A user is authenticated to a mobile computing device using an NFC tag. An apparatus including a head, and an applicator coupled to the head, the applicator having dimensions suitable for contacting localized areas of human skin. The guide system can also include an accessory, such as an accessory guide tube through which a medical instrument can be carried external of the endoscope. The fuel supply structure includes a threaded inner surface formed from a first material. A coupling control layer is formed between two fibers to control the evanescent coupling for optical switching operations.