Optimization of traction force microscopy for micron-sized focal adhesions

Abstract

To understand how adherent cells regulate traction forces on their surroundingextracellular matrix (ECM), quantitative techniques are needed to measure forces atthe cell–ECM interface. Microcontact printing is used to create a substrate of1 µm diameter circles of ECM ligand to experimentally study the reconstruction oftraction stresses at constrained, point-like focal adhesions. Traction reconstructionwith point forces (TRPF) and Fourier transform traction cytometry (FTTC)are used to calculate the traction forces and stress field, respectively, at isolatedadhesions. We find that the stress field calculated with FTTC peaks near thecenter of individual adhesions but propagates several microns beyond the adhesionlocation. We find the optimal set of FTTC parameters that yield the highest stressmagnitude, minimizing information lost from over-smoothing and sampling of thedisplacement or stress field. A positive correlation between the TRPF and FTTCmeasurements exists, but integrating the FTTC stress field over the adhesion area yieldsonly a small fraction of the force calculated by TRPF. An effective area similarto that defined by the width of the stress distribution measured with FTTC isrequired to reconcile these measurements. These measurements set bounds on thespatial resolution and precision of FTTC measurements on micron-sized adhesions.