This work was supported by the Committee Technology and Innovation (CTI) Switzerland grant 14796-PFLS-LSCTI. MicroPost arrays were generously provided by MicroDuits GmbH.

1. Culturing Cells on Micropost Arrays
Note: All steps must be performed in a biosafety cabinet to ensure sterility. The volumes given here are for T25 cell culture flasks. The cell culture medium recipe and cell seeding density are optimized for bone cancer cell lines HuO9 and M132.
<ol>
	<li>Prepare a cell culture for seeding onto a micropost array.
	<ol>
		<li>Inspect the cell culture quality by placing the culture flask on a standard light microscope. Ensure that the cells show a typical...

<ol>
	<li>Sharma, R. I., Snedeker, J. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biochemical+and+biomechanical+gradients+for+directed+bone+marrow+stromal+cell+differentiation+toward+tendon+and+bone.">Biochemical and biomechanical gradients for directed bone marrow stromal cell differentiation toward tendon and bone.</a> Biomaterials. 31 (30), 7695-7704 (2010).</li><li>Suresh, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomechanics+and+biophysics+of+cancer+cells.">Biomechanics and biophysics of cancer cells.</a> Acta Biomater. 3 (4), 413-438 (2007).</li><li>Bartalena, G., 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=A+novel+method+for+assessing+adherent+single-cell+stiffness+in+tension:+design+and+testing+of+a+substrate-based+live+cell+functional+imaging+device.">A novel method for assessing adherent single-cell stiffness in tension: design and testing of a substrate-based live cell functional imaging device.</a> Biomed Microdevices. 13 (2), 291-301 (2011).</li><li>Tan, J. L., 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=Cells+lying+on+a+bed+of+microneedles:+An+approach+to+isolate+mechanical+force.">Cells lying on a bed of microneedles: An approach to isolate mechanical force.</a> PNAS. 100 (4), 1484-1489 (2003).</li><li>Akiyama, Y., Hoshino, T., Iwabuchi, K., Morishima, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Room+Temperature+Operable+Autonomously+Moving+Bio-Microrobot+Powered+by+Insect+Dorsal+Vessel+Tissue.">Room Temperature Operable Autonomously Moving Bio-Microrobot Powered by Insect Dorsal Vessel Tissue.</a> PloS ONE. 7 (7), (2012).</li><li>Biais, N., Ladoux, B., Higashi, D., So, M., Sheetz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cooperative+retraction+of+bundled+type+IV+pili+enables+nanonewton+force+generation.">Cooperative retraction of bundled type IV pili enables nanonewton force generation.</a> Plos Biology. 6 (4), 907-913 (2008).</li><li>Wuang, S. C., Ladoux, B., Lim, C. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+the+Chemo-Mechanical+Effects+of+an+Anti-Cancer+Drug+Emodin+on+Breast+Cancer+Cells.">Probing the Chemo-Mechanical Effects of an Anti-Cancer Drug Emodin on Breast Cancer Cells.</a> Cell Mol Bioeng. 4 (3), 466-475 (2011).</li><li>Schoen, I., Hu, W., Klotzsch, E., Vogel, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+Cellular+Traction+Forces+by+Micropillar+Arrays:+Contribution+of+Substrate+Warping+to+Pillar+Deflection.">Probing Cellular Traction Forces by Micropillar Arrays: Contribution of Substrate Warping to Pillar Deflection.</a> Nano Lett. 10 (5), 1823-1830 (2010).</li><li>Lemmon, C. A., 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=Shear+Force+at+the+Cell-Matrix+Interface:+Enhanced+Analysis+for+Microfabricated+Post+Array+Detectors.">Shear Force at the Cell-Matrix Interface: Enhanced Analysis for Microfabricated Post Array Detectors.</a> Mech Chem Biosyst. 2 (1), 1-16 (2005).</li><li>Lam, R. H. W., Weng, S. N., Lu, W., Fu, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live-cell+subcellular+measurement+of+cell+stiffness+using+a+microengineered+stretchable+micropost+array+membrane.">Live-cell subcellular measurement of cell stiffness using a microengineered stretchable micropost array membrane.</a> Integr Biol-UK. 4 (10), 1289-1298 (2012).</li><li>Roure, O., 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=Force+mapping+in+epithelial+cell+migration.">Force mapping in epithelial cell migration.</a> PNAS. 102 (39), 14122-14122 (2005).</li><li>Papenburg, B. J., Rodrigues, E. D., Wessling, M., Stamatialis, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Insights+into+the+role+of+material+surface+topography+and+wettability+on+cell-material+interactions.">Insights into the role of material surface topography and wettability on cell-material interactions.</a> Soft Matter. 6 (18), 4377-4388 (2010).</li><li>Badique, F., 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=Directing+nuclear+deformation+on+micropillared+surfaces+by+substrate+geometry+and+cytoskeleton+organization.">Directing nuclear deformation on micropillared surfaces by substrate geometry and cytoskeleton organization.</a> Biomaterials. 34 (12), 2991-3001 (2013).</li><li>Sniadecki, N. J., 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=Magnetic+microposts+as+an+approach+to+apply+forces+to+living+cells.">Magnetic microposts as an approach to apply forces to living cells.</a> PNAS. 104 (37), 14553-14558 (2007).</li><li>Weng, R. H. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+silicone-based+stretchable+micropost+array+membrane+for+monitoring+live-cell+subcellular+cytoskeletal+response.">A silicone-based stretchable micropost array membrane for monitoring live-cell subcellular cytoskeletal response.</a> Lab Chip. 12, 731-740 (2012).</li><li>Desai, R. A., Yang, M. T., Sniadecki, N. J., Legant, W. R., Chen, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfabricated+Post-Array-Detectors+(mPADs):+an+Approach+to+Isolate+Mechanical+Forces.">Microfabricated Post-Array-Detectors (mPADs): an Approach to Isolate Mechanical Forces.</a> J Vis Exp. (8), e311 (2007).</li><li>Yang, M. T., Fu, J. P., Wang, Y. K., Desai, R. A., Chen, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Assaying+stem+cell+mechanobiology+on+microfabricated+elastomeric+substrates+with+geometrically+modulated+rigidity.">Assaying stem cell mechanobiology on microfabricated elastomeric substrates with geometrically modulated rigidity.</a> Nat Protoc. 6 (2), 187-213 (2011).</li><li>Sniadecki, N. J., Han, S. J., Ting, L. H., Feghhi, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micropatterning+in+Cell+Biology.">Micropatterning in Cell Biology.</a> Methods in Cell Biol. 121, 61-73 (2014).</li></ol>

