## 7 Conclusions  
This review shows an overview of the NFs benefits applied in three specific areas: machining, solar energy, and biomedical engineering. * In the machining area, the use of NFs has helped to obtain high-quality machined surfaces, has improved tool life, and reduced costs in all branches of the sector, namely in turning, milling, drilling, and grinding. * In the area of solar energy, NFs have been evaluated in heat exchangers for cooling solar systems. The results have shown that nanoparticles added to the base fluid increase heat transfer and reduce the operating temperature of the panels. Furthermore, due to the superior thermal absorption performance of the nanofluid, it has been used to improve the efficiency of capturing solar energy in direct absorption collectors. * In the biomedical field, the use of the NFs has helped improve the diagnostic and treatment of several diseases. Due to the superior thermophysical properties of these colloidal mixtures, the number of studies and interest in the subject has grown massively in recent years. If, on the one hand, there is a consensus regarding its thermal transport capacity due to its higher thermal conductivity compared to base fluids, on the other hand, a list of problems has limited large-scale applications. However, this review sought to highlight studies that inspired solutions to overcome these challenges. Advantages of nanofluids:  
* NFs can have their properties adjustable depending on the concentration of nanoparticles used, including the increase of thermal conductivity in relation to their base fluids being the main attraction for most applications;
* The heat transfer from the NFs presents higher values in relation to the respective base fluids, due to enhancement specific surface area between nanoparticles and liquid;
* Some results from the literature have shown a significant increase in thermal conductivity when an extremely low concentration of NPs was used, reducing production costs. * The nanoparticles in the base fluid change the optical properties of the NF, benefiting light absorption for applications in solar collectors. * Many kinds of nanoparticles can be used to form NFs, such as metallic, with magnetic properties; or non-metallic, as used in cancer theranostics. * Disadvantages of Nanofluids:
* The low stability of NFs for long periods of time and their rapid sedimentation;  
Figure 4: Schematic representation of NPs hemocompatibility evaluation. (**A**) Experimental setup. (**B**) Microchannel geometry. (**C**) Condition profiles present in the hyperbolic channel. Reprinted from [136]. 2016 Springer Nature. * The effects of ultrasonic vibration, the addition of surfactants, and control of the pH value of NF suspensions are still poorly understood;
* The two main techniques for obtaining nanoparticles need to be improved. One is too expensive (one-step), and the other (two-step) does not favor the stability of NFs;
* Enhancement of the pressure drop in systems that require pumping due to the increased viscosity of NFs. During the last decade, several solutions have been proposed to overcome the NFs limitations. An alternative to solving the problem of the low stability of NFs is by modifying the surface of the nanoparticles before forming the NFs. Hence, NPs surface modification can contribute to an increase of the electrostatic force between the NPs, preventing them from agglomerating and, as a result, may reduce sedimentation. Encouraging results have been achieved by new NP production techniques, such as the hydrothermal synthesis process shown in this review. A general trend of NFs is that viscosity decreases with increasing temperature and decreasing concentration of NPs. For this reason, regarding pumping systems that are subject to high temperatures, NFs should be an effective option. Another interesting approach is the use of magnetic NPs under the influence of magnetic fields to prevent the sedimentation of NPs.