# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 4 3D Bioprinting Techniques
### Laser-Based Bioprinting
#### 4.1.3 Digital Light Processing (DLP)  
DLP was developed with the help of SLA biosignature technology. DLP light sources, combined with multiple lenses, allow 3D scaffolds with a resolution between 25 and 150 pixels [101; 105; 122]. Unlike traditional printing systems, DLP adopts the top-down method, which includes a beam source placed under the DLP system and cured bioink. Therefore, the structure system or carrier can be immersed in the bio-substrate and moved up after the layers are polymerized. During this process, bioink may also be delivered directly to the substrate via capillary action, and each repeated step can be completed within 15 s [101; 123]. The DLP system can be run again spontaneously by the user without changing the parameters [109]. Due to its rapid and high-resolution printing capabilities, the DLP method is suitable for tissue engineering and fabricating wound dressings. For example, a CH/acrylamide (AM)-based hydrogel was successfully produced using the DLP system [123]. The bioink was prepared by dissolving CH in acetic acid solution, then adding methacrylic anhydride (MA) at 60 \({}^{\circ}\)C. Furthermore, the addition of AM to the bioink caused an increment in the maximum stress at fracture from 97 kPa to 345 kPa, resulting in an increase of 356% in compressive strength. Subsequently, the tensile stress increased from 47 kPa to 183 kPa. To assess the cytotoxicity of the hydrogel, CH/AM bioinks were placed on well plates and solidified with photocuring. Then, HUVECs were seeded on the surface of hydrogels and cultured for a few days. Although an increase in polymerized AM content resulted in less cellular attachment and activity, which limited the ability of cells to adhere to and proliferate on the surface, in vitro biocompatibility tests demonstrated adequate biocompatibility qualities. For personalized disease management, micro-pyramid-decorated gelatin methacrylate (GelMA) hydrogels were introduced using the DLP technique [124]. GelMA was prepared by the methacrylation of gelatin type A, and the bioink was produced by dissolving GelMA in a solution involving phosphate buffered saline (PBS) and PEGDA. According to the mechanical tests, printed hydrogels exhibited a compressive modulus of 53 kPa, and the swelling ratio was approximately 400% in 5 h. The pyramids were designed to be 1.5 mm in length to ensure the dressing directly contacts the wound. The SEM results show that printed hydrogels kept their shape for 7 days in PBS immersion. To assess in vitro cytocompatibility tests, mouse fibroblasts were seeded on printed hydrogels. Furthermore, the cytotoxicity assays revealed 90% viability after 72 h of incubation. In order to evaluate the in vivo wound healing performance of the hydrogels, wound areas on rat skin were treated with GelMA hydrogels for 0, 7 and 14 days. Following this, an immunohistochemistry (IHC) assay for CD31 was carried out to further explain the healing mechanism. Consequently, the IHC assay confirmed more vascularization in hydrogels with heparin. In vivo tests demonstrated that hydrogels without heparin achieved 33% wound closure, while hydrogels conjugated with heparin achieved 67% wound closure after 14 days of treatment. Overall, the results indicate that the DLP method has significant potential in wound dressing production. Besides the remarkable success achieved in the production of hydrogel-based biomaterials with DLP bioprinting, the most promising use of such biomaterials is the smart dressings combined with sensors. Recently, auxetic hydrogel wound dressings integrated with paper-based colorimetric sensors were developed to detect the pH and glucose concentrations of the wound. A 50 mL volume of hydrogel resin was prepared by mixing HEMA, PEGDA, and diphenyl- (2,4,6-trimethylbenzoyl)phosphine oxide (TPO) at a 1:1:0.05 ratio. In post processing, DLP-printed auxetic structures were washed with isopropyl alcohol, subjected to ultrasonication, and then UV-cured to remove residual monomers.