The lead author (NG) is co-founder of MicroDuits GmbH, the commercial supplier of the micropost arrays used in this study. No other authors have any potential conflicts of interest with this work.

This work seeks to advance the field of traction force microscopy by substantially lowering technical and practical barriers to entry. These barriers come from two sides. First and foremost are the numerous non-trivial technical challenges that must be overcome to reproducibly manufacture and experimentally commission a micropost array. Second, is the similarly non-trivial need for a reliable semi-automated analysis of single cell forces &#8211; keeping in mind that a typical experiment may require the analysis of hundre...

Mechano-sensitive cell-based assays allow for investigating adherent cells, with a broad range of applications that reflect the central role that mechanics can play in cell biology. These applications often focus on the underlying mechanisms that drive subcellular processes or whole-cell behavior. On the one hand, external environmental factors such as extra cellular matrix composition or matrix stiffness can dramatically affect the mechanical and biological response of a cell.1 The same can be observed after use of many classes of pharmaceutically active compounds, the effects of which are often characterized using cell culture models.2 On the other hand genotypic properties, such as those caused by spontaneous or experimentally induced genetic mutations, can induce marked changes in cell phenotype that are associated with alterations in the cytoskeleton structure and function.3 These examples are just a few of the many possible topics for which mechanical phenotyping of cells is relevant, and all of these have been usefully investigated with micropost arrays.
At time of this writing, approximately 200 articles have been published describing cell-micropost interaction. These works discuss theoretical aspects of micropost deflection principles as well as practical instructions on their manufacture. The first article describing the interaction of cells and flexible micropost arrays was published by Tan and colleagues in 2003.4 In contrast to classic traction force microscopy (TFM) where continuous soft substrates are used to estimate nanonewton-scale cell contractility, Tan et al. described a method using multiple closely spaced vertical beams made of silicone elastomer. The main advantages of this technique emerge from two major features. First in order to change the cell-apparent substrate stiffness one only needs to change the micropost dimensions while keeping the substrate composition otherwise constant and thus avoiding differences in surface topology and chemistry. Second microposts act like individual springs that can be discretely analyzed with force and spatial resolutions on the order of individual focal adhesions and can reduce the analytical challenges that are inherent to analogous analysis by standard TFM.
Today the range of applications for micropost arrays greatly exceeds just the mapping of forces for a few single cells. For example, Akiyama reports the use of an isolated dorsal vessel tissue from a moth caterpillar as an actuator for a micropost array, in order to develop an insect muscle-powered autonomous micro-robot.5
However, most published applications of microposts have focused on studies of medical conditions like infection or cancer. For instance, micropost arrays have been used to study the force generation of bundled type IV pili of Neisseria gonorrhoea colonies that is associated with signal cascades enhancing infection.6 Others have used microposts to study breast cancer cells treated with pharmaceutical compounds targeting the cytoskeleton.7
Deflection of a micropost is often described using classical beam theory for a cantilever with an end load assuming the cell attaches only to the very tip of the micropost. Here the applied force F that causes a deflection &#948; depends on the micropost&#8217;s &#8220;bending stiffness&#8221; k and is calculated by:
<img alt="Equation 1" src="/files/ftp_upload/53350/53350eq1.jpg" /> (1)
with E, I, and L being the Young&#8217;s modulus, area moment of inertia and beam length respectively. However, results from this equation only give a general approximation of the forces at work since beam shearing and bending as well as substrate warping are not taken into account. Considering that microposts are typically made from soft materials like polydimethylsiloxane (PDMS)-based silicone rubber these factors need to be included. Schoen et al. demonstrated that there is such a correction factor based on the aspect ratio of the micropost (L/D) and the corresponding polymer&#8217;s Poisson ratio v.8 It is given by:
