Emulating nature for better engineering

UK researchers describe a novel approach to making porous materials, solid foams, more like their counterparts in the natural world, including bone and wood in the new issue of the International Journal of Design Engineering.

According to Carmen Torres-Sanchez of the Department of Mechanical Engineering, at Heriot-Watt University, Edinburgh and Jonathan Corney of the Department of Design, Manufacture and Engineering Management, at the University of Strathclyde, Glasgow in the natural world, the graduated distribution of porosity has evolved so that nature might transfer forces and minimise stresses to avoid whole structure failure. For instance, a crack in the branch of a tree will not lead to the felling of the tree in the same way that a broken ankle will not lead to collapse of the whole leg. “Porosity gradation is an important functionality of the original structure that evolution has developed in a trial and error fashion,” the team explains.

It is not just tree trunks and bones that have evolved graduated porosity, beehives, marine sponges, seashells, teeth, feathers and countless other examples display this characteristic. Researchers would like to be able to emulate the way in which nature has evolved solutions to the perennial issues facing engineers. In so doing, they will be able to develop structures that use the least amount of material to gain the lowest density structure and so the maximum strength-to-weight ratio.

“Many engineering applications, such as thermal, acoustics, mechanical, structural and tissue engineering, require porosity tailored structures,” the team says. If materials scientists could develop porous materials that closely mimic nature’s structural marvels, then countless engineering problems including bridge building and construction in earthquake zones, improved vehicle and aircraft efficiency and even longer-lasting more biocompatible medical prosthetics might be possible.

Unfortunately, current manufacturing methods for making porous materials cannot mass-produce graduated foams. The collaborators in Scotland, however, have turned to low power-low frequency ultrasonic irradiation that can “excite” molten polymers as they begin to foam and once solidify effectively trap within their porous structure different porosity distributions throughout the solid matrix. This approach allowed the team to generate polymeric foams with porosity gradients closely resembling natural cellular structures, such as bones and wood. The technology opens up new opportunities in the design and manufacture of bio-mimetic materials that can solve challenging technological problems, the team adds.

The researchers anticipate that using more sophisticated ultrasound energy sources as well as chemical coupling agents in the molten starting material will allow them to fine tune the formation of pores in the material. This is an area of current interest because it would facilitate the design of novel texture distributions or replicate more closely nature porous materials, the team concludes.

Sanchez, C., & Corney, J. (2011). A novel manufacturing strategy for bio-inspired cellular structures International Journal of Design Engineering, 4 (1) DOI: 10.1504/IJDE.2011.041406

A radical approach to understanding polymers

Polymerization is used to make a whole range of materials but understanding exactly what happens during synthesis when it involves free radicals is difficult. Now, New Zealand chemists have uncovered important clues by following the rates of reaction and the termination steps involved.

Radical polymerization is commonly used to make half the world’s polymers and now Greg Russell and his colleagues at the University of Canterbury have investigated the kinetics of the process that shuts off polymerisation, the termination rate coefficient, for some very common reactions. They have revealed that the diffusion behaviour of short polymer strands, oligomers, in the reaction system is the critical factor.

“The majority of chemists simply try to bring about reactions by mixing different chemicals together under different conditions,” Russell explains. “However it is also important, especially for those who make chemical products on a large scale, to have precise quantitative descriptions of the speeds at which reactions occur. Chemical kinetics is the field of work that develops such descriptions. It is therefore an area where chemistry and mathematics intersect.

Greg Russell
Greg Russell

He adds that the most difficult problem in radical polymerization is the termination step that stops the polymer growing longer. Diffusion of the growing polymers is affected by their size, concentration, temperature and so on, which makes termination a complicated process to study, one that even after 60 years of intensive investigation is still not fully understood.

Russell, who is on sabbatical at the University of Goettingen, Germany, worked with Philipp Vana there and have found that although highly specialized techniques for measuring termination rate coefficients under precisely controlled conditions have been employed the results they found on attempting to replicate earlier studies were inconsistent. “I have taken this information and attempted to see whether it is consistent with systems where many different termination reactions occur at once, as is the case in commercial processes,” Russell says. “For the monomer styrene [used to make polystyrene] I find there is consistency, but for methyl methacrylate [used for polymethylmethacryalate, PMMA] there is not.”

In trying to explain this result, he eliminated most of the conventional views, and came to the conclusion that the answer lies with the oligomers in each system, which seem to have slightly different diffusional behaviour.

LINKS

Macromol. Chem. Phys. 2010
http://dx.doi.org/10:1002/macp.200900668

Greg Russell
http://www.chem.canterbury.ac.nz/people/russell.shtml

Philipp Vana
http://www.fpm.chemie.uni-goettingen.de/pvana.htm

Pathological proteins produce polymers

Deposits of distorted or otherwise errant proteins are key to understanding various brain diseases including Alzheimer’s, Parkinson’s, and the prion disease variant-CJD, they are also implicated in the pathology of type II diabetes. However, while such amyloids are a medical nightmare, researchers in Israel suggest that outside the body, synthetic versions of these substances could help us design a whole new range of nanomaterials and biomimetic plastics.

“The potential applications of these supramolecular assemblies exceed those of synthetic polymers,” explains Ehud Gazit of Tel Aviv University, writing in the current issue of Angewandte Chemie with co-author Izhack Cherny. “The building blocks may introduce biological function in addition to mechanical properties, he adds.

Prof. Ehud Gazit

Prof. Ehud Gazit

While the focus on amyloids is usually on their pathology, even in nature they are not always abnormal, misfolded proteins, they do have physiological roles in some organisms. For example, amyloids are an important protective material in the egg envelopes of insects and fish, they help form bacterial biofilms to protect a colony from natural antimicrobial substances that the bacteria may encounter, and they also allow such blooms to attach themselves to surfaces more effectively.

Technically speaking, amyloid fibrils are usually bundles of highly ordered protein filaments composed of ladder-like strands that can stretch to several micrometres in length. In cross-section, amyloids look like ribbons or like hollow cylinders. But, it is their resemblance to synthetic polymers (plastics) rather than their proteinaceous properties that drew the attention of the researchers in Israel.

Building amyloid polymers as templates for nanowires (Credit: Adapted from Angewandte)

Building amyloid polymers as templates for nanowires (Credit: Adapted from Angewandte)

Amyloids, for instance, are almost as strong mechanically as spider silk and by turn spider silk, weight for weight, is stronger than steel. They can also be stretched to many times their original length without splitting. Both properties are inaccessible to scientists working with synthetic polymers, but both properties are highly desirable for a wide range of engineering and technologies.

“The self-assembly properties of amyloids, together with their observed plasticity, makes them attractive natural building blocks for the design of new nanostructures and nanomaterials,” Gazit explains, “These building blocks can be broadly varied by means of simple molecular biological techniques.” The products might be used in novel sensors, tailored, biocompatible coatings, as enzyme mimics for speeding up chemical reactions, and for constructing nanoscale wires filled with silver and coated with gold for molecular electronics applications.

Further reading

Angew. Chem. Int. Edn, 2008, 47, 4062-4069
http://dx.doi.org/10.1002/anie.200703133

Prof. Ehud Gazit homepage
http://www.tau.ac.il/lifesci/departments/biotech/members/gazit/gazit.html