Chapter 5: Neuroprosthetics

Objectives

Introduction

Definition

Neuroprostheses (NPs) are electronic devices that stimulate nerves in order to improve bodily functions lost as a result of damage to the peripheral or central nervous system. This approach is also called functional electrical stimulation (FES) or functional neuromuscular stimulation (FNS). NPs include both surface stimulators, which deliver current through the skin to target nerves, and implanted stimulators, which deliver current directly to the target nerves. NPs range from simple "muscle" stimulators used to increase muscle bulk to complex devices implanted in the spinal cord and brain. The aim of this chapter is to describe the numerous types of NPs that have been developed since the early days of therapeutic electrical stimulation (TES), with a particular emphasis on those NPs that are useful in functional restoration after spinal cord injury (SCI). The spectacular advances that have occurred in the neurosciences and in bio-medical engineering in the last two decades have led to numerous technical advances and innovations. Surface and implanted NPs are now available to assist with a wide variety of functions, including hand function, postural control, standing, walking, respiration, micturition, and pain control. This chapter will address issues related to the benefits and limitations of these devices and factors affecting the choice of an NP for a particular individual.

History

Electrostatic machines capable of generating single high-voltage pulses were invented in the 1740s. Clinicians very soon began using them to apply single stimuli through pairs of surface electrodes, more to impress than to provide therapy.^1,2 Michael Faraday's invention of the induction coil in the mid-19th century allowed continuous trains of stimuli to be delivered to nerves and muscles. Faradic stimulation quickly became an important means of experimentally stimulating the brain, spinal cord, and peripheral nerves (see "Mechanisms" below).

The first detailed manual of motor points, that is, locations at which faradic stimulation through the skin activated muscles at the lowest thresholds, was published in 1867.³ Electrical stimulation continued to be used at the fringes of medicine until the 1960s, when the advent of the transistor allowed stimulators to become portable enough to be used in activities of daily life (ADL).

Mechanisms

Electrical Stimulation of Nerves

Faradic stimulation consists of delivering trains of very brief pulses of electrical current through pairs of electrodes applied to bodily tissues. Electrodes applied to the skin surface are made of a conductive material, for example, metal or carbonized rubber, often ...