<img alt="Equation 2" src="/files/ftp_upload/53350/53350eq2.jpg" /> (2)
With Ttilt(v) being a tilting coefficient that includes fitting parameter a = 1.3 as can be found in the same article:
<img alt="Equation 3" src="/files/ftp_upload/53350/53350eq3.jpg" /> (3)
That means a micropost&#8217;s corrected stiffness kcorr is the product of the pure bending stiffness k=kbend and the correction factor corr given by:
<img alt="Equation 4" src="/files/ftp_upload/53350/53350eq4.jpg" /> (4)
Therefore, cell force calculations should be performed using the more refined variation of equation (1) now reading:
<img alt="Equation 5" src="/files/ftp_upload/53350/53350eq5.jpg" /> (5)
The impact of the correction becomes more obvious as soon as typical values for micropost dimensions are used. For example, a 15-micron long micropost with a circular cross section and a diameter of 5 &#181;m made of PDMS-based silicone rubber leads to a correction factor of 0.77 and therefore an uncorrected calculation would overestimate the exerted cell forces by 23%. This becomes even more severe for microposts with smaller aspect ratios.
Traditionally, micropost image analysis has also been based on idealized beam bending theory. In 2005 the group that pioneered the use of micropost arrays published an image analysis software suited for micropost analysis.9 The software requires a software license and the user must take three images for each position; one each from the micropost&#8217;s top and bottom planes in transmission mode and another one in fluorescence mode with the stained cell. After comparing the top and bottom positions for each micropost the software determines a force vector field and calculates related parameters like force per post. Other software packages exist and their analysis principles are briefly mentioned in the corresponding articles that describe them, but these analytical software packages are generally not publicly available.10,11
The micropost arrays designed for mapping cell forces can be classified as either being in an orthogonal micropost layout or a hexagonal one, the latter of which have the advantage of equidistant gaps between all neighbor microposts. Typical microposts have a circular cross section and their dimensions range from 1.0 &#181;m to 10&#160;&#181;m in diameter and 2 to 50 &#181;m in length. 4 However, microposts with elliptic or square cross section have also been reported.12,13
The use of PDMS-based silicone mixtures as micropost material allows for adding nanoparticles into the mixture. For example adding cobalt nano-rods enables a magnetic activation of the micropost and thus gives another degree of freedom to potential experimental designs.14 Most groups produce their micropost arrays on flat rigid substrates like cover glass or inside a Petri dish. However, Mann and co-workers recently reported a micropost array formed on a stretchable membrane.15 This allows the application of cell stretching forces to adherent cells while studying live-cell subcellular dynamic responses in terms of cell contractility.
The widely employed and most established process for making micropost arrays is based on soft lithography as described in the insightful protocols of Sniadecki and colleagues.16-18 In short standard cleanroom processes are used to generate the microstructures on top of a silicon wafer using SU8 photoresist. This is followed by a copying process wherein the silicone rubber is cast over the structures transferring them into molds. In a second step these molds are used to replicate the initial microstructure using silicone rubber on top of a chosen substrate. However despite the large and growing number of publications related to their application, establishing a manufacturing process for microposts takes considerable amount of time even for micro-engineering experts; there are many process steps that require optimization and adaptation to the specific lab environment and micropost layout to yield an acceptable quality level.
