Biomechanics of Human Plantar Skin: Experimental and Constitutive Analysis

Purpose:

Plantar skin is a highly specialised tissue which protects the foot from injuries and adapts to external stresses. However, it can be subjected to diabetic plantar ulcers, which are among the most difficult and costly wounds to treat. Although this is a crucial topic, few studies have focused on the mechanical properties of foot skin and how disease alters them. In this context, this work aims to fully describe the mechanical behavior of plantar skin through experiments and constitutive analysis.

Methods:

Different experimental tests (failure tensile tests, unconfined compression at different strain rates, stress relaxation tests) were conducted on human plantar skin samples cut along the posterior-anterior (PA), lateral-medial (LM), and cranial-caudal (CC) directions. Then, experimental results were used to identify, through an inverse analysis, the parameters of the anisotropic visco-hyperelastic constitutive model adopted to describe the skin’s mechanical response.

Mechanical tests were performed with the Biomomentum testing machine (Model Mach-1 v500csst, ©Biomomentum Inc.) using the uniaxial load cell with a capacity of 250 N and resolution of 0.0125 N. The dog-bone shaped samples were clamped by C-shaped tensile grips with textured pattern surfaces. To avoid axial slippage, ensuring effective load transmission along the loading direction, each end of the samples was interposed and glued (Glue Loctite 4013 Prism, Hisco, Houston, TX, USA) between two small Velcro strips [26, 27]. For uniaxial compression tests, the testing machine was equipped with a stainless steel 316L flat indenter, directly screwed into the load cell, with a diameter of 31.75 mm, and with a stainless steel 316L plate above where the sample was positioned for the test. This condition ensures minimal friction at the indenter–specimen interface, allowing lateral expansion and approximating a predominantly uniaxial stress state. Before the mechanical tests, a contact criterion of 0.1 N was applied to each specimen to prevent slack or misalignment of the sample (for tensile tests) and to ensure contact between the flat indenter and the specimen (for compression tests). Following this step, all samples underwent a preconditioning phase consisting of 10 cycles of sinusoidal displacement, with a total applied strain of 5% and a frequency of 1 Hz. The number of preconditioning cycles was selected based on previous studies of collagen-rich soft connective tissues comparable to skin [28, 29], as well as on the analysis of the force–time response during preconditioning, which was shown to be stable after the seventh cycle [30]. This phase is crucial because it stabilises tissue behaviour and reduces the influence of the plantar skin’s unknown prior loading history by inducing progressive microstructural changes through the preconditioning strain. Then, the tissue was allowed to recover during a resting phase of 30 s before starting the effective test.

Results:

Plantar skin’s non-linear, anisotropic, and time-dependent behavior, with differences between the anterior and posterior foot’s regions. In addition, the constitutive model adopted is able to capture the mechanical behavior of the plantar skin Failure tensile tests showed that PA directions exhibited higher elastic modulus than LM directions in both posterior (22.05 vs 12.91 MPa) and anterior (17.39 vs 12.82 MPa) regions, while the unconfined compression tests revealed that compressive elastic moduli in the posterior region increased with increasing strain rates.

The proposed model provides new insights into the mechanics of plantar skin, being a valuable tool for applications such as diagnosing skin diseases and developing skin substitutes.

Keywords:

Constitutive model; Ex vivo analysis; Foot; Human plantar skin; Mechanical tests.