Paradoxical replacement tissue for medicine

Nature shows us how to do it: 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. Empa scientists have now demonstrated the astonishing auxetic properties of nanofiber membranes developed specifically for this purpose. The study published in "Nature Communications" points to a wide range of applications for auxetic materials, including the use of auxetic membranes to regenerate human tissue after injuries.

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 micrometers, 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 suprising properties could be produced: When the membrane was exposed to gentle tensile forces, stretching it by about 10 percent, instead of becoming thinner the material increased about 5-fold by volume and even 10-fold by thickness. "An auxetic effect of this magnitude is almost a world record," enthuses Alexander Ehret of Empa's Experimental Continuum Mechanics lab. Ehret and his team had first predicted the extraordinary effect using mechanical modelling and simulated it on the computer before analyzing membrane samples experimentally. "We ran the simulations on the computer several times because the results were so astonishing," says Ehret. 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. "So far, values around -20 have been achieved. Our results were well below -100," says the biomechanics expert.

And sure enough: In the tensile tests, the polymer membranes behaved as simulated on the computer. 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.

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 researcher Alexandre Morel has now 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.