Chronic neural interfaces are vital for sustained neural communication but face challenges such as foreign body responses and encapsulation failures. Polymer-based soft interfaces offer promise for long-term use due to their softness and compliance, yet water and ions permeability remain concerns. Traditional metal and ceramic barriers are unsuitable for soft interfaces, but thin film encapsulation provides a potential solution for effective protection while maintaining flexibility.
This thesis aims to develop and evaluate hermetic thin-film encapsulation for chronic implantable bioelectronics. The encapsulation must exhibit robust barrier properties and mechanical durability. Its performance must be systematically assessed, including responses to mechanical loading, resistance to water side permeation, and in vivo performance.
The first part of this work focuses on designing and characterizing a hybrid organic-inorganic multilayer encapsulation. This multilayer, composed of chemical vapor deposited (CVD) Parylene C and atomic layer deposited (ALD) alumina/titania (Al2O3/TiO2), is fabricated in a single chamber to avoid ex-situ contamination. A 6 dyads multilayer achieves a crack onset strain (COS) of 1.4% and an equivalent lifetime of 6.7 ± 0.8 years at 37°C. The organic interlayers effectively decouple defects or cracks in the inorganic layer, creating tortuous diffusion paths. Consequently, increasing dyads extends barrier lifetime while maintaining critical strain and the partially cracked multilayer exhibits delayed barrier failure.
Furthermore, innovative methodologies are developed to systematically characterize thin film encapsulation. Magnesium (Mg), sensitive to water and compatible with microfabrication, enables quantitative assessment of water permeation through thin films. The electrical Mg test allows real-time monitoring of water surface permeation under mechanical loading by measuring Mg resistance. This analysis demonstrates that cyclic bending reduces lag time, while stretching increases water permeation. Optical Mg methods track Mg color changes to quantitatively assess water side permeation. Validation with micrometric polyimide (PI) demonstrates a 4.5-fold ratio between side and surface permeation, emphasizing the critical role of the PI-PI interface in lateral diffusion. Guidelines for designing flexible, hermetic neural interfaces include widening encapsulation, refining interfacial interactions, and incorporating high-barrier layers like SiC to enhance lateral hermeticity.
Additionally, a chronic optoelectronic cuff array for spasticity inhibition is developed and tested on the sciatic nerve in a mouse model. In vitro and in vivo aging tests demonstrate the PI-SiC cuff's lifespan exceeds two months. Validation shows reduced EMG signals in opsin-injected muscles, confirming its effectiveness in disrupting spinal cord-muscle communication.
In conclusion, this work advances the single chamber CVD/ALD technique for organic-inorganic multilayer, demonstrating extended barrier lifetimes due to tortuous diffusion paths in partially cracked layers. Novel Mg-based methods are introduced to quantify mechanical loading impacts and water side permeation through thin film encapsulation. The high-barrier PECVD SiC film is optimized to effectively block surface and side water permeation. Additionally, the PI-SiC multilayer demonstrates promising potential for chronic optoelectronic array to inhibit spasticity.