A tetraplegic volunteer was implanted with percutaneous intramuscular electrodes in hand and forearm muscles. Furthermore, a sensory nerve cuff electrode was implanted on the volar digital nerve to the radial side of the index finger branching off the median nerve. In laboratory experiments a stimulation system was used to produce a lateral grasp (key grip) while the neural activity was recorded with the cuff electrode. The nerve signal contained information that could be used to detect the occurrence of slips and further to increase stimulation intensity to the thumb flexor/adductor muscles to stop the slip. Thereby the system provided a grasp that could catch an object if it started to slip due to, e.g., decreasing muscle force or changes in load forces tangential to the surface of the object. This method enabled an automatic adjustment of the stimulation intensity to the lowest possible level without loosing the grip and without any prior knowledge about the strength of the muscles and the weight and surface texture of the object. Index Terms-Functional electrical stimulation (FES), hand grasp, natural sensory feedback, nerve cuff electrode, neural prostheses I. INTRODUCTION M OTOR function of a paralyzed hand can be partially restored by means of functional electrical stimulation (FES) [1]-[3]. The present systems are feedforward-controlled, and the user relies on his/her experience and on visual feedback. When picking up an object, the user has to estimate the appropriate stimulation intensity, which produces enough force to hold the object. This is difficult and causes many patients to use excessive force [4]. Further, the system is not able to adapt to slow changes in force output of the stimulated muscles caused by, e.g., fatigue, electrode drift, and length-tension properties of the muscles at different hand orientations. To deal with these problems, it has been proposed to use force sensors, position sensors, or combinations thereof to provide closed-loop control of the grasp. This provides more linear control of grasp force and grasp opening [5]. Several investigators have looked into the possibilities of using artificial sensors for measurement of finger position and grasp force

A closed-loop control system for controlling the key grip of a C6 tetraplegic patient was developed. Natural sensors served as the source of the feedback signal. The neural signals from cutaneous receptors were picked up by an implanted cuff electrode placed around the radial branch of the median nerve innervating the lateral part of the index finger. Mechanical stress applied to the skin, like pressure and slips, resulted in an increase in amplitude of the recorded neural signal. The goal of the study was to determine whether the recorded neural signals were able to indicate the slip of an object during lateral grasp and whether the slip could be stopped by increasing the grasp force through functional electrical stimulation of the thumb adductor and flexor.

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