THz Sensing of Human Skin: A Review of Skin Modeling Approaches

Abstract

The non-ionizing and non-invasive nature of THz radiation, combined with its high sensitivity to water, has made THz imaging and spectroscopy highly attractive for in vivo biomedical applications for many years. Among them, the skin is primarily investigated due to the short penetration depth of THz waves caused by the high attenuation by water in biological samples. However, a complete model of skin describing the THz–skin interaction is still needed. This is also fundamental to reveal the optical properties of the skin from the measured THz spectrum. It is crucial that the correct model is used, not just to ensure compatibility between different works, but more importantly to ensure the reliability of the data and conclusions. Therefore, in this review, we summarize the models applied to skin used in the THz regime, and we compare their adaptability, accuracy, and limitations. We show that most of the models attempt to extract the hydration profile inside the skin while there is also the anisotropic model that displays skin structural changes in the stratum corneum.

In Vivo Assessment and Monitoring of Burn Wounds Using a Handheld Terahertz Hyperspectral Scanner

“…[ 35 ] Depending on the frequency and the anatomical location, the penetration depth in healthy skin is between 0.1 and <1 mm. [ 41 ] We expect the penetration depth for burned skin to be comparable to that of the healthy skin. The thickness of the dermis layer is typically between 1 and 3 mm.…”

Monitoring the Effect of Transdermal Drug Delivery Patches on the Skin Using Terahertz Sensing

“…Finally, further work should be done to develop a model that can be used to investigate the effect of patch application on the diffusivity of water within the skin, such as that performed by Sun et al [26]. However, this would require more complex models, as the skin is often assumed to be made of water and a biological background; if drugs and excipients are entering the skin then the models would need to be adapted to account for this [39].…”

Terahertz (THz) biophotonics technology: Instrumentation, techniques, and biomedical applications

“…70,151 In our recent review, we show a large variation in the dehydrated properties from different studies, which could lead to a large difference in the extracted water concentration. 9 It should also be noted that applying EMTs is not the only way to extract the water concentration. Nuclear magnetic resonance (NMR) is another approach to evaluate the water content, 153 which has been used to verify the origin of contrast for THz images.…”

“…Several reviews related to the field of THz biophotonics have been published over the last ten years. There are topical reviews on THz cancer diagnosis by Yu et al, 5 Peng et al, 6 and Zaytsev et al 7 Reviews on skin include those by Nikitkina et al 8 and Wang et al, 9 and there are more general reviews on biomedical applications by Fan et al, 10 Yang et al, 11 and Sun et al 12 However, most of these articles have been focused on the application aspect or a specific topic. In this manuscript, we aim to provide a comprehensive overview of THz biophotonics, the scope of which is illustrated in Fig.…”