## 4 Self-Healing Techniques
### Bacteria  
In recent times, the study of bacteria-activated autonomously healing materials generated renewed interests [24; 59]. Meanwhile, the generated cracks in concrete were efficiently filled using the mixed and isolated bacterial cultures [60]. It was argued that the metabolicactivity of the bacteria can induce the calcium carbonate precipitation, thus, filling the cracks in concrete [61]. Autonomous healing can be catalyzed by the injection of a bacterial culture onto the concrete surface [34]. Additionally, in a parking garage, the bacterial cultures were sprayed directly on the concrete surface containing several cracks [62], wherein the autonomous healing was found to substantially decrease the water permeability into the cracks, thus, preventing further decay of concrete surface. Nonetheless, there was no substantial restoration of the CS. It has been reported that the CS and durability of the concrete and cement mortar could be improved through the microorganisms' incorporation [63]. The use of admixtures in concrete in combination with bacteria was shown to cause additional enhancement of the CS [64] up to 36% [65]. Driven by these ideas, many studies have been conducted to explore concrete durability and strength enhancement via the incorporation of bacteria. A distinct method of injection and spraying was enacted to place bacteria directly into the concrete mix, which led to the ureolytic activity and provoked microbial precipitation, thus, filling the formed cracks. Although this method outperformed many other techniques, the life span of bacteria was, however, shortened because the concrete matrix environment was intolerant for the bacteria. Consequently, the efficacy of autonomous healing was lowered with the passage of time [12]. The presence of nucleation sites, the concentration of the calcium ion, pH, and concentration of dissolved inorganic carbon were the main reasons that led to the microbial precipitation. The added bacteria were substantially impacted by the high alkalinity in the concrete mix, making it a harsh habitat. It was concluded that the life span of bacteria can only be enhanced though their safeguarding [13]. The bacterial encapsulation was proposed to be an effective method for bacteria protection, which could improve the autonomous healing properties of the concrete mix. Through the use of polyurethane, a bacterial culture was immobilized in the concrete matrix. This, in turn, led to the gradual reduction of enzymatic activity and prolonged the bacteria lifespan [66]. In addition, the stability of the enzymatic activity was provoked for a prolonged time. Immobilized bacteria in the concrete structures were found to cause about 60% restoration in the CS value [26]. To achieve this goal, the ureolytic bacteria was incorporated into the concrete matrix [67], leading to a gradual filling of the formed cracks via the calcium carbonate precipitation within the cracks. In addition, the performance of the immobilized bacteria became stronger with the passage of more time [68]. Concrete was combined with a hydrogel containing encapsulated bacterial spores and a crack measuring 0.5 mm wide was healed in its entirety, wherein nearly 70% decrease in the water absorption was observed [30]. It was also demonstrated that a crack width of almost 1 mm was c filled in its entirety via the improved performance of immobilized bacteria within the microcapsules [25]. Furthermore, the graphite nanoplatelets were used to encapsulate bacteria [21], wherein the formed crack at 7 and 3 days was plugged entirely when a crack of width 0.81 mm secreted a healing agent from the capsule after being stimulated by the crack formations. An achieved even distribution of nanoplatelets throughout the concrete matrix was beneficial for effective filling of the nanoscale cracks and enhancement of the CS by almost 10%. The conditions, including efficacy of the healing agents' in-situ rupture, processing survivability, and thermal stability, must be met by the encapsulated materials when used for the healing purposes.