Mechanical properties of the MDN hydrogels. (a) Stress-strain curves and (b) Young’s modulus of the double-network hydrogels with different contents of PVP (1, 3, 5, and 7 wt%). (c) Stress-strain curves of PVA hydrogel, double-network hydrogel, and MDN hydrogel. Diagrammatic sketch of the MDN hydrogel (d) twisting, (e) knotting, and (f) stretching after making a knot. The MDN hydrogel bears the sharp pressure of a knife (g), a metal cylinder (h), and scissors (i). (j) A sample loads 500 g weight.