A plug capable of providing information relating to a physical or chemical property of a body fluid, or the presence or amount of a molecular component therein in a living organism is disclosed. Specifically, one embodiment plug is capable of being inserted into a portion of a human eyelid in order to provide information relating to tear fluid is disclosed. This embodiment plug includes a body having a passage which allows for the natural flow of tear fluid therethrough. In addition, a sensing mechanism is provided which is capable of measuring, for example, glucose levels in the body of a patient through the analysis of the tear fluid. Such plug may further be designed so as to double as a punctual plug useful in preventing dry eye. Methods of utilizing and implanting such plugs are also disclosed.