Doesn't really help if the path to the brain is leaking like a sieve. Also one of the goals with blood substitutes was to make them physically smaller than red

Doesn't really help if the path to the brain is leaking like a sieve. Also one of the goals with blood substitutes was to make them physically smaller than red blood cells to get past the second issue which is swelling that can restrict blood supplies. The idea was a smaller O2 carrying molecule could squeeze through where a red blood cell couldn't.