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Time-dependent mechanical characterization of poly(2-hydroxyethyl methacrylate) hydrogels using nanoindentation and unconfined compression

Published online by Cambridge University Press:  31 January 2011

Jessica D. Kaufman ,
Gregory J. Miller ,
Elise F. Morgan  and
Catherine M. Klapperich
Jessica D. Kaufman*
Affiliation:
Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215
Gregory J. Miller
Affiliation:
Department of Aerospace and Mechanical Engineering, Boston University, Boston, Massachusetts 02215
Elise F. Morgan
Affiliation:
Department of Aerospace and Mechanical Engineering, Boston University, Boston, Massachusetts 02215; and Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215
Catherine M. Klapperich
Affiliation:
Department of Manufacturing Engineering, Boston University, Boston, Massachusetts 02215; and Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215
*
a)Address all correspondence to this author. e-mail: jdk21@bu.edu
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Abstract

Hydrogels pose unique challenges to nanoindentation including sample preparation, control of experimental parameters, and limitations imposed by mechanical testing instruments and data analysis originally intended for harder materials. The artifacts that occur during nanoindentation of hydrated samples have been described, but the material properties obtained from hydrated nanoindentation have not yet been related to the material properties obtained from macroscale testing. To evaluate the best method for correlating results from microscale and macroscale tests of soft materials, nanoindentation and unconfined compression stress-relaxation tests were performed on poly-2-hydroxyethyl methacrylate (pHEMA) hydrogels with a range of cross-linker concentrations. The nanoindentation data were analyzed with the Oliver–Pharr elastic model and the Maxwell–Wiechert (j = 2) viscoelastic model. The unconfined compression data were analyzed with the Maxwell–Wiechert model. This viscoelastic model provided an excellent fit for the stress-relaxation curves from both tests. The time constants from nanoindentation and unconfined compression were significantly different, and we propose that these differences are due to differences in equilibration time between the microscale and macroscale experiments and in sample geometry. The Maxwell–Wiechert equilibrium modulus provided the best agreement between nanoindentation and unconfined compression. Also, both nanoindentation analyses showed an increase in modulus with each increasing cross-linker concentration, validating that nanoindentation can discriminate between similar, low-modulus, hydrated samples.

Keywords

BiomaterialTribologyViscoelasticity
Type
Articles
Information
Journal of Materials Research , Volume 23 , Issue 5 , May 2008 , pp. 1472 - 1481
Copyright
Copyright © Materials Research Society 2008

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