Figure 1.

Schematic of coupled microrheology and particle-tracking experiments. (A) Cartoon of an actin network cross-linked by NeutrAvidin (red dots) and doped with filaments with interspersed labeled segments (yellow) for tracking. Three phases of experiments shown: equilibration (no trap movement), strain (middle, trapped probe (white) moves 10 μm through the network at 8 μm/s), and relaxation (no trap movement, probe remains trapped). White arrows show unbinding/binding of NeutrAvidin during the strain/relaxation phases. (B) Measured force traces for networks with R = 0.01 and 0.07 during three experimental phases. Dashed lines during strain phase indicate the times at which images of labeled filaments are captured. Highlighted region corresponds to time depicted in (D). (C) Sample 122 × 140 μm image displaying all filament tracks (rainbow colors) measured for 85 individual measurements. Data for R = 0.07 is shown. Image is sectioned into concentric annuli, each 4.5 μm wide, with increasing radii d centered on the center of the strain path. (D) Probability distributions of tracked particle velocities parallel to the strain P(v_x) at a single window of time (highlighted in B) for R = 0.01 and 0.07 networks. Distributions for annuli near (d = 9 μm, top) and far (d = 40 μm, bottom) from strain path are shown. To see this figure in color, go online.