3 Dimensional Cellular Force Microscopy in Fibrin Gels

The mechanical forces exerted and detected by living cells play integral roles in diverse biological phenomena, including growth and development, wound healing, and cancer metastasis. In the past decade, techniques such as traction force microscopy and micropost arrays have proven to be powerful tools for measuring the forces generated by cells. In particular, traction force microscopy has recently been extended to three-dimensional cell culture environments by embedding tracer beads in either a synthetic polyethylene glycol hydrogel (PEG; Legant et al., Nat. Meth. 2010) or in collagen gels (Koch et al., PLoS ONE 2012). The embedded beads move in response to cell-generated distortions of the matrix, allowing cell-generated forces to be calculated. We sought to develop an experimental system that would exhibit the excellent mechanical properties of the PEG hydrogel while using a naturally occurring biological matrix. Fibrin gels fulfill both of these requirements: fibrin is elastic up to ∼50% strain (Brown et al., Science 2009) and is also widely used for 3D cell culture. Here we describe the use of fluorescently labeled fibrin gels to measure the forces generated by cells in 3D culture. We observe dramatic but elastic deformations of the fibrin matrix surrounding cells as they grow, divide, and migrate. Further, we find that the dynamic forces generated by the cell can be measured using the deformations of the matrix itself, providing a direct observation of how the cell modifies its surroundings. We discuss the use of this new technique in studying matrix remodeling and cell migration.