Physicochemical Tools for Visualizing and Quantifying Cell-Generated Forces

Abstract

To discern how mechanical forces coordinate biological outcomes, methods that map cell-generated forces in a spatiotemporal manner, and at cellular length scales, are critical. In their native environment, whether it be within compact multicellular three-dimensional structures or sparsely populated fibrillar networks of the extracellular matrix, cells are constantly exposed to a slew of physical forces acting on them from all directions. At the same time, cells exert highly localized forces of their own on their surroundings and on neighboring cells. Together, the generation and transmission of these forces can control diverse cellular activities and behavior as well as influence cell fate decisions. To thoroughly understand these processes, we must first be able to characterize and measure such forces. However, our experimental needs and technical capabilities are in discordwhile it is apparent that we should study cell-generated forces within more biologically relevant 3D environments, this goal remains challenging because of caveats associated with complex "sensing−transduction−readout" modalities. In this Review, we will discuss the latest techniques for measuring cell-generated forces. We will highlight recent advances in traction force microscopy and examine new alternative approaches for quantifying cell-generated forces, both of individual cells and within 3D tissues. Finally, we will explore the future direction of novel cellular force-sensing tools in the context of mechanobiology and next-generation biomaterials design.

Recent advances in label-free imaging of cell–matrix adhesions

“…To visualize the miniscule level of force ranging from pN to nN, different methods have been developed, such as traction force microscopy (TFM) [69][70][71] and single molecule force spectroscopy (SMFS) including atomic force microscopy, [72][73][74] optical or magnetic tweezers, 75,76 etc. Among them, TFM is only sensitive to forces at the nN level, while the SMFS methods fail to capture the mechanics of a whole cell because of their low throughput.…”

Combining atomic force microscopy with complementary techniques for multidimensional single‐cell analysis

“…Pillar arrays are typically made of poly(dimethylsiloxane) (PDMS) and characterised by pillar dimensions and spacing 187 . As cells spread and migrate across the tops of the pillar arrays, the contractile forces generated by the cells cause the pillars to bend laterally, and the cellular traction forces can then be directly calculated using classical beam theory after capturing the pillar deflections by optical microscopy 188,189 . The second method is based on the use of tiny fluorescent beads which are embedded in or attached to the surface of an elastic hydrogel substrate (polyacrylamide (PAA) or silicon‐based gels are commonly used substrates) 190 .…”

Integrating mechanical sensor readouts into organ-on-a-chip platforms

“…Several methods have been developed to quantify and characterize the mechanical microenvironment that cells live in, and have been designed to measure both cell-generated forces and mechanical properties of cells and tissues. Overviews of force measurement and mechanical characterization methods are extensively reviewed elsewhere (Polacheck and Chen, 2016;Roca-Cusachs et al, 2017;Nguyen and Kilian, 2020;Obenaus et al, 2020), and here we focus specifically on the opportunities and challenges that come with integrating these sensors into OoC and microtissue engineered models, and how real-time, multiplexed, and spatially-defined information about tissue function can be obtained.…”