The Structural and Mechanical Basis for Passive‐Hydraulic Pine Cone Actuation

• Published in: Advanced science Vol. 9; no. 20; pp. e2200458 - n/a
• Main Authors: Eger, Carmen J., Horstmann, Martin, Poppinga, Simon, Sachse, Renate, Thierer, Rebecca, Nestle, Nikolaus, Bruchmann, Bernd, Speck, Thomas, Bischoff, Manfred, Rühe, Jürgen
• Format: Journal Article
• Language: English
• Published: Germany John Wiley & Sons, Inc 01-07-2022; John Wiley and Sons Inc; Wiley
• Subjects: Contact angle; Environmental conditions; Equilibrium; Experiments; finite element simulation; Humidity; hydration measurement; kinematical and structural analysis; model for water absorption; pine cone movement; Scanning electron microscopy; tissue mechanics; µ‐CT scans; µ-CT scans; kinematical and structural analysis; tissue mechanics; finite element simulation; pine cone movement; hydration measurement; model for water absorption
• ISSN: 2198-3844; 2198-3844
• DOI: 10.1002/advs.202200458

The opening and closing of pine cones is based on the hygroscopic behavior of the individual seed scales around the cone axis, which bend passively in response to changes in environmental humidity. Although prior studies suggest a bilayer architecture consisting of lower actuating (swellable) sclereid and upper restrictive (non‐ or lesser swellable) sclerenchymatous fiber tissue layers to be the structural basis of this behavior, the exact mechanism of how humidity changes are translated into global movement are still unclear. Here, the mechanical and hydraulic properties of each structural component of the scale are investigated to get a holistic picture of their functional interplay. Measurements of the wetting behavior, water uptake, and mechanical measurements are used to analyze the influence of hydration on the different tissues of the cone scales. Furthermore, their dimensional changes during actuation are measured by comparative micro‐computed tomography (µ‐CT) investigations of dry and wet scales, which are corroborated and extended by 3D‐digital image correlation‐based displacement and strain analyses, biomechanical testing of actuation force, and finite element simulations. Altogether, a model allowing a detailed mechanistic understanding of pine cone actuation is developed, which is a prime concept generator for the development of biomimetic hygromorphic systems.
This contribution describes the hygroscopic opening and closing mechanism of pine cones in terms of mechanical and hydraulic properties of the individual seed scales. Through a combination of mechanical, structural, and chemical analyses, a mechanistic model of pine cone actuation and hydraulics is proposed.