Quantitative Assessment of the Mechanical Properties of the Neural Interface

Recent studies provide compelling evidence that besides the implant material, a combination of implant size and implant stiffness determines the long-term integrity and functionality of implantable neural interfaces. Mechanical parameters are now a design imperative for the neural interface engineers. However, despite the compelling empirical evidence, neural engineers face several daunting challenges: First, there are several significant gaps in our knowledge of the underlying mechanisms of how mechanical properties of the neural interface influence stability and reliability of the interface in the long term. Second, there are no standard methods, similar to electrochemical impedance spectroscopy (EIS) or cyclic voltammetry (CV) that characterize the interface's electrical properties, to characterize the mechanical properties of the brain tissue-implant interface quantitatively. In this chapter, we attempt to summarize what we know about the mechanical properties of the brain tissue-implant interface from our studies over the last several years and from a diversity of imaging, neurophysiology, and neural engineering studies. We also propose an improvised, penetrating "microindentation" technique as a possible standard to help neural engineers quantitatively benchmark the mechanical properties of their designs against others. We lay out the neural interface design problem in the framework of two rather broadly contradicting design constraints - "stiff enough" to penetrate and "soft enough" to match the surrounding brain tissue under chronic conditions.