Name: measuring the mechanical properties of living cells using atomic force microscopy
Uploaded: 2013-06-27T14:00:00
Description: Watch this Scientific Journal Video about Measuring the Mechanical Properties of Living Cells Using Atomic Force Microscopy at JoVE.com

The authors thank Dr. Paul Janmey at University of Pennsylvania for providing cell lines used in this paper. QW also acknowledge J.F. Byfield and Evan Anderson for their insightful discussions on AFM techniques.

1. Calibrate the Spring Constant of Cantilever
<ol>
	<li>Load the cantilever into the AFM according to the manufacturer's instructions. It is necessary to clean the cantilever holder with ethanol before any experiments. This will help limit bacterial contamination to culture during the AFM measurements.</li>
	<li>Calibrate InvOLS (Inverse Optical Lever Sensitivity). This parameter describes the amount of photodiode response (Volts) per nanometer of cantilever deflection.</li>
	<li>Load a clean glass slide onto the samp...

<ol>
	<li>Discher, D. E., Janmey, P., Wang, Y. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tissue+cells+feel+and+respond+to+the+stiffness+of+their+substrate.">Tissue cells feel and respond to the stiffness of their substrate.</a> Science. 310, 1139-1143 (2005).</li><li>Wagner, O. I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Softness,+strength+and+self-repair+in+intermediate+filament+networks.">Softness, strength and self-repair in intermediate filament networks.</a> Exp. 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E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Filamin+A+is+essential+for+active+cell+stiffening+but+not+passive+stiffening+under+external+force.">Filamin A is essential for active cell stiffening but not passive stiffening under external force.</a> Biophysical Journal. 96, 4326-4335 (2009).</li><li>Elson, E. 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The AFM indentation method has advantages to characterize mechanical properties of living cells. Albeit less sensitive than the magnetic twisting cytometry and optical tweezers, which can measure forces on the piconewton level32, the AFM can detect resistance force from samples ranging from tens of pico-Newton to hundreds of nano-Newton, comparable to range of force that can be applied to cells using a micropipette19. This range of force fits the needs to create measurable deformations in all types ...

Mechanical properties, especially stiffness, of individual cells and their surrounding extracellular matrices (ECM) are critical for many biological processes including cell growth, motility, division, differentiation, and tissue homeostasis.1 It has been demonstrated that cell mechanical stiffness is mainly determined by the cytoskeleton, especially the networks of actin and intermediate filaments and other proteins associated with them.2 Results from mechanical tests on in vitro networks of actin and intermediate filaments suggest that the cell mechanics is largely dependent on the cytoskeletal structure and the pre-stress in the cytoskeleton.3-5 Stiffness of live cells is then regarded as an index to evaluate the cytoskeletal structure6, myosin activity7 and many other cellular processes. More importantly, changes in cell mechanical properties are also often found to be closely associated with various disease conditions such as tumor formation and metastasis.8-10 Monitoring the mechanical stiffness of living cells can therefore provide a novel way to monitor cell physiology; to detect and diagnose diseases8 ; and to evaluate the effectiveness of drug treatments.11,12
Multiple methods including particle-tracking microrheology,13-16 magnetic twisting cytometry,17 micropipette aspiration18,19 and microindentation20-22 have been developed to measure the elasticity of cells. Particle tracking microrheology traces the thermal vibrations of either submicron fluorescent particles injected into cells or fiducial markers inside the cell cytoskeleton.23 Elastic and viscous properties of cells are calculated from the measured particle displacements using the fluctuation-dissipation theorem.14,23 This method allows simultaneous measurements of local mechanical properties with high spatial resolution at different places in a cell. However, injecting fluorescent particles into cells may lead to changes in cellular function, cytoskeleton structure, and hence the cell mechanics. The micropipette aspiration method applies negative pressure in a micropipette of diameter ranging from 1 to 5 &mu;m to suck a small piece of cell membrane into the pipette. Cell stiffness is calculated from the applied negative pressure and cell membrane deformation.18 This method, however, cannot detect the heterogeneous distribution of stiffness across the cell. Magnetic twisting cytometry (MTC) applies magnetic field to generate torque on super paramagnetic beads attached to the cell membrane.17 Cell stiffness is derived in this method from the relationship between the applied torque and the twisting deformation of the cell membrane. It is difficult to control the location of magnetic beads in the MTC method, and it is also challenging to characterize the twisting deformation with high resolution. Microindentation applies an indenter with well-defined geometry to punch into the cell. The indenting force and the resulting indentation in cells often follow the prediction of the Hertz model. Young's moduli of cells can be calculated from the force-indentation curves by fitting them to the Hertz model. This method has been widely applied to test the mechanical properties of tissue and cells despite of its limitations such as uncertainty in contact point determination, applicability of the Hertz model, and the potential to physically damage the cells. Among the many devices for microindentaion20, the Atomic Force Microscope (AFM) is commercially available and has been widely applied to characterize mechanical properties of living cells and tissues21,24-27.
This paper demonstrates the procedure of using an Asylum MFP3D-Bio AFM to characterize cell mechanics. AFM not only provides high-resolution topography of cells but also has been widely applied to characterize the mechanical properties of tissue cells. The principle of AFM indentation is illustrated in Figure 1. The AFM cantilever approaches the cell from a few micrometers above; makes contact with the cell; indents the cell so that the cantilever deflection reaches a preselected set point; and pulls away from the cell. During this process the cantilever deflection is recorded as a function of its location as shown in Figure 1. Before making contact with the cell, the cantilever moves in the medium without any apparent deflection. When indenting on the cell, the cantilever bends and the deflection signal increases. The cantilevers are modeled as elastic beams so that their deflection is proportional to the force applied to the cell. By setting the maximum cantilever deflection, the maximum magnitude of force applied to the sample is limited to avoid damage to cells. The portion of the force curve from point b to point c in Figure 1, where the tip indents into the cell, is fit to the hertz model to extract the cell stiffness.
<img src="/files/ftp_upload/50497/50497fig1.jpg" alt="Figure 1" fo:content-width="4.75in" fo:src="/files/ftp_upload/50497/50497fig1highres.jpg"/> 
Figure 1. Illustration of AFM microindentation and interpretation of the force curve. The top panel shows the motion of AFM cantilever driven by the piezo scanner. The vertical location of cantilever z and the cantilever deflection signal d is recorded during the process. The cantilever starts from point a, a few micrometers above the cell. While approaching the cell, the sample indentation &delta; remains zero until it reaches point b, where the tip comes into contact with the cell. The coordinates of point b in the plot are critical values for data analysis, denoted by (z0, d0>). From b to c, the cantilever indents into the cell until the cantilever deflection reaches a set point, which is set to be the ratio between the targeted maximum indenting force and the cantilever spring constant. Once the deflection signal reaches the preset maximum value, the cantilever is then withdrawn from the cell to point d, where it often be pulled downwards due to tip-sample adhesion, detaches from the cell and returns to its initial location at e. The right panel illustrates the relationship between the indentation and the recorded z and d signal. In on the lower left panel is a plot of a representative force curve, the maximum indentation of a cantilever, of which the spring constant is measured to be 0.07N/m, is set to be 17 nm so that the maximum indenting force applied to sample is 1.2 nN. The key locations during the indentation are marked.

