RESOURCES

Abstract
Bone tissue engineering (BTE) has proven to be a promising strategy for bone defect repair. Due to its excellent biological properties, gelatin methacrylate (GelMA) hydrogels have been used as bioinks for 3D bioprinting in some BTE studies to produce scaffolds for bone regeneration. However, applications for load-bearing defects are limited by poor mechanical properties and a lack of bioactivity. In this study, 3D printing technology was used to create nano-attapulgite (nano-ATP)/GelMA composite hydrogels loaded into mouse bone mesenchymal stem cells (BMSCs) and mouse umbilical vein endothelial cells (MUVECs). The bioprintability, physicochemical properties, and mechanical properties were all thoroughly evaluated. Our findings showed that nano-ATP groups outperform the control group in terms of printability, indicating that nano-ATP is beneficial for printability. Additionally, after incorporation with nano-ATP, the mechanical strength of the composite hydrogels was significantly improved, resulting in adequate mechanical properties for bone regeneration. The presence of nano-ATP in the scaffolds has also been studied for cell-material interactions. The findings show that cells within the scaffold not only have high viability but also a clear proclivity to promote osteogenic differentiation of BMSCs. Besides, the MUVECs-loaded composite hydrogels demonstrated increased angiogenic activity. A cranial defect model was also developed to evaluate the bone repair capability of scaffolds loaded with rat BMSCs. According to histological analysis, cell-laden nano-ATP composite hydrogels can effectively improve bone regeneration and promote angiogenesis. This study demonstrated the potential of nano-ATP for bone tissue engineering, which should also increase the clinical practicality of nano-ATP.

Abstract
Bone tissue engineering scaffolds have made significant progress in treating bone defects in recent decades. However, the lack of a vascular network within the scaffold limits bone formation after implantation in vivo. Recent research suggests that leonurine hydrochloride (LH) can promote healing in full-thickness cutaneous wounds by increasing vessel formation and collagen deposition. Gelatin and Sodium Alginate are both polymers. ATP is a magnesium silicate chain mineral. In this study, a Gelatin/Sodium Alginate/Nano-Attapulgite composite hydrogel was used as the base material first, and the Gelatin/Sodium Alginate/Nano-Attapulgite composite polymer scaffold loaded with LH was then created using 3D printing technology. Finally, LH was grafted onto the base material by an amide reaction to construct a scaffold loaded with LH to achieve long-term LH release. When compared to pure polymer scaffolds, in vitro results showed that LH-loaded scaffolds promoted the differentiation of BMSCs into osteoblasts, as evidenced by increased expression of osteogenic key genes. The results of in vivo tissue staining revealed that the drug-loaded scaffold promoted both angiogenesis and bone formation. Collectively, these findings suggest that LH-loaded Gelatin/Sodium Alginate/Nano-Attapulgite composite hydrogel scaffolds are a potential therapeutic strategy and can assist bone regeneration.

Abstract
The treatment of bone defects caused by diseases, trauma, or tumor has always been a great clinical challenge. Implantation of bone biomaterials into bone defect areas is an effective method for bone injury repair. In this study, we used three-dimensional (3D) printing technology to prepare nano-hydroxyapatite (nHA)/sodium alginate (SA)/gelatin (Gel) hydrogel scaffolds loaded with different ratios (0, 0.13, 0.26, 0.39, 0.53, and 0.79‰) of emodin (EM) (EM/nHA/SA/Gel). Scanning electron microscopy showed that the scaffolds had a smooth surface without fracture and nHA was evenly distributed on the surface. The cell proliferation and migration results showed that the 0.39‰ EM group, in particular, could significantly promote the proliferation and migration of mouse embryonic osteoblast precursor (MC3T3-E1) cells and significantly increase the mRNA expression of osteogenic differentiation-related genes (bone morphogenetic protein/BMP-2, BMP-9, osteocalcin). In addition, the 0.39‰ EM group exhibited the best effect on osteogenic differentiation-related proteins (alkaline phosphatase, Runx 2, OSX). The expression of M2 polarization-related genes (arginase-1, CD206) also significantly increased after the treatment with the 0.39‰ EM group. Micro-CT showed that in the rat skull defect model, the EM/nHA/SA/Gel scaffold group significantly promoted bone regeneration after being implanted into the skull for 30 days. Our results indicate that the EM/nHA/SA/Gel hydrogel scaffolds can not only directly promote the proliferation and differentiation of osteoblasts but also indirectly promote osteogenic differentiation by supporting M2 polarization of macrophages. EM/nHA/SA/Gel hydrogel scaffolds are potential bone tissue engineering materials for bone regeneration.

