Development and validation of a low-cost, wearable near-infrared spectroscopy system for quantifying tissue water fraction in the human brain

Cerebral water fraction is physiologically relevant to brain fluid dynamics, glymphatic function, cerebral edema, and dehydration, yet it is not routinely measured in wearable neuroimaging.

We aimed to develop and validate a low-cost, wearable, light-emitting diode (LED)-based continuous-wave near-infrared spectroscopy system capable of concurrently monitoring changes in cerebral hemoglobin concentration and water fraction from the forehead.

Our device, called the GS-FlexNIRS, contains nine LEDs spanning 740 to 1050 nm mounted on a flexible printed circuit board. Three of these wavelengths (i.e., 910, 940, and 1050 nm) fall within the water absorption window. A wavelength-weighted fitting algorithm incorporates measured LED emission spectra and photodiode responsivity to enable accurate chromophore recovery despite spectrally broad LED illumination. Performance was characterized in liquid phantoms and in five healthy participants during respiratory maneuvers.

The system achieved a dynamic range of $\sim 100\mathrm{dB}$ , gain linearity ( $R^2>0.99997$ ), and signal-to-noise ratio of 67 to 90 dB. Phantom experiments confirmed independent tracking of hemoglobin deoxygenation and water fraction changes. Hemoglobin responses in human measurements were consistent with prior work.

The GS-FlexNIRS ( $\sim \$350/\text{unit}$ ) and its wearable design may support future longitudinal and naturalistic monitoring of cerebral hemodynamics and brain fluid dynamics, enabling investigation in sleep, glymphatic research, and cerebral edema monitoring.