Plant leaf folding leads to nature-inspired engineering

As published in Proceedings of the National Academy of Sciences (PNAS), researchers from the University of Birmingham, Nanyang Technological University in Singapore, and Brown University, along with colleagues from the University of Oxford, have unveiled findings which provide insight into dehydration-induced corrugated folding behaviour in the leaves of Rhapis Excelsa, commonly known as broadleaf lady palm or bamboo palm and have created a set of biomimetic foldable devices using these observations.

Foldable structures often rely on mechanical hinges, from the retractable wings of an aircraft and deployable solar panels to flip screens and umbrellas - all are foldable structures which expand and retract upon actuation.

The researchers quantitatively characterized the leaf folding and water loss process during dehydration. They found that water loss in the leaves is directly related to the folding angle with a linear correlation. This observation provides insights into how water could shape the morphologies of plants and the kinetics of plant shape morphing. The study also delved into the underlying mechanisms of water loss kinetics in excised leaves, distinguishing between water loss through the edges and the surfaces of the leaves, predicted by a simple mathematical model.

The study identified the critical player, "hinge cells", in the actuation of folding. These cells are distributed to one side of major leaf veins and undergo significant size changes during dehydration, inducing incompatible strain in the leaves and resulting in sharp folding with the leaf veins as the folding hinges. Numerical simulations demonstrated how the distribution and shrinking of these cells contribute to the corrugated folding observed in the leaves.  This suggests that the folding behaviour may have potential biological significance in maintaining stiffness and preserving water.

This also sheds light on the intricate mechanisms behind this natural phenomenon and its potential applications in bionic engineering. Based on the folding mechanism observed in the R. Excelsa leaves, the researchers created soft biomimetic folding devices using Ecoflex -polyacrylamide composite materials - demonstrating potential uses as flexible humidity sensors and a soft deployable "umbrella", showcasing the potential of nature-inspired folding mechanisms for advanced engineering solutions.