# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Self-Healing Concrete
By ensuring a narrower width of the crack, the fibers facilitate the autogenous self-healing process, which is often limited to smaller fracture sizes [8,33]. Combined with the higher tensile properties, this significantly reduces the probability of structures' failures. In order to accelerate the healing time and slow down crack propagation, the addition of encapsulated repair agents has become a well-established self-healing strategy for concrete [34]. The liquid repair agents are embedded in microcapsules, which should break during a damage event and release the agent into the damage zone. At the same time, the microcapsules have to be sufficiently strong to withstand the mixing process and to be homogenously distributed through the concrete matrix. This makes the synthesis and design of the microcapsules quite challenging. Encapsulation materials are typically based on polystyrene methyl methacrylate [35], urea-formaldehyde [36], phenol-formaldehyde [37], glass [38] or ceramics [39]. Along with the type of materials, the mechanical properties, permeability and crack resistance of the capsules are also adjusted by wall thickness and surface chemistry [40]. Once broken, the physical and chemical repair of the crack in the concrete is induced by a chemical reaction of the healing agent (e.g., polymerization, curing), which solidifies the liquid at room temperature. In a single capsule approach, only one type of repair agent is used, which starts to react with water or components of the mortar as soon as it is released. A prominent example is the curing of cyanoacrylate monomers, which undergo moisture-induced anionic polymerization [41]. The curing proceeds rapidly at room temperature and is accelerated by the alkaline concrete matrix. In its cured state, cyanoacrylate polymers form a strong bond with cementitious materials and are able to prevent the formation of new cracks during reloading. Along with curing, moisture was also used to induce a foaming reaction of encapsulated polyurethane prepolymers, which has been successfully applied to heal damage in concrete. Due to volume expansion, larger defects could be efficiently healed [38,42]. Other single capsule strategies include the encapsulation of bacteria for the precipitation of calcium carbonate [43] or the encapsulation of sodium silicate solutions, which yield calcium silicate hydrate (C-S-H) gels by the reaction with calcium hydroxide present in the concrete matrix [44]. In contrast, in a dual-capsule approach, the repair agents are encapsulated in two different microcapsules separately. For autonomous self-healing, both capsules have to be broken and to release the repair agents (e.g., epoxy resins and amine hardeners) [45]. The use of microcapsules provides a fast way to heal cracks within concrete without an additional external trigger (alongside the damage event). Thus, this healing mechanism is not limited to buildings aboveground but can also be applied in self-healing concrete for dams and underground structures. However, it has to be considered that multiple healing events at the same damage location are typically not possible, as the healing agent is irreversibly consumed during the healing process. A repeated healing of concrete, even if the damage occurs at the same site, can be realized by using hollow microchannels instead of capsules [46]. The channels mimicking vascular systems are able to store a larger amount of the healing agent and to transport/release it at the damage zone [47]. While the same healing mechanism and healing agents can be used as in the microcapsule approach, a low viscosity of the healing agent is crucial to filling it in an interconnected hollow fiber network. For healing larger cracks, wires made from shape memory alloys (SMA) such as NiTinol (a nickel-titanium alloy, see also Section 2.3) were added to assist the physical closure of the crack, which was followed by chemical healing via the broken hollow channels [48].