Magnetomicrometry

Abstract

We live in an era of wearable sensing, where our movement through the world can be continuously monitored by devices. Yet, we lack a portable sensor that can continuously monitor muscle, tendon, and bone motion, allowing us to monitor performance, deliver targeted rehabilitation, and provide intuitive, reflexive control over prostheses and exoskeletons. Here, we introduce a sensing modality, magnetomicrometry, that uses the relative positions of implanted magnetic beads to enable wireless tracking of tissue length changes. We demonstrate real-time muscle length tracking in an in vivo turkey model via chronically implanted magnetic beads while investigating accuracy, biocompatibility, and long-term implant stability. We anticipate that this tool will lay the groundwork for volitional control over wearable robots via real-time tracking of muscle lengths and speeds. Further, to inform future biomimetic control strategies, magnetomicrometry may also be used in the in vivo tracking of biological tissues to elucidate biomechanical principles of animal and human movement.

Figure 1:. Free-Space Control of a Robotic Prosthesis via Muscle Magnetomicrometry.

Passive magnetic beads (highlighted here in yellow) implanted in muscle can be used to wirelessly track muscle length via an array of magnetic field sensors (blue) mounted to the outside of the body. The pair of magnetic beads highlighted here is placed in a single muscle, in line with the muscle fiber orientation. Muscle length data can be streamed to a control unit, which can in turn be used to stream commands to neuroprosthetic devices such as exoskeletons, muscle stimulators, or the robotic hand shown here. In a free-space control methodology, agonist and antagonist muscle states (box indicated in yellow) volitionally commanded by the user are mapped through a model of an intact biological limb to control joint angles (indicated here in purple) by modulating motor torque. This control strategy can be extended beyond free-space control by incorporating muscle activation or direct force measurement.

Figure 2:. Real-Time Muscle Length Tracking.

(A) Two magnetic spheres (highlighted in yellow) were implanted in the gastrocnemius muscle (red) in four turkeys. A motor was used to apply a mechanical frequency sweep to the ankle that ranged from 0.7 to 7 Hz, with a spring to provide an opposing force. A laptop computer and a magnetic field sensor array (blue) mounted external to the turkey’s leg were used to track the distance between the magnetic beads in real time. Two X-ray sources (orange, above turkey) and image intensifiers (orange, below turkey) were used to record stereo X-ray video of the magnetic beads. (B) The distance between the magnetic beads as measured by magnetomicrometry (plotted in blue) is shown against the X-ray stereo videofluoroscopy (fluoromicrometry, plotted in orange). The absolute difference between magnetomicrometry and fluoromicrometry is plotted in green. Sample is from the right gastrocnemius of turkey B (see supplementary material for all trial data from all four turkeys).

Figure 3:. Difference Between Magnetomicrometry and Fluoromicrometry Gastrocnemius Frequency Sweep Measurements, in Micrometers.

Histograms show the probability distribution of the difference between magnetomicrometry and fluoromicrometry for each of the four turkeys (turkeys A through D, show from top to bottom alternating between left and right legs), for all trials with each leg. The table shows the offset and standard deviation (SD) for each of the trials, giving a representation of the accuracy and intra-trial precision. Across all trials, the mean absolute offset (MAO) was 229 μm, and the measured precision was 69 μm, root-mean-square (RMS), with an adjusted RMS precision of 37 μm (accounting for the noise from fluoromicrometry). Note that the left gastrocnemius of turkey A was omitted from these trials, as discussed in the results of the migration study.

Figure 4:. Histology for a Single Magnet.

This histology image from turkey D shows a cross section of the muscle through the implantation site after removal of the magnet. The fibrous capsule is marked between the two black arrows. The scale bar indicates a distance of 1 mm.

Figure 5:. Long-Term Implant Stability of 3mm-Diameter Magnet Pairs Against Migration in Muscle.

(A) Pairs of 3mm-diameter magnets were implanted with various separation distances into the gastrocnemius and iliotibialis cranialis muscles of all four turkeys. (B) Separation distances were monitored over time via computed tomography scans. Note that there is a cutoff point at 21.5 mm for the 3-mm-diameter magnets used where magnets should not be implanted any closer to one another to ensure stability against migration.

Figure 6:. Magnetic Field Sensing Array.

Two 6x8 magnetic field sensor grids were custom designed and held together using a 3d-printed fixture and nylon nuts and bolts.