Nature builds extremely durable structures from the weakest compounds. The key element is a responsive material with a complex nanoscale structure – a potential design template of the materials of the 21st century.
Plants and paper draw their strength from the very same material: cellulose. But paper is so weak that a child can rip it into pieces easily, while trees have been the symbol of strength and endurance for thousands of years. A tree can yield to strong wind but not break. A tree can defy gravity and reach the sky. A tree persists.
How can we explain this very significant difference?
The devil is in the details: plants and paper are structured differently. As we zoom in further and further on our plant, the building bricks keep changing: organs build up the plant but tissues form the organs, cells unite in tissues, and the cell wall – the part of the plant that contains cellulose – surrounds every single cell. The building elements of plants are organised hierarchically, forming different units at different lengthscales. The complex structure enhances the material properties in subtle ways – the whole object is more than the sum of its elements. For example, hollow structures are more resistant to bending forces, because the stress is better distributed. Nature often uses this principle to build materials, while man-made materials are often simple and appear homogeneous on these scales – just like paper.
The heart of the differences lies deeper. Looking even closer, we finally see the structure of paper: an arrangement of rope-like bundles. The bundles are made of very fine fibres of cellulose. In the cell wall we also find these very fine fibres, but not in bundles. Here the fibres are embedded in a jelly, and some of the fibres are linked together by even finer ones. Combined this way, the cell wall is more difficult to break and much easier to stretch. The mixing has to be on a nanoscopic level: putting jelly on paper does not work!
The cell wall has a crucial role: it protects the cell from bursting but also allows it to grow. Many factors can change the properties of the cell wall: for example, if the fibres align with each other, we get a material that stretches much more easily in one direction than another. The exact properties of the jelly and the morphology of the links between the cellulose fibres also matter. But the cell wall is not simply a physical object: it is also affected by the biological and chemical changes inside the cell.
This wonderfully complicated object is the focus of our research group. Understanding how the nanoscale structure translates to macroscopic properties is the key to designing better-suited materials for everyday use. Studying how the (bio)chemical environment changes the structure – and therefore the material – gives us a recipe to build responsive materials – intelligent, adaptable materials. The materials of our future.
NanoDTC PhD Student Cohort 2013
Plant & Algal Mechanics Group, The Sainsbury Laboratory
Cover Image credit- AliExpress