Hydrogels as hydrated polymeric structures are promising biomaterials for biomedical applications. In particular, hydrogels inherent analogy with soft tissues (water-swollen solid network) has made them an interesting candidate as regenerative substitute for damaged tissues. However, these synthetic equivalents usually lack the desired functional properties as load bearing biomaterials. Apart from biological key features, a demanding set of mechanical properties including stiffness, elasticity, strength, dissipation, permeability, toughness, and fatigue resistance is required to sustain load. Indeed, an ideal biomaterial might have different characteristics corresponding to physiological stress/strain range for the target load bearing tissues. In case of cartilage for instance, a fairly stiff and tough biomaterial, contrary to soft but highly stretchable and tough, is required to avoid a long period of in vitro culture in bioreactor before implantation. The insufficient dissipative capacity is known as the main reason for low toughness and inability to resist defect growth during loading. Therefore, combining simultaneously high dissipative capacity and stiffness can lead to a load-bearing and tough biomaterial. Yet, there is no guarantee for fatigue resistance performance of the merely stiff and dissipative hydrogels. Indeed, the different sources of dissipation must be well designed for a robust behavior under fatigue load.
By following the sacrificial bonds principle, a single network (SN) system can be developed via a dual crosslinking strategy. Indeed, SN hydrogels created by block copolymers and ionic cross-linking systems demonstrated high stiffness and partial fatigue resistance owing to simultaneous incorporation of soft and stiff bonds. Our lab has recently focused on synthesizing poly [2-hydroxyethyl methacrylate] (PHEMA) hydrogels with tunable dissipative capacity. Crosslinked PHEMA based hydrogels present interesting properties for designing SN systems. In one hand, different reversible bonds including chains entanglement, hydrogen bonding and hydrophobic associations can be formed throughout the network intrinsically. On the other hand, the stiffness and dissipation level can be significantly enhanced if the network crosslinked by short length ethylene glycol dimethacrylate (EGDMA) molecule.
In this study, we show that by combining physical reversible soft bonds with stiff covalent bonds in p(HEMA-co-EGDMA) based hydrogels, different sources of flow independent dissipation can be designed. In parallel, by controlling the morphological architecture of the hydrogel, different extent of fluid frictional drag dissipation and load support are achievable. We will show that a careful combination of these two sources of dissipation can lead to a system presenting optimized stiffness, dissipation and fatigue resistance performances.