A material that thickens when you pull on it seems to contradict the laws of physics. However, the so-called auxetic effect, which also occurs in nature, is interesting for a number of applications. A new study by Empa shows how this amazing behavior can be improved – and even used to treat injuries and tissue damage.
Nature shows how it’s done: A calf sucking milk from the udder of a mother cow uses a fascinating physical property of the teat, which consists of an auxetic tissue. Paradoxically, such tissues do not become narrower under tension, such as a rubber band, but wider, transverse to the direction of pull. Therefore, the cow's milk can flow unhindered through the teat. Scientists from Empa have now demonstrated the astonishing auxetic properties of nanofiber membranes developed specifically for this purpose. The study  points to a wide range of applications for auxetic materials, including the use of auxetic membranes to regenerate human tissue after injuries.
Skin injuries or tissue damage to internal organs heal by, among other things, migrating cells that settle and form a healthy replacement tissue. What is usually done without further ado in the case of a small incision in the fingertip can exceed the possibilities of the human body, for instance when complex wounds such as burns occur, or when a more widespread tissue regeneration is required.
However, tissue regeneration can be facilitated: if a suitable scaffold is provided, the desired cells settle down more easily and grow along the predefined structure. Empa researchers at the Biomimetic Membranes and Textiles lab in St. Gallen/Switzerland have developed novel matrix systems with auxetic properties. By electrospinning, dissolved polymers are spun as wafer-thin filaments in a form similar to the human extracellular matrix. This makes it possible to produce multi-layer membranes from nanofibers that are biocompatible and can be implanted into the human body. If biopolymers such as polylactic acids are used in the spinning process, the membranes can even be degraded by the body. In addition, bioactive substances or drugs can be incorporated into the fibers for controlled and minimized release.
Attractive pore size
One of the challenges thus far has been to make the pore size in the spun membrane as attractive as possible for the desired body cells to adhere. In the original membranes, the polymer threads only formed tiny pores of a few micrometers. With its 20 µm, however, a tissue cell that is to colonize the scaffold is far too large to fit snugly in the membrane.
After the researchers had optimized the spinning parameters a polymer network with surprising properties could be produced. When the membrane was exposed to gentle tensile forces, stretching it by about 10 %, instead of becoming thinner the material increased about 5-fold by volume and even 10-fold by thickness. The auxetic effect, which can be quantified mathematically by the ratio of transverse to longitudinal strain (the Poisson's ratio), is characterized by negative values for the Poisson's ratio.
In the tensile tests, the polymer membranes indeed behaved as simulated. The effect can be explained by fibers that realign themselves under tension and thereby exert pressure on their transverse colleagues in the network. Depending on their length and thickness, the fibers under pressure are forced to bend up or downwards and thus lead to an increase in volume.
"Expand on demand"
Basically, electro-spun membranes are suitable for the treatment of wounds and tissue damage in locations as diverse as on the skin, in blood vessels and in internal organs or even in bone injuries. An appropriate selection of polymers and optimized spinning parameters allow the polymer membrane to be adapted to the properties of the target tissue. Thanks to the larger volume caused by the auxetic effect, the matrix structures are now even more attractive for the body's cells and could facilitate the healing process.
In addition to its use in biomedicine, the concept, which is already patent-pending, can also be applied in numerous other areas. According to the researchers, membranes that can be activated by stress to release enclosed particles, adjustable filters or filling material that only expands to its final volume at the point of use, i.e. quasi "expand on demand", are potential future applications.
Structure of nanofibers
The internal structure of individual nanofibers has a major influence on the properties of the membranes. If nanofibers are treated with certain solvents, the structure of the nanofibers can be elucidated. Empa researchers have discovered that varying the spinning parameters results in different fiber structures, such as fibrillary or shish kebab phases. In the electron microscope shish kebab structures appear as stacked layers resembling a kebab spit. They have a large influence on the membranes' mechanical properties and thus also on the auxetic effect.
 Domaschke, S.; Morel, A.; Fortunato, G.; Ehret, A.E.: Random auxetics from buckling fibre networks; Nature Communications (2019); doi.org/10.1038/s41467-019-12757-7
 Morel, S.; Oberle, S.C.; Ulrich, S.; Yazgan, G.; Spano, F.; Ferguson, S.J.; Fortunato, G.; Rossi, R.M.: Revealing non-crystalline polymer superstructures within electrospun fibers through solvent-induced phase rearrangements; Nanoscale (2019); doi:10.1039/C9NR04432A