The human body is held together by an intricate cable system of tendons and muscles, engineered by nature to be tough and highly stretchable. An injury to any of these tissues, particularly in a major joint like the shoulder or knee, can require surgical repairs and weeks of limited mobility to fully heal. MIT engineers have engineered small coils lined with living cells, that they say could act as stretchy scaffolds for repairing damaged muscles and tendons. The coils are made from hundreds of thousands of biocompatible nanofibers, tightly twisted into coils resembling miniature nautical rope, or yarn.

The researchers coated the yarn with living cells, including muscle and mesenchymal stem cells, which naturally grow and align along the yarn, into patterns similar to muscle tissue. The researchers found the yarn's coiled configuration helps to keep cells alive and growing, even as the team stretched and bent the yarn multiple times.

In the future, the researchers envision doctors could line patients' damaged tendons and muscles with this new flexible material, which would be coated with the same cells that make up the injured tissue. The "yarn's" stretchiness could help maintain a patient's range of motion while new cells continue to grow to replace the injured tissue.

"When you repair muscle or tendon, you really have to fix their movement for a period of time, by wearing a boot, for example," says Ming Guo, assistant professor of mechanical engineering at MIT. "With this nanofiber yarn, the hope is, you won't have to wearing anything like that."

Guo says biomedical engineers have embedded muscle cells in other stretchy materials such as hydrogels, in attempts to fashion flexible artificial tissues. However, while the hydrogels themselves are stretchy and tough, the embedded cells tend to snap when stretched, like tissue paper stuck on a piece of gum.

"When you largely deform a material like hydrogel, it will be stretched just fine, but the cells can't take it," Guo says. "A living cell is sensitive, and when you stretch them, they die."

Going forward, the group plans to fabricate similar coils from other biocompatible materials such as silk, which could ultimately be injected into an injured tissue. The coils could provide a temporary, flexible scaffold for new cells to grow. Once the cells successfully repair an injury, the scaffold can dissolve away.

"We may be able to one day embed these structures under the skin, and the [coil] material would eventually be digested, while the new cells stay put," Guo says. "The nice thing about this method is, it's really general, and we can try different materials. This may push the limit of tissue engineering a lot."