Currently, neurostimulation holds the capability of treating symptoms associated with epilepsy, essential tremor, depression, migraine, incontinence, Parkinson's, Tourette's, and other diseases and disorders. Given the constant evolution in the field of biomedical technology and the increasing demand for advanced solutions in neural interface technology, addressing challenges associated with conventional neural electronic implant packaging becomes crucial. Conventional packaging often results in bulkiness, limited proximity to the target tissue, and potential complications, prompting an emerging need to miniaturize and soften the packaging. While flexible substrates like polyimide, parylene C, polyurethane, and silicone elastomers have been explored by the neural implants industry, the ongoing shift towards fully implantable, biocompatible, and flexible active implants calls for a more tailored packaging approach.
This Ph.D. research aims to provide a comprehensive investigation and overview of utilizing polymers as substrate and encapsulation materials for neural implants, examining both the advantages and challenges associated with their use. In particular, the study will look into the latent potential offered by thermoplastic polymers, with a specific focus on thermoplastic polyurethane (TPU) and liquid crystal polymer (LCP), as these polymers offer a unique blend of properties that make them promising candidates to significantly impact neural interface technology.
In Chapter 2, a thorough literature review investigates polymers commonly used in neural implants. This chapter not only explains the reactions happening when implants are put into the body but also emphasizes the basic requirements for implantation. The chapter focuses on the main properties and advantages of various polymers, distinguishing between thermoset and thermoplastic polymers. Some examples of using these polymers as substrate and coating materials for passive neural interfaces, together with the insights into the associated processing steps, are presented in this chapter. Furthermore, the chapter looks into the ways of integrating electronic chips into passive implants, presenting a detailed review of bonding techniques, bumping technologies, and adhesive types, as well as showing examples of existing active neural implants.
Chapter 3 continues the exploration by focusing on thin film encapsulation materials on flexible LCP substrates. Using HfO2-based atomic-layer-deposition multilayers, a hybrid ParC-ALD multilayer, and an LCP coating layer, this chapter systematically evaluates how well these coatings work through various testing methods. T-peel, water-vapor-transmission-rate (WVTR), and long-term electrochemical-impedance-spectrometry tests give valuable insights into the effectiveness of these coatings, emphasizing the advantage that can be offered by thermoplastic LCP-LCP coating-substrate interfaces.
Chapter 4 presents the fabrication method for a thermoplastic polyurethane-based electrode array with high-resolution gold interconnects employing the following techniques: thermocompression bonding, electroplating, laser direct imaging-based lithography, and laser ablation. The integrity of this electrode array is evaluated under conditions simulating the human body environment, involving soak tests at different temperatures and in-vivo tests. The extended evaluation includes electrochemical and optical transparency tests to further enhance our understanding of how well the electrode array performs in different situations.
Chapter 5 shows the integration of ASICs into the previously described polyurethane-based electrode array. Using flip-chip bonding technology, this integration involves connecting ultra-thin chips to gold metallization tracks using an anisotropic conductive adhesive. The successful combination of these components represents a significant step toward creating polymer-based active neural interfaces.
The concluding Chapter 6 summarizes the key findings and contributions of the thesis. It not only highlights the scientific progress made in using thermoplastic polymers for neural interfaces but also emphasizes the successful integration of ASICs into a polyurethane-based electrode array. The chapter ends with suggestions for future research directions and improvements.
In essence, this thesis provides an exploration of polymer-based flexible neural interfaces, particularly focusing on the unique properties of LCP and TPU thermoplastics. This work introduces polyurethane as a novel addition to the portfolio of biocompatible polymers used as both substrate and coating material for neural interfaces. The combination of biocompatibility, flexibility, microfabrication compatibility, and optical transparency, together with the developed fabrication process technology for high-density and high-resolution soft neural implants, contributes to and expands the toolkit available for developing fully implantable soft neural active implants.