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Plant structures may inspire new materials.

In Brief: From an engineer's perspective, plants are examples of precise engineering on a microscopic scale. The cell walls form the structural supports of all plants. Depending on how the cell walls are arranged, and what they are made of, a plant can be as flimsy as a reed or as sturdy as an oak. Understanding plants' microscopic structures may help engineers design new, bio-inspired materials.

Lorna Gibson, the Matoula S. Salapatas Professor of Materials Science and Engineering at MIT, has compiled data on the microstructures of a number of different plants, from apples and potatoes to willow and spruce trees, and has found that plants exhibit an enormous range of mechanical properties depending on the arrangement of the cell walk' four main building blocks: cellulose, hemicellulose, lignin, and pectin.

To Gibson, a cell wall's components bear a close resemblance to certain manmade materials. For example, cellulose, hemicellulose, and lignin can be as stiff and strong as manufactured polymers. A plant's cellular arrangement can also have engineering parallels: the cells in wood, for instance, are aligned like engineering honeycombs, while polyhedral cell configurations, such as those found in apples, resemble industrial foams.

To explore plants' natural mechanics, Gibson focused on three main plant materials: woods, such as cedar and oak; parenchyma cells, which are found in fruits and root vegetables; and arborescent palm sterns, such as coconut trees. She compiled data from her own and other groups' experiments and analyzed two main mechanical properties in each plant: stiffness and strength.

Among all plants, Gibson observed wide variety in both properties. Fruits and vegetables such as apples and potatoes were the least stiff, while the densest palms were 100,000 times stiffer. Likewise, apples and potatoes fell on the lower end of the strength scale, while palms were 1,000 times stronger. This large range in stiffness and strength arises from an intricate combination of plant microstructures: the composition of the cell wall, the number of layers in the cell wall, the arrangement of cellulose fibers in those layers, and how much space the cell wall takes up.

In trees such as maples and oaks, cells grow and multiply in the cambium layer, just below the bark, increasing the diameter of the tree. The cell walls in wood are composed of a primary layer with cellulose fibers spread randomly through it. Three secondary layers lie underneath, each with varying compositions of lignin and cellulose that wind helically through each layer. The cell walls occupy a large portion of the cell, providing structural support. Taken together, the cells are organized in a honeycomb pattern--a geometric arrangement that gives wood its stiffness and strength.

Parenchyma cells, found in fruits and root vegetables, are much less stiff and strong than wood cells. The cell walls of apples, potatoes, and carrots are much thinner than in wood cells and are made up of only one layer. Cellulose fibers run randomly throughout this layer, reinforcing a matrix of hemicellulose and pectin. Parenchyma cells have no lignin, Gibson says, and this lack of lignin, combined with the thin cell walls and the random arrangement of cellulose fibers, may explain the cell walls' low stiffness. The cells in each plant are densely packed together, similar to the industrial foams used in mattresses and packaging.

Unlike woody trees that increase in diameter over time, the stems of arborescent palms, such as coconut trees, maintain similar diameters throughout their lifetimes. As the stem grows taller, palms support this extra weight by increasing the thickness of their cell walls. A cell wail's thickness depends on where it is along the palm stem. Cell walls are thicker at the base and periphery of stems, where bending stresses are greatest.

Gibson sees plant mechanics as a valuable resource for engineers designing new materials. For instance, she says, researchers have developed a wide array of materials, from soft elastomers to stiff, strong alloys. Carbon nanotubes have been used to reinforce composite materials, and engineers have made honeycomb-patterned materials with cells as small as a few millimeters wide. But researchers have been unable to fabricate cellular composite materials with the level of control that plants have perfected.

"Plants are multifunctional," Gibson says. "They have to satisfy a number of requirements: mechanical strength, as well as growth, surface area for sunlight, and transport of fluids. The microstructures that plants have developed satisfy all these requirements. With the development of nanotechnology. I think there is potential to develop multi functional engineering materials inspired by plant microstructures."

For more information, contact the writer. Jennifer Chu at the MIT News Office, j-chu@mit.edu, or Lorna Gibson, ljgibson@mit.edu.
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Title Annotation:update
Author:Chu, Jennifer; Gibson, Lorna
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Jul 1, 2013
Words:771
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