Figure 5: Cyclic hardening in actin/α-actinin networks depends on protein concentrations.

The cyclic hardening behavior is very sensitive to protein concentrations. Actin/α-actinin networks at c_a=9.5 μM, R=1 at 18 °C show a clear cyclic hardening (red—different strain amplitudes are indicated by different red colors) while it is hardly observable for actin/α-actinin networks at c_a=9.5 μM, R=0.5 at 21 °C (blue—different strain amplitudes are indicated by different blue colors). Thick lines mark the first upward shearing to the respective maximum strain. The responses corresponding to the protocols shown in (a) to upward and downward shearing (indicated by an arrow) are shown in (b, c) and (d, e; the inset in d shows a zoom in at low strains), respectively. A shear rate of 1.4%/s and pauses of 120 s are used for both experiments. It can be seen in d, e that for the network showing the strong cyclic hardening, distinctly higher negative stresses are reached during the downward shearing. (f) The stress at the lower cyclically applied strain 35% as a function of cycle number is shown for both networks. (g) For c_a=9.5 μM, R=0.5 at 21 °C no negative prestrain results if the network is cyclically sheared to 35%/s. The micrograph shows an overlay of confocal images (x projections 21 μm before (green) and after (red) the first cycle as indicated in a)). Scale bar denotes 25 μm.