The goal of this chapter is to provide the researcher with a general context for the design of neuroprosthetic devices. Specifically, the practical issues surrounding the design and development of devices that will actually be used are presented. Concepts are given and exemplified by referring to the development of powered prosthetic elbows. In short, this chapter attempts to describe the overall design challenges (the "forest") rather than the details of any specific area (the "trees").

Neuroprosthetic devices are objects that interact with the nervous system to perform a function that maintains or improves the health and quality of life of an individual. They can be broken down into three classes: those that make use of neural measurements as an input to control a mechanical device that replaces a function of the body (for example, a prosthetic hand that uses myoelectric based control); those that, based on some action or situation, create an output signal that is transmitted to a nerve to have a desired action (for example, neuroprosthetic bladders); and those that have both neural inputs and neural outputs (for example, devices aimed at bypassing spinal cord damage).

Even when neural signals can be reliably measured, interpreted, and transmitted, a device must be constructed to perform the required function. The technical demands on such devices may be significant, as they typically require their own power sources and cannot take advantage of the ability of the body to repair damage and compensate for wear. Neuroprosthetic organs that process material may require supplies of material and the ability to discharge waste material. Further, internal devices must work within strict temperature limits or else surrounding tissues will be destroyed.

To be useful (in the eyes of the user) the device must improve both health and quality of life, it must be reliable, the performance of the device must be predictable, the device must be safe and not create other health hazards as a result of its operation, and the effort and concentration required in training and day-to-day use must be reasonable. These are significant challenges. The use of the device will be compared to the ability of the user to "make-do" and adapt to life without using the device. The ability of people to adapt and find innovative solutions to challenging situations after the loss of function of a limb or organ is significant. This is, however, the level of function that must be surpassed if the device is to be used. Such mundane features as the appearance, operating life, or time between recharging batteries of the device often have a significant impact on the very practical decision that a user makes about using a device.

This chapter will present a general classification of different types of devices and the challenges faced in their practical design. A review is presented of current approaches to the design of a number of current neuro-prosthetic devices: limbs, the bladder, and the ear. Finally, the impact of current technologies on the design of practical devices is briefly discussed. The primary example used in this chapter will be the powered, myoelectri-cally controlled prosthetic elbow.

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