Self-healing Materials – What humans can learn from botanical characteristics

Self-healing materials: sounds more like science fiction than a focus of scientific research. At the Sustainability Center Freiburg (LZN), researchers investigated if and how principles found in nature could be implemented in technical systems. The main goal was to examine self-healing characteristics which can be found in various animal and plant species, in such minute detail that they may be imitated. The idea behind this examination was to then transfer these characteristics into technical systems to create self-healing materials. The researchers were able to model the fundamental functionality of the self-sealing mechanism and thereby create the basis for a transfer to technical systems.

Particular animal and plant species have the ability to heal themselves after being wounded. Some can even heal drastic wounds such as certain lizards including the American Chameleon (Anolis carolinensis) which can detach its own tail in order to flee from danger. This wound not only heals itself but is replaced by a new tail which grows back shortly after. Similar cases can be found in certain plant species. Dandelions (Taraxacum sect. Ruderalia) for example are able to close wounds inflicted upon them with help from a milky white fluid. This capability of healing one’s own wounds is an important component of “resilience”. Nowadays, in which resilience plays an increasingly important role, this characteristic is highly interesting for scientists. If the functionality behind these mechanisms were identifiable, understandable and reproducible, what kind of technical applications could be made possible with this knowledge? With this research question in mind, Dr. Olga Speck and Dr. Linnea Hesse from the Biology faculty of the University of Freiburg joined Dr. Matthias Boljen and Hartmut Klein from Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut EMI in the LZN pilot project “Bio-inspired self-healing materials for sustainable development”.

The first step for the researchers was to choose a suitable species as their model plant. Their goal was to find a plant with self-healing principles which were hopefully “technically” applicable. Speck, Hesse, Boljen and Klein decided on the Pink Carpet species (Delosperma cooperi) as depicted in Figure 1. This species has many properties which makes it relevant and interesting for the project. This includes the finger-like, raised leaves as well as the fact that, as a member of the Aizoaceae family, Pink Carpet can grow in arid areas. To this effect, they have inner fluid-filled cells which not only give them a certain dry-resilience but was also the key feature why they were chosen as the model plant for this project.

The researchers knew that they wanted to inspect the self-healing mechanism experimentally but also with help from computer simulations and models. “We thought about how we could numerically represent the behavior of the plant in a computer. We looked at an individual leaf of the plant, created a simple physical model of it in the simulation, and then varied different parameters, to see what would happen,” explains Hartmut Klein from Fraunhofer EMI. He and his colleagues were primarily focused on the modelling and simulation of this self-sealing activity. The actual healing process is comprised of two parts: the closing or sealing of the wound and the actual healing. For this particular project, only the former process was taken into consideration.  Dr. Speck and her fellow biologist colleagues dealt with the close-up photography of the plants in the laboratory, in order to closely examine the inner structures and the behavior thereof during an injury. Through the combination of the parallel gained results, the researchers were able to reach new conclusions.

The first research question to be addressed was, in which way the plant can actually seal a wound. The researchers of the University of Freiburg, with help from their close-up photographs, were able to see in detail how the plant pulled the separated edges back together to seal the incision. “A further question we posed was how the inner structures behave during this process,” remarks Dr. Boljen. “Our colleagues from the biology department cleanly cut through the various components of the leaf including the epidermis, chlorenchyma, parenchyma and the vascular tissue and then carried out a parameter analysis.” The close-up photographs clearly showed how the tissue was pressed together and partly destroyed during the incision (see Figure 2). Two important elements in the leaf had to be closely inspected: the water loss due to the evaporation from the leaf itself and the tension in the leaf which is regulated by the vascular tissue. Klein and Boljen produced an FE-model for an individual leaf of average size in order to further investigate these two properties (see Figure 3). In order to describe the inner structures as well, they furthermore created a rotation-symmetrical structure in the form of a cylinder and a half-ellipsoid which was able to simulate the properties of the individual layers in the plant tissue. In this way, they were able to simulate the epidermis, chlorenchyma, parenchyma and vascular tissues in their model.

The research team wanted to artificially replicate various environmental factors and botanical characteristics in order to see to what magnitude the self-sealing ability of the plant and of the model would be influenced. Four parameters were examined: the humidity, the reflection coefficient, the elasticity module (the stiffness of the cell tissue) and the permeability. The results showed, that there was a direct correlation between the evaporation of water at the point of injury and the humidity. The lower the humidity, the more evaporation was recorded. The reflection coefficient or the retention capability of the membrane for a dissolved substance determined how large the osmotic potential in the leaf is. During the variation of this parameter, the team discovered that the reflection coefficient caused a large effect on the wound sealing: the higher the coefficient, the quicker the sealing process ensues. Parts of the plant, especially the tissue thereof, have a specific elasticity module. This describes the change in turgor, the osmotic pressure in the cell, per relative volume change of the cell. Changes in this parameter did not result in any significant effects on the self-sealing process. In comparison, the variation of permeability resulted in great effects (see Figure 4). Overall, it could be determined that the permeability and the reflection coefficient caused the greatest changes in the self-sealing process.

In order to validate that the developed model effectively represented the reality, the researchers conducted a component test. They followed a series of points in the simulation and compared these to the behavior of the plant in the laboratory. They could thereby assess in which ways model and reality coincided with one another. The result was positive and indicated, that the model worked well. “Of course there is potential to improve our process but we are very happy with these results,” praises Dr. Boljen.

The interdisciplinary work between the biologists of the University of Freiburg and the engineers of Fraunhofer EMI was viewed as interesting and lucrative by the colleagues. The inner structures of the plants and their behavior during the self-sealing process could be examined in fine detail from different disciplinary perspectives. “Our work has formed a very solid basis for future research, particularly in the biology field,” reports Klein.

A transfer of the results onto technical systems was determined to be too complex to complete within the time frame of this project. The self-healing mechanism works very well by Delosperma cooperi, particularly because the plant is not restricted by its dimensional stability. Plants are flexible and can continually function even when their form or structure is suddenly completely different. Technological systems are in comparison still very shape dependent. That means a constructional component, in contrast to plants, cannot bend and twist itself to seal a crack or rip and still smoothly function. “Therefore we can say that the development of a material which is flexible and self-healing like the Pink Carpet flower is a goal which still lies a good ways in the future,” concludes Dr. Boljen.