Device picks up friction in ‘stick-slip’ joints

UC SANTA BARBARA (US) — Identifying the type of friction that damages cartilage and leads to osteoarthritis could lead to cheaper, non-invasive tests that “listen in” on painful joints.

Imagine going to the doctor for your aching knees. For some, this may involve uncomfortable needle sticks to draw blood for lab tests, or the extraction of the fluid surrounding the aching joint. But what if your doctor could actually listen to your body, monitoring the way your knees sound as they bend and flex?


For their work, the researchers used an instrument called a Surface Forces Apparatus (SFA), a device that measures the adhesion and friction forces between surfaces, in this case cartilage—the pad of tissue that covers the ends of bones at a joint. The degeneration of cartilage is the most common cause of osteoarthritis—the pads wear away, leaving bone grinding against bone.

What the researchers found is that it isn’t just any kind of friction that leads to the irreversible wear and tear on the material. “It is currently believed that a high-friction force, or ‘coefficient of friction,’ is the primary factor in surface wear and damage,” says Jacob Israelachvili, professor of chemical engineering and materials science at University of California, Santa Barbara. “What we found is that this is not the case.”

The critical feature is not a high-friction force, but what is known as “stick-slip” friction, or, sometimes, “stiction.” Both are characterized by surfaces that initially stick together, and then accelerate away quickly once the static friction force is overcome. With stick-slip friction, the surfaces eventually pull slightly apart and slide across each other, stick again, and pull apart, causing jerky movements.

“That’s when things get damaged microscopically,” says Israelachvili.

Stick-slip is a common phenomenon. It is responsible for everything from computer hard drive crashes and automobile failures, to squeaking doors and music.

“The same thing happens with a violin string: Even if you’re pulling the bow steadily, it’s moving in hundreds or thousands of little jerks per second, which determine the sound you hear,” Israelachvili says.

Each little jerk, no matter how submicroscopic, is an impact, and over time the accumulation of these impacts can deform surfaces, causing irreparable damage—first microscopically, then growing to macroscopic (large, visible) dimensions. That’s when you have to change the string.

But it’s not easy to tell the difference between types of friction at the microscopic level, where it all begins. Smooth-sliding joints might feel the same as those undergoing stiction, or the even more harmful stick-slip, especially in the early stages of arthritis.

But, when measured with an ultra-sensitive and high-resolution instrument like the SFA, each type of friction revealed its own characteristic profile: smooth-sliding joints yielded an almost smooth constant line (friction force or friction trace); joints with stiction showed up as a peak, as the “sticking” was being overcome, followed by a relatively smooth line; while stick-slip shows the jagged saw-tooth profile of two surfaces repeatedly pulling apart, sticking, and pulling apart again.

According to the scientists, these measurements could be recorded by placing an acoustic or electric sensing device around joints, giving a signal similar to an EKG.

“With a well-designed sensor, this could be a good way to measure and diagnose damage to the cartilage,” says co-author Xavier Banquy, a postdoctoral researcher. It could be used to measure the progression, or even the early detection of symptoms related to arthritis.

For a country that’s facing a geriatric population explosion, dealing with issues associated with old age is no small matter. Early detection of conditions like arthritis has been a priority for many years, as baby boomers advance in age.

However, the functioning of joints is more complicated, says Israelachvili. The scientists will continue their work by studying synovial fluid—the lubricating fluid between two cartilage surfaces in joints—that also plays a major role in whether or not the surfaces wear and tear, and the synergistic roles of the different molecules (proteins, lipids, and polymers) that are all involved in lubricating and preventing damage to our joints.

“There are a number of directions to take, both fundamental and practical,” Israelachvili says. “But it looks as if we need to focus our research on finding ways to prevent stick-slip motion, rather than lowering the friction force.”

Graduate student researcher Dong Woog Lee also contributed to the study, which appears in the Proceedings of the National Academy of Sciences.

Source: UC Santa Barbara

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