You can't, or shouldn't be able to get a normal knife made of steel past a metal detector. The intent and design of an undetectable knife IS to be able to get it past a metal detector. As a plastic blade (G10, carbon fiber, whatever), it does not have better edge retention than steel, does not cut better, is not more efficient in anything a knife does EXCEPT have the ability to pass through a metal detector. As the intent of the undetectable knife is to defeat measures intended to help keep the public safe, it is not unreasonable to make such things illegal. Don't expect weapon laws to make sense. Nunchakus are illegal in many places, not because of the vast amount of people who have be killed by them, but because of their reputation.