The extracellular matrix (ECM) is known to provide instructive cues for cell attachment, proliferation, differentiation, and ultimately tissue regeneration. The use of decellularized ECM scaffolds for regenerative-medicine approaches is rapidly expanding. In this study, cartilage acellular matrix (CAM)-based bioink was developed to fabricate functional biomolecule-containing scaffolds. The CAM provides an adequate cartilage tissue-favorable environment for chondrogenic differentiation of cells. Conventional manufacturing techniques such as salt leaching, solvent casting, gas forming, and freeze drying when applied to CAM-based scaffolds cannot precisely control the scaffold geometry for mimicking tissue shape. As an alternative to the scaffold fabrication methods, 3D printing was recently introduced in the field of tissue engineering. 3D printing may better control the internal microstructure and external appearance because of the computer-assisted construction process. Hence, applications of the 3D printing technology to tissue engineering are rapidly proliferating. Therefore, printable ECM-based bioink should be developed for 3D structure stratification. The aim of this study was to develop printable natural CAM bioink for 3D printing of a tissue of irregular shape. Silk fibroin was chosen to support the printing of the CAM powder because it can be physically cross-linked and its viscosity can be easily controlled. The newly developed CAM-silk bioink was evaluated regarding printability, cell viability, and tissue differentiation. Moreover, we successfully demonstrated 3D printing of a cartilage-shaped scaffold using only this CAM-silk bioink. Future studies should assess the efficacy of in vivo implantation of 3D-printed cartilage-shaped scaffolds.

The need for organ and tissue regeneration in patients continues to increase because of a scarcity of donors, as well as biocompatibility issues in transplant immune rejection. To address this, scientists have investigated artificial tissues as an alternative to transplantation. Threedimensional (3D) bioprinting technology is an additive manufacturing method that can be used for the fabrication of 3D functional tissues or organs. This technology promises to replicate the complex architecture of structures in natural tissue. To date, 3D bioprinting strategies have confirmed their potential practice in regenerative medicine to fabricate the transplantable hard tissues, including cartilage and bone. However, 3D bioprinting approaches still have unsolved challenges to realize 3D hard tissues. In this manuscript, the current technical development, challenges, and future prospects of 3D bioprinting for engineering hard tissues are reviewed.