Spring in your step? The forces in cartilage

Why healthy cartilage is so resilient–and how disease and aging erode that resilience–is poorly understood on the molecular level. Now Christine Ortiz and Alan Grodzinsky at the Massachusetts Institute of Technology and their colleagues are starting to unravel such details.

Although other researchers have studied cartilage’s behavior on the macroscopic scale, the several forces that scientists believe to be active on the molecular level had never been teased apart, says Ortiz.

To begin doing that, she and her colleagues took cartilage specimens and isolated rodlike polymers called glycosaminoglycans, which scientists believe are responsible for many of the tissue’s mechanical properties. They bonded one end of each 32-nanometer-long polymer to a gold-coated silicon wafer, which created a microstructure reminiscent of a toothbrush with 100 molecular bristles.

Ortiz’ team then turned to a device that measures molecular forces. After chemically coating the tip of the instrument’s probe with negatively charged ions, the researchers lowered it toward the polymer bristles and measured the forces as the probe tip and the bristles repelled each other. Combining those results with theoretical calculations, they determined that the so-called electrostatic and steric forces are the primary ones involved in the molecules’ ability to resist compression, one of cartilage’s most important properties. Ortiz and her colleagues report the results in an upcoming issue of Macromolecules. The scientists expect to repeat the experiment using cartilage of arthritis patients.