(A-40) Histological and Mechanical Analysis of Porcine Skin during Puncture Loading

Introduction:: Understanding the structure-function relationship of skin is the foundation for applications in forensic science, plastic surgery, cosmetics, and protective clothing design. Porcine skin has been accepted as a surrogate for human skin for mechanical experimentation, but variation in skin thickness and pre-conditioning may affect properties. Tension and indentation studies have classified skin as hyperelastic and viscoelastic [1]. However, a rate-dependent hyperelastic constitutive model of skin has not been established for loading in puncture. The objective of this study was to investigate the biological structure and mechanical response of porcine skin from pre-conditioning and puncture loading. Isolated skin testing provides mechanical data for material characterization, which may be used to develop computational models of puncture and ballistic impact.

Materials and Methods:: Frozen (-40°) porcine tissue was thawed at 4°C for two days. Full-thickness skin samples (n=94) of diameter 98.5 ± 6.9 mm were excised and a circle of diameter 25.0 ± 0.5 mm was stamped on the skin during its relaxed state. The sample was bi-axially stretched with a custom pulley apparatus (250 g on four sides) to simulate natural, anisotropic tension of skin, and pre-strain was calculated in the major (29.3 ± 14.6 %) and minor (12.9 ± 9.9 %) axes of the resultant ellipse using percent difference of circle diameter. Surface temperature (20.0 ± 1.3 °C), sample mass (48.1 ± 12.8 g), cutis thickness (2.15 ± 0.50 mm), and total thickness (4.49 ± 1.68 mm) were measured. An electrohydraulic piston actuator with a custom 5 mm stainless steel sphere impactor was fastened to the in-line load cell. With constant bi-axial pre-tension, skin was preconditioned to a displacement of 15 mm at 5 mm/s while unclamped for eight cycles. Strain energy, hysteresis, and reduced elastic modulus were calculated. After mechanical testing, skin samples were fixed, processed, and stained with hematoxylin and eosin (H&E). Epidermal thickness and qualitative differences between 6 samples were analyzed.

Results, Conclusions, and Discussions:: For the first cycle of pre-conditioning, strain energy and hysteresis were 29.0 ± 17.1 J and 50.7 ± 29.8 J, respectively. For the eighth cycle of pre-conditioning, the strain energy and hysteresis were 15.5 ± 9.3 J and 34.1 ± 20.1 J, respectively (46.5 ± 10.0 % hysteresis reduction from cycle 1). Eight pre-conditioning cycles were sufficient since hysteresis minimally reduced from cycle 7 to 8 (Figure 1). Mean maximum force and reduced elastic modulus were 10.93 ± 6.67 N and 548 ± 307 kPa, respectively. Cutis thickness was linearly correlated with maximum force (p< 0.001, R²=0.81) and elastic modulus (p< 0.001, R²=0.83). Indentation literature reported maximum forces and elastic moduli of 0.2 N and 1–15 kPa in human skin, respectively [1,2]. Since forces were two orders of magnitude higher in this study, we can expect a similarly increased response in elastic modulus. From the histological analysis, mean porcine epidermal thickness was 35.9 ± 9.5 μm, which was consistent compared to mean human epidermal thickness of 36–63 μm [3]. Porcine cutis thickness (2.15 ± 0.50 mm) was consistent with reported human cutis thickness (1.5–3 mm) [3]. Collagen fibers appeared more aligned after pre-conditioning, and failure resulted in detachment of the stratum corneum and increased gaps between fiber families (Figure 2), which has also been observed after bulge testing of porcine skin [4].

For material characterization of skin, applied tension and pre-conditioning are essential for accurate constitutive modeling. Based on histological validation, in vitro skin should be pre-conditioned to reduce hysteresis and may be used as an acceptable surrogate to human skin for loading in puncture. Computational models will evaluate the effect of impactor shape and loading rate on failure thresholds.


Acknowledgements (Optional): : This work was partially supported by DEVCOM ARL and VA Medical Center Research.


References (Optional): : [1] Mostafavi Yazdi, S. J., & Baqersad, J. J Biomech 2022, 130.
[2] Zahouani, H., et al., Skin Res Tech. 2009, 15(1), 68–76.
[3] Todo, H. Pharmaceutics 2017, 9(3).
[4] Lakhani, P., et al., J Mech Behav Biomed Mat 2020, 104.