# State of the Art Review about Bio-Inspired Design and Applications: An Aerospace Perspective
## 2 Review on Materials Used in Bio-Inspired Design  
Bio-inspired materials mimic natural materials in terms of properties or functions, these are made of synthetic materials. Bio-inspired materials are a combination between biology and physical sciences. The combination of the various bio-inspired materials provides templates for performance enhancement. Hair-like structures consisted of a network of microfibrils, protruding from a surface. This inspired various devices, such as sensors, adhesion, and geometrical capturing arrangements [10]. Deep-sea glass sponges are capable of producing complex skeletal structures which were reconstructed using the laser additive manufacturing (LAM) technique. These reconstructed biomimetic structures displayed high buckling resistance for a particular material [14]. Nacre and other hybrids consisted of multiscale architecture which displayed high strength, high stiffness, and high toughness. These serve as great alternatives to synthetic materials, that improved structural integrity and provided ease of robust assembly [20]. In nature, the silk from the spider's web has the most common occurrence. These structures were made of microfibrils and the micro/nanostructures consisted of spindle knotted fibers. The interlinking of these amorphous chains were the domains of the supramolecular structure. The development of artificial biomaterials displayed water-controlling properties and impeccable strength [21]. One of the earliest bio-inspired structures was inspired by turtle shells which consisted of Ca, P, Cl, and Na, etc. These multilayer structures consisted of porous structures which were used in composite damping material. Moreover, the inspiration for composite structures that we see today was drawn from these porous shells [22]. Bone-inspired microstructures include bone regeneration to provide cell attachment and ingrowth. These materials were designed using a bottom-up approach and a hierarchical atomic structure [23]. Bio-inspired materials display impeccable material properties, such as self-repair, crack deflection, strengthening mechanisms, toughness, and lightweight structures [24]. These are used in aerospace, automotive, energy, architecture, agriculture, robotics, transportation, and healthcare. Table 1 shows the bio-inspired materials, cellular structures, properties, and microstructures. [PERSON] et al. highlighted the tree trunk's internal structure [25]. It closely resembled a ball-socket joint structure arranged in a lay-up structure consisting of fibrils. In conclusion, the utilization of bio-inspired materials in composite and epoxy layers demonstrate properties, such as improved ductility and damage tolerance. Figure 3 shows the analysis of the T joint performance with respect to the buckling load to study the displacement for bio-inspired tree trunk material. Figure 2: Methodology adopted in bio-inspired design review. Honeycomb structures reduce the material density and honeycomb structures optimize the space to reduce deflection. Figure 4 displays the cellular structure of honeycomb, ladybug, and beetle which progressively show high strength properties [26]. The AM technique was employed in the materials; for bio-inspired design, the infill techniques in AM used ABS (Acrylonitrile Butadiene Styrene) it was used to vary the geometry. It offered the freedom to expand the various materials in specific designs [27]. The beetle structure with tubular showed the maximum strength performance, due to even weight distribution. [PERSON] et al. further emphasized the use of AM techniques to produce bio-inspired materials [28]. Figure 5 shows the methods for additively manufacturing bio-inspired materials and their subsequent properties. \begin{table}
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Table 1: Bio-inspired material properties [24]. The material properties inherit the design principles rather than duplicating the material design [23].