**4.2 Reversible cross-links**

 In chemistry, cross-linking is the linking of one polymer chain to another one. The cross-linking is an irreversible process that can change the polymers' physical properties such as elasticity, mechanical behavior, and surface characteristics [ 42 ]. But highly cross-linked materials have the drawback of brittleness and tend to crack. Some approaches, including Diels–Alder (DA) and Retro-DA reactions as well as the use of ionomers and supramolecular polymers, are available to bring reversibility in cross-linked polymeric. Reversibly cross-linked polymers exhibit self-healing properties but they need an external trigger such as thermal or chemical activation. Thus, these systems show a non-autonomous healing phenomenon [ 34 ].
