Polyimide based neural implants with stiffness improvementKeekeunLee, et al.António Filipe SousaNº64427 MBioNano

Neural implants A novel structure for chronically implantable cortical electrodes using polyimide bio-polimer. These devices have been designed to provide a conformal coverage when placed upon the curved surface of the brain. The polyimide surface chemistry is amenable to modifications and preparations which allow a host of bioactive organic species to be either adsorbed or covalently bonded to its surface.This type of polymer based intracortical neural implants present several attractive features: flexible, biocompatible and easily manufactured using existing microfabrication technology.

Objectives• Stiffnessrequired for penetrating into the brain tissueHybrid device• Flexibilityto accommodate micromotionchronic implant Neuroprosthetic applications This paper describes the design, fabrication, and initial performance feasibility studies of the latest prototype polyimide-based intracortical implant.

Electrode Design and Fabrication• Siliconbackbonelayer, fromsilicon-oninsulatorsubstrate, isattached to thetipandconnectorregionsoftheelectrode to increasestiffness• Therecording sites are interfaced to theexternalcircuit via a 15-channel connector, wichisespeciallydesigned to facilitateprocessingof neural signals.Fig. – SimpleSchematicdiagramofthe PI based neural implant

1 – Fabrication starts with a 4 in. silicon-on insulator (SOI) substrate with varying device silicon thickness from 2 to 10μm and buried oxide thickness of 1 μm.2 – Top device silicon layer was selectively etched away for flexible region using a 2000Å thick gold masking layer (Fig. a)3 – The first layer of polyimide was spin-coated, exposed, and then developed as shown in Fig. b.4 – A reactive ion etch (RIE) was used to clean and microroughen the polyimide surface prior to depositing the metal layers. After RIE, a 2000Å thick gold layer was deposited for recording sites, followed by wet etching (Fig. c).

5 – The top polyimide layer was spun, exposed, and developed to encapsulate or reveal the desired conducting surfaces (Fig. d).6 – Backside silicon etching was performed for 10 h in RIE with SF6. Clean and uniform silicon backside etching was obtained (Fig. e).7 – After complete removal of backside silicon, the buried SiO2 was etched away in 49% HF acid solution (Fig. f).8 – Several rinses with de-ionized water were performed to remove any unwanted etchant products.

ResultsFig. - The fabricated device was visualized through optical microscopy and scanning electron microscopy (SEM). The device has tri-shanks with five recording sites (20μm × 20 μ m). The stiff segment has a silicon backbone layer that is 1.5mm in length and 0.2mm in width for implantation intoratbrain.Electrical ImpedanceSaline tests were performed by immersing the shafts and connecting cable of the devices into a 0.9% saline solution at room temperature in a holding chamber sealed from room air.

More studiesPatric J. Rousche, et al.Bioactive species such as NGF (neuronal growth factor) can be selectively pipetted into the via to provide neural in-growth toward the local electrode site region.

Concluding“We have demonstrated that our electrode design with a silicon backbone layer of 5–10 um is robust enough to penetrate the rat’s pia without buckling.” KeekeunLee, et al.“…thereiscontinuingevidence that a neural interface providing reliable and stable long-term implant function could be used for the realization of clinically useful cortical prostheses for the blind…”.Patrick J. Rousche, et al.