Scientists have been looking to nature for inspiration for a new generation of plastics. As E&T discovers, the catalyst for their development has been the advances in synthetic biology techniques.
Invertebrate zoologist James Weaver was out on a fishing trip collecting specimens when a Humboldt squid latched on to his arm and secured itself with its powerful tentacles.
While the first thought of most of us would have been the pain, Weaver began to wonder how it was that the squid could generate such enormous power through the soft tissue of the suckers which line the tentacles.
His curiosity could lead toward a new generation of stronger and more abrasion-resistant materials. It's also just one example of how multidisciplinary teams are looking at proteins as potentially very important structural materials. Bio engineers believe they could produce strong, light material cheaply using proteins or mimicking their structures.
The Humboldt squid is an aggressive predator whose main weapon is its suckers. Each of these contains a rigid disc lined with triangular teeth.
Weaver says: "These structures have been known about for many years. In the Humboldt they are exceptionally well developed and I just thought about looking at them from an engineering perspective."
The specimens were studied at the Bourns College of Engineering at the University of California at Riverside (UCR) under Prof David Kisailus. What they found surprised them.
"Most of the things we study, like the abalone shell, have minerals in them," says Prof Kisailus. "Other types - say, the lobster cuticle - have lots of crystalline materials. The squid rings are purely protein.
"So we're looking at how you can get these unique multifunctional properties from such a soft material. Based on how it is designed you can harden it or increase its impact resistance which opens up doors for designing real engineering."
Just studying the squid required different techniques from those used on hard tissue or engineered materials.
The team, which included colleagues from the University of California at Santa Barbara (UCSB) and Aarhus University in Denmark, found that the sucker rings contained a network of nano-porous channels. These were orientated parallel to the long axis of the teeth. The channels are between 100nm and 250nm in diameter, increasing from the outer surface to the inner core of each tooth.
It makes them abrasion-resistant on the outside and flexible inside solely by altering the local pore fraction.
Professor Kisailus said: "In terms of how the squid produces this, it is a highly complex process which many people are trying to understand.
"We aren't so much interested in how the protein is formed and self-assembled. All we do is look at the structure and what it is that gives it properties such as hardness or impact resistance, and then try and come up with an engineering solution to basically mimic the structure.
"We are looking at different types of material instead of using a protein such as polymers, metals and ceramics. The design principles will be the same.
"Using this micro structural architecture with pores and having a radiance or distribution of pore sizes throughout the structure will give these materials their multifunctional properties."
Such materials might be used in the automotive, aerospace, sports and medical industries.
Back on land and the ability of one particular animal to run rapidly in any direction along a vertical surface has long fascinated scientists. It is not just the gecko's grip, it's the swift detachment and re-engagement that is intriguing.
A gecko's 'stickiness' comes from the millions of microfibres - called setae - on its feet. The end of each seta consists of hundreds of spatulae plates about 100nm in diameter. The setae and spatulae are mostly made from beta keratin, a hard protein which is similar to our fingernails or hair. These lightly grip though van der Waals contacts as they brush a surface and have the advantage of being self-cleaning. Compare this with adhesive tape, which needs to be pushed onto a surface to form a bond and picks up any loose material when it comes away.
At the University of California at Berkley's (UCB) Biomimetic Millisystems Lab they developed a similar self-cleaning adhesive using microfibres which prefer contact with a large surface than dirt particles.
At Northwestern University, the Messersmith Research Group under Dr Phillip Messersmith has looked at where geckos perform well - and less well - in their ability to constantly connect and release.
The group developed a nanoadhesive called geckel by combining some of a gecko's properties with those of mussel adhesive proteins. Geckel has nanopillars coated with this mussel-mimetic polymer to make it work repeatedly in wet and dry conditions. Work is ongoing to scale-up production of the adhesive for mass production.
