Fibrin fiber fluctuation

Understanding force propagation through the fibrous extracellular matrix
can elucidate how cells interact mechanically with their surrounding
tissue. Presumably, due to elastic nonlinearities of the constituent
filaments and their random connection topology, force propagation in fiber
networks is quite complex, and the basic problem of force propagation in
structurally heterogeneous networks remains unsolved. We report on a new
technique to detect displacements through such networks in response to a
localized force, using a fibrin hydrogel as an example. By studying the
displacements of fibers surrounding a two-micron bead that is driven
sinusoidally by optical tweezers, we develop maps of displacements in the
network. Fiber movement is measured by fluorescence intensity fluctuations
recorded by a laser scanning confocal microscope. We find that the Fourier
magnitude of these intensity fluctuations at the drive frequency
identifies fibers that are mechanically coupled to the driven bead. By
examining the phase relation between the drive and the displacements, we
show that the fiber displacements are, indeed, due to elastic couplings
within the network. Both the Fourier magnitude and phase depend on the
direction of the drive force, such that displacements typically propagate
farther, but not exclusively, along the drive direction. This technique
may be used to characterize the local mechanical response in 3-D tissue
cultures, and to address fundamental questions about force propagation
within fiber networks.