A Method for Systematic Electrochemical and Electrophysiological Evaluation of Neural Recording Electrodes

Summary

Different electrode coatings affect neural recording performance through changes to electrochemical, chemical and mechanical properties. Comparison of electrodes in vitro is relatively simple, however comparison of in vivo response is typically complicated by variations in electrode/neuron distance and between animals. This article provides a robust method to compare neural recording electrodes.

Abstract

New materials and designs for neural implants are typically tested separately, with a demonstration of performance but without reference to other implant characteristics. This precludes a rational selection of a particular implant as optimal for a particular application and the development of new materials based on the most critical performance parameters. This article develops a protocol for in vitro and in vivo testing of neural recording electrodes. Recommended parameters for electrochemical and electrophysiological testing are documented with the key steps and potential issues discussed. This method eliminates or reduces the impact of many systematic errors present in simpler in vivo testing paradigms, especially variations in electrode/neuron distance and between animal models. The result is a strong correlation between the critical in vitro and in vivo responses, such as impedance and signal-to-noise ratio. This protocol can easily be adapted to test other electrode materials and designs. The in vitro techniques can be expanded to any other nondestructive method to determine further important performance indicators. The principles used for the surgical approach in the auditory pathway can also be modified to other neural regions or tissue.

Introduction

Neural implants are being used increasingly for research, controlling prosthetics and treatment of disorders such as Parkinson's disease, epilepsy, and sensory loss^1,2. Measuring and/or controlling both the chemical and electrical composition of the brain is the basis for all neural implants. However, it is important to administer a treatment only when the neural tissue is in the aberrant state to reduce side effects³. For instance, deep brain stimulators for epilepsy treatment should only apply an electrical pulse to the brain during a seizure. Some side effects may be dystonia, loss of memory, disorientation, impaired co....

Protocol

The protocol has been approved by the La Trobe University (09-28P) and RMIT University Animal Ethics committees (1315).

1. Electrode Preparation and Preliminary in vitro Testing

1. Prepare electrode coating deposition solutions; for instance 10 mM 3,4-ethylenedioxythiophene (EDOT) and 0.1 M sodium para-toluene sulfonate (Na₂pTS) to form poly-3,4-ethylenedioxythiophene-pTS (PEDOT-pTS).
2. Connect the electrode array to a potentiostat.
3. Carefully place the electrode array into the deposition solution and clamp into place.
4. Place a platinum mesh counter electrode and Ag/AgCl refere....

Results

A typical electrode array used for this experimental protocol is shown in Figure 1. There are 32 iridium electrodes on 4 shanks with 413 μm² nominal geometric area and a 200 μm pitch. Every second electrode on the array has been coated with one of four different electrode coatings, labeled 1-4. The coating materials have been carefully chosen for their chemical, mechanical and electrochemical properties. As mentioned previously¹⁰, increased deposition times will in.......

Discussion

This protocol provides a method for comparing neural recording electrode coatings within one animal. The electrode design used is ideal for implantation into a rat inferior colliculus (IC), with dimensions of a similar scale. Variations of this electrode such as more space between shanks would prevent all shanks being in the rat IC at the same time, while longer shanks and a larger pitch between electrodes increase the risk that the shank tips will come in contact with the base of the skull during insertion.
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Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Programmable Attenuator	TDT	PA5	Controls the amplitude of the acoustic signal across frequencies
Electrostatic speaker driver	TDT	ED1	Drives the electrostatic speakers (EC1)
Coupled electrostatic speaker	TDT	EC1	Delivers sound to the animal
Processing base station	TDT	RZ2	Records neural activity from electrode array (using PZ2 preamplifier)
Preamplifier	TDT	PZ2-256	256-channel high impedance preamplifier
Multifunction Processor	TDT	RX6	Used to generate acoustic stimuli
Multichannel electrode	NeuroNexus Technologies	A4 × 8–5mm-200-200-413	4-shank 32-channel electrode array
Potentiostat	CH Instruments	CHI660B	Deposits electrode coatings and performs cyclic voltammetry and EIS (used with CHI684)
Multiplexer	CH Instruments	CHI684	Switches between electrodes on the potentiostat
Disodium phosphate	Fluka	71644	Used in the test solution
3,4-Ethylenedioxythiophene (EDOT)	Sigma Aldrich	483028	An electrode coating material
para-Toluene sulfonate (Na2pTS)	Sigma Aldrich	152536	An electrode coating material
Urethane	Sigma Aldrich	U2500	Used to anesthetize the animal
Silver/Silver chloride electrode	CH Instruments	CHI111	Used for testing the electrode in vitro
Platinum electrode	CH Instruments	MW4130	Used for testing the electrode in vitro
Motorized microdrive	Sutter Instruments	DR1000	To control the electrode array position during surgery
Enzymatic cleaner	Advanced Medical Optics	Ultrazyme	Cleans the protein off the electrode array after implantation
Acoustic enclosure	TMC Ametek	83-501	Isolates the animal from acoustic and electrical noise
Stereotaxic frame	David Kopf Instruments	1430	Secures and positions the animal
Temperature controller	World Precision Instruments	ATC1000	Controls the animal temperature
Bone drill	KaVo Dental	K5Plus	Used to perform the craniectomy
Aspirator	Flaem	Suction pro	Used to perform the craniectomy