Every year in the US, about 1.3 million people undergo bone graft surgeries to repair defects left by accidents or disease. While natural bone tissue is the ideal choice for grafts, using a patient's own bone means additional surgery and risks, and donor bone grafts can be rejected.

Engineered grafts that use ceramic, glass or even metal to provide a scaffold for new bone cells to grow are thought to be a promising alternative to natural bone. Some researchers are looking to a more unexpected source of material for bone grafts - the larva of Bombyx mori, also known as the the domesticated silkworm.

Silk is a promising material for bone grafts because it is the strongest naturally occurring fiber, according to David Kaplan, a Tufts University biomedical engineer. A strand of spider silk is five times stronger than a steel fiber of the same diameter, and almost as strong as Kevlar, one of the strongest manmade fibers.

The material is also easy to sterilize, and researchers can also control how quickly the silk graft degrades. But while a sponge-shaped silk matrix allows bone cells to grow, it isn't strong enough to endure the stresses of movement and compression.

It supports the biology, but not the mechanics, said Kaplan.

Usually researchers have attempted to shore up a silk matrix with other materials like collagen, but this often means sacrificing either strength or flexibility in the final graft, according to Kaplan.

Now, Kaplan and a group of international colleagues say they've been able to strengthen the silk matrix by using tiny silk fibers scattered throughout the walls of the silk sponge, akin to how a length of steel bar is used to reinforce a concrete structure .

They described their results in a paper that appeared Monday in the journal Proceedings of the National Academy of Sciences.

Kaplan and his colleagues used a new method to create the tiny tendrils of silk to reinforce the matrix: they washed silk fibers with water and lye, producing tiny fibrils with a length between 10 and 20 micrometers - about one two-thousandth of an inch - within one minute. Conventional silk processing methods produce a much longer fiber of more than 100 micrometers after 12 minutes, according to the authors.

Those tiny fibers, strewn about within the walls of the silk matrix, make a big difference. In addition to being stronger than the unenhanced silk matrix, the composite material also mimicked a number of the features of natural bone in terms of stiffness and roughness.

That roughness may encourage bone growth. In testing, stem cells derived from human bone marrow were more likely to form bone-like tissue on the silk fiber-reinforced matrix than on the unreinforced silk sponges, according to the paper.

When the composite material was implanted in mice, it provoked a minimal immune response, boding well for more testing in live subjects, the researchers found.

Though still inferior to native bone grafts, silk composite scaffolds could be useful to provide temporary support for native bone cells to proliferate, the authors say.

In a statement released Monday, Kaplan said supporting microfibers could be used for many other tissue systems where control of mechanical properties is useful and has broad applications for regenerative medicine, Kaplan said in a statement on Monday.

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