Many a biohacker has encountered the challenge of reliably reading and writing to an implanted T5577 credential (such as the xEM implant from Dangerous Things... https://dangerousthings.com/product/xem )
Given the fact that the T5577 chip doesn't have tear protection, the prospect of bad or incomplete writes can be harrowing, as nobody wants to accidentally damage the configuration of a chip that they have implanted and cannot easily replace.
This 125kHz LF antenna, designed by Tom Harkness and the Proxmark3 hacker community, features a ferrite core and optimized geometry for interacting with biohacking implants such as the Dangerous Things xEM and NExT tags.
The Proxmark3 RDV4 has a modular antenna system, allowing for antennas to be swapped depending on your engagement and distance requirements. Changing the antenna array on a Proxmark3 RDV4-series unit is a user-serviceable operation and relatively east with either a small screwdriver or hex tool, depending on the type of fasteners installed on one's kit. You can do this yourself!
NOTE: While this antenna will optimize a Proxmark3 RDV4 for interaction with small-format embeddable credentials, it is not as efficient for coupling with conventional card or fob form factor RFID tags. Also, while the LF and HF antennas connect to an RDV4 Proxmark unit at different points, it can be a challenge to install this in parallel to a stock High Frequency antenna, although Iceman has tweeted photos of Deviant Ollam's modification of one of his Proxmark units where this was achieved... https://twitter.com/herrmann1001/status/1225295696405811200