**3. Passive self-healing based on extrinsic techniques**

In passive mode, healing is generated by incorporation of foreign functionalities. The extrinsic healing process is based on the use of a healing agent contained in the matrix as a separate phase. The healing agent is usually in the liquid state, placed into reservoirs which may be microcapsules or hollow fibers or microvascular network. In most approaches, the healing agent is used with a catalyst, which can also be encapsulated or dissolved in the matrix. Different extrinsic healing approaches are explored. In some cases, the catalyst is not required to initiate the healing process; the healing agent can also react to itself. The extrinsic healing concept is based on the response after or at the onset of damage. Current research is concerned with the improvement of healing agents in terms of compatibility and catalyst-free system with some new encapsulation techniques.
