Figure 4: Stepwise shearing of actin/α-actinin networks.
Shearing an actin/α-actinin network at ca=4.75 μM, molar ratio R=1 at 18 °C stepwise allows to image the network with a confocal microscope during cyclic loading. Indeed cyclic hardening can be observed if the network is sheared up to 56% then back to 0% and then up to a higher strain (Supplementary Fig. S1). In between the steps of 14% the strain is hold fixed for 2 min and a confocal z-stack is taken. The time points where the micrographs 1–5 have been taken are indicated in the protocol (a). (b) During the shearing of the network to 56% many bundles lose contact to the lower surface (confocal images at time points 1 and 2). An irreversible reorganization is observed when the network is sheared back (3). If the network is sheared up again, distinctly higher stresses are reached. Shearing to a higher strain results in a peak in the stress-strain relation roughly at the previous maximum strain 56%. Interestingly the network now looses contact mainly from the upper surface (5). Confocal images are x-projections of 35 μm. Scale bars denote 50 μm. A video showing one image per step can be seen in the Supplementary information (Supplementary Movie S2). (c) Strain, stress and the network volume normalized on the initial value are shown as a function of step number. As the network shrinks only in z-direction, the volume is proportional to the thickness of the network. The thickness is obtained from an intensity line profile along the z-direction and averaged over ≈250 μm in x and y direction, respectively. The position where the fluorescence intensity reaches half of the maximum value is defined as the boundary of the network.
