Biomimetic design of mechanochromic single-chain nanoparticle polymer networks

Hydrogels are a class of polymer networks that show promise in many biomedical applications but are typically limited in use due to their brittle nature. Synthesizing hydrogel networks that are comparable to biological materials has long been an area of interest. In this study, we are working to create a polymer network inspired by the muscular protein, titin. Titin can unfold under mechanical tension, releasing stored length, making the muscle material extensible and durable. To mimic this behavior, we propose creating a polymer network with similar stored length using single chain nanoparticles (SCNPs) cross-linked with phenolphthalein (PP) and β-cyclodextrin (β-CD). We hypothesize that the polymer chains will fold due to the hydrophobic host-guest interaction between the two molecules, resulting in an SCNP. These SCNP will be used as the crosslink in a polymer network, and when mechanical force is applied, we hypothesize the interactions between PP and β-CD will break, unfolding the SCNP and mimicking the titin protein. Here we report the successful synthesis of acrylamide (Aam) PP and β-CD monomers for use in polymerization reactions. We also report on attempts to demonstrate a mechanochromic nature of the PP-β-CD complex, including the synthesis of various concentrations of both single-network and double-network hydrogels and attempts to control the rate of association of the PP-β-CD complex via the use of a competitive guest. Finally, we report on the successful synthesis of various single polymer chains using reversible addition-fragmentation chain-transfer (RAFT) polymerization, including the precursor polymer to be used as the SCNP crosslinker. Future work will include the further characterization of the SCNP precursor polymer, the optimization of the polymerization reaction, and the integration of the SCNP as a crosslinker in a hydrogel network. Subsequent mechanical testing of the material will indicate whether the SCNP hydrogel exhibits mechanical properties comparable to that of the titin protein.