2 links have been added on this idea.
  1. Check out this patent for a surgical sponge with several special features, including turning a different colour when it comes into contact with blood.
    https://patents.google.com/patent/US4098728A/en

  2. Check out this patent for a surgical sponge with several special features, including turning a different colour when it comes into contact with blood.
    https://patents.google.com/patent/US4098728A/en