Abstract
Tissue-engineered bone material is one of the effective methods to repair bone defects, but the application is restricted in clinical because of the lack of excellent scaffolds that can induce bone regeneration as well as the difficulty in making scaffolds with personalized structures. 3D printing is an emerging technology that can fabricate bespoke 3D scaffolds with precise structure. However, it is challenging to develop the scaffold materials with excellent printability, osteogenesis ability, and mechanical strength. In this study, graphene oxide (GO), attapulgite (ATP), type I collagen (Col I) and polyvinyl alcohol were used as raw materials to prepare composite scaffolds via 3D bioprinting. The composite materials showed excellent printability. The microcosmic architecture and properties was characterized by scanning electron microscopy, Fourier transform infrared and thermal gravimetric analyzer, respectively. To verify the biocompatibility of the scaffolds, the viability, proliferation and osteogenic differentiation of Bone Marrow Stromal Cells (BMSCs) on the scaffolds were assessed by CCK-8, Live/Dead staining and Real-time PCR in vitro. The composited scaffolds were then implanted into the skull defects on rat for bone regeneration. Hematoxylin-eosin staining, Masson staining and immunohistochemistry staining were carried out in vivo to evaluate the regeneration of bone tissue.The results showed that GO/ATP/COL scaffolds have been demonstrated to possess controlled porosity, water absorption, biodegradability and good apatite-mineralization ability. The scaffold consisting of 0.5% GO/ATP/COL have excellent biocompatibility and was able to promote the growth, proliferation and osteogenic differentiation of mouse BMSCs in vitro. Furthermore, the 0.5% GO/ATP/COL scaffolds were also able to promote bone regeneration of in rat skull defects. Our results illustrated that the 3D printed GO/ATP/COL composite scaffolds have good mechanical properties, excellent cytocompatibility for enhanced mouse BMSCs adhesion, proliferation, and osteogenic differentiation. All these advantages made it potential as a promising biomaterial for osteogenic reconstruction.

Abstract
A new prototype of hybrid silk fibroin and sodium alginate (SF-SA) based osteogenic hydrogel scaffold with a concentration of 2.5% magnesium phosphate (MgP) based gel was prepared with the assistance of an extrusion-based three-dimensional (3D) printing machine in this study. To determine the optimum ratio of MgP-based gel in the hydrogel, a series of physical and biochemical experiments were performed to determine the proper concentration of MgP in two-dimensional hydrogel films, as well as the cell compatibility with these materials in sequence. The SF-SA hydrogel with 2.5wt% magnesium phosphate (SF-SA/MgP) stood out and then was used to fabricate 3D hydrogel scaffolds according to the consequences of the experiments, with SF-SA hydrogel as a control. Then the morphology and osteogenic activity of the scaffolds were further characterized by field emission scanning electron microscope (SEM), calcium mineralization staining, and reverse transcription-polymerase chain reaction (rt-PCR). The SF-SA/MgP hydrogel scaffold promoted the adhesion of rat mesenchymal stem cells with higher degrees of efficiency under dynamic culture conditions. After co-culturing in an osteogenic differentiation medium, cells seeded on SF-SA/MgP hydrogel scaffold were shown to have better performance on osteogenesis in the early stage than the control group. This work illustrates that the 3D structures of hybrid SF-SA/MgP hydrogel are promising headstones for osteogenic tissue engineering.

Abstract
Hydroxyapatite (HA) is a promising scaffold material for the treatment of bone defects. However{,} the lack of angiogenic properties and undesirable mechanical properties (such as fragility) limits the application of HA. Nanoattapulgite (ATP) is a nature-derived clay mineral and has been proven to be a promising bioactive material for bone regeneration due to its ability to induce osteogenesis. In this study{,} polyvinyl alcohol/collagen/ATP/HA (PVA/COL/ATP/HA) scaffolds were printed. Mouse bone marrow mesenchymal stem/stromal cells (BMSCs) and human umbilical vein endothelial cells (HUVECs) were used in vitro to assess the biocompatibility and the osteogenesis and vascularization induction potentials of the scaffolds. Subsequently{,} in vivo micro-CT and histological staining were carried out to evaluate new bone formation in a rabbit tibial defect model. The in vitro results showed that the incorporation of ATP increased the printing fidelity and mechanical properties{,} with values of compressive strengths up to 200% over raw PC-H scaffolds. Simultaneously{,} the expression levels of osteogenic-related genes and vascularization-related genes were significantly increased after the incorporation of ATP. The in vivo results showed that the PVA/COL/ATP/HA scaffolds exhibited synergistic effects on promoting vascularization and bone formation. The combination of ATP and HA provides a promising strategy for vascularized bone tissue engineering.

