Polyimide-based intracortical neural implant with improved structural stiffness

Abstract

A novel structure for chronically implantable cortical electrodes using polyimide bio-polymer was devised, which provides both flexibility for micro-motion compliance between brain tissues and the skull and at the brain/implant interface and stiffness for better surgical handling. A 5–10 µm thick silicon backbone layer was attached to the tip of the electrode to enhance the structural stiffness. This stiff segment was then followed by a 1 mm flexible segment without a silicon backbone layer. The fabricated implants have tri-shanks with five recording sites (20 µm × 20 µm) and two vias of 40 µm × 40 µm on each shank. In vitro cytotoxicity tests of prototype implants revealed no adverse toxic effects on cells. Bench test impedance values were assessed, resulting in an average impedance value of ∼2 MΩ at 1 KHz. For a 5 µm thick silicon backbone electrode, the stiffness of polyimide-based electrodes was increased ten times over that of electrodes without the silicon backbone layer. Furthermore, polyimide-based electrodes with 5 µm and 10 µm thick silicon backbone layer penetrated pia of rat brain without buckling that has been observed in implants without silicon reinforcement.

AFM and Cell Staining to Assess the In Vitro Biocompatibility of Opaque Surfaces

“…Neural interfaces using SiO 2 and Si 3 N 4 have shown gliosis also, but polyimide, a chemically resistive polymer material has shown excellent biocompatibility (Boppart et al, 1992;Rousche et al, 2001). Although this material offers excellent biocompatibility, displaying a fibroblast adherence, growth, and spread comparable to polystyrene, it has major drawbacks that hinder its acceptance as the material of choice for NI devices (Lee, K. K. et al, 2004). The material has a relatively high water uptake which decreases electrical impedance, and it is a very flexible material which add complications during NI implantation (Polikov et al, 2005).…”

Insertion of Flexible Neural Probes Using Rigid Stiffeners Attached with Biodissolvable Adhesive

“…One class of designs modifies the polymer probe geometry to increase stiffness in certain sections or axes while maintaining compliance in other parts. This has been accomplished by incorporating ribs or layers of other materials 9,10 . Another approach integrates a 3-D channel into the polymer probe design that is filled with biodegradable material 11 .…”

Removable silicon insertion stiffeners for neural probes using polyethylene glycol as a biodissolvable adhesive

“…However, this negates the flexibility originally intended for the polymer probe. More complicated designs modify the polymer probe geometry to incorporate ribs or layers of other materials such that the device has increased stiffness in certain sections or axes, while maintaining compliance in other parts [10,11]. Yet another approach integrates a 3-D channel into the polymer probe design that is filled with biodegradable material [12].…”