# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 3 Overview of Wound Pathophysiology and Wound Healing Process
### 3D Bioprinted Wound Dressings
#### 3.2.1 Bioinks for Wound Dressings  
In recent years, 3D bioprinting has become one of the most efficient technologies for producing wound dressings, as it can print cells and biomaterials on the exact print path [66; 67]. The functionality of bioink depends on three factors: printability, shape accuracy, and biocompatibility [68]. These aspects not only determine the efficacy of the bioink but also influence the subsequent tissue development process [69]. For instance, during the degradation of a material scaffold, incorporated cells release proteases, initiating a cascade of events that ultimately leads to the synthesis of ECM proteins. These proteins play a crucial role in shaping the intricate architecture of the emerging tissue structure. Additionally, the material's suitability for use as a bioink relies on its rheologicalcharacteristics, such as its ability to be extruded at low pressures and to maintain the desired shape. If 3D bioprinting involves the use of cells or bioactive agents, the rheological behavior should also support cell viability due to clinical and regulatory concerns [70; 71]. According to the Food and Drug Administration's (FDA) regulations, antimicrobial wound dressings are classified as unclassified products with the code FRO (dressing, wound, drug). However interactive wound and burn dressings that promote or accelerate wound healing are classified as Class III devices. There are 24 types of polysaccharide-based hydrogel wound dressings registered by the FDA. These products are composed of materials such as alginate (ALG), cellulose, chitosan (CH), and hyaluronic acid (HA). Specifically, ALG-based hydrogels have dominated approximately 70% of the wound dressing market. These dressings provide a moist wound environment and facilitate autolysis, which is the natural breakdown of dead tissue. For instance, Algicll(tm) Algicll(tm) is a hydrogel dressing that contains silver ions. The antibacterial properties of silver ions make them useful for preventing infections [72]. Various other examples of wound dressings (*US FDA-approved indications) include 2 nd Skin(r) by Spenco Medical, Ltd. (Waco, TX, USA), Carrasyn(r) developed by Carrington Laboratories (Irving, TX, USA), Clearsite(tm) from ConMed Corporation (Utica, NY, USA), Elasto-Gel(tm) by SW Technologies (Kansas City, MO, USA), FlexiGel(tm) and Transigel(tm) produced by Smith & Nephew (London, UK),, Kendall(tm) Curafil(tm) and Kendall(tm) Curagel(tm) both manufactured by Covidien (Dublin, Ireland), Normligel(tm) and Hypergel(tm) from Molinycke (Gothenburg, Sweden) [73] Health Care, Nu-gel(tm) by Systgenix (Quincy, MA, USA), Tegagel(tm) offered by 3M (St. Paul, MN, USA), and Vigilon(tm) by C.R. Bard (Murray Hill, NJ, USA), among others. In addition to providing a suitable environment for wound healing, traditional hydrogel wound dressings can be further enhanced by incorporating biomimetic components into bioprinted products. The studies on this method have demonstrated that biomimicry could positively influence the attachment, function, migration, and proliferation of both endogenous and exogenous cells [74]. By putting more effort into designing or upgrading 3D printers to protect cells from exposure to extreme conditions during the printing process, in combination with enhancing the performance of the materials by adding or changing the polymeric backbone, it is possible to produce biocompatible, mechanically stable, and non-immunogenic dressings with a proposed shape. A wide variety of bioprinted materials is currently presented in the literature using bioinks. Inks are classified into two main groups based on the presence or absence of cells embedded in the materials' matrix. The most widely used type is a scaffold-based biomaterial, in which cells are loaded into hydrogels and bioprinted into 3D structures. Cell-loaded hydrogels allow cell proliferation and growth, facilitating tissue formation.