— Scientists are finally closing in on the answers to a puzzle that has perplexed them for centuries: How does a spider spin a web of silk that is five times stronger, on a weight-to-strength basis, than steel?
If we could spin silk as strong as a spider's, we could create all sorts of biomedical devices of extraordinary strength, and make bulletproof vests that might stop a bomb, and create a whole new line of materials for everything from high-performance aircraft to household appliances.
Over the past decade scientists have figured out parts of the puzzle, including the key proteins used by spiders and silkworms to work their magic, but they have been stumped in their efforts to translate their successes into techniques that would allow industrial-scale production of synthetic spider's silk that is as strong as the real stuff.
The hangup was pretty basic. While they understood the substances used by these inventive little animals, they couldn't figure out the mechanics of how spiders combine those substances to make silk. When they tried it in their labs, they got inferior products.
But it turns out that a key part of the answer to this complex question is really pretty simple. It's all in the timing.
Tiny Globular Structures
"I hate to say it's so simple, but timing is everything," says David Kaplan, professor and chair of biomedical engineering at Tufts University near Boston. Kaplan and his former postdoctoral fellow Hyoung-Joon Jin unraveled part of the mystery with the help of some creative scientific sleuthing and a little good luck, as is so often the case.
"We stumbled on it, really, through a back door," says Kaplan, who published their findings in a recent issue of the journal Nature.
Kaplan had been looking for the answer for more than a decade, dating back to the days when he worked as a staff scientist at the Army's Natick Research, Development and Engineering Center in Massachusetts. But that wasn't what he was looking for when he and Jin were trying to artificially spin some regenerated silkworm silk in Tufts Bioengineering Center, which Kaplan directs.
That line of research is sort of an end-run around spiders and silkworms. If we can't spin the silk, maybe we can take silk spun by the critters and add some non-silk polymers to it to enlarge the sample and make interesting things out of it.
By adding the polymers, Kaplan and Jin produced a solution that was a little like syrup because the amount of water, or water content as scientists call it, had been reduced. They were able to use that technique to produce a silken film, but when they looked closely at the new material they were a bit mystified.
"That was when we first observed some unusual features," Kaplan says. The features were tiny globular structures called "miscelles" which combined together to form larger and larger gel-like structures. Could it be, the researchers wondered, that they were seeing the precursors to silk fibers?
To find out, they went back to nature, examining samples from spiders and silkworms, and they found the same kinds of structures. That suggested, quite strongly, that they had stumbled onto a big part of the secret. Spiders control the water content of the gel to prevent the proteins from crystalizing until they are ready to spin the silk fibers. If the proteins crystalized too soon, the process would fail.
So timing, indeed, is everything.
Key Questions Ahead