A substrate includes a double-network polymer system including a cross-linked, covalently-bonded polymer and a reversible, partially ionicly-bonded polymer, wherein the substrate has a moisture level less than or equal to 15 percent of the total weight of the substrate, and wherein the substrate includes a latent retractive force. A method for manufacturing a substrate includes producing a double-network hydrogel including a cross-linked, covalently-bonded polymer and a reversible, ionicly-bonded polymer; elongating by force the double-network hydrogel in at least one direction; dehydrating while still elongated the double-network hydrogel to form a substantially-dehydrated double-network polymer system; and releasing the force to produce the substrate.