A few months ago Hackaday covered the xNT crowdfunding campaign which aimed at making an NTAG216 based NFC implant for different purposes. I actually backed it, found that standard NFC readers don’t perform well and therefore decided to try using a standard coil as an antenna for better reading performances.
Most NFC readers typically only have a small sweet spot where implant reading is possible. This is due to what we call coupling factor which depends on the reading distance and reader & NFC tag antenna geometries. Having a smaller antenna diameter increases the coupling factor and makes implant positioning easier.
In my detailed write-up you’ll find a good introduction to impedance matching, a process where a few passive components are added in series/parallel with an antenna to bring its complex impedance close to a RF signal transmitter’s. This usually requires expensive tools but allows optimal power transmission at a given frequency.
You may find our xNT coverage here.