A simple method for reconditioning epoxy-coated microelectrodes for extracellular single neuron recording

https://doi.org/10.1016/S0165-0270(02)00365-5Get rights and content

Abstract

Epoxy-insulated tungsten microelectrodes can be used once or twice in our lab before the impedance becomes too low. Dipping the electrodes in epoxy followed by curing restores their initial high impedance which is associated with good isolation of single neurons. It is a cost effective and simple procedure.

Introduction

Tungsten microelectrodes have been widely employed in extracellular single neuron electrophysiological recordings for many decades (see e.g. Hubel, 1957). The metal is stiff, easily etched and the microelectrodes yield high signal-to-noise ratios and are robust (see e.g. Baldwin et al., 1965). Several materials have been used to coat tungsten microelectrodes to obtain the small area of uninsulated tip, with an impedance of several Megaohms (MΩ), necessary for single-neuron isolation (for overview see Sugiyama et al., 1994), e.g. Insl-x and vinyl lacquer (Hubel, 1957), parylene (Loeb et al., 1977), glass (Merril and Ainsworth, 1972), and epoxy resin (Freeman, 1969, Ciancone and Rebec, 1989).

Glass is an excellent insulator with a smooth surface and high durability (Sugiyama et al., 1994). However, glass can be time-consuming to apply to the tungsten. These microelectrodes have the advantage that the uninsulated tip size and shape can be seen under a microscope, and adjusted by use of a glass bead to remove glass or by etching to decrease the tip size as required (Merril and Ainsworth, 1972). Epoxy-coated tungsten microelectrodes are more easily constructed, and if bent (for example when several microelectrodes must pass through the same guide tube for simultaneous recordings) the epoxy does not crack or separate from the tungsten. However, epoxy-coated microelectrodes suffer from low durability due to loss of coating, which reduces the impedance and amplitude of the action potentials and the ability to isolate single neuron activity (Sugiyama et al., 1994).

We have used commercial epoxylite-(6001M) coated tungsten microelectrodes (FHC Inc, Bowdoinham, ME) for single neuron recordings in cortical brain regions. The microelectrode is sterilized by being held in 70% ethanol for 20 min and typically travels for 20–30 mm through the parenchyma before reaching the neural target structures. The microelectrode is protected by a stainless-steel guide tube which just passes through the dura mater. Good isolation and high signal-to-noise ratio is obtained with these microelectrodes for one or two recording tracks (each taking 3–6 h, and performed typically on consecutive days) after which the impedance has dropped from 5–10 MΩ (new, unzapped) to <2 MΩ. Zapping is an electrical means of lowering the impedance after the insulation has been applied. We suspect that during electrode movement mechanical erosion reduces the extent and thickness of insulating layer at the tip, thereby increasing the size of the electric double layer at the tungsten–electrolyte interface, hence reducing the impedance (Robinson, 1968). Glass-insulated tungsten microelectrodes made according to the method of Merril and Ainsworth (1972), last for typically 10–15 tracks when used under the same conditions.

As the microelectrodes do not last long and have a cost of approximately $10 per microelectrode, their usage can result in a considerable annual cost. Here we describe a method that allows laboratories to reuse low-impedance microelectrodes several times. The method takes about as much time for 24 microelectrodes to be reconditioned as it takes to make one microelectrode from initial materials, and has a success rate of approximately 90%.

Section snippets

Methods

Used, straight epoxylite-coated tungsten microelectrodes (FHC, type ‘uewlffsmnnne’, no zap) were reconditioned when their impedance had fallen below approximately 2 MΩ. These microelectrodes have a 200 μm diameter shank, 15–20° straight-tapered angle with a rounded and unplated tip.

The impedance of the microelectrodes was measured twice using a commercial impedance meter (FHC, impedance check module/digital display, which employs a 1000 Hz 10 nA current) at 20 mm depth in 0.9% saline. Proper

Results and discussion

The measured impedances of all 48 used microelectrodes before and after reconditioning are presented in Fig. 1. Before reconditioning the impedance range was 0.1–3.5 MΩ (mean 1.4±0.8 MΩ). Conditioning significantly increased this (P<0.001, paired 1 sided t-test) to a range of 0.9–9.0 MΩ (mean 4.3±2.2 MΩ, Fig. 1). Thus the first conditioning cycle increased impedances by an average of 3.3±2.1 MΩ, being 5.4±8.4 times the initial impedance before reconditioning. A small but significant correlation

Note added in proof

We note that although heat exposure (130 °C) of used electrodes above the glass temperature of the epoxylite (120 °C for 6001M, Epoxylite Ltd) increases the impedance more than at 80 °C, nevertheless heating to 130 °C is less effective than dipping in resin as well.

Acknowledgements

This work was supported by an MRC Programme Grant to E.T. Rolls PG9826105.

References (8)

There are more references available in the full text version of this article.

Cited by (15)

  • Facile preparation of robust and multipurpose microelectrodes based on injected epoxy resin

    2023, Electrochimica Acta
    Citation Excerpt :

    The use of epoxy resin as insulating material is not new, being reported frequently in particular to improve the seal between carbon fiber and glass for carbon microdisks preparation [4] or for customized electrodes, such as double barrel ones [10]. Other examples include the preparation of microelectrodes arrays [6,11], for electrophoresis/flow analysis [12], electrophysiology (protruding electrodes) [13,14]. However, the here presented method has the great advantage of leading to MEs that are or that can be identical (in shape and response) to the ones that many laboratories already use, particularly as SECM tips, but with all the above-mentioned advantages.

  • A comparison of neuroinflammation to implanted microelectrodes in rat and mouse models

    2014, Biomaterials
    Citation Excerpt :

    Further, identifying key mechanistic pathways that contribute to neuroinflammation could aid in the development of more favorable long-term therapeutic targets [42,43]. To date, most studies trying to elucidate mechanisms resulting in neuroinflammation following microelectrode implantation have been conducted in rat models [14,31,39,44]. However, given the ease of genetic alteration, the use of mouse models could be critical in decoupling the mechanistic pathways resulting in neurodegeneration.

  • Silicon-based wire electrode array for neural interfaces

    2014, Journal of Micromechanics and Microengineering
View all citing articles on Scopus
View full text