Rheological properties of cysteine-containing elastin-like polypeptide solutions and hydrogels

The rheological properties of cysteine-containing elastin-like polypeptide (Cys-ELP) solutions and Cys-ELP hydrogels are reported. The Cys-ELP solutions exhibit a surprisingly high apparent viscosity at low shear rate. The high viscosity is attributed to the formation of an interfacial cross-linked "skin" at the sample surface, rather than the bulk of the Cys-ELP solution. At higher shear rate, the interfacial cross-linked film breaks, and its influence on the viscosity of the Cys-ELP solution can be ignored. Cys-ELP hydrogels are formed by mixing Cys-ELP and hydrogen peroxide (H(2)O(2)). At fixed concentration of Cys-ELP, the gelation time can be tuned by the concentration of H(2)O(2). Cys-ELP hydrogels have the typical characteristics of covalent cross-linked networks, as the storage moduli are larger than the loss moduli and are independent of frequency in dynamic oscillatory frequency sweep experiments. The plateau moduli obtained from linear frequency sweep experiments are much lower than those estimated from the number of thiol groups along the Cys-ELP chain, indicating that only a small fraction of thiols form elastically active cross-links. From the small value of the fraction of elastically active cross-links, the Cys-ELP hydrogel is concluded to be an inhomogenous network. Under steady shear, a 2.5 wt % Cys-ELP hydrogel shear thickens at shear rates lower than that necessary for fracture.