Playground veterans appreciate the marvel that is the Superball, the hard plastic sphere that rebounds almost completely when dropped and then just seems to keep on bouncing. Scientists have recently synthesized nature's version of the Superball. Called resilin, the ultraenergy-efficient elastic protein enables fleas to make their leaps, flies to flap their wings and cicadas to sing their songs. An artificial version might find use in biomedical or industrial applications--perhaps as new stents, heart valves, spinal-disk implants, nanohinges, even running-shoe soles.
First identified some 40 years ago, resilin has long been a target of biomimetic engineers--researchers who work to imitate or copy biological systems. One similar biomaterial that has achieved some practical success is elastin, a protein that allows tissues such as blood vessels to resume their shape after being flexed.
Elastin and resilin share a common structural arrangement that accounts for their rubbery properties--each contains randomly coiled amino acid chains tied together at intervals with molecular cross-links, explains Julian Vincent, a professor of biomimetics at the University of Bath in England. When resilin is swollen with water, its coils can rotate freely, which allows the proteins to unwind as they elongate. Both substances stretch substantially without breaking--elastin by 100 percent, resilin by at least 300 percent.
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Soon after scientists identified the gene that codes for resilin in the fruit fly genome in 2001, Christopher M. Elvin of CSIRO Livestock Industries near Brisbane, Australia, started planning to make it himself. As he and his team reported in the October 13, 2005, Nature, they began by transferring the functional part of the resilin gene into Escherichia coli bacteria. The researchers then grew batches of the microbial factories that expressed the fruit-fly gene and produced the raw protein as a runny solution called pro-resilin.
Converting pro-resilin into a rubbery solid required forming molecular cross-links between the spiral peptide strands. After trying several techniques, Elvin and his colleagues finally succeeded by briefly exposing the pro-resilin to light in the presence of a metal catalyst and an oxidant. This measure initiated a photochemical reaction that generated the needed cross-links.
The solid recombinant resilin features properties matching those of the natural version, Elvin says. The group is now working to better understand the material's basic function so it can synthesize novel polymers that incorporate as building blocks the protein sequence responsible for elasticity.
Several problems need solving before practical applications emerge, however. Natural resilin is extremely stretchy, but it is not very stiff. Nature handles this drawback by weaving it with tough chitin; the same kind of approach may be needed to bolster the man-made version. In addition, because the protein is biodegradable, it will most likely require modification if it is to be used in wet environments, such as in the body. Finally, alternatives to water will be required to maintain resilin's elasticity in dry conditions. "Although the potential seems enormous, now we have to make it work," Elvin emphasizes. "And that lies perhaps 10 years down the track." Should that occur, the next big playground fad might just be "UltraBalls."
