Implantable FES stimulation systems: What is needed?

Since their initial development, the performance gains in functional electrical stimulation (FES) systems have been modest. Conceptually, the replacement of normal neural function by artificial electronic systems is attractive, considering the continued technologic advancements in electronics, communication, and control. It is likely that efficacious FES systems will require complete implantation and activation of large numbers of motor units. One approach is to develop a neural interface that has a one-to-one relationship between stimulating electrodes and lower motor neurons. While technology may offer solutions to the design of miniaturized implantable stimulators, the high-density neural interface remains more elusive. During the past 20 years, research in the stimulation of peripheral motor systems has been primarily constrained by progress in two areas of research: strategies for the control of paralyzed muscle and sophistication of implantable stimulation systems. Often, a debate concerning which of these two areas is a "critical path" element yields no strategic ideas. It has been stated that a need must be demonstrated for a specific number of electrode channels before it is warranted to invest effort into the engineering of implantable systems that are capable of driving large numbers of electrodes. Indeed it is a logical approach to problem solving that the need should drive the development of function. Even study sections, in review of FES grant applications, often resort to this logic. In our opinion, when applied to FES, this argument is often fallacious and ignores the reality that research frequently requires that a threshold of experimental methodology be reached before any meaningful work can be accomplished. Practical trials of stimulation control strategies, long-term patient acceptance, and achievable function for FES systems cannot begin without the capability of stimulation. And, in order to determine whether or not stimulating large numbers of muscle groups can lead to more natural control of movement, suitable stimulation hardware must first exist, and be reliable. In the specific case of lower extremity FES research, it is likely that without a quantum advance in technologic capabilities, the practical utility of FES systems will continue to only be marginally close to normal function. To reach the level of being considered a routine treatment for spinal cord injury, FES systems should be able to offer improved functionality, ease of use, and near-equal reliability, compared to wheelchairs. At present, no FES systems attain this combination. The functional reliability of FES systems must approach 100%. As a trivial example, consider that for a standing, or walking, system, the perspectives of bioengineers and users may be quite different. While an engineer might be pleased to design a system that functions, as intended, 99% of the time, if a user falls down I time out of every 100, this is likely to be unacceptable. The minimal threshold of functional utility for FES systems is unclear, and will not be addressed here. Rather, we consider the issues of what features and capabilities are desirable for next generation implantable systems, and to what degree these desires approach engineering feasibility.