Multifunctional polymer optical fiber-based neural devices

Implantable neural interfaces serve as important tools for comprehending the functional networks in the deep brain region and treating neurological diseases. As the functioning of the brain relies on the concurrent transmission of electrical, chemical, and mechanical signals at the cellular level, there is a significant drive from many research groups around the globe towards the development of implantable multi-functional neural tools for simultaneous neuromodulation and neural activity recording. Among all of the neural interfaces, the emergency of polymer optical fiber-based neural implants has led to the evolution in the field of implantable neural interfaces in several ways: Firstly, polymer optical fibers are more flexible compared with the traditional used semiconductor or silica glass implants, which is critical for reducing neuronal death and inflammatory response that derive from mechanical mismatch between the probes and the neural tissues and the achieving long-term neuromodulation and recording. Additionally, with the thermal drawing process, all the functional elements can be integrated into the fibers during the one-time fabrication with a production of hundreds of meters of multi-functional polymer optical fibers. This simple fabrication method can largely reduce the cost compared with the conventionally used methods such as microfabrication, and Photolithography. Besides, their low thermal processing temperature allows the use of a wide palette of materials for developing novel multi-material structures. To explore the full advantage of the polymer optical fiber-based neural interfaces, this thesis includes several critical studies as follows: i) Despite the wide application of the optical polymer fiber-based neural interfaces, the effect of the functional structure parameters such as size, position, and material on the probe’s mechanical properties has never been investigated. In this thesis, the mechanical properties of the polymer optical fiber with integrated microstructures were studied using the finite element method to explore the optimized parameters in terms of probe design to avoid bulking during the insertion process and minimize the inflammatory response after the implantation. We aimed to provide a general guideline in the design considerations for the development of novel fiber-based neural interfaces. ii) During the fabrication process of polymer optical fibers with the thermal drawing method, the involved materials have to be thermally compatible with each other. This imposed a limit on the available integrated function materials. To overcome this limitation, a modified thermal drawing method has been developed. By feeding the metal wires into the two holes in the tube of the polymer preform during the thermal drawing, the metal electrodes with a melting point up to thousands of Celsius can be integrated into the polymer fibers with a drawing temperature of several hundreds of Celsius. iii) Several new polymer optical fiber-based brain implants have been developed for different applications: polymer optical fibers with enlarged illumination angle for optimized optical neuromodulation, polymer optical fiber with functional Bragg gratings at its tip for temperature measurement in the deep brain region, soft multifunctional infrared neural interface for simultaneous neurostimulation and electrophysiology, and angled tip fiber based neural device for depth-resolved neural activity recording in optogenetics. In this thesis, the comprehensive study of the multifunctional polymer optical fiber-based brain implant, ranging from the optimization of the implant design, modification of the fabrication methods, and the development of new implants for different applications has been presented. We anticipate the polymer fiber-based neural devices illustrated in this thesis will open new possibilities for studying neural networks and brain functions in deep brain regions.