Chapter 6 – Forces  223

for measuring the orientation and magnitude of forces experienced in tissues, and how these

vary with mechanical strain. The usual reporters for these mechanical changes are birefrin­

gent protein fibers in either connective tissue or the cytoskeleton. In particular, collagen

fibrils form an anisotropic network in cartilage and bone tissue, which has several important

mechanical functions, largely responsible for tensile and shear stiffness. This method has the

advantage of being label-​free and thus having greater physiological relevance. The resulting

anisotropy images represent a tissue force map and can be used to monitor damage and

repair mechanisms of collagen during tissue stress resulting from disease.

FRET (see Chapter 4) can also be utilized to monitor mechanical forces in cells. Several

synthetic molecular probes have been developed, which undergo a distinct bimodal

FIGURE 6.4  (See color insert.) More complex optical tweezers applications. (a) Cell stretcher, composed of two

juxtaposed optical beams generating a stable optical trap that can optically stretch single cells in suspension. (b) Rotary

molecular motors, here shown with the F1-​ATPase component of the ATP synthase that is responsible for making ATP in

cells (see Chapter 2), can be probed using optical trapping of fused bead pairs. (c) Trapping a fluorescence excitation beam

paths can be combined (d) to generate optical traps combined with fluorescence imaging. (e) A three-​bead “dumbbell”

assay, consisting of two optically trapped beads and a fixed bead on a surface, can be used to probe the forces and

displacements of “power strokes” due to molecular motors on their respective tracks.