title: Negative Poisson's ratios in tendons: An unexpected mechanical response.
abstract: 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 as well as 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 dilapidating consequences. For instance, 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 a suitable diagnostic tool to assess tendon health.
  
  
  CONCLUSIONS
  We report 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 we performed which clearly confirm auxeticity in this living material for strains which correspond to those experienced during most normal everyday activities. We also show 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.
authors.affiliation: Metamaterials Unit, Faculty of Science, University of Malta, Msida MSD 2080, Malta. Electronic address: ruben.gatt@um.edu.mt.
authors.forename: Ruben
authors.initials: R
authors.lastname: Gatt
authors.affiliation: Metamaterials Unit, Faculty of Science, University of Malta, Msida MSD 2080, Malta.
authors.forename: Michelle
authors.initials: M
authors.lastname: Vella Wood
authors.affiliation: Department of Podiatry, Faculty of Health Sciences, University of Malta, Msida MSD 2080, Malta; Faculty of Health Sciences, Staffordshire University, Science Centre, Leek Road ST4 2DF, UK.
authors.forename: Alfred
authors.initials: A
authors.lastname: Gatt
authors.affiliation: Department of Radiography, Faculty of Health Sciences, University of Malta, Msida MSD 2080, Malta.
authors.forename: Francis
authors.initials: F
authors.lastname: Zarb
authors.affiliation: Department of Podiatry, Faculty of Health Sciences, University of Malta, Msida MSD 2080, Malta; Faculty of Health Sciences, Staffordshire University, Science Centre, Leek Road ST4 2DF, UK.
authors.forename: Cynthia
authors.initials: C
authors.lastname: Formosa
authors.affiliation: Metamaterials Unit, Faculty of Science, University of Malta, Msida MSD 2080, Malta.
authors.forename: Keith M
authors.initials: KM
authors.lastname: Azzopardi
authors.affiliation: Department of Anatomy, Faculty of Medicine & Surgery, University of Malta, Msida MSD 2080, Malta.
authors.forename: Aaron
authors.initials: A
authors.lastname: Casha
authors.affiliation: Department of Physiotherapy, Faculty of Health Sciences, University of Malta, Msida MSD 2080, Malta.
authors.forename: Tonio P
authors.initials: TP
authors.lastname: Agius
authors.affiliation: Department of Anatomy, Faculty of Medicine & Surgery, University of Malta, Msida MSD 2080, Malta.
authors.forename: Pierre
authors.initials: P
authors.lastname: Schembri-Wismayer
authors.affiliation: Department of Orthopaedics, MaterDei Hospital, Msida MSD 2090, Malta.
authors.forename: Lucienne
authors.initials: L
authors.lastname: Attard
authors.affiliation: Faculty of Health Sciences, Staffordshire University, Science Centre, Leek Road ST4 2DF, UK; Department of Podiatry, Faculty of Health Sciences, University of Malta, Msida MSD 2080, Malta.
authors.forename: Nachiappan
authors.initials: N
authors.lastname: Chockalingam
authors.affiliation: Metamaterials Unit, Faculty of Science, University of Malta, Msida MSD 2080, Malta; Department of Chemistry, Faculty of Science, University of Malta, Msida MSD 2080, Malta.
authors.forename: Joseph N
authors.initials: JN
authors.lastname: Grima
mesh_terms: D000125:Achilles Tendon / Q000473:pathology / Q000503:physiopathology*; D000328:Adult; D000818:Animals; D004548:Elasticity; D005260:Female; D006801:Humans; D008297:Male; D012756:Sheep; D013314:Stress, Mechanical; D013552:Swine; D052256:Tendinopathy / Q000473:pathology / Q000503:physiopathology*
keywords: Auxetic; Poisson’s ratio; Tendons