Kumar Ganesan and colleagues from University of Melbourne think they may have found the perfect material from which to build bionic eyes—diamond. They are using the ultra-strong, biocompatible material to build the electrodes needed to pass light signals to the optic nerve. And they are already testing their devices.
In fact, diamond seems so well suited to life inside the eye that the team also plan to seal the bionic eye’s light-sensing chip inside a diamond box, which should protect its contents for a projected 80 years.
Diamond Penetrating Electrode Array for Bionic Eye
Kumaravelu Ganesan1, Alastair Stacey1, Hamish Meffin2, Samantha Lichter1, Una Greferath3, Erica L. Fletcher3 and Steven Prawer1
1. School of Physics, University of Melbourne, Parkville, VIC 3010, Australia.
2. NICTA,Victoria Research Laboratory, Parkville, VIC 3010, Australia.
3. Department of Anatomy and Cell Biology, University of Melbourne, Parkville, VIC 3010, Australia.
This paper presents application and characterization methods of diamond penetrating electrode arrays for Bionic Eye. The design and histology of electrode arrays made from polycrystalline diamond implanted into rat retinae are discussed.
Two broad families of retinal diseases are the current focus o f Bionic Eye work: inherited retinal degenerations such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD). Both diseases cause loss of vision through loss o f photoreceptors, but leave neurons in the inner nuclear and ganglion cell layers relatively intact . In this study we present results characterizing an electrode array for use in a retinal implant which has a high electrode density and is also capable of penetrating the retina to allow for close proximity to the target neurons. The array i s constructed from polycrystalline diamond, which displays several material properties which are essential for implantable electrode devices. These include a high degree o f biocompatibility and hermeticity and an ability to spatially manipulate the electrical conductivity over many orders o f magnitude, using the semiconductor dopant boron or nitrogen. This diamond penetrating electrode would be implanted in the retina. The conductive tips of each electrode reside in the ganglion cell layer, allowing for passage of current into the tissue. The electrodes themselves comprise an electrically conductive core, surrounded by a sheath of electrically insulating diamond.
Arrays of 2μm, 5 μm, 10 μm, 20 μm and 30 μm diameter cylindrical electrodes were fabricated, with heights 60μm – 100μm. Some were structurally reinforced with pyramidal or tapered bases.
A conventional microwave plasma chemical vapour deposition (MPCVD) apparatus was employed to synthesize the high quality polycrystalline diamond films, using a standard gas mixture of m ethane (CH4) and hydrogen (H2). The crystalline quality of these films was verified using a Raman spectrometer (Renishaw) with a 532nm excitation laser, where the sharp line at 1332cm-1 is a signature of SP3 bonded diamond.
To test whether the arrays are able to penetrate the retina we implanted them in freshly dissected rat retinae that had been fl at mounted in a petri dish and bathed in physiological saline. Horizontal and vertical 1 μm sections were sectioned and were inspected for evidence of penetration. Scanning electron microscopy (SEM) was used to examine the samples for broken pillars before and after implantation.
Horizontal and vertical retinal sections obtained following implantation of the array were investigated and found that the electrodes penetrates the retina. Implantation was also performed with arrays with electrode diameters of 10, 20 and 30 um at 100 um pitch. Histological results are unavailable at the time of paper submission, however SEM analysis revealed that less than 5% of the diamond electrodes were broken post implantation. It is unclear whether this was due t o implantation or handling.
Diamond penetrating electrodes with diameters of 2μm – 30μm and pitches of 50 μm – 400 μm were fabricated using an MPCVD technique. Their heights were measured to be approximately 60 – 100 μm. An implantation trial was performed on freshly dissected rat retina that were subsequently studied with histological techniques. The high mechanical strength of the diamond electrodes was verified, with less than 5% loss of the electrodes during the experiment.
REFERENCES Humayun MS, Weiland JD, Fujii GY, et al. Visual perception in a blind subject with a chronic microelectronic retinal prosthesis. Vision Res. 2003;43(24):2573-2581.  S. Y. Kim, S. Sadda, M. S. Humayun, E. de Juan, Jr., B. M. Melia, and W.R. Green, “Morphometric analysis of the macula in eyes with geographic atrophy due to age-related macular degeneration,” Retina, vol. 22, pp. 464– 470, 2002.
Kumar Ganesan, email@example.com