  Most tough hydrogels are reinforced by introducing sacrificial structures
that can dissipate input energy. However, since the sacrificial damages cannot
recover instantly, the toughness of these gels drops substantially during
consecutive cyclic loadings. Here, we propose a new damageless reinforcement
strategy for hydrogels utilizing strain-induced crystallization (SIC). In
Slide-Ring (SR) gels with freely movable cross-links, crystalline repetitively
forms and destructs with elongation and relaxation, resulting in both excellent
toughness of 5.5 - 25.2 MJ/m$^3$ and 87% - 95% instant recovery of extension
energy between two consecutive 11-fold loading-unloading cycles. Moreover, SIC
occurs "on-demandly" at the crack-tip area where strain amplification and
stress concentration take place and forces the crack to turn sideways. The
instantly reversible tough hydrogels are promising candidates for applications
in artificial connective tissues such as tendon and ligament.
