Here's what to do:
1 - Purchase an AVR ATTiny10 or similar small chip
2 - Store the key inside the chip
4 - Place the chip inside the capsule
5 - Seal the capsule and sterilise it
6 - Purchase an RFID injector needle, scrub your skin, inject the capsule
3 - Purchase an empty schott transponder capsule here: http://www.schott.com/epackaging/english/glass/transponder.html
Document that you did this in your will
You could also use an NFC tag or similar, but this poses the risk that the data could be read without your knowledge.
The flash memory in the AVR chip should last for far longer than your body will, and by not connecting any other electronics you make sure it can't be read from without removing it from your body surgically.
You could also use a printed out key if it fits inside the capsule.