DEVICE FOR THE IMPLANTATION OF NEURAL ELECTRODE ARRAYS Samuel D. Bredeson and Philip R. Troyk Illinois Institute of Technology Department of Biomedical Engineering Paper WA12.11 Introduction Implantation of electrode arrays for use in neural recording and stimulation applications should be implanted to ensure maximum effectiveness and minimal tissue damage. An insertion tool has been designed for use in implantation surgery to: Control implantation velocity Control electrode array insertion depth Previous studies have shown that for our 16-electrode arrays, a 1 meter/sec implantation velocity is optimal. Desired goals are: Minimize damage to surrounding neural tissue. Eliminate damage to electrode arrays. Device design To minimize damage to the surrounding tissue: No part of the insertion device may contact neural tissue Must be able to sense when arrays have been successfully implanted. (Sensor – future work) Must stall forward motion and retract upon successful implantation. To minimize damage to the electrode arrays: Array collets are held in place by spring clips. Pulling a handle near the tip ejects an empty collet and allows another to be loaded. The device includes a chamber between the motor and the tip that can be used to house sensors to aid in the accurate measurement of the velocity, position, and force output of the motor. Actuator – Voice Coil Motor Motor requirements: High velocity Rapid directional change Small enough to fit in handheld device A voice coil motor was selected as the actuator for this device. Motors of this type use an induction coil and a set of permanent magnets to create linear movement corresponding to the polarity of a voltage applied to the coil. Voice coil motors are lightweight and are well suited to short bursts of speed and force and can have their direction and amount of travel easily controlled. Voice coil motor shown in its retracted (top) and extended (bottom) state. The shaft of this motor is connected to the electrode array through a sensor housing. Spring clips Sensor housing Collet ejection handle Cross-section of the device, showing the internal components. This view of the device shows the spring clips, ejection handle, and the sensor housing chamber. a b c Electrode array shown in metal collet (tool side) Array Collet System Since the insertion device is intended to be used multiple times during each implantation surgery, a method to reload and insert multiple arrays was necessary. The system devised here allows for each array to be placed into a dedicated collet in advance. During the implantation procedure, each collet can be loaded into the tip of the device, which places an array in the proper position for implantation. After implantation, the empty collet is released, readying the device for another implantation. Collets protect thin wire electrodes, eliminating risk of damaging electrodes by handling the arrays. Collets fixed into insertion device during implantation. Collets can be quickly ejected from the tool to allow for rapid implantation of multiple arrays. Device tip Collet in place After collet ejection Testing To measure the motor performance when contained within the device, a set of tests were performed. Velocity measurement Position sensor measures end of stroke. Velocity determined from known displacement and time measurements. a b c a, b, c – Test setup used to measure motor velocity at the end of stroke. Velocity was determined by measuring the time difference between the driveshaft contacting the spring (b) and terminating on the copper plate (c), which were a fixed distance apart. Motor velocity as a function of input voltage (green) and input current (red). Force measurement Weights attached to motor, voltage applied to motor coils until movement. Activation voltage recorded from retracted and extended starting positions. Forward voltage, bias voltage, and pulse width modified. Next Steps The maximum travel of our motor barely meets requirements. Future versions will include a custom-made motor with significantly larger displacement capability (25mm vs 5mm). This will allow for the following improvements: Insertion device can be held farther away from tissue. Wider range of electrode lengths can be implanted. Reduced risk of tissue damage due to tool contact. In addition, a method of determining resistive force applied to the motor is being developed, and will likely consist of either: A sensor package. Recording the back-EMF from the drive motor. Input voltage required to drive an applied force. Voltage necessary: to lift weight from the retracted position (green) to maintain the extended position (red). Motor force output was measured by hanging weights from the motor, and recording the voltage necessary to move the weights. Retracted (left); extended (right). This work was supported by the Telemedicine and Advanced Technology Research Center, U.S. Army Medical Research and Material Command, Contract W81XWH-12-1-0394. SD Bredeson and PR Troyk are with the Illinois Institute of Technology, Chicago, IL 60616 USA. Email: sam.bredeson@hawk.iit.edu and troyk@iit.edu