Miniaturisation of neuroprosthetic implants

Abstract

Electrical stimulation of neural tissue is known to elicit sensations and ac- tuations within the human body. Throughout the last decades, the ability to communicate with neurons was grown and the process become more sophisticated. Todays matured microprocessor technologies and architec- tures allow complex multi-channel stimulation and recording. However, chronic studies of implanted neuroprosthetics comprising hundreds of stim- ulation channels have never been conducted due to the lack of miniaturised encapsulation technologies which can be implanted within the smallest spaces while still withstanding the harsh environment for a long-period of time. In this study, steps towards achieving this goal have been success- fully carried out. Ongoing research was reviewed such that not only the materials, but also their combinations at the joining interfaces are able to withstand decades of implantation. Metallising alumina with platinum was of particular interest due to its established presence in neurostimulators for several decades. In depth knowledge of the bonding mechanism is essential to the understanding of this interface because it a_ects the hermeticity and biocompatibility of the device. Platinum to platinum and titanium to alu- mina interfaces where investigated for structural assembly and long-term stable interconnections. Further studies focused on hermeticity testing of microdevices using helium leak detection. These studies allow to determine possible methods for non-destructive, case-by-case testing to estimate the theoretical life-time of each encapsulation. Microfabrication methods were developed and a design for a neurostimulator encapsulation was outlined to allow the implantation into human body where space constraints are highly restrictive. An illustrative example of the how the above studies can be applied was provided by the complete design-to-prototype development of a visual prosthesis incorporating 98 stimulation channels. The results of testing of the method to miniaturise neuroprosthetic implants indicate advantages of allowing more complex stimulation methods by providing increased numbers of stimulation channels, increased life-time of the im- plants, and miniaturisation to target newly accessible implantation sites.