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Hermetic, Radio-Transparent Micropackaging for Implantable Neural Prosthetics
Keywords: Hermetic, Radio-transparent, Implantable
Hermetic protection is deemed a requirement for implanted electronic circuitry intended to operate for longer than 2 years. The first implanted, hermetic, devices were pacemakers manufactured about 1970 employing welded metal cans, primarily titanium. Some of the rechargeable battery-powered and nuclear-powered pacers from that era continue to function after more than 35 years of implant life. Those devices had relatively large internal free volumes at more than 2 cc, compared with modern pacers or cochlear implants in which that volume is less than 0.5 cc. Today there are micro-stimulators and micro-sensors designed to be implanted by means of catheters or needle-like introducers having an internal free volume of 0.05 cc or less. As the enclosed hermetic space shrinks, the allowable leak rate for moisture must be accordingly reduced to maintain an equivalent lifetime. For implantables with multi-decade lifetimes, leak rates approach the immeasurable. For systems designed with power and data antennas located inside the package, a large percentage of the enclosure is glass or ceramic instead of metal to avoid severe energy transmission losses. The challenge is to design the package to be structurally sound, primarily non-metal, and small enough to be practical for the implant site. The RF Microstimulator developed at AMF has been implanted in 12 patients. It is 2.4 mm in outer diameter and 13 mm long, and is comprised of a zirconia tube with titanium end caps. Although the longest human implantation is just over 6 1/2 years, it has survived accelerated lifetime tests of more than 100 years in water. Packaging innovations that made such hermetic cases possible and helped miniaturize the circuitry for these small devices are discussed.
Charles L. Byers, Principal Scientist
Alfred Mann Foundation
Santa Clarita, CA
USA


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