# Advances in Three Dimensional Bioprinting for Wound Healing: A Comprehensive Review
## 5 Recent Advances in Hydrogel-Based 3D Printing for Wound Healing
### Drug Delivery
According to wound healing assays, Gel/PCL/PDA scaffold-treated wounds contained more blood vessels and hair follicles than control and PCL samples, and cytotoxicity analysis showed no adverse effects on mice [15]. Another study introduced 3D-printable ink for treating diabetic wounds, including citric acid and sodium CMC solution (Figure 3A) [248]. The ink was printed in a cylindrical shape and then freeze-dried for 24 h at \(-\)80 \({}^{\circ}\)C (Figure 3A) [248]. The dried structure was then crosslinked by thermal treatment at 120 \({}^{\circ}\)C for 7 min. The printed CMC scaffold was incubated with platelet-rich plasma (PRP) for 2 h at 22 \({}^{\circ}\)C. SEM analysis demonstrated that a sponge-like structure was obtained due to the homogeneously dispersed microporous structure of the scaffold. After 2 h of soaking in water, the swelling rates of platelet-rich plasma and 0.9% NaCl were 40% and 15%, respectively. The swelling test achieved 17.5% and 55.2% weight loss in the NaCl solution after 2 and 24 h, respectively (Figure 3(A3-A5)) [248]. The growth factor releases of the structure prepared in NaCl solution was investigated. In the first 6 h, burst release was observed. After 72 h of incubation, the total protein release was 232 ug per mg scaffold. On the seventh day, the dressing had completely disintegrated, so all content was assumed to have been released. It was reported that the wound closures in cell experiments were almost at their maximum values at 24 h. To evaluate in vivo wound healing, healthy rats were injected with streptozotocin to promote diabetes. Later, full-thickness wounds were incised on rats and scaffolds were applied accordingly. The in vivo wound healing experiments revealed that control, CMC and CMC-PRP scaffolds had a wound closure rate of 35.8%, 44.1% and 50.2% after 3 days, respectively. After 7 days, the healing rates of the treated wounds were less than 50%, while the CMC and CMC-PRP scaffolds were 58.0% and 62.8%, respectively (Figure 3(A1,A2)) [248]. As a result of the experiments, the CMC-PRP material exhibited the best performance, with a wound healing rate of 88.6%. Figure 3: _Cont._  
Figure 3: **(A)** The effect of 3D-printed carboxymethyl cellulose scaffolds for autologous growth factors’ delivery in a wound healing application. Reproduced/adapted with permission from Ref. [248]. **(B)** In situ cartilage regeneration monitoring by MRI with the help of GelMA-based hydrogel-loaded kartogenin. Reproduced/adapted with permission from Ref. [249]. **(C)** 3D-printed large-scale and highly porous biodegradable tissue engineering scaffolds from poly(trimethylene-carbonate). Reproduced/adapted with permission from Ref. [250]. 3D-printed large-scale and highly porous biodegradable tissue engineering scaffolds from poly(trimethylene-carbonate). **(A1)** Graph representing the wound healing efficiency after 3, 7, 10 and 14 days of wound formation, [PERSON]’s honestly significant difference test (HSD) and one-way analysis of variance (ANOVA) were conducted for evaluation and comparison of the data ((*) for \(p<0.05\), (\(n=7\))); and **(A2)** a representative image of wound healing in treated and untreated groups. Images taken on day 0 indicate fresh wounds that have not been treated with scaffolds. The scaffolds remained transparent on day three. **(A3)** Swelling kinetic plot for CMC scaffolds when incubated in water, NaCl 0.9% or PRP for 24 h. **(A4)** The graph illustrates the pore size changes after incubation in NaCl 0.9% \(w/v\) for 1, 2 or 24 h. **(A5)** SEM images of freeze-dried CMC scaffolds after incubation with the same incubation times. (**B1**) T2-weighted MR image of articular cartilage, (**B2**) R2/S-1 (One-way ANOVA test was performed to obtain significance analysis ((*) for \(p\) < 0.05) and (**B3**) R2*/S-1 value derived from MR imaging ((*) for \(p\) < 0.05). (**B4**) H&E, (**B5**) Masson, (**B6**) toluidine (**B7**) PAS staining images of in vivo cartilage regeneration at 6 and 12 weeks.