Polyimide based neural implants with stiffness improvement

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 enough stiffness for successful surgical implantation into the brain tissue. 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 three shanks with five recording sites (20 μm×20 μm) and two via holes (40 μm×40 μm) to promote tissue attachment on each shank. Each recording site was connected to the external circuitry via a 15-channel connector, which is especially designed to facilitate processing of neural signals to the external circuitry. In vitro cytotoxicity tests of prototype implants revealed no adverse toxic effects on cultured cells. The implant with a silicon backbone layer of 5–10 μm was robust enough to penetrate the rat’s pia without buckling, a major drawback with polymer. The averaged impedance value at 1 KHz was ∼2 MΩ.