Agreed with @wadevb1, I have set up 8' 2x4 spike strips (x4) to wrap around a tree and have has ZERO issues. It's effective, cheap and I'd say not very harmful to the bear. I've set it up so that the bear needs to be on it's hind legs to feel a poke and to a) not hurt any other animals that may pass by b) not hurt any humans around the strips.
