It might not look much like armour, but the skin, hair, glands, nerves and sometimes a tough keratinized layer are the structures that protect animals, including ourselves, against external damage. These thin layers protect us from a wide variety of threats, including puncture wounds and bites that might cause damage. Bingyang Zhang from the University of Illinois Urbana-Champaign, USA, and colleagues were curious how this collection of very thin and soft layers, sometimes only 0.1 mm thick, protect us against punctures.
To test how a structure composed of thin layers can protect us from damage, the team used pig chin skin, with the attached fatty layer beneath, as well as artificial skin – made of two layers of elastic silicone. By using artificial skin with different layer properties and thicknesses, the team could test the impact of the thickness of each of the two layers on the skin's fragility to identify how each layer contributes to minimizing damage. They fired a conical shaped spike, simulating the effect of teeth, nails or a rose thorn, at the real and simulated skin samples at different speeds while filming the spike with highspeed video (10,000–20,000 frames s−1). They then collected detailed 3D microscope images of the area where the damage occurred, the so-called fracture surfaces. From these images, they obtained the depth of the puncture wound, as well as hand-measuring the depth directly.
The team found that the different layers in the natural and artificial skin samples behaved differently as they drove the conical spike into the samples. Impressively, the pig skin, complete with fatty layer beneath, was far more resistant to the spike than the artificial silicone skin samples. The second layer of silicone was often damaged at impact speeds that failed to penetrate the fat beneath the pig skin. The team suggests that this occurs because of unique fibrous microstructures in natural skin, which provide extra strength and flexibility, compared with the artificial skin samples. And when the team compared the damage to the artificial skin with that caused to the pig skin, the outer and deeper layers of the artificial skin exhibited greater puncture damage. The researchers suspect that the pig skin is tougher than the artificial skin, thanks to the skin's natural elasticity and the cushioning effect of the fat beneath. The elasticity of natural skin allows animals to be protected against a broad variety of sharp objects, whether they penetrate slowly or in the blink of an eye.
This study highlights that mammalian skin has the unique ability to respond when stabbed or bitten, something researchers currently cannot recreate with artificial skin. However, the team shows that artificial skin with a stiffer thin and rough outer layer reduces the damage incurred. This suggests that the unique interplay between a pair of artificial skin layers could play a key role in damage control. Learning how animal skin is so tough, while still being flexible, could help us to develop pig skin-inspired materials for puncture-proof tyres or even rip-resistant clothing, that would keep you dry and safe from thorn scratches while searching at night for kangaroo rats in California's Mojave desert, from my own experience of field work!
