Human skin has evolved to achieve maximum durability, flexibility

BINGHAMTON, N.Y. — It takes a strict regimen of stretching to keep muscles and joints limber and flexible, but evolution has us “covered” when it comes to our skin. Scientists from Binghamton University say human skin has evolved to achieve “maximum” durability and flexibility.

This latest research covers both the overall structure of human skin and the maximum amount of damage it can sustain. Study authors created new membranes derived from polydimethylsiloxane (PDMS), which is an inert and nontoxic material used frequently in biomedical research. This approach allowed researchers to recreate or mimic the structure of mammalian skin by covering a soft, compliant layer with another thinner, stiffer outer later.

Next, the newly created “artificial skin” underwent numerous tests gauging how much stress it could take before breaking. Under the pressure of a sharp or blunt rod, the samples indented, forming huge divots before breaking. That’s not all, as the research team made another noteworthy discovery.

“There’s a certain structural formation that is optimal,” says Associate Professor of Biomedical Engineering Guy German in a university release.

“We found that when the artificial skin has the same outer (stratum corneum) and inner layer thickness (dermis) as mammalian skin, the rubber membranes maximized both their puncture toughness and deformability. We believe that mammalian skin has evolved or adapted itself to offer the toughest option to mechanical threats while also remaining as deformable as possible.”

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For context, the research team explains that most organisms on Earth have a tough outer layer that serves to protect a softer layer underneath from environmental threats. We’re not just talking about animals here; nuts, fruits, insects, and even microorganisms use this system of protection.

“Mammalian skin offers maximum locomotion and maximum mechanical toughness,” Prof. German adds. “If it went one way, it would be less flexible, or the other way you would get more flexibility but less toughness. So it’s optimized.”

Study authors also observed a new type of failure, called “coring.” Typically, if something punctures a material, the fracture starts below the indenter tip (like piercing a piece of paper with a pencil). However, when it comes to hyper-elastic, two-layered materials like human skin or the artificial skin membranes used for this project, the fracture appears far from the indenter tip at large indentation depths. The rupture appears where the membrane is most stretched, on the sides of the divot, consequently leaving a cylindrical core in the membrane. As far as researchers can tell, scientists have never documented this phenomenon before.

Study authors explain that a more comprehensive understanding of our skin, and artificial skin, will prove useful across a multitude of industries and technologies – from flexible electronics and medical devices to product packaging, bulletproof vests, and even burn treatments.

“Scientists and engineers are attracted to studying skin because it’s difficult to understand. Skin is heterogeneous and structurally very complex,” Prof. German concludes.

“Traditional materials like steel and cement are uniform in composition and easy to characterize. Nowadays, engineers are using their computational know-how to study really complex materials such as skin.”

The study appears in the journal Soft Matter.

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