The myokinetic stimulation interface: activation of proprioceptive neural responses with remotely actuated magnets implanted in rodent forelimb muscle

Abstract

Objective. Proprioception is the sense of one's position, orientation, and movement in space, and it is of fundamental importance for motor control. When proprioception is impaired or absent, motor execution becomes error-prone, leading to poorly coordinated movements. The kinaesthetic illusion, which creates perceptions of limb movement in humans through non-invasively applying vibrations to muscles or tendons, provides an avenue for studying and restoring the sense of joint movement (kinaesthesia). This technique, however, leaves ambiguity between proprioceptive percepts that arise from muscles versus those that arise from skin receptors. Here we propose the concept of a stimulation system to activate kinaesthesia through the untethered application of localized vibration through implanted magnets. Approach. In this proof-of-concept study, we use two simplified 1-DoF systems to show the feasibility of eliciting muscle-sensory responses in an animal model across multiple frequencies, including those that activate the kinaesthetic illusion (70 - 115 Hz). Furthermore, we generalized the concept by developing a 5-DoF prototype system capable of generating directional, frequency-selective vibrations with desired displacement profiles. Main results. In-vivo tests with the 1-DoF systems demonstrated the feasibility to elicit muscle sensory neural responses in the median nerve of an animal model. Instead, in-vitro tests with the 5-DoF prototype demonstrated high accuracy in producing directional and frequency selective vibrations along different magnet axes. Significance. These results provide evidence for a new technique that interacts with the native neuro-muscular anatomy to study proprioception and eventually pave the way towards the development of advanced limb prostheses or assistive devices for the sensory impaired.

Generating Frequency Selective Vibrations in Remote Moving Magnets

“…[46] The adopted silicone phantom used in this study exhibited mechanical features comparable to those of the muscular tissue [48] and was already assessed in our previous study. [18] Based on our experience with animal tests, gained in vibrating magnets implanted in rodents' paws, [18] we found this methodology appropriate to test the system functionality and we do not expect significant performance deterioration in a real anatomical configuration in terms of tracking accuracy, precision, and frequency selectivity.…”

“…The outcomes from the video analysis test (Test A) confirmed the capability of the system in vibrating a moving magnet fixed onto a silicone phantom at the target frequency with a torsional efficiency always above 0.82, thus extending our previous studies that included static magnets only (Figure 4). [ 17,18 ] Moreover, the video analysis showed that the vibrations at 40 μNm led to magnet displacements up to ≈150 μm, which already demonstrated to be sufficient to elicit muscle sensory neural responses in an animal model. [ 18 ] These displacements are also comparable with those employed in some noninvasive studies to elicit kinesthetic illusions in targeted sensory reinnervated amputees.…”

“…[ 17,18 ] Moreover, the video analysis showed that the vibrations at 40 μNm led to magnet displacements up to ≈150 μm, which already demonstrated to be sufficient to elicit muscle sensory neural responses in an animal model. [ 18 ] These displacements are also comparable with those employed in some noninvasive studies to elicit kinesthetic illusions in targeted sensory reinnervated amputees. [ 13,14 ]…”

“…One of the magnets was vibrated using sinusoidal torque profiles (peak-to-peak amplitude: 40 μNm; 70, 80, 90 Hz frequency range) about the axis perpendicular to the plane of the camera (z axis, Figure 4A), while the other magnet was kept still, akin our previous study. [18] The test was repeated with the magnets moving along a linear trajectory parallel to the camera plane (1.5 cm s À1 speed, x axis), and with static magnets as a control condition (Figure 4A). The videos were processed frame by frame (MATLAB Image Processing Toolbox) to gather magnets motion information via blob analysis.…”

“…We referred to this idea as the myokinetic stimulation interface (Figure 1A). [17,18] Magnetic actuation is achieved by producing motion on (target) magnetic objects, such as magnets or magnetoresponsive materials, through the interaction of their magnetic fields with…”