My University Bike Lock Design - Youtube Video

is it legal to make a lock out of a material, or with a certain layer/filling, that would make cutting it not impossible for the device (i.e. saw) but for the user - i.e. thief?

I'm thinking noxious clouds. Asbestos outer layers with a steel core. Anthrax-impregnated cables. That sort of thing.

Asbestos you would be able to use but due to the way it flakes and stuff and that it could/would take 20+ years to kill a thief after they inhaled it(and even then it's not as likely as people make asbestos out to be) then I doubt it would work. Also if you did market a lock with asbestos in be sure to get sued by customers down the line. You might also need to justify that it's needed for it's industry uses against heat and explain why/how the lock would be used daily near furnaces and things.

Anthrax is more full on illegal as it's pretty much classed as a chemical weapon.

You could probably layer a couple of chemicals so once you cut past one to the other they react and it turns into a flare but then if it did stop the theft would using a flare wrapped around a carbon frame leave you much of a bike to come back to?