The "Artificial Node of Ranvier"

Introduction

Neural prosthesis are devices that get implanted into the human body in order to restore lost functions of organs or limbs, e.g. in patients with disabilities, by interfacing with their nervous system. The interface between technical system and living tissue is called an electrode. An electrode can either be used for electrical excitation of nerves, e.g. in order to control a muscle or for recording of nerve signals which can be used for controlling an implanted nerve stimulator (close loop system). Unfortunately, nerve signals are very weak compared to other electrical signals in the body caused by muscles. The quality and strength of the signal an electrode picks up is directly related to its design, which is continually subject to optimisation. In order to keep the number of in vivo experiments as low as possible, the recording properties of neural electrodes should be investigated and optimised in vitro.

A suitable set up for investigation of electrode recording properties is the so-called artificial node of Ranvier. It mimics the activity of a single node of Ranvier and measures the voltage picked up by the electrode. The artificial node consists of an insulated wire that is bared at one defined position. An electrical current flows from the bared part of the wire through the surrounding saline solution into the counter electrode. This current simulates the membrane current at one particular node of Ranvier of the fibre. The voltage at the nerve electrode is recorded and related to the position of the node relatively to the electrode. Subsequently, the wire is moved until the bared part is positioned at the next (virtual) node of Ranvier, the voltage at the neural electrode is recorded again, and so on.

As a result of the measurements, the transfer function of a neural recording electrode can be determined. It is the ratio of measured voltage and generated current (U/I) for every position (x) of a node of Ranvier of a whole nerve fibre. The maximum of the transfer function is a measure for the maximum voltage that this particular electrode design picks up from a single fibre action potential (figure 1). The shape of the transfer function is needed to calculate the shape of a voltage signal at the electrode caused by a single fibre action potential.

Project Plan

The project is divided into two complementary parts that have to be carried out by two different students: developments in (A) mechanics and (B) electronics. Both parts are joined at the end of the project and a common experiment - at least the proof of principle - has to be done in co-operation.

A: The Mechanical Part of the "Artificial Node of Ranvier"

The mechanics that have to be developed and build consist of a tank with neural electrode holder, counter electrode and a wire positioner. The main challenge of this project part is to develop the wire positioner that has to provide a linear precision of 0.1 mm over a range of 5 cm.

1. A tank has to be build, equipped with a holder that holds the neural electrode in the bath.
2. One section of an insulated wire has to be bared. This section has to be as short as possible.
3. A counter electrode has to be manufactured.
4. A device has to be developed that enables the movement of a wire through a nerve electrode in steps of about 0.1 mm. The exact position of the bared part of the wire relative to the neural electrode has to be displayed.
5. The accuracy of the system has to be verified.
   
6. All mechanical parts have to be combined with the electrical parts (provided by project partner B).
7. The transfer function of two different cuff-type neural electrodes (provided) has to be determined.
8. A report has to be written consisting of the chapters "introduction", "materials and methods", "results", and "discussion and conclusion".

B: The Electronic Part of the "Artificial Node of Ranvier"

The electronics that have to be developed and assembled consist of a current source and a differential voltage amplifier. The main challenge of this part of the project is to develop electronic circuits that deal with very small amplitudes of electrical signals. These signals have to be clearly separated from interference and noise.

1. A voltage to current converter has to be developed that transforms a voltage signal of about 1 V amplitude to a current signal of about 1 mA amplitude.
2. A voltage amplifier has to be developed that amplifies the voltage signal picked up by the neural electrode to an easy displayable (oscilloscope, voltmeter) value. Therefore, the amplification has to be selectable by switches.
3. If necessary, a band pass filter has to be developed that filters the actual signal from noise and interference.
4. The two devices have to be housed in separate metal boxes.
5. The input-output properties of the generator and the amplifier have to be determined using an electrode test set up (provided).
6. The electrical parts have to be combined with the mechanical parts (provided by project partner A).
7. The transfer function of two different cuff-type neural electrodes (provided) has to be determined.
8. A report has to written consisting of the chapters "introduction", "materials and methods", "results", and "discussion and conclusion".

Last update September 2, 2003