Thirsty for a Noninvasive Wearable to Detect Dehydration: A Review

In this article, the significance, clinical needs, methods and principles for continuous human body hydration level monitoring are reviewed. Maintaining hydrated is critical for physical and mental health. A long-term, continuous, convenient and accurate wearable device that can monitor the whole-body water content to provide real-time feedback is needed, especially for infants, the elderly, and people who have special needs in their tasks and occupations. Noninvasive bioimpedance measurements and microwave sensing techniques suitable for conformable wearables are summarized. Microwave probing with deeper field penetrations into the dermis layer of the skin can provide a better assessment of whole-body water content. Frequency choices for the probing signals, the need for realistic phantoms, and evaluation standards suitable for clinical uses are discussed.