Network interactions simultaneously enhance stiffness and lubricity of triple-network hydrogels

Synthetic hydrogels displaying cartilage-mimetic bulk and surface properties may serve as cartilage substitutes. Multi-network, electrostatic hydrogels that leverage intra- and inter-network repulsive and attractive forces represent a promising approach. Herein, triple network (TN) hydrogels were prepared to obtain a combination of desired characteristics (i.e., hydration, stiffness, shear stress, and friction properties). The TN hydrogels were comprised of a negatively charged 1st network and a neutral 2nd network possessing hydrophobic associations. Presumed to significantly influence surface properties, the 3rd network charge was systematically varied as cationic, anionic, and zwitterionic. A double-network (DN) hydrogel, comprised of the same 1st and 2nd network as for the TN hydrogels, was included as a control as well as native cartilage specimens. Micro-indentation was performed with a steel ball, yielding stiffness values as well as the contact area during sliding. The lubrication in both deionized (DI) water and fetal bovine serum (FBS) was evaluated with the micro-indenter wherein the stage reciprocated in a range of speeds. All the TN hydrogels exhibited greater Youngs modulus than the DN hydrogel control. The TN bearing a cationic 3rd network exhibited an exceptionally high Youngs modulus of ≈1.4 MPa, which was even higher than that of the cartilage samples. In both DI water and FBS, for most testing speeds, the TN hydrogels exhibited lower friction coefficient (COF) values and lower shear stresses than DN hydrogel as well as the native cartilage specimens.