Early attempts at manually implanting microelectrode arrays in neural tissue resulted in the deformation of the cortical surface. Even though individual neural electrodes are relatively thin, slow mechanical insertion can lead to intracranial hemorrhage and cortical edema. Therefore, a system capable of rapidly inserting microelectrode arrays allows for circumventing the mechanical dimpling.
Our laboratory developed a high-speed insertion system for the purpose of implanting Wireless Floating Microelectrode Array (WFMA) devices while minimizing damage to the underlying tissue. The system offers greater control over velocity and position compared to previously reported systems, which use either a spring or pneumatic action. The system consists of a handheld insertion device, a control unit, a plastic collet for protection, and a transfer tool for storage and handling.
A voice coil motor is used to provide the required acceleration. The length of the moving shaft can be adjusted, thus providing control over the implantation depth. The insertion device also features a narrow tip, which allows for improved visibility of the underlying tissue. In addition to the insertion device, a collet was designed to provide means to protect, store, and handle WFMA devices. The collet is made of medical-grade plastic and colored surgical white to improve visibility and distinguish it from the brain, blood, vessels, and bone. The top half of the collet is threaded to allow for attaching it to the tip of the insertion device. The thin sidewall of the bottom half can be used as a spacer to implant WFMA devices with minimal, but sufficient, device-to-device separation.
The WFMA is held inside the collet through friction with three internal extrusions. During insertion, upon impact with the accelerating shaft of the insertion device, the WFMA is freed from the holding friction. The WFMA exits the collet at a speed of one meter per second, and the moving shaft comes to a mechanical stop as soon as the WFMA is successfully implanted. The plastic collet is difficult to manipulate by hand, due to its relatively small size. Therefore, a transfer tool was designed to grip the collet pre- and intra-operatively. The transfer tool features a rubber boot, which provides enough friction to hold the plastic collet against gravity. Preoperatively, the transfer tool, the rubber boot, the plastic collet, and the WFMA are assembled and prepackaged in a sterile pack.
During surgery, the transfer tool is brought closer to the tip of the insertion device, thus allowing the external threads on the collet to align with the internal threads of the insertion device. The transfer tool is rotated in the clockwise direction until the collet is fully threaded into the insertion device. The surgeon then positions the insertion device above the desired location, and once the surgeon is ready, an operator is instructed to press an “insert” button located on the control unit. Once the WFMA is fully implanted, the surgeon pulls the insertion device away from the tissue. The transfer tool is used to unthread the remaining empty collet, and the same steps are followed to implant subsequent WFMA devices.
The insertion system was successfully used to implant WFMA devices in the primary motor and occipital cortex of multiple animal models. Histological analysis of neural tissue sections, obtained from chronic experiments, showed minimal damage to the surrounding tissue. Visual inspection showed no sign of bleeding and implanted devices remained electrically functional for the duration of the experiments. The system is also being considered for use in future experiments that aim to develop a wireless Intraspinal Microstimulation (ISMS) system capable of restoring motor function in individuals with complete spinal cord injury.
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