Polyimide based neural implants with stiffness improvement

António Filipe SousaNº64427 MBioNano

Keekeun Lee, et al.

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.

Neural implants

This paper describes the design, fabrication, and initial performance feasibility studies of the latest prototype polyimide-based intracortical implant.

Objectives

• Flexibility to accommodate micromotion

• Stiffness required for penetrating into the brain tissue Hybrid device

chronic implant

Neuroprosthetic applications

Electrode Design and Fabrication

Fig. – Simple Schematic diagram of the PI based neural implant

• Silicon backbone layer, from silicon-on insulator substrate, is attached to the tip and connector regions of the electrode to increase stiffness

• The recording sites are interfaced to the external circuit via a 15-channel connector, wich is especially designed to facilitate processing of neural signals.

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.

Results

Fig. - 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 into rat brain.

Electrical Impedance

Saline 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 studies

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.

Patric J. Rousche, et al.

“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.” Keekeun Lee, et al.

Concluding

“…there is continuing evidence 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.