A team at the Massachusetts Institute of Technology have taken this idea of gripping wet surfaces using gecko-inspired techniques to come up with a biodegradable bandage which might eventually replace sutures.
Some are looking at first analysing what is it about the structure of the material which gives it useful properties and then applying that to their own materials.
Dr Danielle Tullman-Ercek, assistant professor of chemical engineering at UCB, has been working at UCSF's Voight Lab on extracting a natural material in industrial quantities.
Dr Tullman-Ercek is one of many scientists looking at spider silk production and her approach is to produce a natural biomaterial in an artificial - but still biological - host.
Currently spider silk is being produced in transgenic goat's milk. This is rather costly, but is preferable to trying to 'farm' the silk from spiders directly as they are very territorial and tend to eat each other.
"Our idea is to produce the silk using bacteria instead," she explains. "Silk, whether from a silkworm or a spider, is a natural biopolymer of repeating protein sub units. Of course, proteins are polymers themselves, and once we know the amino acid sequence of a protein it would seem a trivial problem to produce it in any biological host using gene synthesis and an appropriate expression vector.
"In the case of spider silks, it was a bit more complicated, because the silk proteins themselves exhibit some toxicity upon expression in bacteria, and may even be self-assembling within the cell.
"This protein aggregation and toxicity kills the bacterial host, which is not good since the bacteria are our factories. To get around this problem, we made use of a natural system (Type III Secretion) that would segregate the spider silks as they are made, and then secrete them completely out of the cell.
"Once out of the cell, it is easy to purify the proteins, and other research groups are working on how to extrude the resulting purified proteins into spider silk threads.
"Since the production of the silk proteins is occurring in each bacterial cell, scale-up of production is achieved by simply growing more cells. Then the bacterial growth and protein expression must be re-optimised for production in bioreactors of large volumes, in which oxygen levels, mixing, pH, and other such conditions are adjustable but quite different from flask-based growth. This is not always trivial but such scaling up is routinely, and successfully, carried out for other biological products. The purification and extrusion processes would also need to be reoptimised, but this is outside my area of expertise."
So why go to the trouble?
The strength of spider silk is well known but it has the added advantage of not generating a response from human immune systems. This makes it ideal for medical applications.
Dr Tellman-Ercek says: "More important to our group is the fact that the properties of silks are entirely dependent on the underlying polymer (amino acid) sequence. Since we are now in control of the sequence produced, we can tailor the properties of the silks by varying the amino acid sequence.
"Nature already does this; spiders produce a variety of different silk monomer proteins for use in different types of thread. The 'spokes' of the web are not the same composition as the dragline, for example. The applications for such tailored material range from stronger but more flexible bullet-proof vests to softer blouses that will not pill up upon repeated washing, and are really limited only by our imaginations."
Back in 2007, James Newcomb, managing director for research at Bio Economic Research Associates, wrote several influential papers considering the future of bioengineering.
He talked of a "new engineering approach to biology" which had "seeded a dynamically expanding network of researchers and start-up companies".
Those seeds are flourishing.
In April, up to 8,000 scientists are expected to attend the Materials Research Society spring meeting in San Francisco with 42 symposia on different aspects of this field.
As Professor Kisailus says: "There is a community and it is growing."
The Synbion Project
While the pharmaceutical industry has dominated research into protein use, other applications are growing.
The Synbion Project is funded by the Biotechnology and Biological Sciences Research Council and the Engineering and Physical Research Council and led by Professor John Ward from University College London. It is looking at whether synthetic biology might be used to develop new electronic devices.
As Professor Ward told E&T when Synbion was launched: "We want to start seeing how we could use proteins that could be arrayed into specific shapes or put onto the surface of objects like bacteriophages.
"We want to get people thinking how you would make components analogous to those used in electronics today."
That might be getting viruses to produce proteins in regular arrangements to harness light or even build displays.
For more information, see www.smb.ucl.ac.uk/synbion/ [new window].
