By a number of measures, spider silk is one of the toughest materials around. It's also light weight and (obviously) biocompatible. Unfortunately, it's also extremely hard to produce in any sort of usable quantity. Now, researchers have figured out a way that might help us make a lot more of something almost as good: they've engineered some DNA that encodes a hybrid of silkworm and spider proteins, and gotten silkworms to produce it.

We've cloned a number of spider silk proteins now, and managed to express them in everything from bacteria to goats. None of these methods end up making much in the way of protein, however, and the material that is made is difficult to purify and form into fibers. Spiders would seem like an obvious choice for making silk but they create a number of issues that we don't normally associate with manufacturing; as the authors put it, "territorialism and cannibalism preclude spider farming as a viable manufacturing approach."

In contrast, we've known how to work with silkworms for centuries and, in recent years, we've developed the ability to insert DNA into their genome. The animals can be grown in bulk, and they very conveniently place all of their silk into a cocoon that's easy to process. For all its appealing properties and commercial value, though, silk from silkworms doesn't have some of the better properties associated with spider silk.

Given all this, it's not a surprise that people have tried to get the genes for spider silk into silkworms. The resulting production, however, has been disappointingly low, and the silk hasn't had many of the properties that made anyone go through the effort of trying this in the first place.

In this case, the authors took a number of steps to integrate the DNA that encoded a spider silk gene a bit better in to the silkworm's regular biology. For starters, they added control sequences that ensured that their DNA construct was only expressed in the organs that make silk for the worm.

They didn't simply express a spider protein on its own, either. Their construct encoded a a hybrid protein that had a central core from a spider silk protein flanked by smaller fragments of a silkworm silk protein. That, they reasoned, would help it get incorporated into the fibers produced by the silkworm a bit better. (They didn't do a control DNA construct without these flanking pieces, so it's hard to know if they were right.)

In any case, it worked. Anywhere from two to five percent of the silk proteins in the cocoons of the transgenic silkworms came from their DNA construct. They even tagged one of their constructs with a fluorescent protein and managed to make glowing silk. Somewhat unexpectedly, the green, glowing silk was just as tough as most of the rest of their engineered silk.

And, as it turned out, the resulting hybrid silk was pretty tough, as measured by the break stress, maximum strain, and, literally, toughness. On average, the transgenic silkworms produced silk that was quite a bit stronger than the material that normally went into cocoons, and the strongest of the material tested was a bit stronger than spider dragline silk.

That's the good news. The bad news was that very few of the transgenic animals produced anything nearly so tough. And there was little consistency among the different transgenic lines—in fact, silk from different animals in a single transgenic line often produced inconsistent results.

Still, there are a lot of different ways the results could potentially be improved. Additional transgenic constructs might increase the percentage of engineered protein in the silk; knocking out some of the silkworm's native genes might do the same. The inconsistency in the quality of the silk might be a rather tough hurdle to clear, though. This is a good first step, but it's a long way from the sort of mass production that the authors were aiming for.

PNAS, 2012. DOI: 10.1073/pnas.1109420109 (About DOIs).