FWIW, today's Philadelphia Inquirer reported that the explosive was found to be based on PETN (consistent with Semtex) and not RDX (consistent with C4). Both are high explosives commonly described as "plastic explosive".

And as high explosives, they need a "primary", like a blasting cap, to set them off, it can't be done with a match as others have noted.

Unclear from the news reports whether a "primary" was included in the shoes.