title: Mechanical interferometry imaging for creep modeling of the cornea.
abstract: OBJECTIVE
  A novel nanoindentation technique was used to biomechanically characterize each of three main layers of the cornea by using Hertzian viscoelastic formulation of creep, the deformation resulting from sustained-force application.
  
  
  METHODS
  The nanoindentation method known as mechanical interferometry imaging (MII) with <1-nm displacement precision was used to observe indentation of bovine corneal epithelium, endothelium, and stroma by a spherical ferrous probe in a calibrated magnetic field. For each specimen, creep testing was performed using two different forces for 200 seconds. Measurements for single force were used to build a quantitative Hertzian model that was then used to predict creep behavior for another imposed force.
  
  
  RESULTS
  For all three layers, displacement measurements were highly repeatable and were well predicted by Hertzian models. Although short- and long-term stiffnesses of the endothelium were highest of the three layers at 339.2 and 20.2 kPa, respectively, both stromal stiffnesses were lowest at 100.4 and 3.6 kPa, respectively. Stiffnesses for the epithelium were intermediate at 264.6 and 12.2 kPa, respectively.
  
  
  CONCLUSIONS
  Precise, repeatable measurements of corneal creep behavior can be conveniently obtained using MII at mechanical scale as small as one cell thickness. When interpreted in analytical context of Hertzian viscoelasticity, MII technique proved to be a powerful tool for biomechanical characterization of time-dependent biomechanics of corneal regions.
authors.affiliation: Department of Ophthalmology, University of California, Los Angeles, California, USA.
authors.forename: Lawrence
authors.initials: L
authors.lastname: Yoo
authors.forename: Jason
authors.initials: J
authors.lastname: Reed
authors.forename: James K
authors.initials: JK
authors.lastname: Gimzewski
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Demer
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002417:Cattle; D003319:Corneal Stroma / Q000502:physiology*; D004547:Elastic Tissue; D004548:Elasticity / Q000502:physiology*; D004728:Endothelium, Corneal / Q000502:physiology*; D019573:Epithelium, Corneal / Q000502:physiology*; D007368:Interferometry / Q000379:methods*; D008857:Microscopy, Interference; D008954:Models, Biological