Spider silk inspires load bearing composite to help heal bones

Silk fibres spun by spiders and moths have inspired the development of a biodegradable composite that can be used to repair broken load-bearing bones.

The advance from a team at the University of Connecticut is claimed to help repair bones without the complications sometimes presented by other materials.

To facilitate the repair of a load-bearing bone, doctors sometimes install a metal plate to support the bone as it fuses and heals. This can be problematic as some metals leach ions into surrounding tissue, causing inflammation and irritation. Similarly, if a metal plate bears too much load in the leg, the new bone may grow back weaker and be vulnerable to fracture.

Seeking a solution to the problem, UConn professor Mei Wei, a materials scientist and biomedical engineer, turned to spiders and moths for inspiration. In particular, Wei focused on silk fibroin, a protein found in the silk fibres spun by spiders and moths known for its toughness and tensile strength.

Silk fibroin is already a common component in medical sutures and tissue engineering because of its strength and biodegradability, yet no one had ever tried to make a dense polymer composite out of it.

Working with UConn associate professor Dianyun Zhang, a mechanical engineer, Wei's lab began testing silk fibroin in various composite forms. The new composite needed to be strong and stiff, yet not inhibit dense bone growth. The composite also needed to be flexible, allowing patients to retain their natural range of motion and mobility while the bone healed.

After extensive tests, Wei and Zhang found the materials they were looking for. The new composite consists of long silk fibres and fibres of polylactic acid - a biodegradable thermoplastic derived from cornstarch and sugar cane - that are dipped in a solution in which each is coated with fine bioceramic particles made of hydroxyapatite. The coated fibres are then packed in layers on a small steel frame and pressed into a dense composite bar in a hot compression mould.

In a study published in the Journal of the Mechanical Behavior of Biomedical Materials, Wei reported that the high-performance biodegradable composite showed strength and flexibility characteristics that are among the highest ever recorded for similar bioresorbable materials in literature.

The team has already begun testing new derivatives of the composite, including those that incorporate a single crystalline form of the hydroxyapatite for greater strength and a variation of the coating mixture to maximise its mechanical properties for bones bearing more weight.