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WP1 Metalisation

WP2 Connection

WP3 Amplifier 

WP4 Stimulation

WP5 Fabrication

WP6 Assembly

WP7 Models

WP8 Global System

Description

This WorkPackage aims to develop laser processes to assist active cuff-on wafer fabrication. Precisely, this WP consists of there tasks: the first task is to develop a laser process to drill holes in silicone so that the platinum pods contact the nerve; the second task is to explore electrode contact surface treatments, such as iridium oxide coating; the third task is using different techniques to characterise the surface of the silicone sheets before and after metallization.

Achievement

Concerning the first task and as shown on Figure 1, holes can be drilled by laser in silicone rubber (PDMS) until the excimer laser beam touch Pt without removing it.

For the second task, coating electrodes of platinum with iridium oxide may lead to lower impedance. This coating is still unsuccessful but it has been shown in some scientific meetings (for example The Eye and The Chip 2006, World Congress on Artificial Vision held in Detroit in June 15-17, contribution of Stuart Cogan), that iridium oxide coating can yield very different results depending on the deposition method and that erosion might still be a limiting factor in chronic implants. At this time however, the prospect of incorporation of iridium oxide coating in the final design seems rather dim.

For the third task, the structure and morphology of untreated and treated (i.e. laser irradiated) silicone rubber (before metallization) have been characterized using optical microscopes, Scanning Electron Microscope (SEM), Analytical Transmission Electron Microscope (ATEM) and Raman spectroscopes.

SEM shows that morphology of irradiated PDMS (Figure 2) is completely different than the one’s of non-irradiated PDMS. The roughness is increased and is favourable to nerve contacts once metallized.

Characterization of silicone rubber after metallization using SEM shows that Pt metallized tracks are constituted of Pt balls (Figure 3) and that there is a strong correlation between the measured DC resistance of the tracks and the size of the Pt balls; which depends on the irradiation conditions.

When Pt balls are bigger, there are observed to be in better contact with each other so that electrical conductivity is higher thus leading to a smaller DC resistance.

We performed traction tests on Pt tracks. We determined DC resistances as a function of the track elongation and we observed that DC resistances become infinite when elongation is about 2 %. However, when tracks are relaxed, DC resistances are again finite and are slightly higher than before elongation. We observed that tracks mechanical rupture occurs when elongation is superior to 53 % (after relaxation DC resistances remain infinite).

In addition to adhesion and traction tests, we performed a test at room temperature in a saline solution to know the evolution of DC resistance with time and maybe erosion or dissolution of Pt in saline solution. For this experiment, we welded with tin 2 tin plated copper wires on 2 Pt contacts and we immersed the track in a saline solution (see Figure 4).

DC resistance is initially equal to 15 Ohms. It decreases for few days and then increases
for 3 months up to 30 Ohms before decreasing for 3 months up to its initial value. This result shows that saline solution does not damage our Pt tracks.