David Tolfree, VP, Micro, Nano and Emerging Technologies Commercialisation Education Foundation (MANCEF), and Dr Alan Smith, materials scientist and independent technology consultant
In this introductory article, the first of three, we show how the study of animals and plants has enabled us to learn to design, develop and manufacture a range of new products, many of which are based on the properties of materials at the micro and nano levels.
Nature has been actively evolving for about four billion years. It is estimated that there could be 3 to 30 million species1 since the exact numbers are not known. There have been many extinctions and new arrivals because of extreme climate and environmental changes. The evolution of life through natural selection has taught us a lot. These processes use the minimum amount of energy, often with ambient heat and pressure, and achieve their objectives with the minimum effluent to develop end-products. Humans are top of the chain of such products but do not necessarily possess all the attributes necessary for sustainable progression and long-term survival. Our knowledge and technology have altered the natural evolutionary process, and we are now challenged on how we should progress. Looking hard at how the other species have and continue to survive is not just interesting but an important element for our own survival. All life is threatened by disease. Bacteria, our greatest friends and enemies, may have the answers.
In these articles, we will give some examples of how, by studying and mimicking nature, our species has been able to design and manufacture a range of innovative products that include drugs and medicines. The name given to this is biomimetics, from the Greek words ‘bios’, meaning life, and ‘mimetikos’, which translates as imitating.
History of biomimetics
Biomimetics is one of few disciplines that can embrace most branches of science and technology, involving biology, chemistry, design, engineering, environmental science, materials science, medicine and physics. This has been illustrated by the increasing number of papers and publications that have appeared in the last twenty years. The term biomimetics was first coined as early as the 1950s by American biophysicist Otto Schmitt who worked on devices that mimic natural systems. However, efforts to imitate nature took place back in the fifteenth century. Leonardo da Vinci (1452–1519) held aspirations for manned flight and both wrote about and produced a great many drawings of birds in flight and flying machines. Although it was not until 1903, when the Wright Brothers built and flew the first aeroplane, that flying became a reality.
A successful project was undertaken by Percy Shaw, an English businessman who realised that the reflection from cats’ eyes could help motorists driving at night. In 1935, he founded the company Reflecting Roadstuds to manufacture reflective lenses, and in 2006, his idea was recognised as one of the top ten design icons alongside Concorde, the Mini, the Spitfire and the World Wide Web.
The Nobel Prize in Physics in 1986 was divided, one half awarded to Ernst Ruska “for his fundamental work in electron optics and for the design of the first electron microscope,” the other half jointly to Gerd Binnig and Heinrich Rohrer “for their design of the scanning tunnelling microscope.” Subsequent commercialisation of this technique has enabled scientists to look down at the nanoscale and understand how molecular processes work.
Some useful products
A commercial biomimicking achievement was made by the Swiss engineer George de Mestral, who in 1941, went on a hunting trip in the Alps and found his trouser legs and his dog were covered in burdock burrs (seeds). He figured out how the burrs managed to stick so well, and in 1955, he took out a patent on the first hook and loop fastener branded as VELCRO and that today is used in various forms across multiple industries, including apparel, construction, medical, packaging and transportation.
one of the best examples of biomimetics is the detailed study of the gecko. They can run up and down walls and along ceilings upside down. At the nanoscale, the surface properties of materials differ widely from those of the bulk material. The reason for this is that at the surface, the physics of molecules is largely determined by intermolecular forces, collectively known as van der Waals forces. Geckos have nanoscale fibrillar structures or hairs on their feet, and each one is attracted by a minute van der Waals force to the surface it is on2. Since there are millions of these hairs, called setae, on each toe, it is possible for the animal to support a lot more than its own weight, even upside down (figure 1). A number of dry adhesive products modelled on geckos have been developed, several examples of which are summarised below.
Figure 1: A gecko can walk upside down on a ceiling, its sticky feet supporting its weight.
•Scientists at the University of Massachusetts Amherst (UMass Amherst) developed a stiff adhesive fabric known as Geckskin. The fabric mimics the feet of the gecko, gripping and peeling away easily from any surface, leaving no residue, and holding over a 300 kg/m2 area. It is being commercialised by the UMass Amherst start-up company Felsuma for hanging devices such as hooks as well as sealant tapes.
•The Defense Advanced Research Projects Agency (DARPA)—an agency of the US Department of Defence—runs a programme called Z-Man for the development of biologically-inspired climbing aids that enable soldiers to scale vertical walls while carrying a full combat load, Spider-Man style. In 2009, the Z-Man programme provided a grant for the aforementioned Geckskin, and in 2012, a proof-of-concept demonstration showed that a 103 cm2 sheet of the fabric adhering to a vertical glass wall could support a static load of up to 299 kg. Z-Man has also supported, and continues to support, the American not-for-profit R&D organisation Draper Laboratory in its development of a gecko-inspired adhesive product.
•The American company nanoGriptech developed a range of dry adhesive products branded Setex as a result of studying geckos. Applications include residue-free masking of components during manufacturing processes and increased friction or gripping of sports gloves for improved ball control.
•In 2008, researchers at the University of Dayton in the US reported a gecko glue capable of supporting 100 N/cm2 and said to equate to 10 times the strength of a gecko's foot.
If it had not been possible to see the geckos’ setae using the microscopes mentioned above to figure out how they seemed to defy gravity, then the many companies marketing sticky coat hangers, tapes, car dashboard pads, etc. may not be in business (figure 2).
Figure 2: A gecko-inspired sticky car dashboard pad.
