Plants as models for self-repairing materials
It’s a great idea: everyday products that can repair themselves. Although it’s still a pipedream, the foundations are already being laid in a series of investigations being carried out by Dr. Olga Speck from the University of Freiburg. Dr. Speck is a botanist who is studying wound-healing reactions in plants to try and find mechanisms that can be used as models for developing materials with “self-healing powers”.
Nature is an inexhaustible source of inspiration for bionics researchers because animals and plants have adapted perfectly to their respective habitats over millions of years. Rather than directly transferring the mechanisms that have been observed into technical applications, bionics is a creative implementation of such mechanisms. Velcro is the best-known example of bionics. The burrs of burdock plants that adhere to the fur of passing animals have inspired the tiny hooks and loops of Velcro. The bionics imitation is popular as a strap for shoes and loved by small children and their parents alike.
However, bionics has much more to offer than well-known examples such as Velcro: “There are many hidden champions,” says Olga Speck from the University of Freiburg’s Botanical Garden. The botanist is involved in a project called “Bio-inspired self-repairing materials for sustainable development” in collaboration with researchers from the Fraunhofer Ernst-Mach-Institut (EMI). The project is investigating wound-healing reactions in plants and assessing the sustainability potential of self-healing products. It is part of the work being done by the High Performance Centre for Sustainability in Freiburg, which was jointly established by the University of Freiburg and the five Fraunhofer Institutes in Freiburg.
Plants as inspiration
The project focuses on plants that grow in arid areas. Speck explains why: “In arid areas, plants need to strictly control their fluid balance so that they do not dehydrate and die. If they are damaged, they lose water and dehydrate. This means that they are under a high selective pressure that forces them to develop appropriate protective mechanisms.” The researchers systematically screened all the plants in the Freiburg Botanical Garden for their ability to protect themselves against injuries. “We incised the leaves with razor blades and observed what happened,” recalls Speck. Most of the plants did not react to the damage. However, a succulent called Pink Carpet or Iceplant (Delosperma cooperi) from South Africa reacted in an unusual way.
If you make an incision in a Delosperma leaf, the injured leaf will start moving. The researchers produced serial images and discovered that the edges of the wound joined and sealed the wound within 60 minutes of making the incision. Speck had never before observed such a phenomenon. “This is really fast for plants,” Speck comments. The researchers used three types of incisions, transverse, longitudinal and ring, and observed that the edges rolled up within a few minutes, thus closing the wound. Exactly how much movement occurs depends on the air humidity: the lower it is, the more pronounced the movements are.
The discovery might not sound very spectacular to most people, but in fact it is. The edges of the wound have to come smoothly together before self-healing processes are triggered. Rapid self-sealing protects the plant from dehydration, and self-healing processes are triggered once the wound has closed. But how does it work? And how can this type of mechanism be turned into a technical application?
The 5-layer model
Speck and her colleagues comprehensively studied the morphology and anatomy of Delosperma leaves and discovered that they consist of five roughly concentric tissue layers of different thickness and mechanical properties. An incision or natural injury, for example caused by thirsty birds pecking on the leaves, leads to the loss of compressive stress in the outer, water-storing tissue layers. This triggers the movement of the leaf. The leaf moves until a new mechanical equilibrium is established, the wound seals and no more water can leak from the wound.
The transformation of the biological mechanisms into technology requires the know-how of biologists as well as the expertise of physicists, bioinformaticians and engineers. Since the first wound-healing phase is mainly triggered by physical and chemical processes, the process can be represented using mathematical formulas and computer models. The interdisciplinary cooperation has led to a digital 5-layer wound-healing model, which, however, has little in common with its natural counterpart. “We have to think differently, and break away from the biological model to get at the functional principle. This is the only way of implementing the principle and transforming it into something technical,” says Speck.
Engineers can now use the model to simulate different sealing mechanisms by varying the thickness, materials and properties of the different layers. The researchers’ findings will soon be published, but a concrete application is not yet in sight. “The area of application is huge. We are curious to discover which product will at some stage be the first to repair itself,” says Speck.
Sustainability through self-repair
The bionics researchers from Freiburg have already brought one bionic product to industrial maturity – a self-repairing foam coating which prevents air from leaking from damaged membranes. Such membranes are used for example in a technology called Tensairity®, an inflatable light-weight support structure. “We received a concrete enquiry and discovered that lianas were ideal models to deal with the problem,” says Speck. While lianas are growing, fissures and ruptures repeatedly appear in their stems. However, these fissures and ruptures close again quickly as cells migrate and swell into the fissures to seal them.
The botanist also hopes that her research can contribute to sustainability. She comments: “The repair, or better still, the self-repair of products promotes more careful use of natural resources.”