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Technical Brief

A Validation Study of the Repeatability and Accuracy of Atomic Force Microscopy Indentation Using Polyacrylamide Gels and Colloidal Probes

[+] Author and Article Information
Donghee Lee

Department of Mechanical and Materials Engineering,
University of Nebraska-Lincoln,
Lincoln, NE 68588
e-mail: donghee.lee@huskers.unl.edu

Sangjin Ryu

Department of Mechanical and Materials Engineering,
University of Nebraska-Lincoln,
Lincoln, NE 68588
e-mail: sangjin.ryu@unl.edu

1Corresponding author.

Manuscript received July 5, 2016; final manuscript received December 18, 2016; published online February 14, 2017. Assoc. Editor: Nathan Sniadecki.

J Biomech Eng 139(4), 044502 (Feb 14, 2017) (7 pages) Paper No: BIO-16-1282; doi: 10.1115/1.4035536 History: Received July 05, 2016; Revised December 18, 2016

The elasticity of soft biological materials is a critical property to understand their biomechanical behaviors. Atomic force microscopy (AFM) indentation method has been widely employed to measure the Young's modulus (E) of such materials. Although the accuracy of the method has been recently evaluated based on comparisons with macroscale E measurements, the repeatability of the method has yet to be validated for rigorous biomechanical studies of soft elastic materials. We tested the AFM indentation method using colloidal probes and polyacrylamide (PAAM) gels of E < 20 kPa as a model soft elastic material after having identified optimal trigger force and probe speed. AFM indentations repeated with time intervals show that the method is well repeatable when performed carefully. Compared with the rheometric method and the confocal microscopy indentation method, the AFM indentation method is evaluated to have comparable accuracy and better precision, although these elasticity measurements appear to rely on the compositions of PAAM gels and the length scale of measurement. Therefore, we have confirmed that the AFM indentation method can reliably measure the elasticity of soft elastic materials.

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Figures

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Fig. 1

The effect of: (a) probe speed (Vp) with the fixed trigger force (Ft), and (b) trigger force with the fixed probe speed, on gel elasticity measured with two colloidal tip sizes (5 μm and 12 μm in diameter). Inset in (b): magnified view for gel 1. Error bar: standard deviation. Used PAAM gel samples: set 1 without allylamine.

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Fig. 2

The Young's moduli of PAAM gels measured by the AFM indentation (set 1), rheometric and confocal microscopy indentation methods. Allylamine was added in PAAM gels for comparison with the confocal microscopy indentation method. +: AFM indentation with the 12 μm-diameter tip. *: Data from the previous study [25]. Error bar: standard deviation.

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Fig. 3

Repeated AFM indentations on PAAM gels of no allylamine across samples and days. Insets: repeated AFM indentations on PAAM gels (set 1, no allylamine) with a time interval of 1 h. Error bar: standard deviation.

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