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Atomic force microscopy probing in the measurement of cell mechanics.

Kirmizis D, Logothetidis S - Int J Nanomedicine (2010)

Bottom Line: Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology.Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles.AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Laboratory for Thin Films-Nanosystems and Nanometrology, Aristotle University, Thessaloniki, Greece. dkirmizi@physics.auth.gr

ABSTRACT
Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology. Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles. AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

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The AFM cell indentation experiment. A) The force curve obtained by measurements of cantilever deflection versus z-position during advancement and retraction of the probe. This curve provides information about the viscoelastic properties of the cell. Once the raw force curve is obtained and the contact point (Z0) identified, cell mechanical properties are obtained from the analysis of the curve of indentation force (F = k × h) versus depth (D = (Z − Z0) − h) B).
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f1-ijn-5-137: The AFM cell indentation experiment. A) The force curve obtained by measurements of cantilever deflection versus z-position during advancement and retraction of the probe. This curve provides information about the viscoelastic properties of the cell. Once the raw force curve is obtained and the contact point (Z0) identified, cell mechanical properties are obtained from the analysis of the curve of indentation force (F = k × h) versus depth (D = (Z − Z0) − h) B).

Mentions: Although both primary forms of AFM imaging, ie, tapping and contact mode, have been used in cell elastography, contact mode is easier and more convenient to use than tapping mode, as long as it is more conducive to switching back and forth between imaging and “force mode,” in which nanoindentation is used to obtain quantitative stiffness measurements, while it can also give high-resolution images with cell viability sustained for several hours.23 The indentation response depends on the spring constant of the probe, the geometry of the tip, and the mechanical properties of the sample. One also can vary the rate of indentation to study viscoelastic properties. Thus, by monitoring the z-position and deflection of the probe (the so-called “force curve”) (Figure 1A), one can obtain an indentation curve of indentation force versus depth (Figure 1B) that can be analyzed to extract the elastic material properties of the sample as discussed below. Force mapping is a hybrid combination of imaging and force probing that involves making a series of in dentations in an array covering a region of interest on the sample and reconstructing an isoforce image from the z-position at which the probe reaches a preset constant deflection (ie, contact force).24 In such images, larger z-values are interpreted as softer regions of the sample because a greater motion of the probe would have been required to achieve the preset force. However, in samples such as living cells, such images are complicated by the highly variable topography of the cell, which also influences the z-position at which a given contact force is achieved. Therefore, it is more accurate to analyze the indentation data and create an image that directly represents the elastic properties obtained at each pixel location. This is the method of AFM elastography. The retraction curve also contains useful information. Differences between the indentation and retraction curves reflect viscoelastic hysteresis of the sample. Upon retraction of the probe, the AFM tip may adhere to the sample and cause negative deflections of the probe. Such retraction events are the focus of experiments on protein unfolding,25 receptor–ligand binding,26 and cell–cell adhesion.27


Atomic force microscopy probing in the measurement of cell mechanics.

Kirmizis D, Logothetidis S - Int J Nanomedicine (2010)

The AFM cell indentation experiment. A) The force curve obtained by measurements of cantilever deflection versus z-position during advancement and retraction of the probe. This curve provides information about the viscoelastic properties of the cell. Once the raw force curve is obtained and the contact point (Z0) identified, cell mechanical properties are obtained from the analysis of the curve of indentation force (F = k × h) versus depth (D = (Z − Z0) − h) B).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2865008&req=5

f1-ijn-5-137: The AFM cell indentation experiment. A) The force curve obtained by measurements of cantilever deflection versus z-position during advancement and retraction of the probe. This curve provides information about the viscoelastic properties of the cell. Once the raw force curve is obtained and the contact point (Z0) identified, cell mechanical properties are obtained from the analysis of the curve of indentation force (F = k × h) versus depth (D = (Z − Z0) − h) B).
Mentions: Although both primary forms of AFM imaging, ie, tapping and contact mode, have been used in cell elastography, contact mode is easier and more convenient to use than tapping mode, as long as it is more conducive to switching back and forth between imaging and “force mode,” in which nanoindentation is used to obtain quantitative stiffness measurements, while it can also give high-resolution images with cell viability sustained for several hours.23 The indentation response depends on the spring constant of the probe, the geometry of the tip, and the mechanical properties of the sample. One also can vary the rate of indentation to study viscoelastic properties. Thus, by monitoring the z-position and deflection of the probe (the so-called “force curve”) (Figure 1A), one can obtain an indentation curve of indentation force versus depth (Figure 1B) that can be analyzed to extract the elastic material properties of the sample as discussed below. Force mapping is a hybrid combination of imaging and force probing that involves making a series of in dentations in an array covering a region of interest on the sample and reconstructing an isoforce image from the z-position at which the probe reaches a preset constant deflection (ie, contact force).24 In such images, larger z-values are interpreted as softer regions of the sample because a greater motion of the probe would have been required to achieve the preset force. However, in samples such as living cells, such images are complicated by the highly variable topography of the cell, which also influences the z-position at which a given contact force is achieved. Therefore, it is more accurate to analyze the indentation data and create an image that directly represents the elastic properties obtained at each pixel location. This is the method of AFM elastography. The retraction curve also contains useful information. Differences between the indentation and retraction curves reflect viscoelastic hysteresis of the sample. Upon retraction of the probe, the AFM tip may adhere to the sample and cause negative deflections of the probe. Such retraction events are the focus of experiments on protein unfolding,25 receptor–ligand binding,26 and cell–cell adhesion.27

Bottom Line: Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology.Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles.AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Laboratory for Thin Films-Nanosystems and Nanometrology, Aristotle University, Thessaloniki, Greece. dkirmizi@physics.auth.gr

ABSTRACT
Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology. Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles. AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

Show MeSH
Related in: MedlinePlus