## 7 Landmarks to be Reached  
The main difficulties against the consolidation of hydrothermal technology as the method to be used for particle generation were, on the one hand, its high operational costs because of its high temperature requirements and, on the other hand, the need to use equipment that is highly resistant to corrosion, since water under hydrothermal conditions is highly corrosive. Considering that the development of new materials for the construction of more resistant and economic reactors is beyond the scope of this review, we will focus on the need to develop new techniques and methods to cut down on hydrothermal synthesis operational costs:  
* Mixing method. In continuous reactors, the method to mix the water with the salt solution is a crucial factor for the viability of the process, as already explained in Section 5.2. This factor determines how rapidly the particles are generated, their homogeneity, and their functionality. It is, therefore, a key factor with regards to process viability and cost effectiveness.
* Hybridization. The hybridization of hydrothermal processes with other industrial processes has been recently explored with the main focus on the reduction of power demand. Professors [PERSON] and [PERSON] have recently studied the possibility of combining the oxidation processes that take place in supercritical water with hydrothermal synthesis processes, so that the particles can be generated thanks to the large amount of heat that is released from the oxidation reactions under these conditions [135]. This is a very attractive field of study, since many other interesting hybridizations may be conceived.
* Heating and cooling rates. These parameters are scarcely studied, particularly continuous reactors, and their effects on the crystallization and purity of the generated particles confirmed their significance [123]. Since there are closely related to reactor sizing, at least with regards to the heating and cooling stages, they represent a substantial factor in terms of cost efficiency and, subsequently, viability of the process.
* Natural resources. The incorporation of natural resources into the process seems to be gaining a lot of momentum at present. Researchers from Mysore University are currently generating particles using natural plant extracts as surface modifiers and stabilizers. They have coined the term bio-hydrothermal synthesis to describe a new process of this nature [53]. It seems that this research path for synthesizing processes presents a promising future and a large number of articles are expected to focus on it.
* Scaling. Industrial scaling is also a crucial aspect to be studied, particularly since larger productions might enhance cost efficiency. As can be seen throughout this review, this matter is still in its early stages.
* General. If these processes viability is to be improved, in addition to what has already been mentioned, it would also be necessary to develop, on the one hand, new typologies, morphologies,and particle uses and, on the other hand, to study and develop new reactor typologies with improved hydrodynamics.
* Product purification. Perhaps this is the key point towards the viability of this technology, since the products obtained are in suspension in the water and must be separated from that water. For a future consolidation of this technology, an optimal and economical method to separate the particles from the water is still to be devised.  
As can be seen, numerous fields of study have been recently opened and remain so. Such issues are to be fully overcome if this technology is to continue developing, improving, and growing (consolidating).