As some of you may have already read on Carlos' blog. We teamed up to do a joint post relating biometrics and fiction. I recommend reading his original post, as I'll only highlight over it. Carlos is more the expert here than I.
In the book and movie, the main chick's dad, Vetra, get's offed and his eye is stolen to be used on a retinal scanner to gain access to a high security area. The is that this can't be done. First, retinas just don't work the same once the tissue is no longer living.
Carlos gives a beautiful explanation of why:
"Consider the retinas a thin piece of tissue paper lining the surface of a sink full of water. When
the water is moved the retina would detach and thus ruin it for scanning. In a younger person
the sink would be full of a material with a consistency closer to jello. However in the book it
mentions that Vetra is elderly and thus his retina would be very fragile."
Also, as Carlos has mentioned on his blog, retinal scanners work by the person being scanned having to focus on a certain point. So, if the eye is dead, the muscles are dead so there is no way that it can focus. In fact, the pupils will dilate. Additionally, if you wait too long after death the eyes will start to cloud over.
Many thanks to Carlos for his work here!!