Implantable Electrodes with Carbon Nanotube Coatings

Abstract

Carbon nanotubes (CNTs), discovered in 1950 (Monthioux & Kuznetsov, 2006; Radushkevich & Luk’yanovich 1952) and rediscovered in 1993 (Iijima & Ichihashi, 1993), came into the limelight as a promising next generation material for standard electronics, computers and aerospace industries. Its ballistic conductance, chemically inert nature (Niyogi et al., 2002), nano size (Ajayan 1999) and ease of chemical functionalization (Shim et al., 2002; Sinnott, 2002) opened the doors for it to become an important biomaterial (Malarkey & Parpura, 2007). The diameter of the nanotubes is of particular interest to neuroscientists: they are of the same order as neuronal processes. The interface of sub-cellular compartments with a material of that same size spawns numerous new ideas. This facilitates an interaction at a molecular level, imperative in the formation of functional neuronal circuits(Lee & Parpura, 2009), was one of the possibilities considered early on. CNTs could be functionalized with one or more bioactive molecules (Mattson et al., 2000). The molecular control of neuronal architecture at focal microdomains seemed possible now. This promoted the candidacy of CNTs out of the variety of available nanomaterials, and onto the main neuroscience stage (Pancrazio, 2008). In this chapter we will discuss the use of CNTs as active coatings for implantable neural electrodes (NEs). We begin with the background and motivation behind NEs, discussing a few examples of electrical stimulation and recording used in treating or ameliorating various nervous disorders. There are various techniques for the evaluation of electrode performance. The two most important are cyclic voltammetry and electrochemical impedance spectroscopy. These techniques are the focus of a section which includes robustness and reliability in neural electrodes. The rationale behind using CNT as a NE coating is discussed next. It throws light on the properties that make them unique. We describe the figures of merit of CNTs with respect to the development and requirements necessary for designing stimulating electrodes. The problems associated with using CNTs fabrication and manipulation are also discussed with the objective of establishing an appropriate view of the challenges faced when using and handling this material. A discussion on CNTs and neuronal interactions then follows. We will limit our approach to a subset of studies relevant to using CNTs as a conductive substrate to stimulate and record