# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 4 3D Bioprinting Techniques
### Extrusion-Based Bioprinting  
The extrusion-based 3D bioprinting system is a technology that allows a system-guided nozzle to print a layer-by-layer structure. The nozzle moves the ink along the X and Y axes by creating a continuous flow and moves up or down in the Z-axis direction (Figure 2E). This technology can be divided into two subgroups: melt-based printing and non-material melt printing [163; 164]. The accuracy and resolution of the extrusion-based bioprinting method with respect to the bioinks are summarized in Table 3, and the advantages and disadvantages of the method are summarized in Table 4.  
Traditional extrusion technology consists of 3D printing by progressively extruding bioinks via the print head with pressure. A previous study was conducted to develop physically cross-linked chitosan-pectin (CS-PEC) hydrogels with good printability and dimensional integrity with this technology, making them suitable for wound dressings [165]. In addition, the integration of an upgraded static mixer to the 3D printer allows the use of bioinks with low viscosity [166]. Similarly, extrusion-based gradient 3D printing technology allows the 3D printing of bioink using two different syringes and two different softwares. It enables a composite bioink with a gradient structure using different syringe speed adjustments [167]. An alternative method is micro-extrusion 3D printing technology, which involves controlling the ink with computer software and extruding it onto the system layer. The bioprinting of the desired structure is created through four or five iterations [168]. In contrast to the methods described above, semi-solid extrusion (SSE) technology utilizes the layer-by-layer deposition of bioink to produce 3D constructions [169]. The primary distinction between SSE and other extrusion methods is in the raw materials used. The beginning material in SSE is a semi-solid or semi-molten substance, whereas the printing material in FDM is a solid filament or powder [170; 171]. Because of the low temperatures necessary for bioprinting, live cell structures such as aortic valves and tissue healing may be created [172; 173; 174; 175]. The distinct features of this biotechnology are currently being investigated in medication development to create novel dosage formulations [176]. SSE is a good contender for inclusion in healthcare settings or research laboratories due to its capacity to generate PrintletsTM (3D-printed tablets) and other devices in minutes in a one-step procedure [177]. Extrusion-based bioprinting technologies with various bioink compositions have been frequently used in wound healing. For example, because of their low cytotoxicity and structural resemblance to ECM, cellulose nanofibrils (CNFs) in hydrogel form are proposed to be suitable for use in bioink formulation for extrusion-based bioprinting. A research group successfully printed low-concentration nanocellulose with 1% \(w/v\) TEMPO-oxidized CNF and less than 1% \(w/v\) GelMA [178]. The bioink was prepared by both mixing GelMA powder and loading GelMA solution into CNF hydrogels. It was noted that CNF and GelMA had a strong physical interaction, and the mechanical strength of the scaffolds was regulated between 2.5 and 5 kPa. Experiments with 3T3 fibroblast cell culture revealed that it evokes non-cytotoxic and biocompatible qualities in formulated inks and printed scaffolds. The incorporation of GelMA into the CNF hydrogel increased fibroblast growth. This study concludes that 3D printing shows considerable promise for wound healing applications. Another research effort focused on the vascularization of 3D-printed full-layer skin grafts to improve current techniques. Dermal-epithelial grafts were implanted into a naked mouse model to evaluate integration with host tissue and wound healing. To produce micro-vessels in skin grafts, human dermal fibroblasts, keratinocytes, and microvascularendothelial cells were developed and constructed using an extruded 3D printer.