New Sustainable Materials for the Future
Materials science is one of the most advanced fields, and its applications are practically immediate. And while its discoveries may not make for splashy headlines, the impact of this discipline on our lives is significant, even if each new discovery is not very visible: researchers are constantly coming up with new materials with surprising properties or improving on those known to us for centuries, and the products of this research end up being commonly used by all of us, almost without us noticing. We may soon see applications from these five recent innovations in our daily lives.
ANTI-ICE SURFACES
Removing frost from a freezer requires additional energy-consuming systems, so preventing ice from forming on interior surfaces is not only good for the consumer, but also for the environment. This is one of the reasons why material scientists are looking for new compounds with microscopic structures that stop water and other substances from adhering.
Numerous examples of hydrophobic surfaces, which repel water, can be found in nature. The most typical case is the lotus leaf, and for almost 80 years we have seen the applications of hydrophobic materials such as Teflon. The microscopic structure of certain surfaces can also confer this property, for example in the plumage of birds or in the tiny hairs that cover the legs or body of aquatic insects.
Another natural example is the carnivorous plant Nepenthes, also known as the pitcher plant, whose inner surface is so slippery for insects that they cannot avoid falling to the bottom, where they are digested. Inspired by the slippery coating of these plants, a team of researchers at Harvard University’s Wyss Institute created the technology of slippery liquid-infused porous surfaces, or SLIPS for short.
This material has a nanostructure to which a special lubricant is bonded that forms a perfectly smooth surface, much smoother than any solid could be, and on which ice will not deposit. In addition, if the coating is scratched, the lubricating fluid flows out, filling the crack and keeping the surface uniform.
The new material is already being commercialised by the start-up Adaptive Surface Technologies, created by the Wyss Institute to exploit its numerous applications, from preventing ice from building up on aircraft wings or other structures exposed to low temperatures, to non-stick food containers that use a safe and sustainable coating. Other research groups are also making progress in the study and properties of SLIPS materials, and it is possible that in the near future our freezers will incorporate such passive anti-frost solutions that consume no energy.
ABSORBING WATER… FROM THE AIR
Some 2.2 billion people worldwide do not have clean running water in their homes. But even purification becomes a secondary issue to the main problem, the water itself, a rare commodity for the more than 2.1 billion people living in arid regions. Researchers are also looking for innovative solutions to solve the water problem, and some of them are based on new materials capable of absorbing moisture from the environment even in the driest climates.
An example of this, also inspired by nature, is a material created by researchers at Rice University in Houston. By mimicking the wings of a beetle able to trap water from the air in the Namibian desert, the scientists built a “hygroscopic scaffold”, a kind of forest of carbon nanotubes capable of attracting water molecules from the air and trapping them inside. And, like a sponge, the water is released by squeezing, and the material is ready to be used again. If commercialised, this material could make it possible to manufacture objects such as moisture-retentive clothing that would allow people to inhabit extremely arid areas, as in the science fiction saga Dune.
But while carbon nanotubes are old news in new materials science, metal-organic networks, or MOFs for short, are not so well known. These are porous materials made of metals and carbon compounds that are currently being investigated for a variety of applications. In one such application, researchers at the Massachusetts Institute of Technology and the University of California have experimented with a low-cost, solar-powered MOF that is capable of harvesting 0.7 litres of water per day per kilo of material in a desert environment. The start-up Water Harvesting, created to develop the product, is working to create devices that can bring water to people in the world’s driest areas without the need for a power supply.
HARDER, MORE SUSTAINABLE MATERIALS
We all know that diamond is the hardest material, capable of scratching any other solid, which is why it is used in industry, mining and construction. However, other materials are challenging diamond for the hardness throne, such as wurtzite boron nitride or the mineral lonsdaleite, although they are so rare and difficult to synthesise that it has not been possible to experiment with them on a large scale. But for years, humans have been able to create synthetic materials harder than diamond, using elements such as carbon nanorods (solid, as opposed to nanotubes) or carbon plate-nanolattices. These materials could be incorporated in the coming years to improve the strength of aircraft and spacecraft structures.
Steel is the king of metals when it comes to toughness. But by manipulating its microscopic structure to resemble that of natural materials such as bamboo or bone, researchers have managed to increase its strength. The idea is to create a gradient in the size of the metal’s grains so that they are smaller on the surface and coarser towards the inside. The resulting material can withstand more stress than normally manufactured material (thanks to the smaller grains on the outside) and is also more ductile as it approaches the breaking point (thanks to the larger grains in the centre), making it possible to detect failure in time to do something about it before the steel part breaks. The authors are studying these materials for their potential applications in structures or mechanical parts such as those used in the automotive industry.
However, although steel is now recognised as one of the most sustainable building materials, it still has a considerable carbon footprint. Industry is looking at various ways to reduce it, but research into new materials is also seeking alternatives that match steel’s mechanical properties while improving its environmental footprint. For example, intermetallic materials, special types of alloys that are harder and lighter than steel, have been studied for years as their strength is comparable to stainless steel. Researchers are working to address their weaknesses, such as lower ductility and higher brittleness, which would clear the way for their use in many important industrial products.
MATERIALS FOR MORE SUSTAINABLE MOBILITY
A key battlefield in sustainability is that of vehicles, especially in road and air transport. Hybrid and electric motors have become more commonplace in cars in recent years, but while propulsion systems are the main way to lower the environmental impact, they are not the only way. New materials science is also studying metal alloys—including intermetallic materials, mentioned above—that can lighten the weight and thus reduce energy consumption. For example, steel has been combined with aluminium for decades to make it lighter, but the resulting metal is more brittle. Recent research has found that by manipulating the nanostructure of the material to disperse the crystals that form, it is possible to obtain a steel-aluminium that is less brittle and as strong as titanium, but much cheaper.
Another area in which vehicle energy consumption can be reduced is aerodynamics. And while design and shape are crucial, new materials can also bring surprising improvements. Inspired by the dimples in golf balls, which cut the drag caused by air resistance in half, scientists at the Massachusetts Institute of Technology have created an object that can alter its surface properties to suit aerodynamic conditions.
They have designed a hollow ball of soft silicone covered by another, stiffer layer of silicone and a mechanism that allows the air pressure inside to be regulated. As air pressure decreases, the surface begins to wrinkle and form golf ball-like dimples that reduce air friction. In fact, these rough textures are also used in footballs and technical clothing for athletics; the advantage of the material built by these researchers, led by engineer Pedro Reis, is that the wrinkling of the surface can be controlled at will to modify the aerodynamic properties.
Other researchers are working on such materials with morphing capability, or that change shape by controllable mechanisms. Using elastic materials, Reis has also developed cylinders with grooves whose depth can be modified to improve the aerodynamics of their shape. In this case, the engineer was inspired by the morphology of the large Saguaro cactus, whose vertical grooves reduce aerodynamic drag, making the plant structure more robust in the strong desert winds.
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