Voyage to the centre of the material for polymers
Understanding, to better reproduce
Researchers working on biomimicry do not simply copy the mechanics found in nature. A fairly recent trend in the field involves seeking inspiration from nature’s working at a molecular level. Every day, new materials, mainly polymers, are tested in laboratories. Scientists have many tools enabling them to dive into the heart of materials, such as the tunnelling microscope. There are many avenues for research: butterfly wings, spiderwebs, shark skin, and more. The aim is to chemically reproduce the surprising biomolecular properties of certain animals on a nanometric scale. However, there is one crucial prerequisite: understanding how nature works! In many cases, quite a lot of mystery remains.
Gecko and polymers, fast friends
or biomimicry specialists, the gecko, a type of lizard found around the world, is iconic. It is sometimes considered one of nature’s greatest inventions. Like other lizards and insects, the gecko is able to walk on ceilings. That is something in and of itself, but its greatest achievement is being able to carry a weight of 40 times more than itself without coming unstuck, despite weighing just under 100g .
Its secret is the way in which its fingers are designed. Simply put, a gecko’s fingers are made up of millions of keratin hairs measuring only a few microns at their base. The ends of the hairs are split into thinner hairs ending in a spatula pattern. At the molecular level, the Van der Walls force comes into play. Roughly-speaking, it is a force able to link atoms or molecules. In short, the gecko’s secret lies in the microscopic structure of its hairs.
SAlthough reproducing the hairs in a synthetic material has become an obsession for researchers, car manufacturer Ford decided to take matters into its own hands and allocated considerable resources to research.
The automotive industry consumes large amounts of plastic parts, and most of them are assembled with glues that are difficult to clean off, complicating plastic recycling processes. The aim is clear: foregoing glues by using the gecko effect to assemble parts together. There is still much work to be done, and Ford has prudently refrained from setting any dates. The manufacturer has partnered with the chemical giant Procter & Gamble in order to review large amounts of polymers and attempt to develop their molecular formula to reproduce the gecko’s famous hairs.
Polyurethane for future Spidermen
Spiders remain fascinating, and their webs in particular, as they were often considered the world’s strongest material. Strong might not be the most appropriate word, given that a small pair of scissors can cut through any web. What impresses scientists is the surprising amount of energy required to stretch the web to its breaking point. The strands seem to stretch “infinitely” before breaking. A French team may have solved the mystery, which is not in any way related to the molecular formula of spider silk. Roughly-speaking, spiders seem to produce droplets of a sort of glue containing a reserve of spider silk, along with the silk itself. The silk unwinds when it is stretched, and winds back up when released. An international team is using this discovery to attempt to chemically reproduce spider silk. The only problem is that the thread’s diameter must be between 2 and 5μ in order to function. A human hair measures 80µ, as a reference.
At this time, ethanol and polyurethane seem to have the same properties. Does this mean that we will soon be seeing the new material? Difficult to say, but it’s a safe bet that it will be used in many applications, including bulletproof vests, sports clothing and medical prostheses, as the fibre is said to be even stronger than Kevlar.
Polymers flutter from one discovery to the next
Butterfly wings are also a subject of endless fascination for researchers due to their ability to play with light. We now know how they do it, and it has to do with the structure of their scales. Each scale is striated, creating a network conducive to the diffraction of light. But that is not all: the scales are not made of a single part, they are in fact stacks of layers of chitin (one of the components of insects’ exoskeletons) which strengthen the iridescence produced by the wings. In other words, the wings change colour depending on the angle from which they are viewed or the angle of the light. Quite the useful phenomenon for escaping predators. The iridescent effect on butterfly wings is of great interest to textile professionals who are seeking to reproduce it. For the time being, they hope to achieve it by weaving two different fibres together, one of which is made up of a semi-transparent polymer that reflects light. All that remains is to develop a weaving technique able to reproduce the striated structure of butterfly scales.
The Greta Oto, a butterfly native to Central America, has quasi-transparent wings which do not reflect light, making it almost invisible.
This uncommon feature is also down to the structure of the wings. Their scales are made up of columns one thousand times thinner than a human hair which are structured chaotically. As a result, there is only a very small surface on which light might be reflected. This discovery should lead to the development of new types of screens which should never inconvenience their users with reflected light. After having learned to conduct electricity, in flexible screens in particular, polymers could end up being the de facto material for the screens of future tablets and telephones if they become able to outmanoeuvre light.
Bacteria-hunting sharks
OEveryone has heard about the ultra-thin polyurethane swimsuit inspired by sharkskin patterns. It triumphed in 2008 when it contributed to around forty swimming records being broken. The suit imitates the roughness of sharkskin which, when in movement, creates turbulence that reduces drag by around 5%. More than enough to break a record! They have since been banned from competitions, as they disadvantage swimmers that do not have such suits. However, sharkskin has other advantages. Along with improving speed, it can also hunt bacteria. Biologists have long wondered why no algae developed on sharks, unlike those found on whales and seals, among others. The final conclusion was that no living organism, such as bacteria for instance, could attach itself to the skin.
After many passes under microscopes, researchers found that sharkskin was impervious to organic attacks because it is made up of millions of minuscule pointy denticles preventing any living organism from settling on it. American company Sharklet was able to exploit this feature by developing a new type of covering made from 3µ denticles. From a technological perspective, the greatest achievement involved reproducing the micro and nano patterns of sharkskin on a polymer film. Unsurprisingly, the nature of the film and the manufacturing technique are kept secret; the only certainty is that an injection-moulding process is used. The new coating, free of chemicals and antibiotics, is already being used in many applications, particularly in hospitals. It is used in operating theatres, on door handles and on medical records that are passed between various people, because bacteria are transferred through physical contact.
