An Osmotic Pressure Sensor for Monitoring the Level of Hydration in Biological Fluids

A miniaturized hydration sensor that records the electrolyte balance in biological fluids and that translates this quantity into an osmotic pressure signal has been developed. The sensor measures 16 mm $\times \,\, 8$ mm and integrates a semi-permeable membrane, a piezoresistive pressure transducer, and an electronic readout platform. The dynamic measurement range of 220–340 mOsm $\text{L}^{-1}$ (280 ± 60 mOsm $\text{L}^{-1})$ covers a serious state of dehydration to a serious state of overhydration (represented by a $\Delta \text {hydration} = \pm 20$ %). Ion exchange membranes were found to be the best candidates for integration with the hydration sensor. Reverse osmosis membranes with physical pores (zero Da molecular cutoff) had high ionic leakage rates, in contrast to gas permeable membranes, which proved to be almost impermeable to water vapor. Consequently, the use of a Nafion NR211 membrane facilitated a sensor response with a time constant of 2.5 h, which conforms to the slow change in osmotic pressure associated with hydration. The low-power front-end circuit architecture is based on an analog pressure-to-frequency converter, consuming only 39.4 $\mu \text{W}$ , and capable of recording changes in hydration with a resolution of 5.71 bits (ENOB $_{\mathrm {SNR}})$ . The small size of the sensor components, combined with the interface low-power architecture, renders it suitable for further miniaturization required for implantation into the human body.