3D printing is one of the more useful ways to securely backup up encryption key’s. Just ensure the only place they exist is inside an object and not only are they durable, but tamper resistant as someone can’t just photocopy the key.
3D printers can make solid objects with internal structures.
It’s the same basic idea as an envelope, as in you need to open it to see what’s inside. However, opening it up inherently breaks the object so if you have it unbroken then it’s obvious that nobody has done so. XRay’s being an obvious risk.
