Mechanical mesoscopic properties monitoring of skin explants regarding the age and the stratum corneum integrity

The monitoring of mechanical properties of the skin is particularly interesting in the field of cosmetics, as it could lead to the validation of product allegations. For example, the impact of an active substance on the skin viscoelasticity could explain its tensing properties. Skin constitutes a complex multilayered material which can be considered as a viscoelastic soft tissue. The uppermost layers range from the micrometer to millimeter thickness, and are composed of either connected viscoelastic cells, or fibrous structure immersed into a ground substance. Usual techniques to characterize skin mechanical properties at macroscopic scale are based on indentation, torsion or suction devices [1-3]. At mesoscopic and microscopic scales, tomography and elastography are preferred [4-6]. The monitoring of the mechanical properties, at mesoscopic scale and including skin multilayer organization, is still an issue. This work proposes another method to monitor and characterize skin mesoscopic viscoelastic properties based on a biosensor consisting of a Thickness Shear Mode transducer on which a three-dimensional ex vivo skin model (named skin explant) is placed [7]. The monitoring of the explant is carried out over 8 days using a specific set-up to ensure renewal of life-support fluid. Viscoelastic parameters evolutions are obtained from a multifrequency impedance measurement ranging from 5 MHz to 45 MHz. The technique has been tested with 18 abdominal skin explants with donors aged between 23 and 57 years old. In addition, 4 stripped explants have been tested to mimic a weakened stratum corneum. With a dedicated fractional calculus model, two relevant parameters have been identified: the apparent structural parameter and the apparent viscosity. The evolutions of these parameters seem to be in accordance with the age of the donor. Furthermore, the shear wave wavelength helps to discriminate between skin uppermost layers. Finally, the identified parameters highlight premature degradation of stripped explants. [1]. Joodaki, H.; Panzer, M.B. Skin Mechanical Properties and Modeling: A Review. https://doi.org/10.1177/0954411918759801 2018, 232, 323–343, doi:10.1177/0954411918759801. [2]. Hendriks, F.M.; Brokken, D.; Oomens, C.W.J.; Baaijens, F.P.T. Influence of Hydration and Experimental Length Scale on the Mechanical Response of Human Skin in Vivo, Using Optical Coherence Tomography. Skin Research and Technology 2004, 10, 231–241, doi:10.1111/J.1600- 0846.2004.00077.X. [3]. Karimi, A.; Haghighatnama, M.; Shojaei, A.; Navidbakhsh, M.; Haghi, A.M.; Sadati, S.J.A. Measurement of the Viscoelastic Mechanical Properties of the Skin Tissue under Uniaxial Loading. http://dx.doi.org/10.1177/1464420715575169 2015, 230, 418–425, doi:10.1177/1464420715575169. [4]. Waigh, T.A. Advances in the Microrheology of Complex Fluids. Reports on Progress in Physics 2016, 79, 074601, doi:10.1088/0034-4885/79/7/074601. [5]. Kearney, S.P.; Khan, A.; Dai, Z.; Royston, T.J. Dynamic Viscoelastic Models of Human Skin Using Optical Elastography. Phys Med Biol 2015, 60, 6975, doi:10.1088/0031-9155/60/17/6975. [6]. Li, C.; Guan, G.; Wang, R.; Huang, Z. Mechanical Characterization of Skin Using Surface Acoustic Waves. Imaging in Dermatology 2016, 327–340, doi:10.1016/B978-0-12-802838-4.00023- 6. [7]. Gauthier, V.; Lemarquand, A.; Caplain, E.; Wilkie-Chancellier, N.; Serfaty, S. Ultrasonic Microrheology for Ex Vivo Skin Explants Monitoring: A Proof of Concept. Biosens Bioelectron 2022, 198, doi:10.1016/j.bios.2021.113831.