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Title

A Wireless Intraspinal Microstimulation Interface for the Recovery of Motor Function Following Spinal Cord Injury

Creator

Tawakol, Omar Sherif Mohamed Safwat

Date

2024

Description

Recovery of function following spinal cord injury (SCI) remains a daunting medical challenge. There are 270,000 people living in the US with...
Show moreRecovery of function following spinal cord injury (SCI) remains a daunting medical challenge. There are 270,000 people living in the US with SCI, and 17,800 additional injuries occur each year. Over half of people with SCI do not recover the ability to walk. In the US, the estimated cost amounts to nearly $14.5 billions of dollars. Attempts to recover function have included surgically implanting polymer scaffolding and stem cells to cause repair of the spinal cord, but to date, these approaches have not been efficacious. Following a SCI, the muscles, their innervating neurons, and the segmental spinal circuitry below the level of injury remain largely intact. Therefore, rehabilitation interventions that activate the surviving neural elements have been investigated. A well-known approach for restoring a large range of functions, even following a SCI, is functional electrical stimulation (FES). FES of muscles, peripheral nerves and spinal sacral roots can restore independent respiration, arm and hand function, and bladder voiding. FES also improves bone and muscle health, increases circulatory function, reduces spasticity, prevents pressure ulcers, and induces functional recovery. Nerve cuff, epimysial, or intramuscular electrodes are used to stimulate nerve fibers innervating various muscles. Notable limitations have compromised their effectiveness in restoring functional walking, including lead breakage, reduced fatigue resistance (due to reversed recruitment order of muscles), widespread implantation of electrodes throughout the legs, and challenges in the activation of multiple muscles. The challenges encountered with applying FES to the periphery for restoring standing and walking led to the investigating the application of electrical stimulation to the lumbosacral enlargement of the spinal cord. While most clinical spinal stimulators are pacemaker-style devices with epidural electrodes, intraspinal microstimulation (ISMS), has emerged as an alternative with the potential to restore functional over-ground walking. ISMS was initially used in improving bladder function after SCI in the 1970s. Mushahwar (one of our collaborators) has pioneered the use of ISMS to restore movement of the legs, and ISMS has produced sustained stepping in severe (ASIA A) SCI in animal models. Moreover, ISMS produced similar movements in other species including pigs and monkeys, strongly suggesting that similar outcomes may also be obtained with ISMS in humans in the future. Current ISMS implant concepts consist of fine penetrating microelectrodes placed with the tips targeting lamina IX bilaterally. Despite experimental results in acute animal models, the literature currently presents no implantable ISMS system capable of providing this type of interface in a form suitable for chronic deployment in a clinical setting. Moreover, emerging ISMS implant designs use wires that connect to the electrodes and cross the dura. Spinal cord electrode wires are known to be problematical due to the chronic adverse tissue responses, tethering forces on the electrodes (with spinal cord movement), and cerebrospinal fluid leakage caused by the transdural conduits. For the latter, it is well known that wires exiting through the dura are a major surgical complication. To overcome the current ISMS limitations, we aimed to develop and test a wireless intraspinal microstimulation interface which eliminates both tethering forces on the electrodes and the transdural conduit. The feasibility of the proposed system is supported using an existing 5mm-diameter wireless floating microelectrode array (WFMA) developed by Troyk and his team (our lab) which is now in use in an FDA-approved clinical trial for intracranial occipital cortex stimulation for visual restoration. Combined with a wireless extension lead (to magnetically reach from the surface of the back to the cord), multiple WFMA devices were implanted into the spinal cord with no transdural lead or conduit; communication across the dura was accomplished via magnetic coupling. The subdural WFMA devices served as a platform for developing a novel wireless ISMS system. Initial experiments demonstrated the system’s mechanical stability and the feasibility of using microelectrode arrays to achieve controlled motor responses. Although early chronic trials led to neural deficits, subsequent surgical refinements—including duroplasty, hemostatic agents, and a redesigned wireless extension wire—successfully preserved motor function in spinally intact animals. These modifications significantly improved neural recovery outcomes, highlighting the impact of precise surgical adjustments on the system's efficacy. The research underscores the spinal cord’s vulnerability and the importance of precise, individualized implantation strategies for ISMS systems. Suggested improvements for future studies include preoperative MRI-guided electrode positioning and use of flexible implant materials to reduce tissue disruption. This work establishes foundational insights for ISMS technology, with potential applications extending beyond motor restoration to include pain management, sensory feedback, and neural regeneration, laying a path toward therapeutic human use.
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