even if your going for a binary chemical compound, if you want that all encapsulated into a single breast implant then your talking about a highly sophisticated device, one that would not set of accidentally and allow for proper mixing of the compounds otherwise your going to just erm... set your breast on fire like the wannabe xmas day bomber, again im just guessing but the kind of fabrication techniques needed for such a device is well beyond the means of standard terrorist
if the second compound is going be inject via a syringe... thats even more a hit and miss affair even if you can get the needle to the correct location (presuming the device had not shifted, more fondling required...) chances are your not going to be able to dump the correct level chemical in a fast enough time for it to go critical
it would seem someone liked the idea of women fondling themselves...