Hi guys :)
Our glass tags use Schott bioglass, biosafe epoxy resin, and biosafe resin coating on the copper antenna coil wire. We've done crush testing and have posted our results to our Facebook page; http://facebook.com/dangerousthings - in short, the tags are extremely fragile when outside the body and placed between two unforgiving metal plates, but once implanted into flesh (we used raw chicken as an analog), they can put up with quite a bit of pressure/force. We maxed out the machine at 500N and still the tags did not break.
We feel the extremely low read range of the tags themselves offer enough privacy protection since a direct read of a tag would need to be very up close and personal. Also, since our xNT uses an NTAG216 chip, the user has the option to protect the memory contents of their tag using a password, so no random passer-by could read it. The NTAG216 also supports elliptic curve authenticity encryption so you can query the tag in a secure way to ensure it is a tag you're talking to and not an emulator. This increases overall system safety.