Our tissues have to handle stress and deformation every day. New research is looking into how it copes so well.

Elephant skin.

Image via Pixabay.

We tend to take our bodies for granted but even the most mundane of actions — walking around, breathing — exerts mechanical stress and subsequent deformation of it. And yet, day after day, our tissues take the brunt of it with grace and even heal themselves without any conscious effort on our part.

Curious to see how it pulls off such a feat, researchers from the Wageningen University & Research (WUR) and the AMOLF Institute found that two components in soft tissue, collagen and hyaluronic acid, work together to underpin how tissues respond to mechanical stress.

Feeling stressed

Pull on your earlobe and it will be soft and flexible. Keep pulling, increase the force you apply, and it will become progressively stiffer. It’s not just your earlobes — skin, muscles, the cartilage in your joints and ribs behave the same way.

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They’re designed to work like that. While soft, such tissues are easily traversed by cells — but they also need to provide mechanical protection to underlying structures, meaning it has to be tough too, and hard to break. Finally, it’s vital that such tissue is able to transition between the two states.

Needless to say, that’s quite a list of demands. But, our soft tissues rise to the challenge through the use of collagen, hyaluronic acid, and the interactions between the two, a new study reports. The findings not only help us better understand how our bodies function, but may also point the way towards new, synthetic polymers that mimic these impressive properties.

It’s the particular way in which collagen proteins order themselves in soft tissues that give them their resilience. They arrange in a structure known as a sparse network (meaning the proteins don’t form the maximum possible number of bonds between each other). Previous research has gauged the physical resilience of this sparse network in in-vitro conditions: collagen networks were extracted from samples of animal skin and reformed inside a rheometer, an instrument which measures the response of a material during deformation. Such efforts, the team explains, only capture part of the image, however.

“Real tissues are far more complex: they are composed of different molecules that have different sizes and interact with each other in still unknown ways,” says Simone Dussi, postdoc in the WUR Physical Chemistry and one of the study’s corresponding authors. “Because of this complexity, real tissues are way more adaptive than the networks studied so far, made of only collagen.”

“[The presence of hyaluronic acid] significantly changed the mechanical response of the composite networks and we were eager to understand why.”

The team reports that, unlike collagen, hyaluronic acid — a polymer made of much smaller and more flexible molecules — is electromagnetically charged. Because of this, electrostatic interactions generate stress between its individual blocks which accumulates as the tissue is subjected to deformation. According to the team, this buildup of stress basically opposes the deformation.

Networks with a larger amount of hyaluronic acid are “already stiffer at small deformation[s]”, explains co-author Justin Tauber. They also become stiffer in response to larger deformation than networks poor in the acid, the adds.

“We managed to construct a theoretical model and performed computer simulations that matched the experimental results. The key ingredients were identified: In addition to the network structure and the bending rigidity of the collagen fibres, the elasticity and the internal stress generated by the hyaluronic acid are crucial,” Tauber explains.

“The model allows us to make a step further in understanding how real tissues exploit the balance of all these effects. In addition, our findings can be translated into material science to create novel synthetic polymeric materials with more tunable properties.”

The paper “Stress management in composite biopolymer networks” has been published in the journal Nature.