New joint repair material employs scaffold of silk cartilage and ceramic bone

An international team of researchers has potentially solved a longstanding obstacle in joint repair by developing a two-part scaffold of materials that mimics the osteochondral tissue of natural joints. The material includes a cartilage scaffold made from silk and a bone scaffold made from ceramics, which allows stem cells to successfully populate the graft and differentiate into cartilage and bone cells.

Lab tests of the new material confirmed its strength and flexibility, as well as its ability to accept and grow stem cells, according to study findings published in the Journal of Materials Chemistry B.

“One of the big problems in cartilage tissue engineering is that the cartilage does not integrate well with host tissue after implantation, so the graft doesn’t ‘take,’” said Rosemarie Hunziker, PhD, Director for the Program for Tissue Engineering at the National Institute of Biomedical Imaging and Bioengineering, in Bethesda, MD, which funded the study. “In this new approach, there is a greater chance of success because the materials have architectures and physical properties that more closely resemble the native tissue.”

For years, bioengineers have been seeking more effective therapies requiring less recovery time for common joint conditions such as osteoarthritis.

In this investigation, researchers at Tufts University, in Medford, MA, and at the Biomaterials and Tissue Engineering Research Unit of the University of Sydney, Australia, teamed up to develop a biphasic scaffold with the correct pore sizes to match the cartilage and bone segments of natural osteochondral tissue to allow cells to enter and populate the scaffold after implantation.

The material also had to be fully degradable over time to remove barriers to tissue regeneration. In addition, a mechanically strong interface between the two layers had to be engineered for the successful implementation of the biphasic graft.

The material they developed—a combination of silk fibroin and bioactive ceramic—met these requirements.

“Mechanical assessment showed that the two phases of the biphasic scaffold imitated the load-bearing behavior of native osteochondral tissue and matched its compressive properties,” the authors wrote.

In vitro tests in cell culture showed that each phase of the scaffold promoted population by human mesenchymal stem cells and their differentiation into the proper cell type—the smaller pore size of the silk segment caused the mesenchymal cells to differentiate into cartilage cells, and the larger pore size of the ceramic segment caused the mesenchymal cells to differentiate into bone cells. Plus, this process was accomplished without the need to integrate any other bioactive molecules into the structure.

“We are extremely encouraged by the outstanding mechanical and bioactive properties present in these materials that also feature relatively simple and reproducible fabrication methods,” said study co-author David L. Kaplan, PhD, Professor and Chair of the Department of Biomedical Engineering at Tufts University.

Following this initial success, the researchers aim to optimize the properties of the scaffold design for eventual in vivo testing in a pig model.