UNCORRECTED PROOF 1 2 Brief communication 3 Mechanical characterization of contact lenses by 4 microindentation: Constant velocity and relaxation testing 5 Sung Jin Lee a , Gerald R. Bourne b , Xiaoming Chen a , W.Gregory Sawyer a , 6 Malisa Sarntinoranont a, * 7 a Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA 8 b Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA 9 Received 21 October 2007; received in revised form 12 February 2008; accepted 19 February 2008 10 11 Abstract 12 Non-destructive methods for testing material properties allow for multiple tests to be performed on the same sample, which will speed 13 up the design and testing process for hydrogel contact lenses. The mechanical properties of contact lenses were investigated by micro- 14 indentation testing. Indenter force responses were recorded for two modes of testing: constant velocity and relaxation indentation. From 15 these tests, we characterized the biphasic properties of a hydrogel contact lens: Young’s modulus of the solid matrix and hydraulic per- 16 meability. Measured indenter force response was fit to finite element (FE) simulation results over a range of Young’s modulus (E) and 17 hydraulic permeability (k) over a short testing time scale (2 s). Estimated hydraulic permeability, 1–5  10 15 m 4 (N s) 1 , was similar to 18 previously measured values for Etafilcon A. However, values determined for Young’s modulus, 50–60 kPa, were lower than previously 19 measured. 20 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 21 Keywords: Hydraulic conductivity; pHEMA-MAA; Poroelastic; Porous media; Polymer Q1 22 23 1. Introduction 24 Mechanical and fluid transport properties of soft con- 25 tact lenses may influence clinical performance, e.g. on-eye 26 movement, fitting, and wettability [1–4], and may be 27 related to the occurrence of complications, e.g. lesions 28 [2,3]. Non-destructive methods for testing these material 29 properties allow for multiple tests to be performed on the 30 same sample, which will speed up the design and testing 31 process for hydrogel contact lenses. In the mechanical 32 assessment of soft hydrated materials, indentation is 33 increasingly being used because of its non-destructive nat- 34 ure, small sample capacity, and ability to hone in on local- 35 ized regions of interest [5–7]. 36 Contact lens hydrogels may be described as a biphasic 37 material. The material properties governing biphasic 38 behavior are the Young’s modulus of the solid phase, Pois- 39 son’s ratio, and hydraulic permeability, which is a measure 40 of fluid conductance in porous media. Previous studies of 41 indentation of biphasic media have been completed by 42 Mow and coworkers [8,9], who solved for displacement 43 during creep for a cylindrical porous indenter. Their ana- 44 lytical solution was used to determine biphasic properties 45 of cartilage. Computational finite element (FE) models 46 have also been developed to account for sample boundary 47 conditions and impermeable indenters. Linear biphasic FE 48 models were developed by Spilker et al. [10] and Hale et al. 49 [7] for stress-relaxation and constant velocity indentation 50 of articular cartilage. Indentation studies for hydrogels 51 have been conducted [11,12]. However, biphasic indenta- 52 tion studies of hydrogels are scarce. Considering a different 53 testing mode, Chiarelli et al. [13] used a poroelastic analysis 54 of the stress-relaxation response of thin strips to estimate 55 poroelastic properties of the hydrogel polyvinyl alcohol– 56 polyacrylic acid. Young’s modulus was estimated to be 1742-7061/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2008.02.021 * Corresponding author. Tel.: +1 352 392 8404; fax: +1 352 392 7303. E-mail address: msarnt@ufl.edu (M. Sarntinoranont). Available online at www.sciencedirect.com Acta Biomaterialia xxx (2008) xxx–xxx www.elsevier.com/locate/actabiomat ACTBIO 389 No. of Pages 9, Model 5+ 21 March 2008 Disk Used ARTICLE IN PRESS Please cite this article in press as: Lee SJ et al., Mechanical characterization of contact lenses by ..., Acta Biomater (2008), doi:10.1016/ j.actbio.2008.02.021