Researchers at the Adolphe Merkle Institute (AMI) at the University of Fribourg have discovered how environmental conditions affect water transport through the waxy skin layer of ivy and olive leaves and have successfully mimicked this function in artificial membranes. Their discoveries could lead to the development of active barriers for use in smart packaging and other applications.
Plant cuticles are composite membranes that protect land plants from dehydration and are vitally important for their survival. While it had long been known that the cuticles of ivy and olive leaves have asymmetric structures, it remained unclear how these architectures affect water transport. To develop a better understanding of the function of these biological membranes, researchers from AMI’s Polymer Chemistry and Materials group led by Prof. Christoph Weder teamed up with plant biologist Prof. Lukas Schreiber at the University of Bonn in Germany. The researchers isolated cuticles from olive and ivy leaves and characterized their structure and water transport properties. They also prepared artificial nanocomposite membranes that mimic the compositionally graded architecture and function of these plant cuticles.

A smart mechanism for water regulation
The researchers demonstrated that the asymmetric architecture of both membrane types can lead to directional water transport characteristics, that is, a higher water permeance in one of the two direc­tions. They further showed that the water transport of both membranes is regulated by their hydration status. They discovered that when the environment is dry, the water permeability through the natural cuticles is relatively low, and this helps the plants to retain water. During fog and rain, the cuticles swell from the outer side and this changes the mechanical and transport characteristics of the membrane, increasing the water flux greatly. This mechanism may allow plants to dissipate excess water or enable dehydrated plants to take up moisture through their leaves.

Valuable insights for materials science

The artificial membranes that were developed in this project displayed similar characteristics. Their composition could be easily varied, which is impossible with leaf cuticles. This allowed the researchers to investigate systematically how different design parameters and system components influence the water transport characteristics. Such membranes also appear to be technologically useful, for example in adaptive packaging films and other applications where regulated and directional mass transport is desirable, such as fuel cells and drug delivery systems. Their results have been published in the leading journal Nature Communications. «We hope that our study will serve as a compelling example for interdisciplinary research at the interface of biology and materials science», said Professor Christoph Weder, AMI’s Chair of Polymer Chemistry and Materials. «It demonstrates that bio-inspired materials research is not only about copying from nature, but that the investigation of artificial systems can improve or even revise our understanding of natural materials», adds Aristotelis Kamtsikakis, the PhD student who spearheaded the work.

A project of international scope

The project was funded by the European Commission through the Innovative Training Network of PlaMatSu (Plant-Inspired Materials and Surfaces). The project saw three institutions collaborate in the field of bio-inspired materials: AMI, the University of Freiburg (Germany), and the University of Cambridge (UK). PlaMatSu brought together plant biologists, polymer chemists, and soft matter physicists to study on a fundamental level the structure and properties of multifunctional plant cuticles, but also to create novel artificial materials and surfaces based on the working principles of cuticles.
This training network provided nine PhD students with the opportunity to pursue their academic training with-in an international multidisciplinary framework along with temporary industrial internships. The aim of the program was to boost scientific excellence and business innovation, as well as to enhance the career prospects of the researchers through developing their skills in entrepreneurship, creativity, and innovation.

Reference:
Kamtsikakis, A.; Baales, J.; Zeisler-Diehl, V.V.; Vanhecke, D.; Zoppe, J.O.; Schreiber, L.; Weder, C. Asymmetric water transport in dense leaf cuticles and cuticle-inspired compositionally graded membranes. Nature Communications, 2021 DOI: 10.1038/s41467-021-21500-0