## 4 Current Limitations and Future Perspectives  
The main goal regarding cultural heritage objects is their preservation (conservation). Restoration appears often necessary, but should always be the subject of professional restorer's decision. As any restorer knows, all the recipes used for the restoration/conservation of cultural heritage objects should meet several critical conditions, among which two are of particular interest: the reversible character of the treatment and authenticity preservation [147]. The specialists working in this area find significant differences between clearly defined terms used (such as _protection_, _preservation_, _conservation_, _consolidation_, _restoration_, _reconstruction_ or _anastylosis_) [147]. These terms are not, at this moment, thoroughly integrated in the vocabulary of the researchers working in the field of materials science (from which comes the large majority of the solutions presented in the current review). This represents a sign of a major (and probably the most important) drawback of the current approach, worldwide. Most researchers perform laboratory (or even in situ) experiments without the assistance (or collaboration) from a conservative/restorer. Although with scientific value for sure, this approach could lead to possibly insignificant results for the restorers. The main goal of materials science researchers should be, in our opinion, to provide the necessary tools and the scientific support for the specialists that have the authority and the knowledge to practically perform the interventions on the cultural heritage objects. Thus, closer collaboration should be established between the two types of scientists, as well as the development of a common criteria preferences regarding the materials/treatments, as some studies pointed out the major differences regarding the selection of materials for the protection of heritage objects [148]. Additionally, as a general remark, the wide spread of nanomaterials in their pure state (without incorporation in polymeric matrixes) can, in some instances, violate the reversibility criteria. This could, on the other hand, be explained by its relative wide criticism from conservationists, that consider it a \"dubious principle\", \"chimera\", \"myth\" or \"Utopian idea\" [149]. From the point of view of materials science, it appears surprising the lack of original materials, implemented at least at laboratory level. As researchers could obtain nanoparticles or nanomaterials (such as, for example, TiO\({}_{2}\), SiO\({}_{2}\), hydroxyapatite, etc.) with the desired morphological characteristics, they should use such synthesized materials instead of commercially available ones. There are a large number of metallic or metal oxide nanoparticles that could exhibit photocatalytic activity (for self-cleaning purposes) or antimicrobial properties. Additionally, the exploration of antimicrobial properties of phytosynthesized nanoparticles could offer new recipes with potential application in this area [78]. The same observations can be made regarding the use of hydroxyapatite/other apatite materials (which could be tuned for multiple applications). These materials could be incorporated in polymeric films, in order to ensure the previously-stated reversible character. Especially when talking about materials for cultural heritage buildings, the treatments should also meet some supplementary criteria (easy and deep penetration, resistant to attack, prevention of humidity penetration, allowing exiting of water, no modification alteration, uniform contraction and expansion with the substrate, inexpensive, non-corrosive, non-reactive, lasting properties, easily applicable, resistance to acid and alkaline attack, etc.) [150], which should be all presented by the authors reporting the evaluation of new materials.