Time-dependent mechanical characterization of poly(2-hydroxyethyl methacrylate) hydrogels using nanoindentation and unco
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Gregory J. Miller Department of Aerospace and Mechanical Engineering, Boston University, Boston, Massachusetts 02215
Elise F. Morgan Department of Aerospace and Mechanical Engineering, Boston University, Boston, Massachusetts 02215; and Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215
Catherine M. Klapperich Department of Manufacturing Engineering, Boston University, Boston, Massachusetts 02215; and Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215 (Received 5 November 2007; accepted 22 February 2008)
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.
I. INTRODUCTION
Many factors contribute to the difficulty of measuring the mechanical properties of biomaterials on the scale of cellular interactions.1 As with all polymers, the mechanical properties of biomaterials depend on loading rate and temperature. In addition, materials incorporating biological monomers such as sugars or proteins may be too
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0185 1472 J. Mater. Res., Vol. 23, No. 5, May 2008 http://journals.cambridge.org Downloaded: 25 Mar 2015
expensive to produce in the large amounts needed for conventional testing. Also, as in the cases of bioMEMs (microelectromechanical systems with biological or chemical applications) and drug delivery devices, the structures fabricated from these materials are themselves smal
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