Mechanics of intermediate filament networks assembled from keratins K8 and K18

We have investigated intermediate filament networks assembled from the recombinant keratins K8 and K18 in vitro at various protein and MgCl2 concentrations using mechanical rheometry. Experimental parameters were chosen such that artifacts from sample surface elasticity or wall slip were avoided, and the gap width did not affect network formation. The modulus G(0) depends weakly on the protein concentration (G(0) similar to c(0.5)) and the critical deformation gamma(crit) at which non-linear response sets in is concentration independent. These findings can be rationalized assuming that the cross-link density decreases with decreasing protein concentration, while the filament contour length between cross-links remains unchanged. Thus, filaments are more stretched at lower protein concentrations and this increase in conformational energy partly compensates the free energy decrease related to the change in cross-link density. G(0) is independent of the MgCl2 concentration indicating that the contribution of stretched filaments decreases when the cross-link density increases. Networks rupture when a critical strain is exceeded, but fully recover within 30 minutes. The non-linear network response is characterized by pronounced strain stiffening with increasing shear stress sigma. Reduced differential modulus K' data obtained at different protein or MgCl2 concentrations collapse onto a master curve. Two scaling regimes K' similar to sigma(alpha) are observed with alpha = 1 at intermediate and alpha = 0.6 at high stresses. These exponents may be rationalized in terms of the glassy wormlike chain model assuming sticky contacts with finite, constant bond strength. Two distinct scaling regimes could also result from the existence of two types of filament contacts with different bond energies or by the compliance of individual filaments.