My research lab needs a low profile implantable antenna. Right now, we just have some vague specs
1) MICS /UWB/ ISM/ WLAN ALL THESE BANDS are acceptable to us (my concern is if you choose higher frequencies, probably you will get more difficulties )
2) low profile: at most 20x20 mm square
3) Can it be packaged in ABS
4) At most we can feed 5mW into this antenna
I think that the biggest problem is not 20x20mm antenna. Depends on your application you probably need to chose the frequency first. It looks like you have a medical application and for medical implantable device there are some critical parameters like power consumption, EMC with other medical and non-medical devices, MTBF and so on. This is the serious work and you must consider all critical factors, not only antenna.