Thanks Amal for the detailed answer!
Well, I have a 3mm diameter chip in my hand and it doesn’t feel that big. It’s certainly not as long though. I just figured 4mm is only one more millimeter - at least in diameter 
I also saw 34mm-long implants for sale on Alibaba (like here for instance). But it’ll be a cold day in hell before I implant something I bought from Alibaba.
There is no ISO15693 Flex.
I didn’t know that was an issue as well. Hmm, it looks like implant-making might be harder than just hygiene and material safety after all
When you say biofouling, you mean like bugs getting into the surface cracks that can’t be removed? Wouldn’t autoclaving take care of them also?
Yes, I did wonder about that. Especially chips that long. Although the 22mm / 4mm ratio (5.5) still makes somewhat compact, and presumably more resilient. The 3x14 in my hand is 4.7 times longer than it’s wide, so it’s not too far off, and it is holding up just fine. The 4x34 format would be 8.5 times longer, and that feels awfully twig-like to me.
Also, I was considering implanting it somewhat deeper along - and close to - a long bone to act as a splint of sorts. Like in the forearm, an inch behind the wrist, between the radius and ulna.
And yeah, wall thickness, and whether it’s completely or partly filled with epoxy, is a big worry also. Although I figured the device is made for industrial applications, so surely it’s tough enough to withstand rough handling. But maybe that’s wishful thinking.
I agree. Too many unknowns.
Thanks for the heads-up!