Commercial micropost arrays are now available in a ready-to-use (&#8220;off-the-shelf&#8221;) format with a consistently high quality. As such they are an alternative to the complex and lengthy manufacturing process required for on-site production. In this paper a commercially available micropost array was used for mapping cellular forces using a single bright-field microscopy image. More importantly this article describes and documents a fully-functional open-source software named MechProfiler, which is available for download as supplementary material to this manuscript. An actively maintained version of the software can also be found at http://www.orthobiomech.ethz.ch.
The combination of an &#8220;off-the-shelf&#8221; assay and a compatible open-source analysis software markedly lowers the entry hurdle to achieve accurate TFM experiments. Researchers without access to either clean room facilities or software development expertise can analyze cellular forces successfully. It enables a user to focus on the mechanosensitivity assay output rather than the technology itself, and makes traction force measurements available to a broader community. Furthermore, this is an important step to pave the way towards fully automatic screening of micropost arrays.
The MechProfiler analysis software processes images in file format tiff, png, bmp and jpg. The images can be taken using fluorescence, phase contrast or bright-field light microscopy. The standalone program runs together with the free Matlab Compiler Runtime (available at: Figure 12) and underlying algorithms allow for streamlined image processing, which enables the user to process images with single or multiple cells in about 1 min. Further, these cells may either be living or &#8220;fixed&#8221;.
The MechProfiler software is able to greatly increase data analysis throughput by relying on reproducibility of quality commercial micropost arrays, more specifically, the default &#8220;non-deflected&#8221; position of each post in the array can be presumed against an ideal grid (manufacturing deviations for the grid in the arrays used for this study were less than 100 nm).
In short one opens a selection of image files for analysis, crops them to the region of interest, defines the posts covered by cells or which need to be discarded, determines the post positions, calculates the deflections/forces against the ideal grid, and finally saves all cell-specific data with a possibility for export, including to a standard office spreadsheet.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>T25 cell culture flasks</td>
			<td>Nunc</td>
			<td>156367</td>
			<td></td>
		</tr>
		<tr>
			<td>1x PBS-buffer</td>
			<td>Sigma</td>
			<td>D8537</td>
			<td></td>
		</tr>
		<tr>
			<td>0.5% Trypsin-EDTA</td>
			<td>Life Technologies</td>
			<td>15400054</td>
			<td></td>
		</tr>
		<tr>
			<td>Medium DMEM/F12</td>
			<td>Sigma</td>
			<td>D8437</td>
			<td></td>
		</tr>
		<tr>
			<td>FBS South America</td>
			<td>Life Technologies</td>
			<td>10270106</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin 100x</td>
			<td>Sigma</td>
			<td>P4333</td>
			<td></td>
		</tr>
		<tr>
			<td>15 ml centrifuge vials</td>
			<td>Sarstedt</td>
			<td>62.554.502&#160;</td>
			<td></td>
		</tr>
		<tr>
			<td>Micropost array&#160;</td>
			<td rowspan="2">MicroDuits</td>
			<td rowspan="2">MPA-col1/FN</td>
			<td>Micropost dimensions:</td>
		</tr>
		<tr>
			<td>pre-coated with Collagen I/Fibronectin</td>
			<td>&#216;= 6.4 &#181;m, l=18.2 &#181;m; grid=&#160;13 &#181;m, kcorr= 2.87 nN/&#181;m</td>
		</tr>
		<tr>
			<td>Ethanol abs p.A.</td>
			<td>Merck</td>
			<td>100.983</td>
			<td></td>
		</tr>
		<tr>
			<td>12-well plate</td>
			<td>Nunc</td>
			<td>150628</td>
			<td></td>
		</tr>
		<tr>
			<td>Formalin solution, neutral buffered 10%</td>
			<td>Sigma</td>
			<td>HT5011</td>
			<td></td>
		</tr>
		<tr>
			<td>Brilliant Blue G-250</td>
			<td>Sigma</td>
			<td>27815</td>
			<td>Coomassie blue</td>
		</tr>
		<tr>
			<td>Methanol ACS p.A.</td>
			<td>Merck</td>
			<td>1.06009</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetic acid</td>
			<td>Sigma</td>
			<td>695092</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass bottom dish</td>
			<td>WillCo Wells BV</td>
			<td>GWSb-3522</td>
			<td>35 mm diameter, aperture 22&#160;mm</td>
		</tr>
		<tr>
			<td>T-100 Eclipse</td>
			<td>Nikon</td>
			<td>n/a</td>
			<td>Inverted microscope</td>
		</tr>
		<tr>
			<td>D3-L3</td>
			<td>Nikon</td>
			<td>n/a</td>
			<td>Camera controler</td>
		</tr>
		<tr>
			<td>DS Fi2</td>
			<td>Nikon</td>
			<td></td>
			<td>Camera</td>
		</tr>
	</tbody>
</table>

