Silk and ceramics offer hope for long-term repair of joint injuries

"It's a challenging problem to tackle," says Rosemarie Hunziker, Ph.D., NIBIB Director for the Program for Tissue Engineering. "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.' 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."

The natural structure at the joint where two bones meet is a two-part material composed of cartilage and bone, called osteochondral tissue: osteo meaning bone and chondral meaning cartilage. This "biphasic" material combines flexible cartilage and more rigid bone, which enables joints to flex and function properly, while maintaining weight-bearing strength.

Existing clinical treatments generally fall into two categories. Non-surgical treatment involves immobilization and restricted weight-bearing, with gradual progression of weight-bearing and physical therapy. Current surgical treatments include debriding (removing injured cartilage and bone), or grafting of new bone and cartilage. Both surgical techniques are aimed at restoring the natural shape and gliding surface of the cartilage. The existing approaches are typically successful at alleviating pain and restoring some function in the short-term, but rarely achieve full restoration of functional osteochondral tissue in the long-term. In particular, poor healing of bones that were cut in surgery is a common problem.

NIBIB-funded researchers at Tufts University in Medford, Massachusetts and researchers at the Biomaterials and Tissue Engineering Research Unit, University of Sydney, Australia, teamed-up to develop materials that mimic the unique and complex nature of osteochondral tissue. The goal was to develop an artificial scaffold with mechanical and bioactive properties that successfully promotes healing of damaged tissue to restore a fully functional joint. Bioactive properties include having a scaffold with the correct pore sizes that allow cells to enter and populate the scaffold after implantation, and being fully degradable over time to remove barriers to tissue regeneration.

Identifying materials that meet the challenge

The biphasic scaffold meets the basic requirements of flexibility and resilience. In addition, the bioactive properties of the two parts of the scaffold are critical because the implanted scaffold must be populated by human mesenchymal stem cells that then differentiate into cartilage and bone cells to integrate the graft securely into the joint.

The researchers developed relatively simple and reproducible processes to form the two materials into the desired scaffold with the different pore sizes needed. Finally, a mechanically strong interface between the two materials was engineeried, which was critical for the successful creation of a biphasic graft.

Testing the properties of the scaffold design

In tests that measure how the materials hold up under stretch and compression forces, the biphasic scaffold maintained its structural integrity under forces that were much higher than would be encountered in the body under physiological conditions. Also, the bonding between the two phases remained intact under very high stretch and compression tests.

Tested in cell culture, the researchers found 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 cartilage-like segment caused the mesenchymal cells to differentiate into cartilage cells. The larger pore size of the ceramic bone-like segment caused the mesenchymal cells to differentiate into bone cells. The attraction of the mesenchymal cells to the pores was accomplished without the need to integrate any other bioactive molecules into the structure, which greatly simplifies the design and fabrication of the scaffold. The interface between the two phases also allowed cell migration and interaction between phases. Tests of gene expression in tissue culture revealed that the genes expressed from the cells populating the silk and ceramic phases were indeed those expected from cartilage and bone, respectively.

Next steps

"We are extremely encouraged," says David L. Kaplan, Ph.D., Professor and Chair, Department of Biomedical Engineering at Tufts University, "by the outstanding mechanical and bioactive properties present in these materials that also feature relatively simple and reproducible fabrication methods."

The team continues to optimize the properties of the scaffold design for eventual in vivo testing in a pig model as they move toward the use of this strategy for dramatically improved and lasting reconstruction of osteochondral defects.