Organic electrode coatings for next-generation neural interfaces

Abstract

Traditional neuronal interfaces utilize metallic electrodes which in recent years have reached a plateau in terms of the ability to provide safe stimulation at high resolution or rather with high densities of microelectrodes with improved spatial selectivity. To achieve higher resolution it has become clear that reducing the size of electrodes is required to enable higher electrode counts from the implant device. The limitations of interfacing electrodes including low charge injection limits, mechanical mismatch and foreign body response can be addressed through the use of organic electrode coatings which typically provide a softer, more roughened surface to enable both improved charge transfer and lower mechanical mismatch with neural tissue. Coating electrodes with conductive polymers or carbon nanotubes offers a substantial increase in charge transfer area compared to conventional platinum electrodes. These organic conductors provide safe electrical stimulation of tissue while avoiding undesirable chemical reactions and cell damage. However, the mechanical properties of conductive polymers are not ideal, as they are quite brittle. Hydrogel polymers present a versatile coating option for electrodes as they can be chemically modified to provide a soft and conductive scaffold. However, the in vivo chronic inflammatory response of these conductive hydrogels remains unknown. A more recent approach proposes tissue engineering the electrode interface through the use of encapsulated neurons within hydrogel coatings. This approach may provide a method for activating tissue at the cellular scale, however, several technological challenges must be addressed to demonstrate feasibility of this innovative idea. The review focuses on the various organic coatings which have been investigated to improve neural interface electrodes.

Keywords: carbon nanotubes; coatings; conductive polymers; hydrogels; living electrodes; material properties.

FIGURE 1

Schematic of coating approaches used for addressing the limitations of metallic electrodes. (A) aligned carbon nanotubes on metallic electrodes; (B) conductive polymers electrodeposited on metallic electrodes; (C) hydrogels polymerised to coat electrode site and device; (D) interpenetrating network of conductive polymer grown through hydrogel coating to form conductive hydrogel over electrode sites; (E) electrode site coated with biologically active molecules; (F) schematic of ideal tissue engineered interface incorporating combined coating approaches of conductive polymers, hydrogels and attachment factors with neural cells.

FIGURE 2

Scanning electron microscope (SEM) image of multi-walled carbon nanotubes (MWNTs) coating a platinum disk electrode, demonstrate that CNTs produce fibrillar surface structures, imparting a high charge transfer area to the typically flat electrode. The platinum disk is not visible, as the entire substrate is covered with CNT bundles. Image produced at 15,000× magnification.

FIGURE 3

Structure of PPy and PEDOT with alternating single and double bonds along the backbone which impart conductivity (Green et al., 2008a).

FIGURE 4

Scanning electron microscope images show that (A) PEDOT doped with PSS produces a coating which is substantially smoother than that produced from (B) PEDOT doped with pTS.

FIGURE 5

Different morphologies of CHs can be produced through varying the hydrogel component. PEDOT grown through 30 wt% heparin has a more cobblestone appearance (left) compared to the nodular PEDOT grown through 20 wt% PVA-Hep (right).

FIGURE 6

Scheme of proposed construct, derived from Green et al. (2013a) bottom, Pt electrode site. Middle, conductive hydrogel coating and top, neural network encapsulated within degradable hydrogel.