Another biomimetic property that has given rise to numerous commercialised products is superhydrophobicity linked to self-cleaning surfaces. It was found that the lotus leaf, as well as the water lily leaf, repels water (figure 3). Close examination of the leaf's surface reveals that it is not flat, but covered in lots of minute bumps. When a droplet of water lands on the leaf, the surface tension effect pushes it away. The droplet is a bit like a hovercraft as it rises above the surface, and when it rolls off the leaf, dirt is gathered up and the surface cleaned (figure 4). The concept of these minute bumps has been copied for several products. For example, stain resistance for a lot of garments and upholstery is achieved by applying a layer of Teflon just a few nanometers in thickness to fabric fibres in order to form a non-stick surface. The layer is so thin that the fabric still has the same feel as the original material.
Figure 3: Lotuses.
Figure 4: A diagram showing the surface tension effect of a lotus leaf on a water droplet.
Superhydrophobicity is also demonstrated by the fogstand beetle (Stenocara gracilipes), which is native to the Namib, a coastal desert in Southern Africa and one of the most arid areas in the world, receiving only 1.4 cm of rain per year (figure 5). The beetle is able to survive by collecting water from early morning fogs on the bumpy surface of its back. As the moisture from the air condenses on the tiny bumps, it forms a large drop that runs down the hydrophobic surface and is channelled straight into its mouth. The South Korean designer Pak Kitae developed a product called the Dew Bank bottle that mimics the beetle’s ability to collect water, thus enabling people in water-starved areas of the world to collect one glass of water overnight.
Figure 5: A fogstand beetle (Stenocara gracilipes) in the Namib, a coastal desert in Southern Africa.
Collecting water is a major issue in many countries. Fog harvesting nets that mimic the way spines and hair on some cacti species collect droplets of fog water are being used in the Atacama Desert in Chile3 and on the edge of the Sahara in Southwest Morocco4. The nets are capable of collecting around 500 l of water overnight.
Sharks’ skin—like that of other marine creatures—is superhydrophobic and numerous product developments are attributable to it. The American biotechnology company Sharklet Technologies found that by replicating the rough, sandpaper-like surface of shark skin, green algae settlement could be reduced by 85 percent compared with smooth surfaces. This led to the company introducing a range of micropatterned antibacterial surfaces for medical products such as catheters and dressings.
Research projects
There is a European Union (EU) project called PHOBIC2ICE aimed at overcoming the accretion of ice on aircraft in sub-zero conditions. This icing is a severe problem for planes, as the presence of just a scarcely visible layer can severely limit the function of wings, propellers, windshields, antennas, vents, intakes and cowlings; this is perhaps best yet most sadly exemplified by the Munich air disaster in 1958, an event forever ingrained in many football supporters’ memories. PHOBIC2ICE is developing deposition technologies and predictive simulation tools for avoiding or mitigating icing. These solutions are expected to enable the design and fabrication of icephobic surface treatments and/or coatings affording icephobic and superhydrophobic properties.
A team of researchers from the University of Adelaide and the University of South Australia (UniSA) is using nano-modification technology to create dragonfly-inspired, titanium medical implants that are expected to reduce the chance of infection. The implants are to have an antimicrobial surface that mimics the nano patterns of the surface of dragonfly wings. This is because dragonfly wings are covered in tiny spikes known as nanopillars that have been found to rip bacteria apart. The nano surface is independent of the chemistry and material properties that it is applied to and so the technology could also potentially be applied to products in other industries, including aeronautical, construction, food and marine.
Shells are another source of inspiration in biomimicry, and some of the applications are based on the colours achieved by certain molluscs. The bright colours from the paua shell, for example, are the result of light bouncing off the microscopic layers of the shell in a similar way to how light is broken down by a prism into the colours of a rainbow (figure 6). Paua shells are used in jewellery, as are the pearl effects from many other shells. A few years ago, Christian Dior introduced a perfume called Pure Poison, and its glass container had nano-thick layers to make it look like pearl.
Figure 6: A paua shell.
Oysters secrete an extremely hard and durable calcium carbonate material known as nacre for both the inner layer of their shells and outer coating of pearls; its formation takes place in cold water and without pressure. In 2008, a team of scientists at the University of Dayton Research Institute (UDRI) received funding from the Air Force Office of Scientific Research (AFOSR) to manipulate oyster blood cells into depositing nacre on metal alloys and thus create multi-layered pearlescent coatings. The objective was to investigate the possibility of developing a non-hazardous (specifically room-temperature and -pressure) manufacturing process for lightweight ceramic aircraft coatings that protect against corrosion and impact.
Biomimetics and nanotechnology are now inexorably linked, and the main properties being exploited are:
• adhesion;
• aesthetics;
• durability;
• light weight;
• sensing (particularly in medical applications); and
• super-hydrophobicity.
The table contains examples of products that are either on the market already or are in development. Many of the examples provided are reliant on manufacturing techniques that can lay down thin films and vary the topography of surfaces, factors that can prove challenging for manufacturers.
We hope this first article stimulates readers’ interest in studying the natural world more closely, since there is still a great deal to learn. With the inevitably of climate change and the rapid development of technology, such studies could help us to meet the challenges we face to sustain our own wellbeing and even our survival.
David Tolfree, VP, MANCEF
Dr Alan Smith, materials scientist and independent technology consultant
References
1Hickman, C., Roberts, L., Keen, S. (2012). Animal Diversity (6th ed.). New York: McGraw Hill; p. 479.
2Tolfree, D., Smith, A. (2018). Sticky feet. Physics Review; Vol. 27, No. 3, pp. 7–11.
3 Eaglescliffe, B. (2018). Harvesting fog for fresh water in Chile and Peru [press release]. August 28. Owlcation. Available at: https://owlcation.com/stem/harvesting-fog-in-peru-for-drinking-water
4Prisco, J. (2016). Desert 'fog catchers' make water out of thin air [press release]. November 18. CNN. Available at: http://edition.cnn.com/2016/11/18/africa/fog-catchers-morocco/index.html