The Histological and Mechanical Behavior of Skin During Puncture for Different Impactor Sizes and Loading Rates

The hierarchical structure of skin dictates its protective function against mechanical loading, which has been extensively studied through numerous experiments. Viscoelasticity and anisotropy have been defined for skin in tensile loading, but most puncture studies utilized skin simulants, which lacked natural tension and varying skin thicknesses. The purpose of this study was to define the mechanical behavior and failure thresholds of skin during puncture with various blunt impactor sizes and loading rates.

After determining natural tension of porcine skin, 232 isolated skin samples were loaded in puncture. Pre-conditioning, sub-failure, and failure trials were conducted with an electrohydraulic piston actuator loading pre-strained skin samples with a 3-, 5-, or 8-mm spherical impactor at rates of 5 to 1000 mm/s. Generalized linear mixed models were used to determine significant factors and predict probability of puncture.

Increased skin thickness significantly increased RIII stiffness (p = 0.002), failure force (p < 0.001), and strain energy at failure (p = 0.002) and significantly decreased displacement at failure (p = 0.002). Significantly greater force, displacement, strain energy, and stiffness (p < 0.05) at failure were observed with the 8-mm impactor. Loading at 1000 mm/s resulted in significantly greater force (p = 0.026) and stiffness (p < 0.001) at failure compared to 5 mm/s and significantly decreased displacement at failure (p < 0.001). 3D-DIC strain maps displayed anisotropic behavior, and larger elliptical wounds resulted from puncture with an 8 mm impactor (p < 0.001). Quantitative histological analyses revealed collagen re-alignment near the impactor from pre-conditioning and minimal structural damage during sub-failure trials. Initial structural failure occurred in the reticular dermis followed by the papillary dermis and epidermis.

The presented failure metrics, with support from histological findings, may be utilized in development of protective clothing, improvement of computational models, and advancement in forensic sciences.