A common occurrence that astronauts face during space flight missions is a decrease in bone mass.To monitor this physiological change, a molecular-based biosensor is being developed to detect extracellular levels of Calcium from the human body.This extracellular biosensor will be non-invasive and the prototype detection unit itself will be a sweat patch. Extracellular detection is possible since calcium is one of the products of human sweat. This biosensor is a modified GFP (green fluorescent protein) that consists of a red- and blue-shifted variant that are linked by a calmodulin binding domain. At the C-terminus of the blue-shifted variant contains a truncated calmodulin molecule.When calcium is present, this molecule binds to calcium and causes the GFP to go through a conformational change.This conformational change can be detected in the fluorescence emission at 505nm and 440nm at an excitation value of 380nm. To assess the correct concentration of calcium needed to make a sensitive enough detection unit, a standard curve is being produced by plotting calcium levels against a ratio of the 505:440 emission.