Functional Neuromuscular Stimulation
Design Features
The design considerations for the fabrication of microminiature devices include: size (diameter and length), device encapsulation, power (source), physical range, number of channels, pulse width control (sec), pulse amplitude control (mA), pulse waveform, and waveform options [5].
The main distinguishing feature of microstimulator-based applications is that multiple stimulators can be implanted with minimal surgery. The size and shape of the device is restricted to a cylindrical capsule to permit injection through a hypodermic needle. A second feature of the device concerns the control of multiple implants. Each device must contain circuitry that is uniquely addressable and capable of delivering a variety of pulse durations and intensities. A third feature is long life, which requires that internal components of the stimulator be electronically isolated from the body fluids, and power and data be transmitted through RF coupling [1].
The microminiature implantable electronic devices are designed such that they can be used in a variety of combinations to stimulate individual nerves and muscles. These devices eliminate the donning and reliability problems of surface and percutaneous electrodes, and do not require long leads between the electrodes and the controller because the receiving and stimulating circuitry are contained within each unit. Their small size allows injection directly into the desired muscles through a hypodermic needle, providing for unlimited combination of a multiple of individually selective channels [1].
Biocompatible borosilicate glass is used to encapsulate the internal components hermetically while adhering to the constraints on size and electromagnetic transmission. Sintered tantalum (Ta) is used as one electrode material because it is able to act as a “capacitor-electrode” with respect to body fluids. The stimulus charge must be accumulated between pulses and stored on the electrode material itself because the limited size of the device does not allow for the inclusion of an internal storage capacitor. Iridium (Ir) is the other electrode material because of its non-polarizing, low-impedance interface when activated. This combination of tantalum and iridium has been found to perform well during extensive electrochemical testing [1]. Ir activation is required in order to increase charge storage and lower the interface impedance. Electrochemical conditioning of both the Ir and Ta electrodes is carried out after the capsule is sealed [5].
Go to BME 240 Homepage
Created by Alison Vass    |    BME 240    |    June 9,