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High tech materials that are flexible but unbreakable, like some kind of military grade soft fabric where you can still move and stretch without being cut in half.
Sin There was a strange crowdfunding project in 2013 for anti-rape underwear. From what I could tell, it was just a pair of elastic undershorts, but with reinforcing strips of UHMWPE (also used in ballistic armor). The whole thing fastened at the front of the waistband with a combination lock.
There was enough interest that the project hit its funding goal, despite it getting a lot of negative attention (especially from pop culture commentators). Unfortunately, after posting sporadic updates for three years, the project fell through and the founders vanished with about $USD50k of backers’ money. Oh well.
But I think the idea is sound: it’s now possible to replace the steel cables/bands of a belt with polyethylene straps to make something a lot more comfy and ordinary-looking.