Introduction to Traction Force Microscopy and Monolayer Stress Microscopy

Traction Force Microscopy and Monolayer Stress Microscopy are methods to measure the force generation and internal forces of cells and cell colonies.

A typical experiment starts by seeding cells on a substrate containing small fluorescence labeled beads. The cells adhere to the substrate, start generating forces and consequently cause deformations in the substrate. These deformations can be measured by tracking the labeled beads. In practice the beads are imaged two times: Once when cells are attached to the substrate, and once when the cells are removed from the substrate. In the first image the substrate is strained by the cells, while the second image shows the completely relaxed substrate.

The deformations in the substrate can be used to calculate the traction forces that acted on the substrate surface. This is done by Fourier Transformed Traction Force Microscopy (TFM) [1]. Any force that is applied from the cells to the substrate surface must be balanced by counteracting force inside of the cells. Forces inside of materials are described by stress.

A cell that generates two opposing forces at its ends, experiences a high stress between the points of force generation. The stress inside of cells and cell colonies is recovered by Monolayer Stress Microscopy (MSM) [2]. In MSM the cells are modeled as a 2-dimensional sheet. Following the argument of force-balance, the traction forces calculated from TFM, with the opposite signs, are applied to the cell sheet. Then, the stresses in the cell sheet are calculated with 2-dimensional Finite Elements Methods

[1]	Traction fields, moments, and strain energy that cells exert on their surroundings James P. Butler, Iva Marija Tolić-Norrelykke, Ben Fabry, and Jeffrey J. Fredberg Am J Physiol Cell Physiol 282: C595–C605, (2002)

[2]	Monolayer Stress Microscopy: Limitations, Artifacts, and Accuracy of Recovered Intercellular Stresses Dhananjay T. Tambe, Ugo Croutelle, Xavier Trepat, Chan Young Park, Jae Hun Kim, Emil Millet, James P. Butler, Jeffrey J. Fredberg PLOS ONE 8(2): e55172 (2013)