Abstract
BACKGROUNDS/OBJECTIVE: Intraspinal microstimulation (ISMS) is a neuromodulation technique for restoring walking after spinal cord injury. The objective of this study was to fabricate a stretchable ISMS device suitable for pigs, a clinically-relevant animal model. METHODS: Polyimide-insulated microwires (50µm, Pt-Ir, 80%/20%) were used for fabrication of electrodes. Their tips were de-insulated (∼0.15mm(2)) and sharpened using nanosecond and femtosecond UV lasers. Microcoils were fabricated from 25μm microwires (Pt-Ir, 80%/20%) to add stretchability to the lead wires. Sixteen microelectrode-leads were connected to a custom, wirelessly controlled stimulator using Medtronic extension cables (Model 37081). The implants were tested in seven domestic pigs and current pulse trains were delivered to various rostro-caudal regions of the lumbar spinal cord (1s, 40Hz, 50µA-300µA) to activate locomotor-related muscle synergies. The kinematics and isometric joint forces of the evoked hindlimb responses were recorded. RESULTS: Graded joint movements were evoked with increasing stimulus amplitude. Changes in the hip, knee, and ankle joints angles evoked by ISMS at 300µA were 17.9±1˚, 28.1±1˚, and 21.6±2˚, respectively. Isometric joint forces evoked by ISMS at 300µA were 12.21±0.91N, 7.4±0.71N, and 1.7±0.15N for knee extension, hip flexion, and ankle flexion, respectively. CONCLUSION: The movements evoked using the developed ISMS implant could generate full ranges of motion in the joints. The graded responses imply a near-physiological recruitment order of motoneurons, which is necessary for achieving long walking distances without muscle fatigue. The results show the capability of the developed ISMS device in generating movements in pigs, and the implants’ potential for future use in humans.