Negative Poisson’s ratios in tendons: An unexpected mechanical response.

Tendons are visco-elastic structures that connect bones to muscles and perform the basic function of force transfer to and from the skeleton. They are essential for positioning in adition to energy storing when involved in more abrupt movements such as jumping. Unfortunately, they are also prone to damage, and when injuries occur, they may have detrimental consequences. For example, there is consensus that injuries of tendons such as Achilles tendinopathies, which are common in athletes, are difficult to treat. Here we show, through in vivo and ex vivo tests, that healthy tendons are highly anisotropic and behave in a very unconventional manner when stretched, and exhibit a negative Poisson’s ratio (auxeticity) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion. Furthermore, since the Poisson’s ratio is highly dependent on the material’s microstructure, which may be lost if tendons are damaged or diseased, this property may provide an appropriate diagnostic tool to assess tendon health.
This study reports that human tendons including the Achilles tendons exhibits the very unusual mechanical property of a negative Poisson’s ratio (auxetic) meaning that they get fatter rather than thinner when stretched. This report is backed by in vivo and ex vivo experiments that were performed which clearly confirm auxeticity in this living material for strains which correspond to those experienced during most normal everyday activities. It is also shown that this property is not limited to the human Achilles tendon, as it was also found in tendons taken from sheep and pigs. This new information about tendons can form the scientific basis for a test for tendon health as well as enable the design of better tendon prosthesis which could replace damaged tendons.

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