Why biological tissues are pliable and tough
ETH engineers have discovered that soft biological tissues deform very differently under tensile stress than previously assumed. Their research findings are already flowing into medical research projects, such as growing skin replacements for burn victims better and faster.
In the womb, the unborn child swims in a container bulging with amniotic fluid. Amniotic sac. The fact that it remains intact is very important for the smooth development of the baby. But it can happen that the protective covering tears after interventions such as amniocentesis and surgery, or quite spontaneously.
Stretched tissue loses volume
Based on such medical problems, researchers in the group of Edoardo Mazza, professor at the Institute of Mechanical Systems at ETH Zurich, have studied how parts of the amniotic sac and other biological Fabric deform under tensile load. One of their most important and surprising results: The tissues lose mass when stretched - about 50 percent on average at a physiological stretch of 10 percent. "This contradicts the previously accepted paradigm, according to which such soft biological tissues can deform strongly, but their volume remains unchanged in the process," explains biomechanist Mazza. Using measurements on tissue samples, his group was able to show that the loss of volume is due to the fact that fluid stored in the tissue between cells and collagen fibers escapes from the stretched area.
Interplay of mechanics and chemistry
Alexander Ehret, team leader in Mazza's group, was able to elucidate the mechanism behind this together with his team and with the help of extensive computer simulations. The basis for this is the alignment of the Collagen fibers in the fabric. The fibers form a kind of three-dimensional mesh in which they run in a plane in all cardinal directions, with only slight deviations upwards and downwards. If one pulls on this mesh, all the collagen fibrils, which lie more or less in the direction of pull, move closer together in a kind of scissors movement and press the fluid out of the tissue. The fibers remain undamaged, as they are mainly shifted into one plane and at most slightly stretched.
The loss of volume is reversible. When the tissue relaxes again, it reabsorbs water from the surrounding tissue. "The reason for this are macromolecules with negative charges that are firmly attached to the collagen fibers," Mazza explains. They cause the water to flow back into the tissue according to the principles of osmosis. The process can easily be repeated many times in experiments.
Direct applications in medicine
But Mazza and Ehret weren't just interested in understanding how tissue behaves under tensile stress. "We are engineers," says Mazza. And as such, they prefer to work on practical solutions in real life. The new findings therefore flow directly into concrete medical issues. For example, in "tissue engineering," the artificial production of biological tissueswhich are intended to regenerate or replace damaged tissue in patients. Based on the new findings, the researchers would like to focus primarily on the carrier materials on which these tissues thrive. "Our goal is to create the most physiological conditions possible for the artificial tissues, in other words, to imitate nature as closely as possible," Mazza explains. He and his colleagues are convinced that cells in the growing tissue receive signals from the carrier material that play an important role in the later properties of the replacement tissue.
The scientists attribute a fundamental role to the interaction between chemistry and mechanics. "It is crucial that the carrier material has the right properties. In particular, this includes the right interaction between charged macromolecules and collagen fibers," Ehret explains.
Faster new skin for burn victims
Specifically, the researchers are planning to participate in a project at the Children's Hospital in Zurich aimed at growing skin substitutes for burn victims (see technical article on first aid for burns in Safety-Plus 4/2017 from page 10) better and faster. The collaboration is to take place within the framework of the Skintegrity project of the Verbund Hochschulmedizin Zürich. The researchers submitted the project application to the Swiss National Science Foundation at the end of September.
Mazza's group is already contributing its expert knowledge to a project at the University Hospital in Zurich that is dealing with the aforementioned ruptures in the amniotic sac. The initial aim was to find out what properties tissues need to have in order to repair any injuries. In the meantime, the focus has shifted to the question of why these injuries occur in the first place. Here, too, the biomechanists are in their element. "Being able to contribute to such projects with medical relevance," says Mazza, "we find very motivating."
Source: ETH Zurich