# State of the Art Review about Bio-Inspired Design and Applications: An Aerospace Perspective
## 3 Review on Manufacturing Methods Used in Bio-Inspired Design
The continuous growth of 3D printing from the nanoscale to the macroscale was aided by various structure formations shown in Figure 14[52]. The CNC wire cut machine was used to impose manufacturing constraints in foam materials for grid fabrication. It highlighted the importance of using conventional manufacturing techniques combined with digital manufacturing [53]. Freeze casting methods were employed to manufacture bio-inspired structures that ensured the lamellar microstructures between ice structures that improved strength and complexities [23]. The beam-based dynamic model was proposed and could accurately transmit the waves and identify the multiparameter efforts. Furthermore, the thin-walled metal tubes provide superior mechanical properties, and these were the horse hoof inspired compared to vertex structure tubes [54]. The AM techniques utilized lattice structures with different process parameters, producing dense lattice structures, which provided successive transfer of load [55]. Conclusively, AM processes, in their futuristic approach, moved towards sandwich composites with re-entrant cores [56]. It ensured the balance between fatigue strength and loading. There is still scope for improvement due to crack development and fatigue failure. The  
Figure 12: Types of filling paths [44]. compressive properties developed in both room and elevated temperatures [57]. The primary shortcomings noted were matrix softening and interfacial debonding. The presence of helicoidal structures reduced these failures. It is further enhanced by impact performance [58]. However, one of the critical concerns is the manufacturing defects due to the 3D printing techniques. Bio-machining is a manufacturing technique that utilizes microorganisms to remove metal from the workpiece. The main advantages are the less utilization of energy and reduced thermal damage. It is a sustainable technique for machining micro components. These micro components are used in aerospace electronics and other technologies. The overall dimensions of the compact parts are around 100 \(\upmu\)m. There are various kinds of machining processes which include physical, chemical, and biological. Bio machining is a  
Figure 14: Bio-inspired Structure [52]. Figure 8:0 Performance Trade-offs in Bio-inspired structures [50]. controlled biological process that uses selective microorganisms to remove metals. It is a consecutive oxidation and reduction reaction, with an extracellular polymeric substance. Bio machining performed on a copper workpiece involved the conversion of Fe\({}^{2+}\) to Fe\({}^{3+}\). Subsequently, the ferric ion acts as an oxidising agent, and Fe\({}^{2+}\) was reduced by the microorganisms. This was a cyclic process inclusive of subsequent reduction and oxidation [59]. [PERSON] et al. studied the machining of copper. The oxygen-free copper (OFC) consisted of a 10 \(\times\) 15 \(\times\) 2 mm disk rinsed with deionized water. The tests consisted of the OFC block in a culture medium, it was placed in a deionized sample and taken out in intervals. Extremophile bacteria was used in bio machining, and the efficiency was increased with the increase in the inoculum. This shows the potential to be implemented at an industrial level [60]. Slip casting was a technique used to manufacture bio-inspired materials and structures. The process consisted of the deposition of fluids into a mould. These were pre-dimensioned moulds, where the fluid is deposited and vacuumed after the wetting. The continuous assembly was used to prepare the bio-inspired structures. This technique when combined with AM can potentially be implemented for complex, nacre-based bio-inspired structures and tubular honeycomb structures [61]. The various manufacturing techniques with a focus on AM are highlighted and used to manufacture bio-inspired structures.