## 3 Nanotechnology in Wound Dressings
### Two Major Nanotechnological Preparatory Techniques Applied in Wound Dressing  
The nanoformulation strategy is a solution to preserve the therapeutic efficacy of essential oils while minimizing their physicochemical limitations. For this purpose, mainly large molecules (e.g., cyclodextrin) and polymers have been used as carriers [93]. As a delivery system for the pharmacological application of essential oils, nanoemulsions are gaining special attention because they are inexpensive and scalable [94]. In fact, the nanoemulsion method is a simple synthetic technique and is applied for a simple composition. Nanoemulsions are isotropic dispersed systems of two immiscible liquids formed by mixing oil and water and adding surfactant/cosurfactant to obtain a droplet size in the nanometer range [95]. Three essential parameters should be considered in the composition, method, and manufacturing conditions to obtain a stable formulation [95]. The emulsion of the two immiscible liquid phases can create an interfacial tension that can be controlled by the addition of surfactants, which allows the dispersion of the particles [96]. Nanoemulsions can be classified as water-in-oil (W/O) and oil-in-water (O/W) emulsions. O/W nanoemulsions are mainly used in the pharmaceutical field because they are easily washed out and are often used in drug delivery systems. The preparation of nanoemulsions starts with the successive conversion of a macroemulsion into a nanometric form by applying ultrasonic emulsification, high-pressure homogenization and microfluidization as high-energy methods, phase inversion and self-emulsification as low-energy techniques [97]. Macroemulsions are thermodynamically more stable than nanoemulsions, but the nanometric size improves lipophilic molecules' loading capacity (like essential oils, natural compounds) [98] and reduces particles sedimentation by decreasing the gravity effect onto their surface [99]. Even if the use of essential oil nanoemulsion as a food biopreserver is more prevalent [100], their wound-healing potential is noteworthy. The development of all kinds of wound dressing also influences the design of essential oil nanoemulsions. Indeed, essential oil nanoemulsions are combined with biopolymers, such as chitosan, with recognized biological effects [101], or they are used to ameliorate another kind of wound dressings, such as hydrogel [102] or both situations, as chitosan/alginate films [103]. The three-dimensional architecture of electrospun nanofiber membranes has an excellent advantage for wound treatment by mimicking the ECM structure. The properties of electrospun nanofibers also improve the wound-healing progress. In particular, the size of nanofibers and their network promote gas exchange and alienation of bacteria, while their high surface-to-volume ratio supports the delivery of therapeutic molecules [104]. Moreover, electrospun membranes show good adaptability to wound shape compared to conventional dressings, and their surface can be functionalized to enhance their bioactivity [105]. In electrospinningning, a positive high voltage is applied to a viscoelastic liquid droplet of the polymer solution, deforming it into a conical shape. A pump ejects the droplets as a jet from a metal needle attached to a syringe and deposits them as fibers on an anodically charged collector plate. During the process, the overlapping of the fibers gradually forms mats. The resulting electrospun fibers are influenced in their morphology and mechanical properties by several parameters associated with the process, such as the applied voltage, the distance between the needle and the collector, the viscosity and conductivity of the solution, the volatility of the solvent, and the molecular weight of the polymer [106]. In particular, the solvent properties and the collector geometry seem to influence the porosity of the electrospun fibers.