Collagen is the main structural protein in the extracellular space in the various connective tissues in the body. As the main component of connective tissue, it is the most abundant protein in mammals, making 25% to 35% of the whole-body protein content. Collagen consists of amino acids wound together to form triple-helices of elongated fibrils.
An animal product is impure (najis) if both of the following are present:
- At the time of slaughtering the animal, its blood comes out with gush, and
- Such animal (as mentioned above) is not slaughtered according to method prescribed by Islam, nor is there a probability that it may have been slaughtered thus.
Now if these conditions apply to Collagen then it is najis, otherwise it is pure (paak).
Reference: See Tawdheehul Masaa'il of both Ayatullah al-Khu'i (may God have mercy on his ruh) and and al-Seestani in the chapter of Najasat under the discussion of "Dead Body".