The appropriate packaging method for a cortical implant depends upon the required implanted lifetime, the number of input-output
electrical connections, and the allowable package volume. During our design of the Visual Prosthesis we have
developed hermetic packaging techniques that minimize the implant volume while allowing for a high number of electrical
outputs.
The Visual Prosthesis packaging is based upon glass-sealed layers of a machinable ceramic. Each 64-channel
Submodule is comprised of a 4-layer hermetic ceramic package. The thickness of each Submodule is 0.06. Stacking
4 Submodules together results in a 256-channel Module thickness of 0.24.
The 256-channel Module
Each Submodule is made from 4 layers of machinable ceramic. The Submodule is sealed by firing a low-temperature
glass around the perimeter of the ceramic layers. Three glass seals are used between: layers #1 and #2, layers #2
and #3, and layers #3 and #4 (lid). Use of a machinable ceramic for the package facilitates the fabrication of the complex
shape of the Submodule. A completed package is shown with the lid removed. The glass seal is visible beneath
the upper layer #3. The Submodule coil, for the transcutaneous link is contained within the seal area, on layer #1
and is not visible.
The Complete Package
The 256-channel Module includes four 64-channel connectors for connection of the electrode wires to the implant
package. These connectors are based upon the principle of pressure sealing. An anisotropic elastomer is placed
between two opposing sets of metal contacts. This design strategy can be seen in the sectional view, at layer #2
in Figure 9, below. Using the 0-80 screws, approximately 300 psi is placed upon the elastomer.
This pressure is sufficient to prevent water from condensing at the interface. Similar connectors have been
designed and tested by BioElectric Corp. under other NINDS contracts. The anisotropic elastomer is comprised of small
gold plated filaments which act as conductors between the connector pads and the pads on the Submodule, but do not
connect to each other. The filaments are embedded in a silicone rubber base. Pressure on the anisotropic elastomer
serves the dual purpose of forcing the filaments into contact with the pads, as well as compressing the silicone rubber.
A magnified view of the connector/Submodule interface shows the compressed gold filaments. In order to obtain the
300 psi pressure the screws must be tightened to over 6 in-oz.of torque.
