Synthetic Spider Silk Capsules Assemble Themselves
- 6:30 am |
In addition to snaring dinner and protecting spider babies, spider silk makes a pretty good shield for bioreactive enzymes. Even when it’s not made by the spiders themselves. Turns out, self-assembling spider silk capsules, crafted by colonies of bacteria, are pretty good at keeping reactive molecules calm.
“We called this ‘Spiderbag’,” said Thomas Scheibel, a protein-chemist-turned-engineer, and coauthor of a study describing the capsules published in Advanced Functional Materials. The tiny spheres, produced by Scheibel and his colleagues at the University of Bayreuth, are about as strong as glass — comparable to the ornamental globes that hang on Christmas trees, “just a few sizes smaller,” Scheibel says.
At once both tough and malleable, the silky containers can sheath proteins that would normally want to react with many things around them. The silk stops the enzymes from unfolding or becoming inactive before they’re needed. Soon, the team says, these capsules will be ready for use in medical diagnostics. Though tiny, the spheres are too large to be injectable. Instead, though he won’t go into details, Scheibel says the capsules could be used as a super-sensitive array capable of detecting performance-enhancing substances in athletes, for example.
“They could be used as an analytical tool, to identify substances in the body, in the blood — like drugs,” Scheibel says.
The capsules aren’t hard to make. Scheibel and his team mix a solution of tiny water droplets into silicon oil, forming what’s called an emulsion. The water droplets carry the dissolved silk proteins, which spring out of solution and self-assemble into wispy, 50- to 70-nanometer thick capsules at the oil-water boundaries. Then, the filmy capsules trap the water-based solution inside. “That’s the trick,” Scheibel said. “You encapsulate anything that’s inside the water droplet.”
So, if you’ve included an enzyme in that original watery solution, it’s now locked up and waiting for the right time to step outside. The team tested the system with enzymes and proteins normally used in lab work, such as beta-galactosidase and serum albumins, but Scheibel says it could be used with just about anything that doesn’t react with spider silk itself.
Modifying the size of the initial droplets allows scientists to make the capsules larger or smaller, in effect customizing the silky spheres for various applications. “You could also do it the other way around, too, and make oil droplets in water,” Scheibel said. Such a reversal would be useful for systems needing oil-friendly enzymes.
“This concept of utilizing silk as a matrix to house or contain enzymes or other bioactive molecules is a fantastic direction to go in,” said David Kaplan, a biopolymer engineer at Tufts University who is working on something similar using silkworm silk. “It offers tremendous control over what you want those containers to do.”
Others would like a bit more evidence that silky capsules offer something that other engineered molecules don’t.
“I don’t see the obvious advantages over other synthetic polymers yet,” said Randy Lewis, a molecular biologist at Utah State University. Lewis’ group recently received funding from the U.S. Navy for a project involving spider silk adhesives – they’re hoping to make something resembling one-sided Velcro that will easily stick to anything, even wet or slimy surfaces.
It’s no surprise that different research groups are examining the potential offered by synthetic silks. Spider silk itself has earned a reputation as a wonder-material: As tough as steel, biocompatible, environmentally friendly, stretchy and antiseptic, the substance can seemingly do pretty much anything you want it to.
“For hundreds of years, there’s been a myth that spider silk is the best performing fiber. Which is actually true,” Scheibel says. “Mechanically, it outcompetes everything.”
Modifying spider silk, by attaching carbon nanotubes, for example, can give it additional properties – like conductivity – that aren’t normally found in nature. But most of its natural properties are more than useful. For centuries, people have even collected spider webs and used them as wound dressings; the webs stick to the skin, forming a barrier, and the silk’s tough surface prevents infiltration by bacteria and viruses.
It’s also kind of smart. “You can design it, and under the right conditions, it knows how to find its corresponding polymer partner and organize itself into a structure that becomes very robust and useful,” Kaplan said. And when you’re done with it, “You could eat it. Or put it in the water or soil — it’s not going to hurt anything,” he says.
But making enough spider silk to use commercially has been a challenge. Spiders, unlike other critters amenable to farming, tend to eat one another when sharing captive spaces. They also don’t produce much silk – it took a million spiders and four years to create a single, gleaming golden cloth.
So, scientists are coaxing other organisms to produce the spider silk. So far, goats, silkworms, E.coli, and alfalfa (yup), have made the strong, sticky substance – or at least, the proteins that go into making the actual fiber. Inside a spider, silk proteins live in a soupy, unstructured jumble that remains goopy until the spider pulls a trigger that snaps the proteins into steely, fiber form. Perhaps not surprisingly, different labs are experimenting with ways to replicate this part of the silk-crafting arachnid experience; so far, methods like pulling the proteins through a fine syringe, and electrospinning (where an electrical charge pulls fibers from a solution), have been the most used. Silky coatings, capsules, gels and foams form readily when other triggers, like salts, are pulled.
Scheibel’s team uses little bacterial factories – colonies of E.coli – to make silk. These bacteria carry the silk protein genes from orb-weaving spiders such as Nephila clavipes and Araneus diadematus. Normally, though, E.coli would look at the genetic sequence for spider proteins and hit the road; it’s tough for a single-celled organism to produce massive, repetitive proteins like the silk’s building blocks. So, Scheibel and his team removed some of the repetitive elements and translated the code into something the bacteria could understand – then let them get to work.
Similar methods have been used to insert spider silk genes into goats, which dump the proteins into their milk, silkworms – which spin the a modified spider silk instead of their own — and alfalfa, the plant Randy Lewis works with.
A few years ago, Scheibel helped found a company called AMSilk, which is beginning to release products on the commercial market that are made from synthetic spider silk and its proteins.
AMSilk’s line includes cell culture materials, a skin cream containing silk proteins, and, soon, shampoos and spider silk bandages.
We don’t know about you, but we definitely like the idea of festooning wounds with spider silk bandages, especially if the marketing jingle includes “Spider-Band” sung to the 1960s Spider-Man theme song.