A finite-element model of the mechanical effects of implantable microelectrodes in the cerebral cortex.

The viability of chronic neural microelectrodes for electrophysiological recording and stimulation depends on several factors, including the encapsulation of the implant by a reactive tissue response. We postulate that mechanical strains induced around the implant site may be one of the leading factors responsible for the sustained tissue response in chronic implants. The objectives of this study were to develop a finite-element model of the probe-brain tissue interface and analyze the effects of tethering forces, probe-tissue adhesion and stiffness of the probe substrate on the interfacial strains induced around the implant site. A 3D finite-element model of the probe-brain tissue microenvironment was developed and used to simulate interfacial strains created by 'micromotion' of chronically implanted microelectrodes. Three candidate substrates were considered: (a) silicon, (b) polyimide and (c) a hypothetical 'soft' material. Simulated tethering forces resulted in elevated strains both at the tip and at the sharp edges of the probe track in the tissue. The strain fields induced by a simulated silicon probe were similar to those induced by a simulated polyimide probe, albeit at higher absolute values for radial tethering forces. Simulations of poor probe-tissue adhesion resulted in elevated strains at the tip and delamination of the tissue from the probe. A tangential tethering force results in 94% reduction in the strain value at the tip of the polyimide probe track in the tissue, whereas the simulated 'soft' probe induced two orders of magnitude smaller values of strain compared to a simulated silicon probe. The model results indicate that softer substrates reduce the strain at the probe-tissue interface and thus may also reduce tissue response in chronic implants.