Periodic Report Summary - BRAIN TOUCH (3D flexible probe for deep brain stimulation and recording)
The objective of this project was the manufacture and in vivo testing of silicon based, yet flexible, neuroprobe for deep brain stimulation and recording research applications on rodents. We chose to focus our work on small animal implants instead of humans, because most of the neurological disorders could be studied on rodents provided that the researcher had the appropriate behavioural and physiological model. The technological advances of this project could be nevertheless transferred to human implant applications.
An important challenge consisted in finding a reliable solution for a flexible, yet stiff enough, probe for precise stereotactic implantation. Given the small size of the animals, the requirements for implantable devices were different than those for bigger animals or humans, with an approximately 10 times dimension scaling factor being necessary. Some of the obvious solutions used for humans, like for example keeping the probe rigid during insertion with a metal stylet, manually removed at the end of the surgery, were difficult to translate to a 100 or 200 microns wide device and virtually impossible to integrate in a wafer level process. Therefore, a great deal of the first project period, i.e. the first year, work focussed on implant design, materials’ choice and introduction and qualification of any new materials in the manufacture facility of the interuniversity microelectronics centre (IMEC).
Given the small dimensions of our implant device, it was decided in the early stages that the use of an internal self-sustaining structure was more suitable for a flexible, yet stiff enough probe for precise stereotactic implantation. A finite element modelling study led to the conclusion that the use of a polymer material for the flexible part of the implant was more suitable in case the polymer material was wrapped around a carefully designed, permanent inner rigid core which was not to be removed after surgery. The polyimide material chosen for the flexible part of the implant exhibited low internal stress and water absorption and was already used for chronic in vivo experiments by some of the most known researchers in the field. The work on the design of the rigid-flex composite structure of the probe pointed out that little was known and published about the mechanical aspects related to the insertion surgery procedure. Therefore, in order to enable better design for future generations, a force measurement sensor was added to the existing stereotactic surgical setup that was available at IMEC.
During the second project period, i.e. the second project year, the polymer process know-how that was previously acquired was transferred on real device manufacture and led to the successful manufacture of both rigid and flexible neural implants. The three-dimensional structure of our flexible implant design, consisting of a polymer material wrapped around a silicon inner core, was achieved by combining deep silicon etch process steps with back side wafer thinning by grinding. A novel front side silicon dry etch process was also developed, capable of creating a two level step structure inside the bulk of the silicon wafer. The force measurements performed during the in-vivo insertion experiments led to the development of an experimental model capable of quantitatively predicting penetration forces and tissue dimpling values based on implant design parameters, such as the tip angle, width and thickness, as well as on surgical procedure ones, such as the insertion speed. To our knowledge, this was the first time that such an accurate quantitative model was ever developed.