Abstract
BackgroundDespite important advancements in control and mechatronics of myoelectric prostheses, the communication between the user and his/her bionic limb is still unidirectional, as these systems do not provide somatosensory feedback. Electrotactile stimulation is an attractive technology to close the control loop since it allows flexible modulation of multiple parameters and compact interface design via multi-pad electrodes. However, the stimulation interferes with the recording of myoelectric signals and this can be detrimental to control.MethodsWe present a novel compact solution for simultaneous recording and stimulation through dynamic blanking of stimulation artefacts. To test the system, a feedback coding scheme communicating wrist rotation and hand aperture was developed specifically to stress the myoelectric control while still providing meaningful information to the subjects. Ten subjects participated in an experiment, where the quality of closed-loop myoelectric control was assessed by controlling a cursor in a two degrees of freedom target-reaching task. The benchmark performance with visual feedback was compared to that achieved by combining visual feedback and electrotactile stimulation as well as by using electrotactile feedback only.ResultsThere was no significant difference in performance between visual and combined feedback condition with regards to successfully reached targets, time to reach a target, path efficiency and the number of overshoots. Therefore, the quality of myoelectric control was preserved in spite of the stimulation. As expected, the tactile condition was significantly poorer in completion rate (100/4% and 78/25% for combined and tactile condition, respectively) and time to reach a target (9/2 s and 13/4 s for combined and tactile condition, respectively). However, the performance in the tactile condition was still good, with no significant difference in path efficiency (38/8%) and the number of overshoots (0.5/0.4 overshoots), indicating that the stimulation was meaningful for the subjects and useful for closed-loop control.ConclusionsOverall, the results demonstrated that the developed system can provide robust closed-loop control using electrotactile stimulation. The system supports different encoding schemes and allows placing the recording and stimulation electrodes next to each other. This is an important step towards an integrated solution where the developed unit will be embedded into a prosthetic socket.
Highlights
Despite important advancements in control and mechatronics of myoelectric prostheses, the communication between the user and his/her bionic limb is still unidirectional, as these systems do not provide somatosensory feedback
The present study describes a novel system designed for simultaneous electrotactile stimulation (16 channels) and myoelectric control (8 channels) with compact form factor
The signals were filtered as explained in Section Signal processing and myoelectric control, and they are reported in arbitrary units
Summary
Despite important advancements in control and mechatronics of myoelectric prostheses, the communication between the user and his/her bionic limb is still unidirectional, as these systems do not provide somatosensory feedback. Even though myoelectric prostheses can partially restore lost functionality following transradial amputations or congenital limb deficiency, approximately 25% of the users abandon their prosthesis [1]. A functional prosthesis without feedback can be viewed as an alien object; a mechanical tool required to accomplish the task rather than a bionic replacement of the missing limb. Demonstrating the functional benefits of feedback is not an easy task [8], several recent studies have shown that providing sensory feedback can improve user performance [9,10,11] as well as experience [12, 13]. Only three commercial prostheses promise to deliver non-invasive tactile feedback, namely, Vincent Evolution 4 (Vincent Systems, Karlsruhe, Germany) [14], Luke Arm (Mobius Bionics, Manchester, USA) [15] and Ability Hand (Psyonic, Champaign, USA) [16]; the benefits of feedback in these devices are yet to be clinically proven
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