Nonlinear elastic brain tissue model for neural probe-tissue mechanical interaction

The reliability of long-term recording of implantable neural probes depends on several factors, including adverse tissue response which forms astroglial sheath scarring around the probe. Brain micromotion is one of the possible causes of this tissue reaction. It creates localized strain around the probe tip, leading to shear-induced inflammation at the implant site. Although the strain induced by brain micromotion may exceed the accepted linear range for biological tissues, linear brain models have been employed in most modeling studies. Hence, the aim of this investigation is to verify the validity of assuming small-strain deformation used for linear brain tissue models in brain micromotion studies. Finite element (FE) models, utilizing both elastic and hyperelastic brain models, are developed to study the strains at the probe-tissue interface, for longitudinal and lateral brain micromotion. The results show that the strain around the probe tip for both material models, exceeds commonly accepted linear strain range. This finding suggests that use of a nonlinear elastic model to model brain micromotion is more realistic. Moreover, the neural `kill zone', denoting distances within which the strains are greater than 5%, is estimated to extend to more than 20 μm from the probe tip for longitudinal micromotion displacement.