The level of hydration in the human body is carefully adjusted to control the electrolyte balance that governs the biochemical processes that sustain life. An electrolyte deficiency caused by de- or overhydration will not only limit human performance, but can also lead to serious health problems and death if left untreated. Since humans can withstand a change in hydration of only ± 20%, frequent monitoring should be performed in risk groups. This article presents an osmotic hydration sensor which can record the level of hydration as a function of osmotic pressure in phosphate buffered saline (PBS) or sodium-chloride solutions that simulate the interstitial fluid in the body. The osmotic pressure is recorded with the aid of an ion-exchange membrane that facilitates the migration of water and cations, in favor of reverse osmosis (RO) or gas separation membranes. The hydration sensor is designed to be coupled to an inductively powered read out circuit designed for integration in a micro-implant that has previously been shown to consume only 76µW of power. The dynamic range spans a state of serious overhydration (220 mOsm L-1) to a serious state of dehydration (340 mOsm L-1) with a response time of approximately 7 hours (for a variation of hydration of 20%).
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