Mechanical properties derived from phase separation in co-polymer hydrogels

• A bi-continuous micro-structure in two-component hydrogel controls performance.
• Reduction in crosslinking density enhances extensibility (1000% failure strain).
• Aggregating phase prevents uncontrolled swelling of under-crosslinked hydrogel.
• Phase separation provides a network of recovery pathways for “bonding” phase.

Hydrogels can be synthesized with most of the properties needed for biomaterials applications. Soft, wettable, and highly permeable gels with a practically unlimited breadth of chemical functionalities are routinely made in the laboratory. However, the ability to make highly elastic and durable hydrogels remains limited. Here we describe an approach to generate stretchy, durable hydrogels, employing a high polymer-to-crosslink ratio for extensibility, combined with an aggregating copolymer phase to provide stability against swelling. We find that the addition of aggregating co-polymer can produce a highly extensible gel that fails at 1000% strain, recovers from large strains within a few minutes, maintains its elasticity over repeated cycles of large amplitude strain, and exhibits significantly reduced swelling. We find that the gel׳s enhanced mechanical performance comes from a kinetically arrested structure that arises from a competition between the disparate polymerization rates of the two components and the aggregation rate of the unstable phase. These results represent an alternative strategy to generating the type of stretchy elastomer-like hydrogels needed for biomedical technologies.

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