<table border="1">
<tbody>
<tr>
<td>Atomic Force Microscope </td>
<td>Asylum Research</td>
<td>MFP3D-BIO</td>
</tr>
</tbody>
</table>

measuring the mechanical properties of living cells using atomic force microscopy

Mechanical properties of cells and extracellular matrix (ECM) play important roles in many biological processes including stem cell differentiation, tumor formation, and wound healing. Changes in stiffness of cells and ECM are often signs of changes in cell physiology or diseases in tissues. Hence, cell stiffness is an index to evaluate the status of cell cultures. Among the multitude of methods applied to measure the stiffness of cells and tissues, micro-indentation using an Atomic Force Microscope (AFM) provides a way to reliably measure the stiffness of living cells. This method has been widely applied to characterize the micro-scale stiffness for a variety of materials ranging from metal surfaces to soft biological tissues and cells. The basic principle of this method is to indent a cell with an AFM tip of selected geometry and measure the applied force from the bending of the AFM cantilever. Fitting the force-indentation curve to the Hertz model for the corresponding tip geometry can give quantitative measurements of material stiffness. This paper demonstrates the procedure to characterize the stiffness of living cells using AFM. Key steps including the process of AFM calibration, force-curve acquisition, and data analysis using a MATLAB routine are demonstrated. Limitations of this method are also discussed.

Figure 2a shows three representative force curves taken from 3T3 fibroblasts cultured on plastic surface, polyacrylamide gel of Young's moduli 3,000 Pa and 17,000 Pa, respectively. After carefully identifying the contact points in the curves, the indenting force as function of cell deformation is plotted in Figure 2b. Under a force of magnitude smaller than 0.3 nN, a pyramid shape tip indents 3 micrometers into a cell cultured on a 3 kPa polyacrylamide gel. In contrast, a force more than...

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Measuring the Mechanical Properties of Living Cells Using Atomic Force Microscopy

This paper demonstrates a protocol to characterize the mechanical properties of living cells by means of microindentation using an Atomic Force Microscope (AFM).

Mechanical  properties of cells and extracellular matrix (ECM) play important roles in many  biological processes including stem cell differentiation, ...

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A Novel Method for Localizing Reporter Fluorescent Beads Near the Cell Culture Surface for Traction Force Microscopy

PA gels have long been used as a platform to study cell traction forces due to ease of fabrication and the ability to tune their elastic properties. When ...

Surface Potential Measurement of Bacteria Using Kelvin Probe Force Microscopy

Surface potential is a commonly overlooked physical characteristic that plays a dominant role in the adhesion of microorganisms to substrate surfaces. ...

Atomic Force Microscopy Imaging and Force Spectroscopy of Supported Lipid Bilayers

Atomic force microscopy (AFM) is a versatile, high-resolution imaging technique that allows visualization of biological membranes. It has sufficient ...

In Situ Mapping of the Mechanical Properties of Biofilms by Particle-tracking Microrheology

In Situ Mapping of the Mechanical Properties of Biofilms by Particle-tracking Microrheology

Bacterial cells are able to form surface-attached biofilm communities known as biofilms by encasing themselves in extracellular polymeric substances ...

Using Synthetic Biology to Engineer Living Cells That Interface with Programmable Materials

We have developed an abiotic-biotic interface that allows engineered cells to control the material properties of a functionalized surface. This system is ...

A Millimeter Scale Flexural Testing System for Measuring the Mechanical Properties of Marine Sponge Spicules

Many load bearing biological structures (LBBSs)&#8212;such as feather rachises and spicules&#8212;are small (&#60;1 mm) but not microscopic. Measuring the ...

Measurement of Force-Sensitive Protein Dynamics in Living Cells Using a Combination of Fluorescent Techniques

Cells sense and respond to physical cues in their environment by converting mechanical stimuli into biochemically-detectable signals in a process called ...