sEEG Electrode Coiler

Background: Wireless, implantable sEEG (stereo-electroencephalography) electrodes are vital for localizing and visualizing seizures. Helical electrode geometries allow for increased electrode flexibility, allowing the implant to move with the brain and decrease the changes for immune rejection. A key challenge lies in the precise coiling of Liquid Crystal Polymer (LCP) ribbons, which encapsulate the electrode leads. LCP is chosen for its superior properties, such as lower water absorption, compared to conventional materials. The objective was to create a mechanically aided, and eventually computer-guided, process to consistently wrap thin LCP ribbons in a helical shape around a removable rod, allowing subsequent silicone filling.

Project Goals: The project aimed to design a system capable of wrapping LCP ribbon at a constant and adjustable pitch, accommodating various rod diameters, and ensuring high precision and repeatability for mass production of sEEG electrodes.

Design and Iteration Highlights:

• Initial Concepts & Challenges: Early manual coiling methods involved heat guns for thermoforming LCP and heat shrink for handling. Design inspirations included stepper motor-driven systems for precise pitch control. Initial manual winding prototypes were developed using 3D-printed parts and explored various rod diameters (2.5mm and 1mm) and bearing systems. Fidelity issues with 3D printing for micron-scale clamping led to exploring clamp/screw systems.

• Pitch Consistency: Analysis revealed that maintaining a constant angle of approach by moving hardware perpendicularly towards the winding rod is crucial for consistent pitch. Experiments with different setup orientations, including a vertical setup, confirmed that consistent tension is a limiting factor, requiring additional weight to prevent coil sliding and maintain pitch.

• Material Selection & Attachment: Extensive research and testing were conducted on rod materials (carbon fiber, high-speed steel, tungsten) to ensure rigidity and compatibility with small diameters (down to 0.8mm). Adhesive trials (epoxy, super glue, silicone-based adhesives with primers like Loctite 7701) were performed to secure the LCP to the rod. Methods of attachment, including tape, magnetic strips, and shrink tubing, were evaluated for ease of use and ability to prevent crimping.

• Proof of Concept & Refinements: Proof-of-concept trials were conducted with string and LCP on various rods to assess coiling consistency. Initial trials with 2.525mm lead width and a 0.83mm rod at a 20-degree approach angle showed semi-consistent pitch, but indicated the need for stronger rod material. Later trials with 1mm OD tubing and 100um LCP at a 20-degree approach yielded a mean pitch of 5.55 mm ± 0.28, with observed pitch consistency once pulled through the casing. Increasing the approach angle to 45 degrees significantly improved observation and control of coiling, yielding semi-consistent pitch at expected values.

• Future Outlook: Future goals include downsizing the tubing to 940um or smaller, refining the design to maintain consistent tension and a 20-degree approach angle during coiling, and further testing the prototype in saline. The project aims to coil 100um LCP at 20° around a 1mm OD rod and compare measured pitch to expected values, progressing towards a robust, repeatable manufacturing process for clinical-length electrodes (390mm coiled length).

Impact: This project directly contributes to the development of advanced implantable medical devices by overcoming manufacturing challenges associated with micro-scale LCP coiling. The iterative design process and rigorous testing of materials and methodologies are crucial for producing precise, repeatable helical electrode leads, ultimately enhancing the capabilities of sEEG electrodes for neurological applications.