Measurement of Bioelectric Current with a Vibrating Probe

Summary

The manufacture, calibration and use of non-invasive vibrating probes to measure bioelectric current in various biological systems is described.

Abstract

Electric fields, generated by active transport of ions, are present in many biological systems and often serve important functions in tissues and organs. For example, they play an important role in directing cell migration during wound healing. Here we describe the manufacture and use of ultrasensitive vibrating probes for measuring extracellular electric currents. The probe is an insulated, sharpened metal wire with a small platinum-black tip (30-35 μm), which can detect ionic currents in the μA/cm² range in physiological saline. The probe is vibrated at about 200 Hz by a piezoelectric bender. In the presence of an ionic current, the probe detects a voltage difference between the extremes of its movement. A lock-in amplifier filters out extraneous noise by locking on to the probe's frequency of vibration. Data are recorded onto computer. The probe is calibrated at the start and end of experiments in appropriate saline, using a chamber which applies a current of exactly 1.5 μA/cm². We describe how to make the probes, set up the system and calibrate. We also demonstrate the technique of cornea measurement, and show some representative results from different specimens (cornea, skin, brain).

Protocol

1. Probe Manufacture

Blank probes are purchased from World Precision Instruments (elgiloy/stainless parylene-coated microelectrodes) (see 'Table of specific reagents and equipment' below). The probe is cut 25-30 mm behind the tip and about 5 mm of parylene insulation at the cut end scraped away with a scalpel (#11 blade) to ensure a good connection. The probe is mounted in a gold R30 connector using electrically-conductive silver-loaded epoxy (e.g. Rite-Lok SL65) [*see note below]. The probe is stored overnight at room temperature to allow the epoxy to harden. Next, the probe tip is plated with gold and then platinum, using a nano-amp power....

Discussion

We describe a low cost, basic, but highly sensitive vibrating probe system for measuring non-invasively electric current in a variety of biological systems.

Possible Modifications

1. If platinum/iridium electrodes (World Precision Instruments; cat # PTM23B20) are used instead of stainless steel, then the gold plating stage can be eliminated.

Applications

We have used the vibrating probe to measure electric current in: ra.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Eligoy-Stainless Electrode	World Precision Instruments, Inc.	SSM33A70	76 mm, 7 MΩ, 1-2μm tip
Gold R30 connector	www.vectorelect.com	R30	Re-usable
Silver-loaded epoxy	3M	SL65	Mix 1-part Resin with 1-part Hardener
Dissecting microscope	Olympus Corporation	SZ40	Magnification x6 to x40
Potassium dicyanoaurate (KAu(CN)2)	Sigma-Aldrich	379867	CAUTION: Toxic
Chloroplatinic acid hydrate (H2PtCl6 x 6H2O)	Sigma-Aldrich	520896	CAUTION: Toxic
Lead(II) acetate trihydrate (Pb(CH3CO2)2 x 3H2O)	Sigma-Aldrich	185191	CAUTION: Toxic
Nano-Amp power source	Home made	-	Powered by six 1.5 V (AAA) batteries
3-dimensional micro-positioner	Line Tool Co.	Model H
Lock-in amplifier	Stanford Research Systems	SR530
Digital I/O interface	National Instruments	PCI-6220
Shielded Connector Block with BNC connections	National Instruments	BNC-2110
Strathclyde Electrophysiology Software	University of Strathclyde Institute of Pharmacy and Biomedical Sciences, UK	WinWCP V4.1.5	Free download from: http://spider.science.strath.ac.uk/sipbs/software_ses.htm
Calibration Chamber	Home made
Constant Current Calibrator	Vibrating Probe Company		Powered by one 9 V (PP3) battery