4/2/2015 Skin tough: study at Advanced Light Source shows why skin is resistant to tearing

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ChemistryMaterials ScienceMarch 31, 2015

Skin tough: study at Advanced Light Source shows why skin is resistant to tearing

Mar 31, 2015 by Lynn Yarris

Enlarge

(Left) Collagen fibrils in the dermis of the skin are normally curvy and highly disordered, but (right) in response to a tear align themselveswith the tension axis (arrow) to resist further damage. Credit: Berkeley Lab

When weighing the pluses and minuses of your skin add this to the plus column: Your skin - like that of all vertebrates - is remarkably resistant to

tearing. Now, a collaboration of researchers at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) andthe University of California (UC) San Diego has shown why.

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Making good use of the X-ray beams at Berkeley Lab's Advanced Light Source (ALS), the collaboration made the first direct observations of themicro-scale mechanisms behind the ability of skin to resist tearing. They identified four specific mechanisms in collagen, the main structural protein in

skin tissue, that act synergistically to diminish the effects of stress.

"Collagen fibrils and fibers rotate, straighten, stretch and slide to carry load and reduce the stresses at the tip of any tear in the skin," says Robert

Ritchie of Berkeley Lab's Materials Sciences Division, co-leader of this study along with UC San Diego's Marc Meyers. "The movement of thecollagen acts to effectively diminish stress concentrations associated with any hole, notch or tear."

Ritchie and Meyers are the corresponding authors of a paper in Nature Communications that describes this study. The paper is titled "On the tear

resistance of skin." The other authors are Wen Yang, Vincent Sherman, Bernd Gludovatz, Eric Schaible and Polite Stewart.

Who among us does not pay close attention to the condition of our skin? While our main concern might be appearance, skin serves a multitude of vital

purposes including protection from the environment, temperature regulation and thermal energy collection. The skin also serves as a host forembedded sensors.

4/2/2015 Skin tough: study at Advanced Light Source shows why skin is resistant to tearing

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EnlargeSequence of images showing how skin when torn does not propagate but progressively yawns open under tensile loading. Credit:Berkeley Lab

Skin consists of three layers - the epidermis, dermis and endodermis. Mechanical properties are largely determined in the dermis, which is the thickestlayer and is made up primarily of collagen and elastin proteins. Collagen provides for mechanical resistance to extension, while elastin allows for

deformation in response to low strains.

Studies of the skin's mechanical properties date back to 1831 when a physician investigated stabbing wounds that the victim claimed were self-inflicted. This eventually led to scientists characterizing skin as a nonlinear-elastic material with low strain-rate sensitivity. In recent years research has

focused on collagen deformation but with little attention paid to tearing even though skin has superior tear-resistance to other natural materials.

"Our study is the first to model and directly observe in real time the micro-scale behavior of the collagen fibrils associated with the skin's remarkable

tear resistance," Ritchie says.

Ritchie, Meyers and their colleagues performed mechanical and structural characterizations during in situ tension-loading in the dermis. They did this

through a combination of images obtained via ultrahigh-resolution scanning and transmission electron microscopy, and small-angle X-ray scattering(SAXS) at ALS beamline 7.3.3.

"The method of SAXS that can be done at ALS beamline 7.3.3 is ideal for identifying strains in the collagen fibrils and watching how they move,"

Ritchie says.

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4/2/2015 Skin tough: study at Advanced Light Source shows why skin is resistant to tearing

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Enlarge

These images compare the reactions of skin and bone to strain. Credit: Berkeley Lab

The research team began their work by establishing that a tear in the skin does not propagate or induce fracture, as does damage to bone or toothdentin, which are comprised of mineralized collagen fibrils. What they demonstrated instead is that the tearing or notching of skin triggers structural

changes in the collagen fibrils of the dermis layer to reduce the stress concentration. Initially, these collagen fibrils are curvy and highly disordered.However, in response to a tear they rearrange themselves towards the tensile-loading direction, with rotation, straightening, stretching, sliding anddelamination prior to fracturing.

"The rotation mechanisms recruit collagen fibrils into alignment with the tension axis at which they are maximally strong or can accommodate shapechange," says Meyers. "Straightening and stretching allow the uptake of strain without much stress increase, and sliding allows more energy dissipationduring inelastic deformation. This reorganization of the fibrils is responsible for blunting the stress at the tips of tears and notches."

This new study of the skin is part of an on-going collaboration between Ritchie and Meyers to examine how nature achieves specific propertiesthrough architecture and structure.

"Natural inspiration is a powerful motivation to develop new synthetic materials with unique properties," Ritchie says. "For example, the mechanisticunderstanding we've identified in skin could be applied to the improvement of artificial skin, or to the development of thin film polymers for applicationssuch as flexible electronics."

Explore further: Making a better wound dressing—with fish skin

More information: Nature Communications , www.nature.com/ncomms/2015/150327/ncomms7649/full/ncomms7649.html

Journal reference: Nature Communications

Provided by Lawrence Berkeley National Laboratory

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