Long-term stability, high strength, and 3D printable alginate hydrogel for cartilage tissue engineering application.

Cartilage damage is one of the main causes of disability, and 3D bioprinting technology can produce complex structures that are particularly suitable for constructing a customized and irregular tissue engineering scaffold for cartilage repair. Alginate is an attractive biomaterial for bioinks because of its good biological safety profile and fast ionic gelation. However, ionically crosslinked alginate hydrogels are recognized as lacking enough mechanical property and long-term stability due to ion exchange. Here, we developed a double crosslinked alginate (DC-Alg) hydrogel for 3D bioprinting, and human umbilical cord mesenchymal stem cells (huMSCs) could differentiate into chondrocytes on its printed 3D scaffold after 4 weeks' culture. We performed sequential modification of alginate with L-cysteine and 5-norbornene-2-methylamine, and the DC-Alg hydrogels were obtained in the presence of CaCl2and ultraviolet light with stronger mechanical properties than those of the single ionic crosslinked alginate hydrogels, which was similar to natural cartilage. They also had better stability and could be maintained in DMEM medium for over 1 month, as well good viability for huMSCs. Moreover, the DC-Alg as 3D printing inks demonstrated a better printing accuracy (∼200 µm). After 4 weeks culture of huMSCs in the 3D printed DC-Alg scaffolds, the expressions of chondrogenic genes such asaggrecan (agg), collagen II (col II), and SRY-box transcription factor9(sox-9) were obviously observed, indicating the differentiation of huMSCs into cartilage. Immumohistochemical staining analysis further exhibited cartilage tissue developed well in the 3D printed scaffolds. Our study is the first demonstration of DC-Alg in 3D printing for MSC differentiation into cartilage, which shows a potential application in cartilage defect repair.