It
is highly desirable to prevent crack formation in polymeric
materials at an early stage and to extend their lifespan, particularly
when repairs to these materials would be difficult for humans. Here,
we designed and synthesized catechol-functionalized polymers that
can self-heal in seawater through hydrogen bonding and coordination.
These bioinspired acrylate polymers are originally viscous materials,
but after coordination with environmentally safe, common metal cations
in seawater, namely, Ca<sup>2+</sup> and Mg<sup>2+</sup>, the mechanical
properties of the polymers were greatly enhanced from viscous to tough,
hard materials. Reduced swelling in seawater compared with deionized
water owing to the higher osmotic pressure resulted in greater toughness
(∼5 MPa) and self-healing efficiencies (∼80%)