Topology-Specific Injectable Sticky Hydrogels

Stimuli-responsive injectable hydrogels based on weak supramolecular interactions may represent safer alternatives to chemically reactive adhesive hydrogels for biomedical applications where weak to moderate adhesion is required. We investigated the linear and nonlinear rheological properties as well as the adhesive properties of two thermoresponsive graft copolymers with inverse topologies, poly(N-isopropylacrylamide)-g-poly(N,N-dimethylacrylamide) (PNIPAM-g-PDMA) and PDMA-g-PNIPAM. Except for their topologies, these copolymers are analogous in terms of chemistry, architecture (graft), and monomer composition (50-50 wt %). Over a wide range of concentrations, they both form injectable homogeneous solutions at room temperature and turn into soft and sticky viscoelastic hydrogels close to body temperature. We find that the linear viscoelastic properties of these two hydrogels are not discernible far above the thermal transition temperature. However, the PNIPAM-g-PDMA hydrogel having long thermoresponsive backbones shows a strain-hardening behavior in large strains both in probe tack tests and in shear. The inverse topology, PDMA-g-PNIPAM, showed no hardening and simply softened until failure. This distinction was observed regardless of the polymer concentration (in the entangled regime). We attribute the hardening to a continuous, load-bearing nanostructure from strong hydrophobic PNIPAM associations, while the softening is due to the easy pullout of short PNIPAM grafts from separate hydrophobic clusters bridged by PDMA backbones. The findings of this work highlight the importance of macromolecular design in determining the nanostructure and thereby the mechanical performance of soft hydrogels for specific applications.