Interesting but I don't really see them catching on as is.

Maybe add a wallet to the device, so they can be used for air gapped Bitcoin transactions as well, that would increase the number of people willing to buy or build their own.

And if has to be connected to the internet and on the same wifi network as your phone to use it? Kinda defeats the purpose as far as I can tell. Still very interesting.

Something may evolve from this that actually does get mainstream adoption