Materials inspired by nature

From imitating the exceptional toughness of Mother of Pearl to the way geckos stick to walls, the burgeoning field of biomimicry is inspiring the design of new, leading edge engineering materials. Laura Hopperton reports.

From the silk of a spider's web to the armour of a turtle's shell, nature's materials fascinate. Natural selection has produced some pretty amazing examples, with extraordinary properties that are still unrivalled by almost anything manmade. So it is unsurprising that engineers are increasingly turning to the planets own 3.8billion year materials development programme for inspiration.

Biomimicry – or biologically inspired engineering – is the study and imitation of nature's best ideas to help solve human challenges. From a materials engineering point of view, it adopts the premise that natural materials (and the processes that lead to them) have evolved, under conditions that favour the efficient use of available resources. It suggests that if a natural material does a better job than an existing synthetic counterpart, then biomimicry should be considered.

One company utilising this approach is aircraft manufacturer Airbus, which has developed a range of aeronautical materials inspired by natural structures as part of its bid to build the fuel efficient aircraft of tomorrow. Taking inspiration from the way the lotus leaf has evolved to keep its surface clean and dry by causing rain water to roll off and take dirt with it, the company recently developed an innovative synthetic compound for its cabin fittings that does exactly the same. The coating sheds water in beads, taking contaminants with it and is designed to improve hygiene and reduce the amount of time, effort and water needed for cleaning.

"We've also been turning to biological systems such as shark skin to see how we can reduce drag," says Airbus UK's head of policy development, David Hill. "The skin of a shark is covered in microscopic grooves that scientists have found actually reduces their drag through the water, allowing them to conserve energy as they search for food. While this ability has proved extremely difficult to emulate in the air, we are working with universities across the world to see if it's something we can deploy in future aircrafts. The benefits or reducing drag, fuel burn and carbon emissions, we believe, will be quite substantial."

Airbus engineers have also been exploring nature inspired manufacturing techniques to create 'bionic bones' for safer, higher performance aircraft structures. According to Hill, part of the company's fascination with taking this approach is to help it improve its environmental performance. This philosophy that mimicing nature will also help deliver environmental drivers is being echoed elsewhere in the academic world.

Researchers at Harvard University's Wyss Institute for Biologically Inspired Engineering, have developed a cheap, biodegradable, biocompatible material called Shrilk. They believe it could one day provide a more environmentally-friendly alternative to plastic. Designed to replicate the exceptional strength, toughness and versatility of insect cuticles, the material takes its name because it is composed of fibroin protein from silk and from chitin, which is commonly extracted from discarded shrimp shells.

The material is thin, transparent, flexible, and according to postdoctoral fellow Javier Fernandez, as strong as an aluminium alloy at half the weight. It was created in the lab after Fernandez and his team studied closely the interactions between the layers that make up insect cuticles, before mimicking their unique chemical and mechanical properties.

"A major benefit of Shrilk is its biodegradability," says Fernandez. "Plastic's toughness and mouldability represented a revolution in materials science during the 1950s and '60s. Decades later, however, the material is raising questions about how appropriate it is for one-time applications such as plastic bags. The great thing about Shrilk is that not only will it degrade in landfill, but its basic components can be used as fertiliser, so it will enrich the soil."

In addition, Fernandez says Shrilk can be produced at a very low cost, since chitin is readily available as a shrimp waste product. It is also easily moulded into complex shapes, such as tubes. By controlling the water content in the fabrication process, the researchers have even been able to produce a wide variation in stiffness.

These attributes could have multiple applications. For example, as a cheap, environmentally-safe alternative to plastic, Shrilk could be used to make rubbish bags, or consumer packaging that biodegrades quickly.

It's also an exceptionally strong, biocompatible material so could be used to suture wounds that bear high loads, such as in a hernia repair.

Fernandez says: "Shrilk has the potential to be both a stepping stone toward significant medical advances and a solution to some of today's most critical environmental problems."

Looking to create a more environmentally-friendly alternative to today's adhesives, scientists at the University of Massachusetts Amherst (UMass) in the US have taken inspiration from geckos in the way they stick to surfaces. They have developed a 'super adhesive' called Geckskin that can hold a maximum force of approximately 320kg while adhering to a smooth surface such as glass.

Duncan Irschick, a biologist on the project says: "Gecko feet can be applied and disengaged with ease with no sticky residue remaining on the surface. It's a tantalising possibility, synthetic materials that can easily attach and detach heavy everyday objects such as televisions or computers to walls."

While previous efforts to synthesise the tremendous adhesive power of gecko feet and pads were based on the qualities of microscopic hairs on their toes called setae, attempts to translate them to larger scales were unsuccessful. The researchers at UMass have been able to demonstrate that setae are not required for gecko-like performance.

The key innovation was to create an integrated adhesive with a soft pad woven into a stiff fabric, which allows the pad to 'drape' over a surface to maximise contact. As in natural gecko feet, the skin is woven into a synthetic 'tendon,' yielding a design that maintains stiffness and rotational freedom.

"It's a concept that has not been considered in other design strategies and one that may open up new research avenues in gecko-like adhesion in the future," says Irschick. "The Geckskin's adhesive pad uses simple everyday materials such as polydimethylsiloxane, which holds promise for developing an inexpensive, strong and durable dry adhesive. Our design for Geckskin shows the true integrative power of evolution for inspiring synthetic design that can ultimately aid humans in many ways."

Laura Hopperton

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