11.3.1　Preparation of polymer fibers

Ideal wound dressings should be non-toxic，non-sensitizing，and they can allow gas exchange，absorb wound exudate to protect the wound，form a microenvironment conducive to wound healing，and inhibit bacterial growth.Commonly used fiber dressings，such as cotton gauze，were used for clinical wound care and treatment by covering the damaged skin.However，gauze is prone to cause adhesion between the wound and the dressing，which causes secondary trauma and bacterial growth.In recent years，the preparation technology of fiber dressings has developed vigorously.The micro-nano structured fiber membrane with high specific surface area and high porosity is a new type of wound dressing.Compared with other dressings，it is lighter，thinner，more breathable，and has various functional directions.There are many technologies for preparing micro-nano structured fiber membranes，such as stretching method，template synthesis，phase separation，selfassembly，and electrospinning technology.

Among the preparative technologies of fiber dressings，electrospinning technology is currently the most widely studied technology.Electrospinning technology is a fiber production method that uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of some hundred nanometers.The electrospinning process is simple，low cost，with wide selection of materials.

Compared with other forms of fiber materials，electrospun fibers show many advantages in medical dressings.Firstly，they have structural advantages.The diameter of the fibers obtained by electrospinning technology ranged from a few nanometers to a few microns，and it has good mechanical strength.Its multi-level geometric size is similar to the microstructure and biological function of the extracellular matrix（ECM）.At present，the electrospun fiber membrane is often used as a scaffold material for tissue engineering.It provides support and guidance for the proliferation and migration of cells and the transfer of active substances，which is conducive to angiogenesis and tissue remodeling.Secondly，electrospun fiber membrane usually has high permeability and porosity，which can quickly absorb the tissue fluid that appears in the wound，facilitating the ventilation of the wound and preventing bacterial infection.Thirdly，the electrospun fiber membrane can also achieve an excellent hemostatic effect by optimizing the fiber material and the electrospun fiber membrane can be used as a drug loading platform to enhance its drug delivery efficacy.For example，it can be loaded with anti-inflammatory drugs，growth factors，antibiotics，etc.，and the drug delivery speed can be adjusted as needed to promote wound healing.Figure 11-12 shows the commonly used strategies for directly preparing drug-loaded electrospun fiber dressings，including blending，coaxial，and emulsion electrospinning.Blended electrospinning usually produces fibers containing active agents dispersed throughout the fiber，while coaxial electrospinning and emulsion electrospinning synthesize core/shell morphologies well to meet different drug loading requirements.(https://www.daowen.com)

In addition，post-treatment of electrospun fibers can also improve wound healing properties.Generally，the surface of electrospun fibers may lack the properties required for specific biological applications.The property can be imparted by electrostatic adhesion，dip coating，layer-by-layer assembly，or surface chemical reaction.For example，PVA electrospun membranes can be coated with chitosan by simply immersing the PVA membrane in a chitosan solution，which can provide instant hemostatic activity.Epidermal growth factor（EGF）can also be fixed on the surface of the fibers through a facile grafting strategy.In vivo wound studies have shown that EGF conjugated fibers can increase keratinocytespecific genes.Therefore，the incorporation of growth factors can promote gene expression，thereby accelerating wound healing.