Silk and Ceramics Offer Hope for Long-term Repair of Joint Injuries

Science Highlights
September 16, 2015
Thomas M Johnson

Bioengineered scaffold combines two materials to retain flexibility, resilience, in cell culture

Complete joint replacement, while highly successful, is major surgery with rigorous and often painful therapy regimens and lengthy recovery time. Driven by the need to develop more effective therapies requiring less recovery time for common joint conditions such as osteoarthritis, an international team including NIBIB-funded researchers has developed an integrated two-part scaffold for implantation into damaged joints -- with cartilage scaffold made from silk, and bone scaffold made from ceramics. This combination of materials mimics the cartilage and bone of natural joints in both mechanical strength and pore structure. It also allows stem cells to successfully populate the graft and differentiate into cartilage and bone cells. The cells fill the damaged areas to reconstitute the original structure of the joint, after which the scaffold biodegrades, leaving the smooth surface required for a pain-free, functioning interface. The scaffold is a significant step towards improved and lasting treatment of common and often debilitating 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.”

diagram of injured joint cartilage with picture of scaffold used to repair joint defect
Biphasic silk and ceramic scaffold is designed to mimic properties of cartilage and bone for grafting into damaged joint. Source: Springer Science1

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.

sidebar listing characteristics of silk and ceramic parts of scaffoldNIBIB-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.


Close-up of scaffold showing upper small pores of silk component, interface and lower large pores of ceramic component
Detail of biphasic scaffold showing smaller pores in flexible silk cartilage-like phase and larger pores in the more rigid ceramic bone-like phase. Source: Springer Science1

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.

The research was supported by the National Institutes of Health through grant P41EB002520 from the National Institute of Biomedical Imaging and Bioengineering and grant R01DE106525 from the National Institute of Dental and Craniofacial Research. Additional funding was provided by the Australian National Health and Medical Research Council, the Rebecca Cooper Award, the Australian Postgraduate Award, and the Vice-Chancellor's Research Scholarship.

1. A biphasic scaffold based on silk and bioactive ceramic with stratified properties for osteochondral tissue regeneration. Li JJ, Kim K, Roohani-Esfahani SI, Guo J, Kaplan DL, Zreiqat H. J Mater Chem B Mater Biol Med. 2015 Jul 14;3(26):5361-5376