Mm. It would have to be the neck not only so it's definitely fatal when it explodes, but also so they can't just cut their arm off and escape. The reason I prefer collars over chips is that chips can be influenced by certain types of body control powers. Then again, so could collars. In reality, I'm sure it would be based on the power and what the scientists think that power can eventually do. Someone who can control their own biological chemicals or bioelectricity could easily short out a chip, whereas a collar would be more difficult. Collars are also larger to allow for more tech and functions as well as to help protect against outside influence. If you have a chip, all it takes is an emp to disable it or trigger it, but a collar can be designed to have fail-safes in it due to its increased size. A 2 or 3 inch thick collar made of specific materials designed in a specific way, possibly even specific to the mal based on their power(s), would pretty well protect against outside interference and mal tampering. Sure it costs more, but they're trying to get insanely loyal living superweapons out of it. That would be worth a lot of investment.