Silk is an extremely strong and stretchy material and is safe to use inside the human body. For that reason, researchers are engineering silk spun spiders and silkworms so it can one day heal our wounds, hold up our bones, and become part of our bodies. One very intriguing thing that silk make help us do is replace tendons. These are hard to replace precisely because of their need to stretch, but it could soon be possible to use a silk-nanocellulose hybrid as a replacement.