Non-Invasive in Vivo Quantification of Directional Dependent Variation in Mechanical Properties for Human Skin

Abstract

Skin is the body’s largest organ, and it shows non-linear and anisotropic behavior under the deformation. This behavior of the skin is due to the waviness and preferred orientation (in a particular direction) of collagen fibers. This preferred orientation of collagen fibers results in natural pre-tension and anisotropy of the skin. The knowledge of natural skin pre-tension and anisotropy is essential during incisions and surgery. The available suction-based devices quantify the anisotropy through the displacement field and cannot measure the stress-strain relation in particular directions. Therefore, in the current study, an in vivo full-field measurement suction apparatus was developed to measure the stress and strain of skin in all planar directions through a single experiment. First, this apparatus was tested on silicone substrates of known properties, and then it was used to test the skin of 12 human forearms. Further, to check the effect of hand stability on the measurements, the obtained results of the skin were compared with the results of a standard test performed in the same skin using a steady setup. The consistency between these two results confirms that the stability of the hand does not influence the measurements of skin properties. Furthermore, using the developed apparatus, the skin’s anisotropy and its relation with the Kraissl’s lines orientation was quantified by measuring the toe and linear moduli at an interval of one degree. The minimum and maximum values of the toe and linear moduli were 0.52 ± 0.09 and 0.59 ± 0.11 MPa, and 3.09 ± 0.47 and 5.52 ± 1.13 MPa, respectively. Also, the direction of maximum moduli was found almost similar to Kraissl’s lines’ orientation. These results confirm the contribution of skin pre-tension on the anisotropy of the skin. The present apparatus mimics the tissue expansion procedure, where observation of the test may be helpful in the selection of size and shape of the expander.

Crosslinked Collagenic Scaffold Behavior Evaluation by Physico-Chemical, Mechanical and Biological Assessments in an In Vitro Microenvironment

“…Compared with intact human skin, the mechanical and visco-elastic behavior of the studied scaffold registers modest values (with at least ten-fold less stress accommodated at the same applied strain) [54]. However, the role of the collagenous scaffold is not to directly replace the skin, but to progressively substitute it, by facilitating the healing processes in which is involved.…”

Effect of collagen fibre orientation on the Poisson's ratio and stress relaxation of skin: an ex vivo and in vivo study

“…Despite the new results presented related to the skin tissue, there are numerous limitations associated with this study, First, all the calculations performed in this study are based on the linear elastic theory; however, the skin tissue shows nonlinear elastic response during the stretching [ 62 , 63 ]. The assumption of linear elastic behaviour of skin may have induced some error in the presented results of this study.…”

Bilayer Stiffness Identification of Soft Tissues by Suction

“…The amount of tissue aspirated is related to tissue stiffness. The height of the aspirated tissue is generally estimated by ultrasound methods [23][24][25][26], mechanical stops [27,28], optical coherence tomography [26] or cameras usually associated with mirrors or prisms [29][30][31][32][33][34][35][36].…”

“…Additionally, as far as the authors are aware of, only the methods proposed in [12,28,36] were tested on reference phantoms to be validated; all other studies directly applied the proposed methods directly in vivo to human tissue for which mechanical properties were complex and unknown. Implementing a validation on multi-layered phantoms made from known reference materials is a tedious task that yet seems necessary prior to in vivo application.…”