Closed-loop neuromuscular electrical stimulation using feedforward-feedback control and textile electrodes to regulate grasp force in quadriplegia

Abstract

Background: Transcutaneous neuromuscular electrical stimulation is routinely used in physical rehabilitation and more recently in brain-computer interface applications for restoring movement in paralyzed limbs. Due to variable muscle responses to repeated or sustained stimulation, grasp force levels can change significantly over time. Here we develop and assess closed-loop methods to regulate individual finger forces to facilitate functional movement. We combined this approach with custom textile-based electrodes to form a light-weight, wearable device and evaluated in paralyzed study participants.

Methods: A textile-based electrode sleeve was developed by the study team and Myant, Corp. (Toronto, ON, Canada) and evaluated in a study involving three able-body participants and two participants with quadriplegia. A feedforward-feedback control structure was designed and implemented to accurately maintain finger force levels in a quadriplegic study participant.

Results: Individual finger flexion and extension movements, along with functional grasping, were evoked during neuromuscular electrical stimulation. Closed-loop control methods allowed accurate steady state performance (< 15% error) with a settling time of 0.67 s (SD = 0.42 s) for individual finger contact force in a participant with quadriplegia.

Conclusions: Textile-based electrodes were identified to be a feasible alternative to conventional electrodes and facilitated individual finger movement and functional grasping. Furthermore, closed-loop methods demonstrated accurate control of individual finger flexion force. This approach may be a viable solution for enabling grasp force regulation in quadriplegia.

Trial registration: NCT, NCT03385005. Registered Dec. 28, 2017.

Conflict of interest statement

Competing interestsCEB holds various patents in the bioelectronic medicine field and is a shareholder in Sanguistat, a company focused on bleeding reduction/suppression, and is a founder and shareholder of Neuvotion, LLC, a company focused on movement restoration in paralysis, related to the reviewed work.

© The Author(s) 2019.

Figures

Fig. 1

The sleeve is comprised of a nylon base material and silver thread forming circular electrode sites (128) that cover the forearm and facilitate stimulation of flexor and extensor muscles to produce a wide variety of wrist and hand movements

Fig. 2

The feedforward-feedback control architecture included a feedforward block (fi) which uses the desired force (Fdesired) as an input to produce a computed current output. The output of the feedforward block is added to the feedback controller output and this sum is provided to the Alpha Omega Stimulator as the command input. The feedback controller is a proportional-integral (PI) type, providing current commands to the Alpha Omega stimulator

Fig. 3

Textile-based electrodes in sleeve form. (a) Natural hand position when stimulation is off (b) Stimulation turned on, evoking index finger flexion and pinch-type movement (subject NMES05). (c) and (d) Stimulation evoked a cylindrical grasp with sufficient force to hold a full (750 mL) water bottle against gravity in two study participants with quadriplegia (NMES04 and NMES05 respectively)

Fig. 4

Open and closed-loop responses during neuromuscular stimulation. a Force produced for various stimulation amplitudes and linear fit (blue). b and c Example open-loop responses to neuromuscular electrical stimulation. d Representative closed-loop response to a desired force step input; between 8 and 10s the controller automatically increases the current delivered to the target muscle to compensate for muscle fatigue

Fig. 5

Closed-loop response to increasing and decreasing target (desired) forces. a The target force is increased from 2.5 to 5 N causing a rapid response that overshoots the final desired value, but settles within +/− 10% of the final value in less than 1 s. b The target force is decreased from 5 to 2.5 N associated with a rapid response, settling within 1 s