Is the Apple Watch Sport nickel free?

I want an Apple Watch, but I have a nickel allergy. I believe the Apple Watch Sport, which has an anodized aluminum case, would be the safest choice. My main concern is the composite back. It would be extremely helpful if someone from Apple could elaborate on what exactly the composite back is made of. Is the composite back nickel free? I'm sure I won't be the only customer concerned to know this.