## 7 Use of Biomaterials in the Release of Extracellular Vesicles for Treatment of Cutaneous Wounds
Another important characteristic of hydrogels is their porosity. It must be high, to facilitate the transport of nutrients, waste products and gas exchange, as well as the migration and survival of skin cells in wound beds [18]. Furthermore, because rheological properties of hydrogels are similar to those of the extracellular matrix of the skin, they can be used to mimic their functions [18]. Within the different types of hydrogels, injectable ones are well suited for wound treatment and nanoparticle or EV release. Injectable hydrogels are low viscosity fluids, with high penetration capacity into beds and crevices of ulcers. Once injected, chemical or physical hydrogel crosslinking can be induced in situ. This provides stable hydrogels, in intimate contact with damaged tissues [18]. The incorporation of MSC-EV into biomaterials depends on interactions between them. Therefore, one line of research is to explore new strategies that increase the loading and retention of MSC-EV in these biomaterials. EV can be incorporated into hydrogels in different ways: (i) combining EV with polymers, and then adding crosslinkers to induce gelation; (ii) making hydrogels first, and then incorporating EV through physical methods; and (iii) mixing polymers together with crosslinkers and EV [88]. Of the three possibilities, the second one has the advantage over the others in that polymerization conditions do not affect the properties of EV. Binding can occur through electrostatic or bioactive adhesion interactions [89]. In the former case, negatively-charged phospholipid membranes of MSC-EV and cationic (positively charged) delivery systems, such as those based on chitosan-containing hydrogels, retain MSC-EV through electrostatic forces. For bioactiveadhesion, considering that MSC-EV express adhesion molecules (such as CD29 CD44 and CD73, as well as some \(\alpha\)-integrins), extracellular matrix components such as collagen I, fibronectin and hyaluronic acid are used for biomaterial fabrication [90]. The structure and porosity of the biomaterial matrix also play a very important role in the retention, stability and subsequent release of EV. A very promising technique for manufacturing nanofibers, with applications in regenerative medicine, is electrospinning. This consists of discharging a charged polymer solution through a syringe, at a given rate, in presence of high voltage. That causes electrostatic charges to accumulate at the tip of the polymer-droplet row, producing what is known as a Taylor cone. With increasing voltage, a polymer jet is generated and deposited on a collector, generating nanofibers to form non-woven mats with high porosity. They mimic the extracellular matrix, due to their small pore size [91]. With this technique, gelatin-based nanofibers, as well as others produced by mixing gelatin with other biopolymers, have been generated with the ability to improve wound healing [91]. In addition, it has been demonstrated that these nanofibers have a high capacity to store and release different drugs and bioactive agents, with therapeutic functions. Among these agents are EV. Thus, it has recently been described that electrospun polyvinylpyrrolidone-based nanofibers can preserve up to 12 weeks EV without significant loss of functionality [92]. So far, to our knowledge, no nanofibers-containing MSC-EV have been fabricated by this technique, for the treatment of cutaneous wounds. However, other preclinical studies have been performed using both in vivo and in vitro models. They have shown a high potential of this type of biomaterials in regenerative medicine when applied, for example, to urethral or bone regeneration [93; 94]. Among the most used wound dressings to release MSC-EV are hydrogels based on chitosan, alginate, hyaluronic acid, collagen and Pluronic F127. Other biomaterials produced through tissue decellularization and from other compounds such as polyurethane also stand out, as described below.