Abstract
Objective
To investigate the efficacy of a paeoniflorin-sodium alginate (SA)-gelatin skin scaffold for treating diabetic wound in a rat model.
Methods
Bioinks were prepared using various percentages of paeoniflorin in the total weight of a solution containing SA and gelatin. Skin scaffolds containing 0%, 1%, 3%, 5%, and 10% paeoniflorin were printed using 3D bioprinting technology, and scaffold microstructure was observed with scanning electron microscopy. Skin scaffolds were then used in rats with diabetic wounds. H&E staining, Masson staining, and immunohistochemical staining for IL-1β and CD31 were performed on days 7 and 14.
Results
All skin scaffolds had a mesh-like structure with uniform pore distribution. Wounds healed well in each group, with the 1% and 3% groups demonstrating the most complete healing. H&E staining showed that skin accessory organs had appeared in each group. On day 7, collagen deposition in the 3% group was higher than in the other groups (P＜0.05), and IL-1β infiltration was lower in the 10% group than in the 3% group (P = 0.002). On day 14, IL-1β infiltration was not significantly different between the 10% and 3% groups (P = 0.078). The CD31 level was higher in the 3% group than in the other groups on days 7 and 14 (P＜0.05).
Conclusion
A 3% paeoniflorin-SA-gelatin skin scaffold promoted the healing of diabetic wounds in rats. This scaffold promoted collagen deposition and microvascular regeneration and demonstrated anti-inflammatory properties, suggesting that this scaffold type could be used to treat diabetic wounds.

Abstract
Recently, three-dimensional (3D) bioprinting technology is becoming an appealing approach for osteochondral repair. However, it is challenging to develop a bilayered scaffold with anisotropic structural properties to mimic a native osteochondral tissue. Herein, we developed a bioink consisting of decellularized extracellular matrix and silk fibroin to print the bilayered scaffold. The bilayered scaffold mimics the natural osteochondral tissue by controlling the composition, mechanical properties, and growth factor release in each layer of the scaffold. The  in vitro  results show that each layer of scaffolds had a suitable mechanical strength and degradation rate. Furthermore, the scaffolds encapsulating transforming growth factor-beta (TGF-β) and bone morphogenetic protein-2 (BMP-2) can act as a controlled release system and promote directed differentiation of bone marrow-derived mesenchymal stem cells. Furthermore, the  in vivo  experiments suggested that the scaffolds loaded with growth factors promoted osteochondral regeneration in the rabbit knee joint model. Consequently, the biomimetic bilayered scaffold loaded with TGF-β and BMP-2 would be a promising strategy for osteochondral repair.

Abstract
As cartilage tissue lacks the innate ability to mount an adequate regeneration response, damage to it is detrimental to the quality of life of the subject. The emergence of three-dimensional bioprinting (3DBP) technology presents an opportunity to repair articular cartilage defects. However, widespread adoption of this technique has been impeded by difficulty in preparing a suitable bioink and the toxicity inherent in the chemical crosslinking process of most bioinks. Our objective was to develop a crosslinker-free bioink with the same biological activity as the original cartilage extracellular matrix (ECM) and good mechanical strength. We prepared bioinks containing different concentrations of silk fibroin and decellularized extracellular matrix (SF-dECM bioinks) mixed with bone marrow mesenchymal stem cells (BMSCs) for 3D bioprinting. SF and dECM interconnect with each other through physical crosslinking and entanglement. A porous structure was formed by removing the polyethylene glycol from the SF-dECM bioink. The results showed the SF-dECM construct had a suitable mechanical strength and degradation rate, and the expression of chondrogenesis-specific genes was found to be higher than that of the SF control construct group. Finally, we confirmed that a SF-dECM construct that was designed to release TGF-β3 had the ability to promote chondrogenic differentiation of BMSCs and provided a good cartilage repair environment, suggesting it is an ideal scaffold for cartilage tissue engineering.

Abstract
The recent advent of 3D bioprinting of biopolymers provides a novel method for fabrication of tissue-engineered scaffolds and also offers a potentially promising avenue in cartilage regeneration. Silk fibroin (SF) is one of the most popular biopolymers used for 3D bioprinting, but further application of SF is hindered by its limited biological activities. Incorporation of growth factors (GFs) has been identified as a solution to improve biological function. Platelet-rich plasma (PRP) is an autologous resource of GFs, which has been widely used in clinic. In this study, we have developed SF-based bioinks incorporated with different concentrations of PRP (12.5%, 25%, and 50%; vol/vol). Release kinetic studies show that SF-PRP bioinks could achieve controlled release of GFs. Subsequently, SF-PRP bioinks were successfully fabricated into scaffolds by bioprinting. Our results revealed that SF-PRP scaffolds possessed proper internal pore structure, good biomechanical properties, and a suitable degradation rate for cartilage regeneration. Live/dead staining showed that 3D, printed SF-PRP scaffolds were biocompatible. Moreover, in vitro studies revealed that tissue-engineered cartilage from the SF-PRP group exhibited improved qualities compared with the pure SF controls, according to histological and immunohistochemical findings. Biochemical evaluations confirmed that SF-PRP (50% PRP, v/v) scaffolds allowed the largest increases in collagen and glycosaminoglycan concentrations, when compared with the pure SF group. These findings suggest that 3D, printed SF-PRP scaffolds could be potential candidates for cartilage tissue engineering.