled patients. FES has been applied to aid in standing, walking, and basic handgrips and for restoring bowel and bladder function [13][14][15]. Many other electrical stimulation applications exist, including the restoration of somatosensation and vision, the treatment of hypertension and gastroparesis, and the promotion of nerve regeneration [16][17][18][19].While these neurostimulation devices have demonstrated some success in bypassing, replacing, or treating damaged neural circuits, they are far from being able to reliably restore natural function in all patients. In order to understand the variability and limitations of traditional stimulation electrodes, we must first appreciate how electrical potential differences lead to efficacious activation of nearby neurons and examine the basic electrochemical mechanisms of charge transfer at an electrode/electrolyte interface. This chapter will first lay out our current knowledge of these areas and afterwards will cover the advantages and current challenges of emerging micro-/nanostructured electrode materials for ...

Elastomeric and soft conducting microwires for implantable neural interfaces

“…The impedance of our soft electrodes was lower than that of iridium metal and iridium oxide electrodes and, more importantly, the values are in the middle-to-lower portion of the acceptable range of impedance values for central nervous system electrodes [79], and in some cases, even lower. This suggests that further reduction of the active area of the soft electrodes is possible even with the expected increase in impedance [34]. In addition, the ability to adjust the formulation filler loading to a higher or lower fill ratio provides a mean to tune the impedance value.…”

“…These results indicate that the interaction between neurons and the fluorosilicone surfaces is favorable and sustainable in the presence of micromovement. However, variations in surface roughness (physical texture) rather than simply chemical composition may have an impact as well [34, 80]. Importantly, increases in neuronal attachment and density at the neural electrode-tissue interface could impact long-term functionality resulting in improved chronic recording with FS-coated soft wires compared to stiff wires.…”

Photoelectric artefact from optogenetics and imaging on microelectrodes and bioelectronics: new challenges and opportunities

“…One challenge with electrical stimulation is the tradeoff between safety limits and spatial selectivity of the stimulated neural population 48 . Stimulation electrodes with small surface areas can result in high charge densities that lead to permanent damage of the electrode or permanent damage to the tissue and nearby neurons 8, 48, 49 . Therefore, large stimulation electrodes are frequently used, which leads to a spatially broad orthodromic and antidromic activation of neurons 50, 51 .…”

Effects of caspase-1 knockout on chronic neural recording quality and longevity: Insight into cellular and molecular mechanisms of the reactive tissue response

“…This variability and unreliability is understood to be the result of complex multimodal failure mechanisms [9]. These include, but are not limited to: material failure such as corrosion, insulation failure, material degradation, electrical lead breakage, electrode delamination and biological responses including biofouling, neural degeneration, and inflammatory gliosis [10]. The present study is focused on dissecting the molecular pathways behind the biological responses that are related to chronic neural recording performance.…”

“…This study also revealed the unpredictability of disrupting or avoiding these large intracortical BBB vessels if only the surface vasculatures are avoided during insertion. More recently, it has been shown that implanting ultrasmall electrodes closer to major penetrating blood vessels leads to increased astrocytic GFAP activity [10, 12]. …”