## 4 Summary and Perspectives  
This review has made clear the relevance of the use of complex fluids in energy dissipating systems. Although the beginning of the research activities on these complex fluids belongs to the last Century, the accumulation of knowledge together with the new findings allowed by more sophisticated experimental devices and more powerful numerical tools have flourished into new applications of complex fluids in energy dissipative systems. Complex fluids have been sorted into active-fluids (ERFs, FFs, MRFs and EMRFs), requiring an external field (electric, magnetic or both simultaneously) to trigger their rheological behavior and, consequently, their increased dissipative capabilities; and passive-fluids (STFs and VEFs), not requiring of any sort of external field and activating immediately upon the applied stress and/or timescale of deformation. The most typical applications have been classified into three groups, i.e.,  
Figure 6: Photographs of neat Kevlar fabric (**a**) and sphere silica/ethylene glycol (EG) suspension treated Kevlar composite fabric (**b**) after ballistic resistance testing. Reprinted with permission from [189], Copyright 2011, John Wiley and Sons. body armor, damping and sandwich structures, all of them having the same final aim, which is dissipating the kinetic energy of an external mechanical load (impact or vibration) by turning it into heat energy through the fluid. Table 1 summarizes the most relevant applications of complex fluids in energy dissipating systems and their pros and cons. The selection between active or passive-fluid for the development of an energy dissipating system will depend on the particular application itself. If one thinks about vibration isolation applications, the passive isolator without external energy due to its simple structure has been widely used in industry practice, due to their good economy and reliability, as well as their ease of implementation. However, passive vibration isolation is not adaptable. Then, if adaptability is relevant, like in the context of artillery [199], it would be preferable to use an active vibration isolation system. However, it is important to keep in mind that active vibration isolation systems are heavy weight and high cost, they consume energy, and they have a complex structure, which may result in a poor reliability and the system require maintenance. In body armoring, although active and passive fluids harden very quickly and reversibly, allowing the production of thinner, lighter and more flexible protective equipment, MRFs shows a couple of serious drawbacks compared to STFs:  
* Whereas the nanoparticles in STFs stay dispersed in the suspension, the particles in MRFs may form aggregates, settle or precipitate, making the fluid ineffective, although proprietary additives can promote particle suspension. For the sake of truth, shear thickening behaviour, can be made from larger particles and, consequently, the fluid would face the same phase-separation issue referred to MRFs. * STFs harden almost instantaneously (few milliseconds) upon impact, while MRFs require a magnetic field to be activated. Thus, apart from being a drawback in light-weight applications, an eventual loss of power or short circuit could deactivate the system, causing the armor to fail. In dampers consisting of an open-cell reticulated or partially closed-cell foam impregnated with an active-field fluid (ERF or MRF) or a passive-field fluid (STF), the impact energy is dissipated through bending and buckling of cell walls stiffened by the fluid. Additional energy absorption comes as a result of fluid-flow that may be generated in the matrix as the solid is compressed [200]. In this sort of system, the same invention is conceived to be used with either active or passive fluids [105; 201; 202]. Thus, it would be again the final application of the damper determining the preference for the active or passive fluid.