A multi-institutional team consisting of Illinois Institute of Technology, the University of Alberta and the University of Chicago is developing an intraspinal microstimulation (ISMS) system as a means of restoring standing and walking for individuals with spinal cord injury. ISMS, a technique pioneered at the University of Alberta, is characterized by reduced fatigue when compared to other modes of functional electrical stimulation, making it an attractive candidate for restoring motor function.
The current approach for ISMS consists of fine microwires implanted in a small region of the spinal cord. By alternating the stimuli through two or more electrodes, multiple populations of motor neurons can be activated with the result being sustained smooth muscle contractions that allow for standing and walking.
Despite the successful restoration of function in acute animal models, technological limitations have restricted the ISMS technique to research laboratories. Current ISMS techniques involve the implantation of multiple electrodes, followed by empirical stimulation to identify optimally placed electrodes that evoke target responses. Placement of the electrodes involves hand delivery that relies on anatomical landmarks. While this technique might be effective for animal models, it is insufficient for clinical translation due to the tissue disruption associated with each electrode insertion.
Shortcomings of existing ISMS techniques are not only limited to the time of surgery. Following implantation, during physiological motion, the tethered electrodes are often displaced from their initial position within the tissue, leading to an increased risk of infection, hemorrhage, and neurodegeneration. In addition, the transdural wires exiting the dura mater add another risk of cerebrospinal fluid (CSF) leakage. Not only these forms of tissue damage diminish the effectiveness of electrical stimulation over time, but they also put the patient’s life at risk and may require additional surgery to remove the implant. Therefore, the successful application of ISMS therapy in humans requires the development of a stable ISMS interface that is suitable for use outside a laboratory setting.
Developing a wireless ISMS microelectrode array will enable us to overcome the tethering restrictions. Our laboratory pioneered the design and fabrication of multi-channel Wireless Floating Microelectrode Array (WFMA) devices for the purpose of stimulating the brain’s visual centers. We also developed a rapid insertion system for the safe implantation of WFMA devices in neural tissue, thus eliminating the need for manual delivery. As such, the WFMA device and insertion system will serve as a platform for developing and testing a novel wireless ISMS system.
Migrating the wireless technology to a spinal application comes with a new set of engineering and optimization challenges. The distance from the skin to the spinal cord is greater compared to that from the skin to the cortex, pushing the limits of wireless magnetic coupling. Increasing the surface area of the device might allow for the signal transmission over a greater distance; however, from a biological standpoint, a larger device will elicit more of a foreign body response. Therefore, multiple design constraints must be taken into consideration when designing and testing a wireless ISMS system. It may be desirable to approach the preclinical stage of this project in three phases. In phase one, a number of stimulator modules will be implanted in acute animal models. The objective of the first phase is to establish a proof-of-principle for the feasibility of using wireless microelectrode arrays to evoke a variety of motor functions. In phase two, the modules will be chronically implanted in animal models to demonstrate the stability of the electrode-tissue interface. In the third phase, we plan to direct our attention to design and fabricate the components that will enable the signal to traverse the layers between the skin and the spinal cord.
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