There is provided a hydration state indicator. The hydration state indicator comprises a watertight shell； a semi-permeable membrane configured to permit the passage of water molecules and to block the passage of molecules of at least one solute； a water-absorbent indicator layer enclosed by the shell and the membrane； and output means configured to provide an output. The water-absorbent indicator layer has a predetermined osmotic strength. The volume of at least one part of the indicator layer is variable in dependence on the water content of the indicator layer. The output is variable in dependence on the volume of the at least one part of the indicator layer.
