## 8 Conclusions and Future Perspectives  
New therapies based on MSC and their secretome, to enhance tissue regeneration, have opened great expectations for chronic-wound treatments. Specifically, secretomes, and especially fractions formed by EV, have shown in recent years to have significant therapeutic potential in this regard. The rationale is that they carry molecules with immunomodulatory, angiogenic, proliferation- and migration-inducing properties. They modulate cells involved in healing, such as endothelial ones, fibroblasts and keratinocytes. However, the short half-life of MSC-EV, when they are directly applied to damaged tissues or via the bloodstream, complicates their clinical application. In order to overcome this limitation, numerous biomaterials have been designed with properties of retaining, protecting and progressively releasing MSC-EV. Thus, encapsulation of MSC-EV in these dressings has a high potential to facilitate tissue regeneration, for the treatment of cutaneous wounds as shown schematically in Figure 3.  
However, currently, the studies performed with this strategy are mainly preclinical, using wound-healing animal models. On the other hand, these studies have been carried out under very different experimental conditions, in terms of cell type and culture conditions used to obtain EV. Other variables are EV concentrations and treatment patterns, as well as animal models used. Not all of them correspond to ulcers that are difficult to heal, such as diabetic ones (Table 1). Additionally, although such data are encouraging, most publications of translational studies are less than four years old, so they can still be considered preliminary. Thus, in clinical trials.gob <[[https://clinicaltrials.gov/](https://clinicaltrials.gov/)]([https://clinicaltrials.gov/](https://clinicaltrials.gov/))> (accessed on 25 January 2023), we have found four studies have been reported proposing the use of EV for the treatment of skin ulcers: (i) NCT05475418 (without starting recruitment), in which they use adipose tissue-derived exosomes; (ii) NCT05078385 (without starting recruitment), which uses BMMSC-derived EV for treatment of burn wounds; (iii) NCT04134676 (completed), in which stem-cell-conditioned medium is used on chronic, ulcer-wounds, albeit without published results at the time of writing; and (iv) NCT02565264 (unknown status), to study the effect of plasma-derived exosomes on cutaneous-wound healing. EV will be applied in hydrogels, in studies NCT05475418 and NCT04134676, but their compositions are not specified. Therefore, in order to understand the real clinical potential of such strategies, it is necessary to carry out clinical trials, to ascertain the promising results obtained so far in animal models. To this end, it is essential to advance mainly in the development of techniques for obtaining MSC-EV. In this regard, it is necessary to define which MSC sources, as well as which culture conditions or manipulations through bioengineering, would be the most appropriate to produce MSC-EV. They should have a high regenerative capacity, without posing a relevant risk to patients. Likewise, they should be obtained in sufficient quantities, to meet the demand required for clinical use. It is also important to standardize concentrations and purity of EV loaded in biomaterials, to guarantee consistent regenerative effects. That may be the main handicap to accelerating the progress of this approach, for the treatment of chronic ulcers in humans. Figure 3: Scheme of therapeutic strategy for treatment of cutaneous wounds. It is based on combination of biomaterials or multifunctional wound-dressings with MSC-EV. Biomaterials, such as hydrogels, can be of different nature and can be supplemented, for example, with bioactive compounds such as ascorbic acid, AgNP and plant extracts. MSC-EV can be produced from MSC derived from different tissues and different culture conditions. Biomaterials allow the creation of adequate microenvironments for wound healing.