Next Generation FES System in the 21st Century

N.Hoshimiya*, Y.Iguchi** and Y.Handa**

*Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

** New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan

Introduction

Functional Electrical Stimulation (FES) is an effective method for restoring motor functions to limbs paralyzed by spinal cord injury or cerebral apoplexy. By applying proper artificial electrical pulse train to the muscles or nerves, desired motion can be restored. Here we will discuss our recent accomplishments and prospects in five topics which are Implantable FES System, FES Estimator, Neuromuscular Control with Natural Sensor Information, New Electrode Material and Self-Learning Capability of the FES System.

[1] Implantable FES System

Though clinical FES system of our group has used the percutaneous electrodes, total implantation of the FES system is desirable because the percutaneous electrodes have problems as the wire breakage, infection at the skin and unfavorable cosmetics. Therefore, we have been developing an implantable stimulation unit for multichannel FES system[1]. The stimulation unit implanted in the body was externally controlled and powered. The information to check operating states of the implanted unit was also returned to the external unit for safety. A new type of RF coil utilizing amorphous magnetic fibers was introduced to realize high efficiency of power transmission[2] with high performance of EMC(electromagnetic compatibility). As one of fail-safe methods to improve the immunity to electromagnetic noise, an application of the Error Correcting Circuit (ECC) was proposed [3]. The ECC based on the Hamming coding, made high-speed error detection and correction feasible, and allowed hardware simplicity. This project has been supported by The Japan Development Corporation.

[2] FES Estimator

We have used the electromyogram (EMG) signals obtained from neurologically intact subjects to determine the multi-channel stimulation intensity patterns in order to realize the desired motion, and it has been called "EMG-based method". The method proved to be useful to restore paralyzed hand and elbow functions and shoulder movements. However, there are also problems in the method. Firstly, it imposes burden to the neurologically intact subjects during the EMG signals measurements. Secondly, the modification process of the stimulation patterns for the tuning sometimes endanger the patients, especially in case of lower extremities. Therefore, we have been developing a high performance computer simulation system of FES-induced motion named "FES Estimator". FES Estimator enabled us to generate initial stimulation patterns or the standard stimulation data without using EMG analysis and helped us modify the parameters without applying electrical stimulation to the patients[4]. We also studied a muscle model whose force was dependent on both the stimulation frequency and stimulation amplitude for use in the FES estimator[5]. The FES estimator is expected to play a role of combining basic research with applied studies and to be an experimental tool for studying a new FES control method.

[3] Control with Natural Sensor Information

One of the major problems in clinical FES is that the open-loop control scheme cannot cope with the external disturbance and the change of musculoskeletal properties, thus the closed-loop control is desirable in the future. The biggest problem of the closed-loop control will be the lack of proper sensors with easy attachment and good reproducibility. In this context, interest in the natural sensors has increased in recent years[6-8]. We have early suggested to utilize a kind of natural sensor information in electrophrenic respiration (EPR) [9-10]. The new type of EPR (EPRS) was designed to function synchronously as an element of the patients' respiratory control systems. Specifically, the feeble nerve impulse trains from the proximal stump of the phrenic nerve trunk was detected with bipolar electrodes and used as a trigger signal to stimulate the distal stump of the phrenic nerve trunks.

[4] New Electrode Material

In the clinical application of the FES system, percutaneous electrode made by SUS-316L has been used in Sendai FES Project. The electrode showed quite good performance for long-term clinical applications. But new electrode materials were investigated to guarantee the more reliable and safety performance for clinical applications. In this presentation, typical characteristics of some new materials (e.g. NAS106N) will be shown.

[5] Self-Learning Capability of the FES System

We could realize rather complicated hand movement by an FES system, e.g. extending the elbow, openibg the hand, grasping a cup, flexing the elbow and then drinking water. But the patient sometimes wish to modyfy or improve the induced finger/wrist/elbow movements in daily usage[11]. Therefore it is desirable to equip learning capabilities in FES system. We proposed one approach in the design of the FES system[11]. This idea was not installed in the clinical system yet, but this approach will be very important to offer a clinically acceptable practical system in the near future.

Conclusion

It is getting possible for the paralyzed patients to use their arms and hands or to stand and walk with the aid of FES, which was thought to be totally impossible before. Though the functions that the present FES supply cannot be said to be enough, it is certain that FES will endow hope to paralyzed patients by enhancing their quality of life (QOL) more and more through the researches toward better FES as introduced in this paper.

Acknowledgement

We would like to express sincere thankfulness to the members of the Sendai FES Project for their great collaborations. This work was partly supported by the Ministry of Education, Science, Sports and Culture of Japan under a Grant-in-Aid for Scientific Research, the Proposal-Based New Industry Creative Type Technology R&D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and Miyagi Industry Development Organization (supported by The Japan Development Corporation).

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