Unlock iPhone 5s with different parts of body?

Do you think it would be possible to unlock 5s with other body parts? Nipple maybe?

Doubt it since fingerprints are based on vectors in the print itself but give it a try:)

Very likely. The home button simply records a high resolution digital scan of what's pressed against it and then compares that to whatever gets held against the home button in future. It doesn't have to be a fingerprint but it does need to be something which is biometrically unique and unchanging.

In practice, a nipple probably lacks adequate detail and appears very different depending upon how hot or cold you are, your state of arousal, and for women it can look sligjtly different at different times of the month. More importantly, they do change in appearance over time unlike a fingerprint which remains the same (barring injuries) from the time you're fully grown to the day you die.