Table A1.
Studies for the measurement of skin hydration using optical techniques.
| Author(s) | Year | Aim | Major Findings | Measurement Technique |
|---|---|---|---|---|
| Ruini, C. [19] | 2022 | Authors conducted an in vivo examination into the effect of moisturisers on human skin measured using both confocal Raman spectroscopy and optical coherence tomography | A decrease in skin roughness, but an increase in epidermal thickness following moisturiser application. A positive correlation of water content was found in the treatment forearm via CRS measurements, in relation to the increase in epidermal thickness; however, a statistical significance was not conveyed. | Optical—Raman Spectroscopy |
| Kelman, Y. T. [20] | 2019 | Authors used optical tissue probing and analysis of temporal tracking of back-reflected secondary speckle patterns to detect human skin hydration | After the application of moisturiser, the optical signature displayed a significant decrease over time from the dry skin signature. The higher the level of moisture, the faster the acoustic wave faded. | Optical |
| Perkov, S. A. [21] | 2020 | Authors conducted studies on both gelatine tissue phantoms and human skin to investigate the monitoring of water content using optoacoustic methods | An evident second peak was discovered at a depth of 2 mm, which signifies that the signals originate from subcutaneous tissue. | Optoacoustic |
| Qassem, M. [13] | 2011 | Authors investigated the properties of the stratum corneum in the NIR region of the EM spectrum and visualised the water characteristics in this layer | In vivo experiments showed peaks at 1450 nm and 1920 nm as expected, with the intensity at a lower magnitude than that of water. A single fibre detector with a six-fibre source allowed for the most reliable readings. | Optical—NIRS |
| Kilpatrick-Liverman, L. T. [24] | 2006 | Authors determined differences in skin water content by measuring the absorption spectra using a fibre optic cable accessory connected to a NIR spectrophotometer | As the RH was decreased, lower water content was recorded in the skin. This result was established due to the area under the 1936 nm band being highest for skin that was equilibrated at 100% RH and lowest at 11% RH. Moisturizing products increased skin water content by increasing the sampling volume due to an increased beam penetration. | Optical—NIRS |
| Arimoto, H. [14] | 2004 | Authors conducted a study investigating measurements of non-contact skin moisture using NIR spectroscopy | Optical—NIRS | |
| Arimoto, H. [25] | 2015 | Authors conducted a study investigating the measurement of water content distribution in the skin | When skin was moist, the water content was shown to decrease from the skin surface to a depth of 5–10 µm. The water content at the skin surface was lower for the immediate recording. Beyond 5–10 µm, both water content recordings were almost identical. At a wavelength of 1300 nm, the pixel value was stagnant through 5 min after removal of the wet pad. At 1462 nm, half the participants presented an increase up to 1 min, then a subsequent plateau. At 1950 nm, all participants displayed an increase up to 1 min, then again, a subsequent plateau. | Optical—NIRS and Confocal Raman Spectroscopy |
| Qassem, M. [26] | 2013 | Authors assessed the optical properties of skin following direct water contact and application of moisturiser using a spectrophotometer with a fibre optic probe attachment | The resulting spectra as an average of all participants recorded prior to water contact or moisturiser application displayed higher peaks of bands near 1450 nm and 1780 nm. This response was similarly seen when comparing individuals who frequently moisturise and do not moisturise. | Optical—NIRS |
| Mamouei, M. [27] | 2020 | Authors designed and developed a multi-wavelength optical sensor to measure dermal water content | The developed sensor was able to present reliable measurements of the water content of skin. 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. | Optical—NIRS |