Charge-balanced current driver and electrode characterization for an implantable neural stimulator

Building an Implantable Neural Stimulator involves engineers across many disciplines. From an electrical engineering standpoint, the primary concern was the electrical components needed to interface with the body's electrical system. This worjc will specifically address the design of analog circuitry intended to inject charge into living tissue. One purpose of injecting charge into a living being is to send sensory data back to the brain from damaged or missing limbs. Two programmable complementary current sources were built to drive charge onto tiny microelectrodes that interface with nerve tissue. A Digital-to-Analog converter (DAC) was designed and fabricated to control the amplitude of the current injected. A Finite State Machine (FSM) controlled whether current was driven onto or taken off of the electrodes and for how long. A Microchip PIC microcontroller (connected off chip) passed data to the FSM and controlled how often stimulation pulses were to occur. Because these systems are going to be operating within a living organism, it is important not to interfere with other biological processes occurring simultaneously. This required that the stimulator be very small and consume very small amounts of power to keep tissue heating to a minimum. To develop all these electronic systems to drive stimulation pulses onto the microelectrodes, it was important to understand how the electrodes appeared electrically to the stimulator. Throughout the remainder of this work we will discuss how data were passed to the stimulator, how the stimulator electronics were developed, and how the microelectrodes, built by other engineers, were characterized and modeled electrically.