New insights into the movement of pine cone scales

Pine cones open when dry and close when wet. In this way, pine seeds are released only under advantageous conditions, namely when it is dry and the seeds can be carried far by wind. Opening and closing is of particular interest to researchers because the actuation is passive, that is, it does not consume metabolic energy. This is why the pine cone has already served as a model for biomimetic flap systems that react to moisture and are used, for example, in building envelopes to regulate the climate.

A team led by Prof. Dr. Jürgen Rühe, Carmen Eger and Prof. Dr. Thomas Speck from the Cluster of Excellence Living, Adaptive and Energy-autonomous Materials Systems (livMatS) at the University of Freiburg, Dr. Simon Poppinga from TU Darmstadt and Prof. Dr. Manfred Bischoff from the University of Stuttgart, in collaboration with researchers from TU Munich and project partners from BASF, has now analyzed in detail which tissue structures are involved in the actuation. The researchers show that the cone scales close due to several tissue layers, all of which absorb moisture. With the help of their findings, biomimetic flap systems with improved function could be created. The team published their findings in the journal Advanced Science.

Previous model assumes two functionally relevant tissue types

Until now, research assumed that the movements of the pine cone were mainly due to the interaction of two tissue layers in the scale: An upper, rigid layer that absorbs no or very little water, and a lower layer that swells and elongates due to moisture. In this process, it pushes the scale upward and the cone closes. If the lower layer dries and shrinks, it pulls the scale down again and the cone opens.

Fiber strands set bending process in motion

The scientists refute this simplified model in their analysis. They show that several tissue layers in the scale absorb water. The so-called sclerenchyma fibers are of particular importance. In the bilayer model, they are considered the main component of the rigid, upper tissue layer. In their analysis, however, the researchers come to the conclusion that although the fiber strands are rigid in the dry state, they soften when exposed to moisture and thus set the bending process in motion in the first place. In addition, although the different tissues are made of chemically identical materials, their arrangement along the scale varies. For example, the scale consists of a bending zone and a longer flap-like periphery that does not contribute significantly to the bending process but is passively moved along with it. In their analysis, the researchers performed, for example, high-resolution 3D structural analyses, 3D deformation measurements, measurements of the water absorption of the different tissues, and biomechanical experiments.

Overview of facts:

• Original publication: Eger, C. J., Horstmann, M., Poppinga, S., Sachse, R., Thierer, R., Nestle, N., Bruchmann, B., Speck, T., Bischoff, M., Rühe, J., The Structural and Mechanical Basis for Passive-Hydraulic Pine Cone Actuation. Adv. Sci. 2022, 2200458. DOI: 10.1002/advs.202200458
• The study was funded by the German Federal Environmental Foundation, the Ministry of Science, Research and the Arts Baden-Württemberg within the project BioElast as well as by BASF SE through the PostDoc network JONAS and the Cluster of Excellence livMatS.
• Jürgen Rühe has been Professor of Chemistry and Physics of Interfaces since 1999. He is the managing director of the Freiburg Center for Interactive Materials and Bioinspired Technologies and a member of the spokesperson team of the Cluster of Excellence livMatS at the University of Freiburg.
• Thomas Speck has been Full Professor of Botany: Functional Morphology and Biomimetics and Director of the Botanic Garden at the University of Freiburg since 2001. He is a member of the spokesperson team of the Cluster of Excellence livMatS at the University of Freiburg.
• Simon Poppinga was group leader on plant movements and biomimetics at the Botanical Garden of the University of Freiburg and has been head of the Botanical Garden of the TU Darmstadt since 2022.
• Manfred Bischoff heads the Institute for Structural Mechanics at the University of Stuttgart and is Vice Rector for Research and Early Career Researchers at the University of Stuttgart.
• Simon Poppinga and Thomas Speck were able to show in the past that fossil cones are still capable of the bending movements of their individual seed scales after millions of years.

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Contact:
Prof. Dr. Jürgen Rühe
Department of Microsystems Engineering
University of Freiburg
Tel.: 0761/203-7160
E-Mail: ruehe@imtek.uni-freiburg.de