Without cartilage, the material that connects our joints, our bones would be forced to rub together, resulting in limited movement and extreme discomfort. Unfortunately cartilage does not heal well by itself, and so when cartilage is damaged in an injury or through disease, science may have to step in.
Researchers at the University of Michigan have recently made a significant breakthrough in repairing cartilage with the creation of nanofibrous hollow microspheres [Liu et al., Nature Mater (2011) doi:10.1038/nmat2999]. These microspheres are able to act as an artificial cellular matrix; the tissue which provides structural support to cells. The cellular scaffold these particular spheres provide mean that cartilage can grow in a much more efficient and natural way. Prof Peter Ma and co-workers found that significantly more chondrocytes (the cells that make up cartilage) attached themselves to the hollow nanofiber spheres than to solid spheres. This difference can be attributed to the greater surface area of the nanofiber bundle, but also to the increased porosity, which allows the nanofiber spheres to absorb key adhesive proteins.
The team compared the hollow nanofiber spheres to regular nanofiber and solid spheres. Chondrocytes were mixed with all three scaffolds and then injected into mice. After eight weeks the hollow spheres resulted in the growth of nearly 40 % more tissue than the regular nanofiber spheres, and 200 % more than the solid spheres. The hollow scaffold outperformed the control group which used no microspheres by over 230 %.
Thanks to the nanofiber make-up and the hollow structure, the microspheres biodegraded faster than their counterparts, meaning that as the cartilage grew, additional space was available for genuine extra cellular matrix and even more cartilage. The biodegradation of these hollow particles also appears to be harmless. According to Ma, “The final hydrolytic degradation by-product is lactic acid, which exists in the body ([and is] harmless) and can further degrade enzymatically into water and carbon dioxide.” “However, large amounts of lactic acid may lower the local pH, which could negatively affect cells. Here the extremely low density of the hollow structure helps too.”
To test the regenerative effects, the suspensions were injected into cartilage defects within the knees of rabbits. Again the hollow nanofiber bundles produced the best results, filling the defect and integrating with the original cartilage.
When asked what the team would be looking at next, Ma told Materials Today that they will “evaluate these nanofibrous hollow microspheres for cartilage regeneration in larger animals (such as dogs, pigs, or goats) and eventually for clinical trials. In addition, we will expand the novel spheres for other tissue regeneration and drug delivery applications.”


Stewart Bland