A monolithic diamond microelectrode array fabricated for a high acuity retinal prosthesis

Abstract

 

Purpose

 

To fabricate and test a biocompatible, chemically inert, hermetically sealed, near-conformable microelectrode array comprised of two kinds of diamond materials

 

Methods

 

Feedthroughs were made on insulating polycrystalline diamond (PCD) substrate using laser milling and electrically conducting nitrogen incorporated ultrananocrystalline diamond (N-UNCD) was grown on top of it using chemical vapour deposition. Finally, the N-UNCD electrodes were electrically isolated by laser milling. An optical 3D profile image of the flat part of a 16x16 electrode array is shown in Fig.1. The electrode array was tapered in order to reduce the space between the electrode and the retina with approximate radius of 12 mm as shown in the Fig. 2. In order to test the stimulation efficacy, diamond electrodes were tested using acute in vivo preclinical models. The hermeticity of the array was tested using a helium leak tester by attaching the electrode array by a Viton® O-ring.

 

Results

 

A high density hermetic diamond microelectrode array was fabricated and implanted in vivo with a PDMS carrier and the average minimum charge density required to produce cortical activity was 151 µC cm^-2. The in vivo electrode impedances were between 2.5-76kΩ and stable over the course of the acute testing phase. The helium leak rate was found lower than the detection limit of the helium mass spectrometer (10-11 mbar L s^-1) for approximately 30s and thereafter helium is found to leak through the O-ring.

 

Conclusions

 

The average threshold charge density exhibited by the diamond arrays presented here is well within the safe charge injection limit. The hermeticity of the high density electrode array means that the array can also function as encapsulation for implantable microelectronics. The use of a single material for construction of the array eliminates any potential materials mismatches that could lead to device failure. The results are evidence that diamond shows potential for both active and passive components of an implantable neural prosthesis.

 

 

Fig. 1: A 3-D optical profile image of the 16x16 diamond micro electrode array.

 

 

Fig.2: A schematic of the near-conformable diamond electrode array and the high acuity implant. (Not to scale). 1. Retina, 2. N-UNCD electrode. 3. Electrical feedthrough. 4. Indium contact bump. 5.Diamond Encapsulation. 6. PCD substrate. 7. microprocessor chip.

 

Keywords: 688 retina • 507 electrophysiology: clinical