Biofidelic Conductive Synthetic Skin Composites

Abstract

Skin is the first point of contact of the human body with the outer environment, and influences the biomechanics of different organ systems in normal and diseased states. Wearable electronics such as fitness tracking equipment, motion sensing devices, and advanced wearables in prosthetics and orthotics are often used to quantify the interaction of the body with the environment during different physical activities, and improve health. These wearable equipment can be bulky and a source of discomfort to the human skin with prolonged wear. To date, very few flexible polymers have been developed which can conduct electricity and be used in wearable devices. In the current work, a novel conductive synthetic skin composite system was developed, which would be indispensable for integration into wearable technologies, and also allow the biomechanical testing of the human skin for different engineering and medical applications. The mechanical behavior of this polymer can be tuned to mimic the human skin from different locations of the body with varying stiffnesses, with a phenomenal degree of accuracy. The composite system is composed of short carbon fibers dispersed in a multi part silicone based matrix material. The volume fraction of the fibers were varied to control the mechanical and electrical properties of the composite. Uniaxial tensile tests were conducted to generate stress versus strain responses of the synthetic skin composites at different fiber volume fractions, and electrical measurements were recorded at different strains. Microscopy was used to understand composite fiber orientations in unstretched and stretched states, and its effects on the electrical conductivity of the material. Additionally, non-linear material characterization models were developed to characterize the composite variants. To the best of our knowledge, such an accurate synthetic skin composite system with tailorable electrical properties has not been developed; making this state of the art in bio mimicking and functionalization of the human skin.

Tissue Anisotropy Modeling Using Soft Composite Materials

“…Similarly, Humphrey model parameters ( c 1 = ±3.7 E − 4 to 2 E − 2, c 2 = ±1 E − 3 to 5.5 E − 1) and Veronda-Westmann model parameters ( c 1 = ±1 E − 4 to 3.8 E − 3, c 2 = ±1.8 E − 1 to 3.8 E − 1) were also higher than the literature values (Humphrey: c 1 = ±3 E − 4 to 1.2 E − 2, c 2 = ±1 E − 3 to 5 E − 1, and Veronda-Westmann: c 1 = ±5 E − 5 to 3 E − 3, c 2 = ±1 E − 1 to 2.5 E − 1) [ 22 , 26 , 34 ]. In other literature testing with skin and pelvic tissues, and elastomers with representative stiffnesses, higher Veronda-Westmann parameter values of c 1 (±0 to 11.8 E − 3) and c 2 (±1 E − 1 to 5 E − 1) were reported [ 18 , 21 , 27 , 28 , 34 , 35 ], which is expected as the current composite mechanical properties were inferior to skin and close to pelvic tissues (discussed in Section 3.5 ). It should be mentioned here though that in future studies, using transversely isotropic models in place of isotropic models used in the current work may improve the accuracy of hyperelastic modelling results.…”

Human Skin-Like Composite Materials for Blast Induced Injury Mitigation

“…Hyperelastic curve-fit formulations such as the Fung model, Mooney-Rivlin's model, and Yeoh's model are used often to characterize the load response of soft materials [38][39][40][41]. Hyperelastic constitutive equations are based on the definition of the material specific strain-energy density functions (symbolically written as ψ) [42][43][44][45][46][47][48]. The strain energy density function typically depends on the principal stretches (λ i , i = 1, 2 and 3) along the Cartesian coordinate axes, or the invariants (I i , i = 1, 2 and 3) of the Cauchy-green tensor, which are the functions of the principal stretches (see Equation 1).…”

“…The elasticity modulus of Kevlar 129 fiber material [49,50], human skin, muscle, and bone were obtained from recent literature [31,47,48,[51][52][53][54][55][56], and are listed in Table 1. The composite material changes due to varying fiber volume fractions [57] have been modeled within the FE system, using different fiber cross-sections and lengths, and not by modifying the global material property of the composite using the rule of mixtures.…”

Biomechanical Modeling of Wounded Skin

“…Two-part silicone material with a shore hardness of 30A (Mold Star 30 from Smooth-On, Inc., Macungie, PA, USA) was mixed in a 1:1 ratio by weight to fabricate biofidelic human skin phantoms [19,[24][25][26][27][28][29][30][31]. Ten test coupons were generated with 50 mm length, 9 mm width and 2.5 mm depth.…”