Methods and devices for chronically-reliable packaging of implantable neural microsystems

Abstract

The current landscape of neurotechnology is thriving with a wide variety of systems and devices to study the brain and treat diseases associated with the nervous system. However, many patients who live every day with a prosthetic limb or with the consequences of a spinal cord injury are still waiting for a solution that could restore their abilities to move freely. In this regard, no established clinical solution exists at the moment, and the majority of the systems proposed and developed by the scientific community fail to translate to commercial products, mainly due to the lack of consistent chronic performances. The design of an implantable system that can reliably work over decades is still a great challenge. The objective of this thesis is to explore the issues that impede long term operation, and to provide possible methods and solutions in the aspects regarding packaging and encapsulation of chronic implantable devices for cortical recording. With such goal in mind, the following work starts by presenting a review of neural implantable devices, with an emphasis on the challenges they face when dealing with chronic implantation. The original contributions presented in the subsequent technical chapters are summarized here: • The identification of a method to assess the level of hermeticity of a micro-packaged implantable device. The proposed solution is the use of a custom designed relative humidity sensor, to detect the presence of water molecules inside the packaged device. The full design of the sensor is described in detail, from the mathematical modelling of the sensing element to the readout circuit architecture, with focus on reducing the circuit footprint, as required by the specific application. The use of a commercially available sensor is also explored for the realization of a bench top hermeticity testing platform, by measuring its ability to withstand the temperatures required in the sealing process of an implantable micro-package. • The quantification of the damages due to corrosion that occur on a silicon-based device encapsulated with a polymer coating. This has been achieved by designing dedicated test structures, and observing their behaviour in accelerated life tests, that have demonstrated the reliability of silicon substrates for chronic implantable devices. The use of an industry standard CMOS process increases the significance of the results. The idea of equipping implantable devices with self-diagnostic capabilities has also been explored. Two instrumentation circuits are proposed to monitor in real time the diffusion of water molecules within an implanted integrated circuit. real time the diffusion of water molecules within an implanted integrated circuit. • The development of an overall structure and fabrication method for the realization of a fully autonomous, wireless, millimetre-sized neural implant, focusing on the hermeticity of the embedded electronics and on the use of penetrating microwires for cortical recording of local field potentials. The mechanical structure of the implant is presented in detail, and a first prototype is realized, together with a dedicated platform to speed up the assembly of the probes. This work proposes devices and methods for the realization of miniaturized neural implantable devices, stressing the importance of chronic reliability and emphasizing the need for a design approach that keeps in mind the clinical applications of future neural interfaces.