Silk has been valued for millennia, but in recent years, scientists have paid attention to the material because it’s extraordinarily strong, making it useful for bulletproof vests and body armor. The potential of silk is more than just protecting the outside of our anatomy, however, and researchers are now engineering silk so it can one day heal our wounds, hold up our bones, and become part of our bodies.
Silk is made by silkworms for their cocoons and by spiders for their webs. The two kinds are spun differently and have some different properties. (For example, the silk made by worms tends to be weaker). But in both cases, the material is strong, stretchy, and safe to use inside the human body — which opens it up to a wide range of medical uses.
One possibility is that silk can heal our wounds faster. In a study published recently in the journal Advanced Science, scientists engineered silkworms to spin a light-activated material that disinfects. First, the researchers identified all the natural proteins that could be activated by a specific type of light to create a chemical reaction that kills pathogens, according to Young Kim, a materials scientist at Purdue University and co-author of the paper.
Then, the scientists genetically engineered silkworms by inserting this protein, called mKate 2, into their DNA. These silkworms then produced a red, glowing silk activated by visible green light, like a regular LED light. When the scientists put some E. coli bacteria on the red silk and shined a green light on it for an hour, the survival rate of the bacteria fell by 45 percent. This process is very similar to using hydrogen peroxide to disinfect a cut, says Kim. The fluorescent silk and the light together generate chemicals similar to hydrogen peroxide.
The silk doesn’t distinguish harmful pathogens (like E. coli) from benign ones, but, as Kim points out, neither does hydrogen peroxide. And we don’t yet know the minimum time the light needs to shine on the silk to be effective. But the discovery is an exciting one, and the material could be used in devices that purify air and water and many areas of health. In another recent paper, Kim and his team figured out the exact physical properties that make silk so cooling, which is useful for treating inflammation. This finding could help us make silk even cooler, or engineer other fabrics to be more cooling as well. Between the self-cooling effects of silk and these bacteria-killing properties, it could be an ideal material for advanced bandages.
Silk can be used to prop up parts of our bodies, too. When we fracture or break our bones, doctors usually implant a piece of metal to stabilize the area until it’s fixed. Most of the time, these metals — like stainless steel and titanium — are very stiff and can cause more fractures themselves, according to Mei Wei, a materials scientist at the University of Connecticut. When the bone is healed, doctors need to do another surgery to take the metal out. Wei and her team created a form of silk that could offer a better solution. It’s strong but also stretchy, and it will degrade inside the body after about a year, removing the need for another surgery. Their results were published this month in the Journal of the Mechanical Behavior of Biomedical Materials.
The team combined a protein found in spider silk, called fibroin, with a form of plastic and a type of calcium that’s found in our bones. The result is much stronger than natural bone. In fact, it has the highest-recorded strength for a material that can also be absorbed in the body, says Wei. The team is still improving it in the hopes that it can get better and even stronger before doing tests on animals and then clinical trials, she adds.
The big problem with silk is that it’s expensive to make. Silkworms are rare and hard to raise, and farms of spiders are generally not an appealing idea. But the material inside trees, called wood-based nanocellulose, is strong and cheap. A possible solution is to combine the two into a cheap and even better material. Nanocellulose is also durable. After all, “when nature builds a tree, it needs to give it a good mechanical structure and performance so that the tree doesn’t fall down,” says Daniel Söderberg, a researcher at KTH Royal Institute of Technology in Stockholm and an author on the ACS Nano paper about this hybrid material. Söderberg’s team put the two together: nanocellulose for strength and silk for even more toughness and stretch. “We’re using nature’s building blocks and combining them,” he says.
The material is on par with Kevlar, and it could be used for bulletproof vests. But one day, it could also be used to replace body parts. This material breaks down in the natural world, but not inside our bodies because they’re unable to process nanocellulose. Still, it’s safe, and cells grow on it and around it. And its stretchiness makes it ideal for certain body parts like tendons.
Tendons are hard to replace precisely because of their need to stretch. Right now, there’s not an easy way to fix them, and replacement often means taking tendons from another part of the body. The silk-nanocellulose hybrid has the necessary properties that could be used to replace a tendon, says Söderberg. To get there, though, he and his team are trying to improve the strength of the material even more and are also looking at the economics of the process, like how to produce it easily and cheaply so that the possibility of silk-as-a-body-part and all the other potentials of a strong silk come to fruition.