Unnatural plastics

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.

Gecko nanoadhesive

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.

Spider silk

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."

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