The mammalian extracellular matrix (ECM) fits the definition of a hydrogel in terms of water content, but its mechanical properties are superior to synthetic hydrogels due to a complex set of interactions between the different macromolecular components of the ECM. To develop synthetic networks with comparable properties, combinations of physical entanglements with covalent crosslinking of two or more components will likely be required. In this work, multicomponent hydrogels based upon the glycosaminoglycans chondroitin sulfate (CS) and hyaluronic acid (HA), major components of the ECM, and synthetic monomers and macromers were developed to cover a broad range of moduli and fracture properties. Several routes were taken to achieve this: copolymerization of photopolymerizable methacrylated CS or HA macromers (MCS, MHA) with monomers to improve crosslinking efficiency, photocrosslinking of pentanoate-functionalized HA (PHA) by thiol-ene chemistry and the creation of CS and HA double network (DN) gels.
The first goal was to improve the crosslinking effectiveness of photopolymerized GAG hydrogels to improve moduli and fracture properties of the gels. Considering the high persistence lengths of GAGs, it was hypothesized that copolymerization of GAGs with small amount of oligo(ethylene glycol) diacrylates would reduce crosslinking inefficiencies resulting from the steric hindrances of the macromers in solution. Copolymerization of 13 wt% MCS in water with 0.5 to 2.0 wt% oligo(ethylene glycol diacrylate) (OEGDA) increased the shear modulus of MCS homopolymer (45 kPa) up to five times and lowered the swelling degree (44 g/g) to a third of its value in water. Similar results were achieved with MHA. The dependence of moduli and swelling on the OEGDA length and the fact that monoacrylates equally increased moduli suggested that crosslinking occurs primarily by the methacrylate kinetic chains rather than the EG linker. This work establishes that copolymerization of MCS or MHA with OEGDA is a simple strategy to tune the moduli of hydrogels to values needed for specific applications. In contrast, photocrosslinking of HA using thiol-ene chemistry enabled formation of gels at much lower macromer concentrations (down to 2 wt%), indicating a more efficient crosslinking reaction. Additionally, the fracture strains of thiol-ene PHA gels were notably improved relative to MHA. This suggests that thiol-ene chemistry results in more efficiently and uniformly crosslinked HA gels.
Despite increased moduli upon copolymerization and use of thiol-ene chemistry, the fracture strain of the homo- and copolymer gels remained nearly independent of composition, suggesting that fracture strain is primarily a function of GAG chain conformation and secondarily due to network homogeneity. This hypothesis was tested by measuring the mechanical properties of MCS, MHA and PHA gels in salt solutions which reduce the polyelectrolyte persistence length. The compressive fracture strain of 13 wt% MCS increased continuously with increased salt content from 18% in water to 60% in 1 M NaCl solution. Increased fracture strain and reduced moduli were interpreted as the result of the chain distribution shifting from non-Gaussian toward Gaussian as a result of the reduced persistence length.
The generality of the double network combination of a brittle polyelectrolyte gel with a ductile nonionic second network to achieve superior properties was demonstrated by synthesizing MCS and MHA-based DNs. This was accomplished by using MCS or MHA as the first network with the second network of polyacrylamide (PAAm). The resulting DN hydrogels have properties comparable to the well-known tough double-network gels of poly(2-acrylamido-2-methylpropane-sulfonic acid)/PAAm developed by J.P. Gong and coworkers at Hokkaido University in Japan. The MCS/PAAm hydrogels show both pre-yielding and yielding regions in tension. In compression, the mechanisms for toughening may be different than in tension. Although energy dissipation mechanisms due the fracture of the first network can increase the toughness of DNs, improved failure properties under compression may result directly reorganization in the entanglements of the two networks. Improved toughness of DNs relative to single networks even at high ionic strengths was consistent with the idea that the nature of the entanglements is an important toughening mechanism in such gels.