Tendons connect bones to muscles so if you want to run during a football game or fight a Trojan War, they are important.   If you didn't understand the clever pun in the title, the only vulnerability of Achilles was his calcaneal tendon (tendo calcaneus), because when his mother Thetis dipped him into the magical river Styx to make him invincible, she held him by the heel.

Naturally, it was his undoing.   So today that tendon of the posterior leg is called the Achilles tendon and a devastating weakness in an otherwise strong group or person is colloquially called the Achilles Heel (see Death Star - should we close that 2 meter port that leads to the fusion core?) 

300,000 per year someone sprains or tears one of their tendons year but the mystery of why something so strong can be so vulnerable has lasted for centuries too.   

Case Western Reserve University researcher say they may have found the weakest links and perhaps a way to prevent tendon damage.   The weakest links are fibrils of collagen.   Sure, you know collagen because Angelina Jolie has it stuffed in her lips, but this is the tougher form.

"The fibrils are about five times stronger and can strain about five times farther than a tendon," said Steven Eppell, a professor of biomedical engineering and senior author of the study.   "About 80 to 90 percent of a tendon is collagen but mechanical properties like strength are probably controlled by the other stuff."

The other stuff is a cement that holds the bundles of fibrils together; it's made of molecules called proteoglycans. This cement or the interface between collagen fibrils and proteoglycans, is most likely the weakest link in the system, the researchers say.  The scientists suspected that's the case but direct testing of cement, which is more complex and less available than fibrils, was difficult. So, they decided to test the strength of just the collagen fibrils and then compare this with the strength of whole tendons.

To test the tensile strength of fibrils, which have a diameter 100 times smaller than the diameter of a human hair, Eppell's team used what is essentially a structural testing lab on a microchip. This allowed them to measure how far fibrils stretch and the pressures they withstand before breaking.

"It's the equivalent of what civil engineers use to test a steel beam under 100,000 pounds of pressure, shrunk to the micro level," said co-author Roberto Ballarini, chairman of civil engineering at the University of Minnesota. 

In the first tests of their kind, the scientists glued one end of fibrils taken from a sea cucumber to a stationary base and the other end to a movable pad. When pulled apart, the fibrils stretched up to 100 percent of their resting length before breaking. A tendon stretches only 10 to 20 percent before breaking.

The new technology, developed by Zhilei Shen during her Ph.D. dissertation work, allowed her to keep the fibrils hydrated in saline to mimic their condition in the body. Earlier testing on dehydrated fibrils and tendons gave her different results.

The investigators believe water actually toughens fibrils and tendons as a whole. Since the proteoglycans in the cement largely control the degree of hydration around the fibrils, the team suspects they may be useful targets when designing drugs to control tendon strength.  Their next steps include more detailed testing of bundles of fibrils and whole tendons as well as computer modeling that connects behavior of individual atoms and their bonds to molecular behavior and finally to mechanics of simulated fibrils and tendons. The models will be tested against experimental data collected in the biomedical engineering department at Case Western Reserve.

Although the work focuses on tendons, the investigation will also provide insight into the workings of collagen in ligaments, skin and even mineralized collagen in bone.

Citation: Zhilei Liu Shen, Mohammad Reza Dodge, Harold Kahn, Roberto Ballarini, and Steven J. Eppell, 'In Vitro Fracture Testing of Submicron Diameter Collagen Fibril Specimens', Biophysical Journal, Volume 99, Issue 6, 1986-1995, 22 September 2010 doi:10.1016/j.bpj.2010.07.021