TSA efforts seem to always focus on some type of explosive. First we had the shoe bomber and then the liquid explosives scare with Abdulmuttalab providing the exception that proves the rule.
I'm not certain how likely inserting explosives under the skin is in reality. It seems like a great way to kill a would be terrorist by messing up the procedure. Even if the procedure works, you still risk the human bomb prematurely detonating.
A far more likely scenario, and one TSA is not able to detect, would be a terrorist who has been intentionally infected with small pox. The individual could fly into say Chicago O'Hare and then proceed to touch as many passengers transiting through the terminals as possible. Passengers would not show signs until much later by which time numerous others would have been exposed. No risk of the bomb going off too soon. Zero chance of being detected by TSA.