title: Mechanical properties of contact lenses: The contribution of measurement techniques and clinical feedback to 50 years of materials development.
abstract: OBJECTIVE
  This review summarises the way in which mechanical property measurements combined with clinical perception have influenced the last half century of materials evolution in contact lens development.
  
  
  METHODS
  Literature concerning the use of in-vitro testing in assessment of the mechanical behaviour of contact lenses, and the mutual deformation of the lens material and ocular tissue was examined. Tensile measurements of historic and available hydrogel lenses have been collected, in addition to manufacturer-generated figures for the moduli of commercial silicone hydrogel lenses.
  
  
  RESULTS
  The three conventional modes of mechanical property testing; compression, tension and shear each represent different perspective in understanding the mutual interaction of the cornea and the contact lens. Tensile testing provides a measure of modulus, together with tensile strength and elongation to break, which all relate to handling and durability. Studies under compression also measure modulus and in particular indicate elastic response to eyelid load. Studies under shear conditions enable dynamic mechanical behaviour of the material to be assessed and the elastic and viscous components of modulus to be determined. These different methods of measurement have contributed to the interpretation of lens behaviour in the ocular environment. An amalgamated frequency distribution of tensile moduli for historic and currently available contact lens materials reveals the modal range to be 0.3-0.6MPa.
  
  
  CONCLUSIONS
  Mechanical property measurements of lens materials have enabled calibration of an important aspect of their ocular interaction. This together with clinical feedback has influenced development of new lens materials and assisted clinical rationalisation of in-eye behaviour of different lenses.
authors.affiliation: Biomaterials Research Unit, Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham, B4 7ET, UK. Electronic address: biostuff@aston.ac.uk.
authors.forename: Tarnveer Singh
authors.initials: TS
authors.lastname: Bhamra
authors.affiliation: Biomaterials Research Unit, Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham, B4 7ET, UK.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Tighe
mesh_terms: D001672:Biocompatible Materials; D003261:Contact Lenses; D008422:Materials Testing / Q000379:methods*; D055595:Mechanical Phenomena; D011108:Polymers
chemical_list: D001672:Biocompatible Materials; D011108:Polymers
keywords: Compression; Contact lens; Mechanical properties; Modulus; Shear; Tension