# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 5 Recent Advances in Hydrogel-Based 3D Printing for Wound Healing
### Tissue Engineering  
In tissue engineering, the 3D-printing of scaffolds has enabled researchers to combine different materials with desired shapes and characteristics for the treatment of various complications. For example, [PERSON] et al. created a keratin-based hydrogel scaffold by formulating a photo-crosslinkable resin involving riboflavin-sodium persulfate-hydroquinone (initiator-catalyst-inhibitor) solution [275]. They applied it in a DLP 3D printer to study for tissue engineering and regenerative medicine applications. The scaffold's compressive modulus was in the range of 5.49-15.45 kPa, between the 4 and 6% keratin hydrogels. An in vitro cytotoxicity assay was conducted by placing the scaffolds directly on top of the wells seeded with L929 mouse fibroblast cells. Under direct exposure to keratin, the metabolic activity of the cell was maintained at 96.3 \(\pm\) 28.8%. Consequently, the study showed that a keratin-based printing resin and printing methodology have minimal cytotoxicity. Further, the fabricated hydrogels behave no differently from other keratin-based constructions and commercially available non-cytotoxic substances, with decent printing resolution, adequate mechanical properties, uptake, and swelling capacities. A subsequent investigation was conducted to determine the biological properties and the use of photosensitive GelMA hydrogel solutions for tissue engineering applications. GelMA hydrogel was fabricated using a 780 nm femtosecond laser by mixing GelMA hydrogel with a VA-086 photoinitiator and Rose Bengal photosensitizer in a solution. For both HPF-1 human lung fibroblasts for the cytotoxicity assay and CCC-ESF-1 human skin fibroblasts for the cell attachment assay, cells were seeded in well plates and afterwards scaffolds were placed in the wells. The photosensitive gel-hydrogel solution produced by TPP technology was utilized to obtain solutions with 66.3% cell viability suitable for cell binding. Further, a micro/nano 3D-printed scaffold with superior biocompatibility was produced using a photosensitive GelMA hydrogel solution. The production of scaffolds by TPP technology was performed at 150 m per second, at a power of 7.8 mW, with a layer distance of 150 nanometers, and at a concentration of 12% [276]. Another line of research successfully 3D-printed an artificial skin structure containing hair follicles, epidermis and dermis using gelatin GelMA/hyaluronic acid methacryloyl (HAMA) hydrogels as the bioink. A DLP printer was used to 3D-print papillary molds, and an extrusion-based bioprinter was used to 3D-print skin equivalents. A micro-colored culture dish was then used to mature the skin equivalents. The average pore sizes of GelMA/HAMA hydrogels were 118.40 \(\pm\) 12.32 \(\upmu\)m. Mechanical tests revealed 59.02 \(\pm\) 22.96 kPa compressive stress and a Young's modulus of 15.72 \(\pm\) 3.9 kPa on average. Further, cytotoxicity assays displayed that 99.6% of the cells were alive. The papillary layer was able to support epidermis-dermis interaction, and to promote the growth of laden cells. Ultimately, these findings suggest that GelMA/HAMA hydrogels can be utilized in tissue engineering as therapeutic scaffolds [277]. Moreover, the formulation of a specialized photosensitive, biodegradable, and biocompatible resin, based on a three-armed methacrylated poly(trimethylene-carbonate) (PTMC-MA) and its printability in 3D bioprinting, have been examined and tested for their biocompatibility (Figure 3(C1)). PTMC-MA macromer was synthesized by polymerizing TMC and then reacting the resulting PTMC oligomer with MA. Afterwards, a PTMC resin was prepared by dissolving PTMC-MA in tetrahydrofuran (THF), adding a photoinitiator, and evaporating the THF. To remove the air bubbles, the solution was placed in a vacuum before dividing into sample holders and the bioink was heated to 50 \({}^{\circ}\)C before printing.