# Modifications of Wound Dressings with Bioactive Agents to Achieve Improved Pro-Healing Properties
## 2 Current Concepts in Wound Dressings  
Accurate wound valuation--including estimation of the wound size, color, type, location, and exudate level--determines the most suitable type of the wound dressing to be used [15]. In contrast to the conventional approaches using gauze and cotton bandages in order to cover the wound, there is a broad range of polymer-based materials in the world market (e.g., gels, foams, and films) that can ensure a faster and more effective wound healing process by facilitating the function of the wound (not just covering it) [16; 1]. An ideal wound dressing should: (1) absorb excessive exudates; (2) control the moisture in the wound bed; (3) possess good mechanical stability; (4) have great gases transmission; (5) protect from microorganism colonization and infections; (6) be non-toxic, biocompatible, and biodegradable; (7) ensure easy and non-painful removal after completed skin regeneration; and (8) be available at an acceptable cost [4,16; 17; 18; 19]. The above-mentioned features are summarized in Figure 2.  
A great challenge for scientists is to create an ideal dressing material that would have all the aforementioned features, providing optimal conditions for the most effective regeneration process. With the development of science, materials used in the production of the wound dressings have changed to provide better conditions for wound healing [20]. Since a moist environment is crucial for accelerated wound healing, modern hydrogel dressings containing 70-90% water have recently attracted a lot of attention. This type of dressing provides an easy application and removal without any tissue damage and can be used for burns, chronic and necrotic wounds, and pressure ulcers [1]. The most common method for hydrogel production involves polymerization of a monomer within  
Figure 1: Schematic comparison of chronic and acute wound microenvironment. a cross-linked hydrogel network, whereby a second cross-linked network or a polymer is formed [21]. Conventional hydrogels are prone to breakages due to their reduced mechanical strength, limiting their clinical applications. To improve their mechanical strength, hybridization-based techniques using different polymers and strong physical and chemical interactions are used [22]. Hydrocolloid dressings are usually made of pectin, a cross-linked gelatin matrix, or carboxymethyl cellulose. This type of dressing forms a soft gel by absorbing wound fluid. They are useful for low-exuding or shallow wounds, and similarly to hydrogels they have the ability to maintain the moist environment at the wound bed [23]. Since hydrocolloids are semi-permeable to water and oxygen, it is not recommended to use mentioned dressings in strongly infected wounds due to the risk of hypoxia and increased infections [24; 25]. In the case of moderately to heavily exuding wounds, it is recommended to use hydrofiber dressings, which combine the features of hydrocolloids and alginate dressings [26]. Alginate is a natural, readily available, non-toxic, and biocompatible biopolymer obtained from seaweed. It is particularly attractive in wound dressing production and application due to its high ability to maintain a moist environment and to reduce bacterial infections [27]. The most common method for alginate dressing preparation is cross-linking of the sodium alginate solution with divalent ions (calcium, barium, cadmium, magnesium, zinc, cobalt, strontium) in order to obtain a gel, which can be additionally processed to obtain a foam-like or fibrous dressing [28; 29]. The absorption capacity of the cross-linked calcium alginate dressing is excellent because it is able to absorb fivefold-seventold more fluid compared to traditional gauze. Due to its specific features (homeostasis ability), it is recommended for the management of bleeding and highly exuding wounds [30]. Films and foams are two other large groups of dressings widely used in skin regeneration.