Strong, Light, and Stretchy Materials
A nanocomposite of aluminum oxide and a polymer is as tough as metals but lighter.
Researchers have dispersed tiny platelets of aluminum oxide in a polymer to make a material that is tough, stretchy, and lightweight. The material could lead to longer-lasting bone and dental implants and lighter, more fuel-efficient car and airplane parts. It could also be used to make bendable, transparent electronics.
In their efforts to create strong yet light materials, chemists and materials scientists have long tried to mimic nanostructures found in nature. Shells, bones, and tooth enamel all consist of stiff ceramic platelets arranged in a polymer matrix like bricks in mortar. These hybrid materials combine the strength of ceramics and the stretchability of polymers.
In 2007, University of Michigan researchers engineered clay-reinforced polymers that were extremely strong but brittle: it takes a lot of energy to deform them, but when they do deform, they break abruptly. Researchers at MIT succeeded in making stiff but less brittle clay-polymer composites, which will tolerate some stretching before they break. (See “Ultra-Tough Nanotech Materials.”)
Ludwig Gauckler, the professor of materials at the Swiss Federal Institute of Technology Zurich, in Switzerland, who led the new work, says that his group’s composite is better still. It’s five times as strong as the material made at MIT, he says, yet it’s still stretchy. A film of the composite is already as strong as aluminum foil, Gauckler says, but if stretched, it can expand by up to 25 percent of its size; aluminum foil would break at 2 percent.
An added advantage of the hybrid material is that it’s light, says Harvard materials scientist Andre Studart, who was involved in the work. The material is half to a quarter as heavy as steel of the same strength, Studart says, and it would make a good substitute for fiberglass, which is commonly used in car parts. Because the material’s strength comes from the platelets diffused through it, Studart says, “it will be strong in two directions and not only in one direction, as in the case of fiber-reinforced material.”
Moreover, while the material is translucent now, its structure could be modified to render it transparent, making it suitable for dental material and transparent electronic circuits.
To assemble their material, the researchers disperse aluminum oxide platelets in ethanol and spread the mixture over water. The platelets arrange themselves into a single layer on the surface of the water. Then the researchers dip a glass plate into the solution, transferring the platelets to the glass. Finally, they deposit a layer of the biocompatible polymer chitosan on top of the platelets. The researchers repeat this process until the thickness of the final composite is a few tens of micrometers, and then they peel the material off the glass plate with a razor blade.
In designing the material, the researchers carefully studied the mechanical structure of nacre, the shiny layer on the inside of seashells, and tried to improve it. Nacre has platelets made of calcium carbonate arranged in layers inside a protein-based polymer. “There’s something very special about the size of these platelets,” Studart says. “Nacre uses specific platelet length and thickness to achieve the high strength and [stretchability] that you see in metals.”
The ratio between the length and thickness of the platelets has to be just right, Studart says. If it is too high, the platelets break when the material is stretched. If it is too low, the material is not very stiff.
The researchers chose to work with aluminum oxide platelets, which are five times as strong as the calcium carbonate platelets found in nacre. They also made their platelets thinner–about 200 nanometers across, as opposed to the 500 to 1,000 nanometers of the naturally occurring platelets–to lower the likelihood of flaws in their structure. The best average length-to-thickness ratio, the researchers calculated, is 40, so they made the platelets 5 to 10 micrometers long. “Stronger platelets allow us to use a higher ratio and therefore achieve higher strength, compared to shells, with a lower concentration of platelets,” Studart says. Low concentrations are important, he says, “because that means the composite has more polymer and has a lot of [stretchability].”
This is the closest anyone has come to duplicating the mechanical structure and behavior of a natural material, says Francois Barthelat, a mechanical-engineering professor and biomimetic-materials researcher at McGill University, in Montreal, Quebec. But before the material can be used, he says, the researchers will have to develop a faster way to make it in larger quantities.
Princeton University chemistry professor Ilhan Aksay thinks that the technique should be easy to modify so that it is suitable for bulk manufacture. “You could make large shapes with this technique,” he says. He imagines that the material could be useful for bone and dental implants.
Gauckler says that the material needs many improvements before it can be practically used. A better polymer would make the composite stronger. The researchers also need to find a way to get better bonding between the aluminum oxide and the polymer. For now, Gauckler says, “we have shown that we can [come close to] doing as good a job as nature.”