One, Tony's heart part of the bomb due to some of the fragments, if left unattended will lead to fragments of the blood flow through the whole body is extremely dangerous, so the chest of the energy block (the Ark reactor) played the equivalent of an electromagnet, adsorption of debris
Second, due to the Ark reactor releases a great deal of energy, so it also plays the role of the armor's energy source
In the end of Iron Man 3 Tony refer to Iron Man 1 villain Obadiah Stanley's cottage version of the Battle Armor made from Mark 1 debris))
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