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Natural hold – adhesive secretions from the animal kingdom as model

Marine animals and insects have extraordinary adhesive forces. An interdisciplinary team from the University of Tübingen is investigating the molecular composition of the adhesive secretions of insects, barnacles and other organisms. The researchers’ goal is to decipher the underlying principles of adhesion and use them for technical applications.

One of the models of nature is the adhesion system of Sagra femorata, a Southeast Asian frog-legged leaf beetle that lives mainly on fresh fruit such as apples, bananas and pears. © O. Betz, University of Tübingen

One thing already seems clear: the adhesion system of such organisms is more than the sum of its individual components. The components of animal adhesive secretions are very heterogeneous and act together perfectly to exert a specific effect, namely the permanent or reversible adhesion to substrate. The researchers hope that an understanding of the animals’ adhesion systems will provide them with ideas as to how such systems can be used in bionics. Working with the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM and a number of Baden-Württemberg companies, the researchers aim to use the results for the development of innovative adhesion systems.

“We are not aiming to copy what we have found or will find in the future, by this we mean using exactly the same components as those used by the animals. We are mainly focusing on understanding the principles in order to be able to produce and use synthetic or natural substances as inexpensively as possible,” said Prof. Dr. Oliver Betz, zoologist at the University of Tübingen and coordinator of the project that is being funded by the Baden-Württemberg Ministry of Science, Research and the Arts for three years.

Half of the funding period is now over and the researchers have been able to overcome important hurdles in the last one and a half years. Initially, the researchers looked for suitable animals to use in their research that had the right kind of adhesive secretions for the intended applications. In this, Betz worked closely with the geologist Prof. Dr. James Nebelsick and the chemist Prof. Dr. Klaus Albert who also performs the chemical analysis of the samples. Numerous species turned out to be suitable providers of adhesive substances, and the researchers selected the most promising. “With regard to insects, we wanted to focus in particular on two types of adhesive systems - smooth and hairy. Our selection criteria included the climbing behaviour and the size of the animals, the latter because meant we could obtain as many adhesive secretions as possible. In addition, we were looking for animals that could easily be bred in the laboratory,” said Betz. The researchers selected the desert locust Schistocerca gregaria, partly because this species had already been well characterised in physiological terms.

First major obstacle: sampling

The searchers selected firefly larvae as models for the hairy type of adhesion system. These insects were accidentally discovered during a field trip to the Federsee, a lake in the south of Germany. Firefly larvae use their pypopodia to adhere to extremely smooth surfaces which makes them particularly suitable for the ongoing project. Another model used by the researchers is the frog-legged beetle Sagra femorata. “Our ambitious goal is to be able to breed the beetle, but this is very difficult to do. At present, we obtain the pupae from breeders in the beetle’s endemic area in Southeast Asia,” said Betz. In addition to the aforementioned terrestrial species, the researchers also use aquatic species such as barnacles and bristle worms. “These marine species secrete proteins that form insoluble complexes as they insolubilise and adhere to the ground,” said Albert referring to the researchers’ initial results.

The marine tube-building annelid Pomatoceros lamarckii also has an interesting way of adhering to substrate and is used in the bionics investigations carried out by the researchers from Tübingen. © J. Nebelsick, University of Tübingen

When they first started their investigations, the Tübingen team experienced major problems with artifacts. They were only able to obtain tiny quantities of adhesive secretions. In addition, the secretions were composed of a broad range of different compounds. The foreign substances analysed along with the sought-after secretions were of much greater quantity and tended to falsify the results.

During their work with locusts, the researchers came up with a simple idea: “We used tape to fix the animals and their legs to the substrate. This does not harm them and it allows us to use a so-called Hamilton syringe to apply aqueous and fat-dissolving solvents in order to solubilise the adhesive secretions.

In addition, we save time as this method enables us to treat between ten and 20 animals simultaneously,” said Betz highlighting that Hamilton syringes have a special advantage over standard plastic syringes. The plastic of standard syringes contains softeners which managed to enter the sample and falsify the results. The use of Hamilton syringes and double distilled water led to a considerable reduction in artifacts.

Use of the entire range of analysis systems

Hamilton syringe for removing the tarsal adhesive secretion. Tarsus (ta), Hamilton syringe (ha), fixation wire (d). © O. Betz, University of Tübingen

The development of a specifically coated needle brought the researchers an important step closer to their goal. The needle takes up the secretions like a sponge, which can then be directly applied to the chromatography systems used for the analyses. In addition, Albert and his team developed special microcoils for use in NMR spectroscopy. “This own development now enables us to process very small volumes of around 1.5 ml,” said Albert pointing out that the coupling of gas and mass spectrometry with NMR spectroscopy now enables the chemists to carry out comprehensive analyses. “Mass spectrometry does not provide us with information about the spatial structure of the molecules; this is why we require NMR spectroscopy as well. We also use electrophoretic systems and electron microscopes and our collaboration with the Fraunhofer IFAM in Bremen provides us with access to infrared spectroscopy data,” said Albert.

“The excellent equipment, in addition to the excellent and friendly cooperation among the project partners, has very much contributed to what we have achieved so far,” confirmed Albert. The geographic vicinity of the partners in Tübingen is advantageous in another way: it only takes the researchers a few minutes to take the samples collected in the Department of Zoology to the Institute of Organic Chemistry, where they are subsequently analysed. “This quick analysis is extremely important, in particular in the case of relatively small sample quantities. It has always been our goal to reduce the number of processing steps and analyse the samples as soon as possible after withdrawal. We do not need to use a courier to send the samples which would mean we would have to use special gas to protect the samples from deteriorating,” said Albert.

Ready for application in the not-too-distant future

The project partners are confident that they will be able to obtain comprehensive insights into the composition of the adhesive secretions by the end of this year. They have already found out that different peptides appear to be involved, and have also identified sugars and lipid components. The question that remains to be answered is whether and which systems are water-in-oil or oil-in-water emulsions. “The subsequent interpretation of the physical and chemical results will be very exciting,” said Albert.

The researchers already have some idea of potential industrial applications that would enable them to put their findings to good use. For example, the IFAM could carry out feasibility analyses and Tübingen-based EMC microcollections GmbH is a potential partner for commercial application. “We have already established informal contact with the company and we believe that we can enter product development in cooperation with EMC microcollections. For example, EMC could produce the protein components of future adhesion systems and potentially also market the products,” said Betz highlighting future prospectives.

Further information:
University of Tübingen
Institute of Zoology
Prof. Dr. Oliver Betz (Project coordinator)
Auf der Morgenstelle 28E
Tel.: +49 (0) 70 71 29/ 72 995
E-mail: oliver.betz(at)uni-tuebingen.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/natural-hold-adhesive-secretions-from-the-animal-kingdom-as-model