ARLINGTON, Va., June 14, 2002---The electrostatic force that makes your hair stand on end in the winter is also at work in the cartilage of your knee, giving it strength and flexibility.

Electrostatic repulsion between like charges can be powerful. For hair to stand on end, the repulsive force between the charged strands must be strong enough to overcome the force of gravity.

This same electrostatic force---only in a wet environment---has now been confirmed as playing a major role in cartilage at the nanomolecular level, according to research that could open new possibilities for treating injury and disease.

Alan Grodinsky, Sc.D., and Christine Ortiz, Ph.D., and their group at the Massachusetts Institute of Technology have taken some of the first nanoscale measurements of cartilage to better understand its toughness, strength, resiliency and ability to act as a durable shock absorber---and how it is affected by age and disease. The study relies on a new nanomechanical instrument called a molecular force probe, which has a tip about 50 nanometers (billionths of a meter) in diameter.

Cartilage is made of a mesh of collagen and a gel of proteoglycans. For the current work, being published in the journal Macromolecules, the group isolated proteoglycans and then separated from these molecules smaller building blocks (glycosamineoglycan chains) that resemble the bristles of a brush.

The researchers attached these individual bristles to a surface of gold, aligning them as they appear in nature. Then they moved the tip of the molecular force probe up and down along the bristles and measured the forces between the two surfaces.

These measurements revealed that electrostatic forces make the bristles repel one another. The measurements also showed that pressure from contact with the probe tip has an effect on the bristles' behavior.

"For many years we've suspected from measurements at the tissue level that these molecules play an extremely important role in cartilage function because of electrical repulsive interactions between the brush 'bristles,'" Grodzinsky said.

The ability to measure these and other forces on such a minute scale is expected to lead to new understanding of how these forces give rise to the many valuable properties of cartilage. This detailed understanding could form the basis of new treatments for cartilage injury and disease.

Other members of the research team at MIT include graduate students Joonil Seog the Department of Mechanical Engineering and Whitaker graduate fellow Delphine Dean of the Department of Electrical Engineering and Computer Science.

Contact:
Alan Grodzinsky, MIT
Christine Ortiz, MIT
Frank Blanchard, Whitaker Foundation