How? Read this description. It's a regular zipper bought from a sewing store... Glued the fabric edging to my face with liquid latex, as flush as possible with my head tilted slightly back, and when I tilt forward it naturally wants to open up.
Red "Blood Effects Gel" used for the inside; heat it up to flow and when it cools it gets hard again and stays put, but still looks wet and goopy.
Zipper fabric was coloured by rubbing/soaking fake blood into it near the metal, and flesh-coloured cream makeup on the outside to make it blend with my skin somewhat. Could have blended it with my skin with better tools and effort.
Done to myself in about an hour. Made it up as I went, there might be better ways.
Photo ended up on Kotaku: http://kotaku.com/5676963/costumes//gallery/2