Calcium and dopa
The tentacles of the Humboldt squid aren't the only part of sea creatures to catch the eye of scientists.
At UCR, Professor Kisailus's team is also studying the inner layer, or nacre, of the abalone shell on behalf of General Motors.
Professor Kisailus says: "The nacre is calcium carbonate in a brick and mortar structure. Calcium carbonate is basically chalk but these things are 3,000 times tougher than chalk and we are investigating why that is.
"What is the design principle behind the nacre's incredible fracture toughness and can we go out and engineer composite structures for, say, body panels.
"We are also looking at bio-minerals that are abrasion resistant such as the magnetic teeth in a particular mollusc which they use to grind away at structures to get at their food.
"The reason we do that is these organisms that form these minerals control their inorganic crystals, the size and shape, and this could help with energy applications."
The team is looking at controlling the morphology of the nano structures at the heart of these organisms so as to provide enhanced catalytic properties which could be used in batteries and solar cells.
Meanwhile colleagues at UCSB have been looking at squid beaks, and in particular how such a tough limb works in conjunction with a relatively flabby body. Scientists liken it to having a knife held by a jelly handle.
The answer is that the beak is a hundred times tougher at the tip than at the base meaning a much smoother fit to the body.
The beak is made up of chitin, water and protein enriched by Dopa. The nearer the tip, the less chitin and the more Dopa-enriched protein there is.
According to Dr Frank Zok at UCSB, mimicking this graded material could lead to much stronger adhesives.
"If you graded an adhesive to make its properties match one material on one side and the other material on the other side you could potentially form a much more robust bond," he says.
"This could really revolutionise the way engineers think about attaching materials together."
The remarkable jumping ability of fleas and the intense effort needed by insects to fly are due to one particular protein.
Resilin has elastic properties that no artificial version can match. It can stretch to twice its length for long periods and still return to its original shape. It's durable enough that it doesn't need to be renewed in insects despite the huge amount of work it undertakes.
Resilin makes up the wing hinge in creatures such as dragonflies which need to perform some 500 million cycles in their lifetime.
Dr Michael Dickinson, who runs the Dickinson Lab aimed at understanding and mimicking how flies fly, calls it "the must complicated joint in the animal world".
Australian scientists have succeeded in copying resilin. They cloned a portion of the resilin gene in a vinegar fly and expressed it in bacteria as a soluble solution. Using a patented process they then created a solid version which showed 97 per cent recovery after stress - well in excess of conventional materials.
It's not the end of the story, though.
Professor Steve Shaw, from Dalhousie University's Department of Psychology and Neuroscience, looked at a creature best known for blowing bubbles out of its bottom.
The spittlebug can leap 100 times its own body length in a sudden explosion of flight.
This is achieved through a contraction of muscles using the energy stored in the spittlebug's resilin joint. This is set against the creature's cuticle body armour.
Much as a bow achieves its effect with a hard back and an elastic string working together, so the spittlebug's leap is due to the resilin and cuticle working in unison.
Scientists are now looking at creating their own version of this joint.
The economics of oil and concerns about the disposal of traditional plastic make it increasingly profitable to look at alternatives. Bioplastics are the result.
All the major plastic companies and many smaller outfits are looking at creating industrial polymers from plants.
DuPont opened its first bioplastic plant in 2006, in Brazil petrochemical giant Braskem is spending £250m on a factory producing sugarcane-based bioplastics, and in Japan they are using fermented plant starches.
Against these are companies such as Metabolix based in Cambridge, Massachusetts.
It creates its product, Mirel, through genetically modified microbes. These feed on sugars from corn and the result is dried and turned into pellets. From this a variety of bioplastic materials can be made, all of which degrade naturally when composted.
These new materials are often twice the price of traditional plastics and there are still 500 times more traditional plastics produced than the greener variety.
However, as production becomes more efficient, and if the current economic model continues, that ratio can only reduce.
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