The only implantable chips I have seen are the 125KHz ones. They are passive chips and carry an 8 digit code. I forget the name but it begins with an M (and it's not myfair thats a 13MHz card) the chip it is based on is the EM4223. While these are the most popular there are many other types. This is not the type I have seen being used with the Arduino.
Most readers output the data in the form of Wiegand data, that's two lines. One pulses with a logic one when there is a zero in the code and the other pulses when there is a one in the code. Actual asynchronous or RS232 type serial is not common but not unknown either.
http://www.rfip.eu/aboutrfid.php
Has some background although it says 132KHz is the standard low frequency which is not correct. This is just for one manufacturer who happen to be the first with this technology about 25 years ago. The modern standard is 125KHz