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Posted: Sep 23, 2011
Self-healing membranes - nature shows the way
(Nanowerk News) Lianas whose stabilization rings of woody cells heal spontaneously after suffering damage serve as a
natural example to bionic experts of self-repairing membranes. Such membranes could find use, for
example, in rubber dinghies. Empa researchers have borrowed this trick from nature and developed a
polymer foam surface coating with a closed cell construction which not only reduces the pressure loss
after the membrane is damaged but also makes the inflatable structure more resistant and giving it a
longer operational life. The scientists report on this work in the current issue of the Journal of Bionic
Engineering ("Self-Repairing Membranes for Inflatable Structures Inspired by a Rapid Wound Sealing Process of Climbing Plants").
A hole in an inflatable boat is only a disaster if the air escapes too quickly to reach the safety of land. It's
somewhat less dramatic but nonetheless uncomfortable to spend the night on a leaky air mattress. Even in
this case, though, you can get some uninterrupted sleep if only the air leaks out slowly enough. In future,
self-repairing layers of porous material should ensure that the membranes of inflatable objects are not only
water and airtight but also that they can plug up any holes on their own, at least temporarily.
A membrane made of polyvinyl chloride-polyester (yellowish colour) is punctured with a 2.5-millimeter diameter needle, and at that moment the polyurethane foam (brown) suddenly expands. (Photo: Empa)
The idea behind this comes from nature. Bionics experts keep on discovering amazing principles of
construction which engineers can adopt for countless technical solutions. This is also the case with selfrepairing
materials. The self-healing process of the pipevine (Aristolochia macrophylla), a liana which grows
in the mountain forests of North America, gave the biologists at the University of Freiburg, Germany, a
decisive clue. When the lignified cells of the outer supportive tissues which give the plant its bending
stiffness are damaged, the plant administers "first aid" to the wound. Parenchymal cells from the underlying
base tissue expand suddenly and close the lesion from inside. Only in a later phase does the real healing
process kick in and the original tissue grows back.
Self-healing inflatable structures
This principle is now being transferred to materials – more specifically, to membranes – in a bionics project
sponsored by the German Federal Ministry of Education and Research. As soon as a membrane suffers
damage, an additional layer provides "first aid", thanks to its mechanical pre-tensioning, closing the hole until a proper repair can be made. This is analogous to the natural process which occurs in lianas. While
researchers from the University of Freiburg under the direction of Olga Speck are busy studying the
biological and chemical aspects of the model provided by liana plants, Rolf Luchsinger and Markus Rampf at
Empa's Center for Synergetic Structures are working on technical solutions for polymer membranes.
Cell repair in a pipevine (Aristolochia macrophylla). Parenchymal cells of the base interior tissue sud-denly expand if the lignified cells of the outside supporting tissue are damaged (a and b), and in a later phase (c) they eventually lignify. (Photos: Plant Biomechanics Group, University of Freiburg im Breisgau)
Luchsinger's impetus, however, concerns neither inflatable boats nor air mat-tresses but rather load-carrying
pneumatic structures for lightweight construction. His tensairity beams serve as elements for quickly erected,
lightweight bridges and roofing.
The study's goal is to understand under which conditions a hole plugs itself up if the foam expands on a
membrane following damage. Within the scope of his dissertation, Rampf is studying this process with the
help of an experimental setup which places a membrane under pneumatic pressure and then punctures it
with a nail. The researchers have already achieved successful interim results. A two-component foam of
polyurethane and polyester suddenly expands when exposed to the excessive pressure which arises when air
rushes out of a hole.
"It works in the lab," notes Luchsinger, "and we're achieving high repair factors." What does this mean in the
real world? Take the case of an air mattress with a volume of 200 litres. Given a certain-sized hole, previously
it was necessary to pump it up every five minutes; it now holds for eight hours – enough time to sleep
through the night. "We now know enough about the foam that we can enter into discussions with
membrane manufacturers about commercializing this technology," according to Luchsinger, when describing
the next steps.