Engineers are looking to build lighter, stronger airplanes out of high-performance fibers that are embedded in polymer sheets, which can be stacked and pressed into extremely lightweight and durable structures. But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer "glue" to bond the layers together, can crack easily on impact, spreading between layers and weakening the material even though there may be no visible damage. Now, MIT engineers have shown they can prevent cracks from spreading between composite layers, using an approach they developed called "nano stitching," in which they deposit microscopic "forests" of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.