Hygroresponsive coiling of seed awns and soft actuators

The seeds of Pelargonium species possess a long slender appendage called an awn, which enables self-burial of the seeds into soil by helically coiling in response to environmental humidity change. Here we analyze morphology of the helix formed by the awn, and fabricate artificial hygroresponsive helical actuators by depositing aligned nanofibers, and show that our theory agrees well with the experimental results of both the biological and the artificial actuators. To go beyond previous analysis of individual cell deformation, which cannot predict the overall awn shape, we model the awn as a trilayer structure consisting of two active sublayers and one inactive layer by noting that the inextensible microfibrils coiling individual active cells are tilted at constant angles in each sublayer. The laminated composite anisotropic plate theory allows us to predict the shape of the biological and artificial helices as a function of the mechanical properties, the layer thickness, the fiber tilt angle, and the environmental humidity. Our tissue-level understanding of the biological actuator and cost-effective nanofabrication schemes can lead to soft actuators capable of versatile helical deformation driven by external stimuli. (C) 2020 Elsevier Ltd. All rights reserved.