The Craig-era 'reboot' Bond series has been largely serious about getting rid of the more ridiculous gadgets.
e.g. tens (or more!) of metres of high-tensile wire packed into a watch,
plus the explosive wherewithal to accurately fire the wire (with a piton-type-thing suitable for embedding itself in any surface), across any desired gap, thus giving a speculative one-use escape-wire
and a working watch as a handy and not noticeably encumbering wrist-mounted package... And that's not the 'worst' of the pre-Craig series items. But it doesn't really matter too much if you accept the "it's Bond, it's
supposed to be OTT" premise.
New-Bond has been debunking this trope, on the whole, though. There's
some dubiousness, still. A fully bulletproof supercar that can still compete well
as a supercar, despite the implied weight... (Still, at least it isn't packed full of an impossible amount of armament. Quite conservative with the weaponry, even if it
had been loaded with ammunition. The flame-throwers probably tapped the existing fuel-tank, and we never saw whether it had a drastically reduced range, afterwards.)
But the nanotech 'smart blood'. Necessary for plot (perhaps... people knowing where Bond was... at least if they were told/tapped into the intel... although Bond consciously tipping off Moneypenny/Q in his calls home could have done that), but practically?
Let's consider the functionality. Foreign bodies, injected into the bloodstream. They need to escape the machinations of the immune-response system. They need to avoid being filtered by (or interfering with) the various organs the blood passes through. They need to generate a signal strong enough to be detected by (presumably) satellites. They need to
avoid broadcasting a signal that is trivial to pick up (accidentally or otherwise) by third parties.
The last point is probably answerable by a rapidly-changing multi-frequency broadcast. Synchronised (one would imagine) across the whole blood-stream load, the broadcast would scan around the spectrum in a way that both encoded the individual (with, it was said but never used for, medical information encrypted on top) but also would have appeared as little more than a nanosecond of static-blip at any particular frequency to any receiver not looking for the likely cross-spectrum signature it should receive (for all such individuals!).
But then there's the
range of frequencies. Available frequencies are limited by the size of the aerial used. Each individual nanotech item has an
extremely small aerial. Perhaps they (virtually?) connect up, like the One Hectare Radio Telescope (large number of small dishes) or other Long Baseline arrays. Only at the scale of nanomachines within a human body. Whilst they're all moving around the body. Whilst the body is moving and running and jumping and... at the very least... breathing and beating its heart... So it's perhaps a couple-of-metres aerial. If all the machines can properly synchronise their individual contributions to the body-wide whole...
And now we're back to the piton-watch. These nano-machines
have to be able to gather full-body information, to work out where each element is, to work out what this means for
this element when (perhaps through collaborative calculation) it has worked out the encoded signal that each needs to contribute towards. With some source of power. While not impairing normal biological function. (Which might also rule out metabolising the body's own normal blood-borne 'fuel', unless Bond doesn't mind having to 'eat for two', for the duration.)
Really, it would have been far more believable to have given him a subcutaneous multi-band cell-phone chip that periodically 'chirped' through whatever local mobile phone coverage it found itself in the vicinity of. They could have worked out a way to have stopped him just gouging it out (probably).
Not that they needed this electronic leash, anyway! I still can't think of any reason for the tracking being necessary, or useful to the plot. Bond always leaves a Bond-shaped path of destruction behind him that
everyone concerned can easily track. Q meeting him in the mountaintop refuge used the tracking, but needn't have. (I think that's really the only instance of him being so tracked... perhaps including that information being passed on to the henchmen who came along as well, but they could just as easily have followed him through more mundane methods. By the time he was met at the station by the Roller, he was well within the probable 'unexpected visitor monitoring' range of the voyeuristic complex he was intentionally trying to get invited to visit). They'd already visited him on the train, but not interrupted him in (or on stepping out of) the Hotel, like they should have done if they were serious about this.
Honestly, it's in the territory of the 'nuclear hiccup pill' from the original
1 Casino Royale visual production.
1 Unless you count the Barry Nelson one... Which I don't, but it doesn't hurt to mention it, to pander to the real Bond-nerds.