Integration of Micro-Patterned Carbon Fiber Mats into Polyimide for the Development of Flexible Implantable Neural Devices

Polymer-derived carbon as neural electrode material has recently been subject of interest due to its ability to act as a multimodal platform for simultaneous recording and stimulation of neural activity and neurotransmitter detection. Its mechanical properties and the inverse fabrication protocol commonly used for the manufacturing of pyrolyzed carbon thin-film devices, however, only allow for its use as the electrode material and not as material for interconnects and other conductive components. In this study -for the first time-a process to fabricate flexible neural devices entirely made of carbon fibers (CFs) and polyimide (PI) was developed. The devices consist of carbonized polyacrylonitrile (PAN) fiber mats embedded in polyimide, which were patterned into the desired shapes using reactive ion etching (RIE). This method allowed for the fabrication of miniaturized, flexible and conductive carbon components with critical dimensions of 12.5 µm. Tensile tests were performed to evaluate the mechanical stability of the CF/PI composite, to detect potential electrical resistance changes due to bending and to study the adhesion of different PI layers onto each other. A strong mechanical interlock between CFs and PI was demonstrated and no significant change in the resistance of the CFs was detected after 100k cycles of tensile bending (r = 3 mm). The fabrication approach proposed here successfully yielded entirely metal-free and entirely flexible electrodes. It opens the door to further studies with the guarantee of highly stable electrodes, both mechanically and electrically.