This is the new phone unlocking mode that vivalnk designed for Moto X phone. The system is rather simple. You stick an NFC-based skin temporary tattoo on your wrist. Once the tattoo is paired with your phone, to unlock the phone you just need to bring the phone in the range of the tattoo. It is possible to unpair a tattoo if it was lost or stolen.
According to vivalnk, the tattoo’s adhesive lasts about five days, even under water. It costs one dollar per tattoo. Currently, it is only available for the Moto X.
This tattoo is a wearable authenticator. I forecast that we will see this kind of authentication method using an NFC start to spread. It may come in ewatches, rings, or key rings. I believe that the ring would be a good device. The mere fact to take your phone in your hand may unlock it.