## 5 Conclusions  
Cellular Materials and Topology Optimization can be considered as approaches to tailor properties by selectively removing material, e.g., stiffness, in a structure eliminating the need of additional elements and maintaining the overall dimensions of the piece. These approaches appear as a possible solution to deal with inaccuracies and limitations of compliant systems. In this work, these two were reviewed regarding their used in tailoring the stiffness of flexure elements. Interest in both has grown in recent years due to the increasing availability of additive manufacturing processes. These allow the fabrication of complex geometries with a controlled distribution of matter, comparable to the ones resulting from these approaches. It was then relevant to overview both approaches, so that the users and readers can compare and contrast prior selecting one of them. Regarding Cellular Materials, stretch-dominated structures are preferred when maximizing stiffness is the objective, while bending-dominated structures are the best option for compliance applications. Two-dimensional bending-dominated lattice arrangements have been more frequently used in compliance applications. The use of bending-dominated 3D closed-cell structures has been studied recently in energy absorption applications, demonstrating their capacity to be used for compliance purposes. Therefore, tailoring stiffness of flexures via Cellular Materials using 3D and closed-cell structures appears to be a promising option, due to the design advantages such as high off-axis stiffness and greater load bearing capacity. Auxetic architectures are the most widely used in compliance applications because of their high deformations under relatively low stresses. However, the use of these type of architectures in flexure elements has not been explored. In addition, cellular structures with a zero-Poisson ratio represent another type of non-explored architecture. This type of meta-structure is characterized for providing deformation in one direction while maintaining unchanged its dimensions in the others. This particular deformation-mechanism could be useful in the design of flexure elements and compliant mechanisms to avoid parasitic motions due to non-desire displacements. Numerous literature has focused on flexure joints for rotational motion, while very few works on joints for other types of motion were encountered. Research has been focused almost entirely on SARFJ leaving unexplored other types of joints such as prismatic or cylindric. The wide variety of Cellular Materials may allow the creation of unexplored compliant joints that commonly need more than one flexure to be conformed. On the other hand, the use of Topology Optimization allows finding the best option of material distribution, and with the use of computational power, this could be done relatively fast. However, it often leads to a complex architecture that requires a postprocessing stage to obtain the final design. Furthermore, the resulting design could be difficult to parametrize and handle analytically, compromising the prediction of the behavior under any other conditions different from the ones used in formulation of the optimization process. As opposed to this, the use of Cellular Materials allows an accurate structure-property relation. Still, the potential tailorability is limited to the Cellular Material topologies. Therefore, combinations of Cellular Materials and Topology Optimization, i.e., TOAUC, MTOCM, and TOFCM, must be explored in the design of flexure elements making used of the advantages offered by both approaches. The use of Topology Optimization in the design of flexure joints has been studied more than that of Cellular Materials. Looking at the evolution of the Topology Optimization of flexure joints, it is possible to see that the tendency is to create designs with distributed compliance through the implementation of repeatable members in their geometrical sapce, avoiding the concentration of deformation in a single element.