Design and Analysis of a Continuous and Non-Invasive Multi-Wavelength Optical Sensor for Measurement of Dermal Water Content

Abstract

Dermal water content is an important biophysical parameter in preserving skin integrity and preventing skin damage. Traditional electrical-based and open-chamber evaporimeters have several well-known limitations. In particular, such devices are costly, sizeable, and only provide arbitrary outputs. They also do not permit continuous and non-invasive monitoring of dermal water content, which can be beneficial for various consumer, clinical, and cosmetic purposes. We report here on the design and development of a digital multi-wavelength optical sensor that performs continuous and non-invasive measurement of dermal water content. In silico investigation on porcine skin was carried out using the Monte Carlo modeling strategy to evaluate the feasibility and characterize the sensor. Subsequently, an in vitro experiment was carried out to evaluate the performance of the sensor and benchmark its accuracy against a high-end, broad band spectrophotometer. Reference measurements were made against gravimetric analysis. The results demonstrate that the developed sensor can deliver accurate, continuous, and non-invasive measurement of skin hydration through measurement of dermal water content. Remarkably, the novel design of the sensor exceeded the performance of the high-end spectrophotometer due to the important denoising effects of temporal averaging. The authors believe, in addition to wellbeing and skin health monitoring, the designed sensor can particularly facilitate disease management in patients presenting diabetes mellitus, hypothyroidism, malnutrition, and atopic dermatitis.

Review of Advances in the Measurement of Skin Hydration Based on Sensing of Optical and Electrical Tissue Properties

“…There has also been an increasing investigation into the use of optical techniques for the measurement of water bands within the absorption spectra of the skin. Research has found that the use of an optical sensor that consists of multiple wavelengths within the near-infrared region exceeds the prediction accuracy of a standard NIR spectrophotometer [ 13 , 28 ]. It expresses an increased sensitivity to variations in skin water content whilst being concurrent with the absorbance values obtained from the spectrophotometer.…”

“…A further study was conducted using the same developed optical hydration sensor [ 28 ]. An in vitro experiment was performed on porcine skin to compare the developed sensor with a spectrophotometer to determine its performance and efficacy, with the reference being an electronic precision balance for gravimetric measurements.…”

“…In vitro tests on porcine skin illustrated a descending trend showing water loss, predominantly with absorbance in the 1450 nm region. Optical—NIRS Mamouei, M. [ 28 ] 2021 Authors designed and developed a multi-wavelength optical sensor to measure dermal water content A high agreement between the absorbance results from the developed hydration sensor and the spectrophotometer, with both weight and absorption presenting a decrease as water content diminished. Optical measurements at the 1450 nm band displayed a higher sensitivity to water content variations.…”

Pilot Study of a Wearable Hydration Monitor in Haemodialysis Patients: Haemodialysis Outcomes & Patient Empowerment Study 02

“…Molecules within tissue have known absorption coefficients (i.e., light is absorbed by these molecules at a specific wavelength) [20]. Near-infrared red light has several water absorption bands near 970 nm, 1,200 nm, 1,900 nm, and 1,450 nm, meaning that optical sensors have the potential to track dermal water [21,22]. While this has been proven using benchtop optical sensors in vivo and vitro studies [21][22][23][24], it is unknown whether near-infrared red lightemitting diodes (LEDs) can be sufficiently engineered to detect changes in interstitial fluid, within the limitations of a wearable device.…”

“…Near-infrared red light has several water absorption bands near 970 nm, 1,200 nm, 1,900 nm, and 1,450 nm, meaning that optical sensors have the potential to track dermal water [21,22]. While this has been proven using benchtop optical sensors in vivo and vitro studies [21][22][23][24], it is unknown whether near-infrared red lightemitting diodes (LEDs) can be sufficiently engineered to detect changes in interstitial fluid, within the limitations of a wearable device. Several companies have created wristworn devices that claim to measure interstitial volume changes by assessing infrared light absorption subdermally [16,25].…”