A dedicated weed wacker I once saw used for brambles had three metal chains instead of the wire. The chains ended in the rotor with both ends, so, even if they broke, most of the pieces wouldn't fly away (I guess the broken ring could still detach, but I think it was a riveted flat chain, so I'm not sure even that was possible). The shaft holding the rotor was very long, so the chains were pretty far from the user. It was a very powerful gas-powered tool, and I think it takes some experience to use it safely; I certainly would wear eye protection.