We run, we breathe, our hearts beat: our body is deforming continuously. You might wonder: how do the tissues in our body deal with all these mechanical stresses? Publishing today in Nature Physics, researchers from Wageningen University & Research (WUR) and AMOLF institute show how the two principal components of soft tissues, collagen and hyaluronic acid, work together to finely tune the mechanical response of our tissues.
This study advances our understanding on how biological matter precisely regulates its function by combining different components. Exploiting not only their individual properties, but also how these components interact, and thus opening the way for synthesizing novel polymeric materials.
Tweaking someone's ears
If you gently pull an earlobe, you will feel it is soft. However, if you are more insistent and apply more force, it will become very stiff. Not only the skin but most of the soft tissues in our body, including earlobes, muscles and cartilage inside our knees, have this extraordinary ability to drastically switch from soft to stiff when they are subjected to large deformation. This ability is crucial for biological functioning: when the tissue is soft, cells can move around. At the same time, the tissue has to protect the cells and should not break, therefore it becomes stiffer when the deformation becomes too large.
Collagen networks in the skin
The physical origin of this special mechanical behaviour is the particular structure formed by the collagen proteins: a sparse network. This was revealed in previous in-vitro studies, where networks of collagen, extracted from the skin of animals, were formed directly inside a rheometer, an instrument that allows researchers to measure the response of a material while deforming it.
Tissues are far more complex
“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 Soft Matter group led by prof. Jasper van der Gucht. “Exactly because of this complexity, real tissues are way more adaptive than the networks studied so far, made of only collagen. We were very excited to see the experimental results obtained at AMOLF by Federica Burla in the group of prof. Gijsje Koenderink. They systematically studied double networks where also the second most abundant component of tissues, hyaluronic acid, was present. Its presence significantly changed the mechanical response of the composite networks and we were eager to understand why.”
Stiffer with hyaluronic acid
“In contrast to the rigid collagen fibres, hyaluronic acid is a much smaller and very flexible polymer. And it is charged. Because of electrostatic interactions, a lot of stress is built up internally during the network formation. This stress becomes relevant when you deform the material, for instance when pulling on it. Firstly, the networks with a larger amount of hyaluronic acid are already stiffer at small deformation and secondly the switch to the even stiffer response occurs at a larger deformation,” explains Justin Tauber, PhD candidate in the same group.
“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, also the elasticity and the internal stress generated by the hyaluronic acid are crucial. 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.”
Researchers are now investigating when and how these networks fracture, in another biology-inspired study from which they might gain inspiration for tougher man-made materials.