Shape-Shifting Scaffold Supports Growing Bone Cells

Researchers have used thin fibers of a shape-changing polymer to create a scaffold for growing bone tissue in the laboratory (ACS Appl. Mater. Interfaces 2014, DOI: 10.1021/am405101k). Such a material could help plug the holes left behind after the screws and plates used to repair broken bones are removed.

Broken bones held together by metal plates and screws are vulnerable to further damage because the difference in stiffness between the metal screw and the bone tissue weakens the bone near the screw. Yanzhong Zhang of Donghua University, in China, and his colleagues imagined reinforcing these surgically altered bones by removing the screws and filling the holes with a material that supports bone cell growth. The researchers also wanted to improve tissue regeneration by using a material that could generate mechanical forces to stimulate that growth.

As a first step toward that goal, the researchers made a biodegradable polymer scaffold and demonstrated that bone cells could grow and function on it. They chose a polymer made of a mixture of D, L-lactide and trimethylene carbonate. The composition of this material can be tuned so that the polymer changes shape at various temperatures. The researchers thought they could take advantage of this shape-changing ability to generate mechanical force by triggering it with heat to expand and press against existing bone.

The researchers used electrospinning to create thin fibers of a polymer containing an 8:2 ratio of D, L-lactide to trimethylene carbonate. Compared to tissue scaffolds that take the form of a foam or gel, the new material more closely resembles the network of collagen fibers that surrounds cells in natural tissue.

To test the polymer’s shape memory properties, the researchers spun the fibers into mats that they rolled into a cylindrical bar. The researchers warmed the plug to 39 °C, slightly above the temperature at which the polymer transitions from a rigid glass to a rubberlike material. They bent the warmed plug into the shape of an S and fixed the shape by cooling the bar to room temperature. The bar recovered its initial straight shape within 12 seconds of being rewarmed to 39 °C.

To test if the polymer scaffolds supported growing bone cells, the researchers seeded mats of the polymer with osteoblasts from newborn mice. The cells migrated, spread, and adhered to the mats faster than cells growing on smooth pieces of glass. The cells on the polymer scaffold also secreted minerals and produced an enzyme enhanced during bone formation.

Aaron S. Goldstein of Virginia Tech says bone engineering is a fairly new application for electrospun shape memory polymers. “Ultimately, it might be interesting to see how the rate and extent of bone healing compares between a deployed electrospun fiber material and an injectable gel or foam,” he says.

Patrick T. Mather, a biomedical engineer at Syracuse University, says that while bone cells appear to function on this scaffold, it is not clear if triggering a deformed scaffold to return to its original shape would affect the rate of mineral formation.