# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 4 3D Bioprinting Techniques
### Extrusion-Based Bioprinting
Human dermal fibroblasts and human microvascular endothelial cells were combined, with a gelatin-sodium ALG composite hydrogel as the dermis and human keratinocytes were mixed with gel as the epithelium. A full-thickness wound formed on the backs of naked mice and was then closed with a double-layer skin graft. The cell survival rate was kept at >90% per day, which indicates that the composite hydrogel meets the requirements for material for printing cells, while the wound contraction was improved by about 10% compared to the control. In the first week after surgery, no significant observations of differences in wound healing performance between the 3D-printed skin graft and control groups with no endothelial cells and no cells were seen. In the fourth week, however, the wound healing rate of the 3D-printed skin graft was relatively lower than both control groups. Histological examination revealed that the new skin looks like normal skin, and has a significant degree of angiogenesis [179]. A 3D-printed self-assembling peptide nanofiber hydrogel scaffold (SLg) combined with UV-cured GelMA possessing interpenetrating polymer networks was examined using the SSE printing method in terms of the elasticity and absorbency [180]. Photosensitive GelMA was prepared by dissolving gelatin in PBS and then MA was poured into the solution. Peptide hydrogels were obtained in the water mixture. The swelling rates of GelMA variations after 4 h ranged 462-729%, while the swelling rate after 24 h was between 472 and 649%. In order to study wound healing and angiogenesis performance, full-thickness wounds were incised on rats. Following this, the IHC test was performed to assess the biocompatibility and vascularization capabilities of the hydrogels. In vivo studies have demonstrated that the modified GelMA/SLg scaffolds promoted the adhesion and proliferation of HUVECs. The growth rate was parallel to that of L929 fibroblasts, and the proliferation of endothelial cells on GelMA/SLg20 exhibited the highest cell activity. Dermal tissue regeneration analysis utilizing a rat model revealed that on the 28 th day, the GelMA/SLg20 group had the greatest regular and collagen fibrous structure, increased collagen type III capacity, and symmetrically distributed fibers. In conclusion, the addition of SLg into the scaffolds improved angiogenesis, vascular regeneration, and dermal tissue growth according to in vivo results. Recently, two promising studies have been conducted on the development of 3D-printed cellulose/\(\epsilon\)-polyslysine (CP) hydrogel and a 3D-printed GelMA hydrogel patch encapsulating VEGF, both decorated with photoactive and antibacterial tetrapodal zinc oxide (t-ZnO) microparticles for wound therapy. To prepare the CP hydrogels, CMC and \(\epsilon\)-polyslysine were placed under weak acidic conditions with glycolidyl methacrylate. Afterwards, both materials were dissolved in the I2959 solution. The first dressing showed swelling rates of CP hydrogels of 253.41% compared to hydrogels that had a lower CP concentration (333.07%), due to the crosslinking of hydrogels resulting in smaller pore size (25-100 \(\upmu\)m) and more balanced internal structures. Degradation time varied between 180 and 250 h, depending on the CP content. CP hydrogels resisted 62%\(\sim\)87% strain and endured 280 kPa compression stress. Antioxidant activity was assessed using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging methods, and different concentrations of CP hydrogels revealed 59 \(\pm\) 5%, 82 \(\pm\) 5%, and 88.5 \(\pm\) 4% antioxidant properties. Cytotoxic evaluation demonstrated no toxicity in CP hydrogels, and cell viability was over 98%. The antibacterial activity of the CP hydrogels also revealed that hydrogels with varying CP contents killed \(\approx\)93 \(\pm\) 2%-96 \(\pm\) 2% of \(E\). _coli_ and 88 \(\pm\) 3%-95 \(\pm\) 1% of \(S\). _aureus_. To evaluate in vivo wound healing performance, full-thickness skin incision was performed on rats, and the wounds were treated with each hydrogel dressings.