easy and accurate mechano-profiling on micropost arrays

Cell culture substrates with integrated flexible microposts enable a user to study the mechanical interactions between cells and their immediate surroundings. Particularly, cell-substrate interactions are the main interest. Today micropost arrays are a well-characterized and established method with a broad range of applications that have been published over the last decade. However, there seems to be a reservation among biologists to adapt the technique due to the lengthy and challenging process of micropost manufacture along with the lack of easily approachable software for analyzing images of cells interacting with microposts. 
The force read-out from microposts is surprisingly easy. A micropost acts like a spring with the cell ideally attached at its tip. Depending on size a cell applies force from its cytoskeleton through one or multiple focal adhesion points to the micropost, thus deflecting the micropost. The amount of deflection correlates directly to the applied force in direction and in magnitude. The number of microposts covered by a cell and the post deflection patterns are characteristic and allow determination of values like force per post and many biologically relevant parameters that allow &#8220;mechano-profiling&#8221; of cell phenotypes.
A convenient method for mechano-profiling is described here combining the first generation of ready-to-use commercially available microposts with an in-house developed software package that is now accessible to all researchers. As a demonstration of typical application, single images of bone cancer cells were taken in bright-field microscopy for mechano-profiling of cell line models of metastasis. This combination of commercial traction force sensors and open source software for analysis allows for the first time a rapid implementation of the micropost array technique into routine lab work done by non-expert users. Furthermore, a robust and streamlined analysis process enables a user to analyze a large number of micropost images in a highly time-efficient manner.

The main advantage of the described technique lies in its simplicity and potential for rapid and effective integration into routine lab work. The combination of high quality commercial sensor arrays paired with open-source software provides information about mechano-sensitivity that would otherwise requires access to cleanroom facilities and in-depth knowledge of image analysis and software development. Figure 1 illustrates the workflow of the presented method. It starts with preparing and seeding cells ...

Watch this Scientific Journal Video about Easy and Accurate Mechano-profiling on Micropost Arrays at JoVE.com

Easy and Accurate Mechano-profiling on Micropost Arrays

Described are protocols for quantifying mechanical interactions between adherent cells and microstructured substrates. These interactions are closely linked to essential cell behaviors including migration, proliferation, differentiation, and apoptosis. The protocols present an open-source image analysis software called MechProfiler, which enables determination of involved forces for each micropost.

Cell culture substrates with integrated flexible microposts enable a user to study the mechanical interactions between cells and their immediate ...

easy-and-accurate-mechano-profiling-on-micropost-arrays

Research

JoVE Journal

Bioengineering

11.0K visualizações.  Balgrist University Hospital. Described are protocols for quantifying mechanical interactions between adherent cells and microstructured substrates. These interactions are closely linked to essential cell behaviors including migration, proliferation, differentiation, and apoptosis. The protocols present an open-source image analysis software called MechProfiler, which enables determination of involved forces for each micropost. 