Mechanical loading has been shown to influence various osteogenic responses of bone-derived cells and bone formation in vivo. However, the influence of mechanical stimulation on the formation of bone organoid in vitro is not clearly understood. Here, 3D bioprinted human mesenchymal stem cells (hMSCs)-laden graphene oxide composite scaffolds were cultured in a novel cyclic-loading bioreactors for up to 56 days. Our results showed that mechanical loading from day 1 (ML01) significantly increased organoid mineral density, organoid stiffness, and osteoblast differentiation compared with non-loading and mechanical loading from day 21. Importantly, ML01 stimulated collagen I maturation, osteocyte differentiation, lacunar-canalicular network formation and YAP expression on day 56. These finding are the first to reveal that long-term mechanical loading is required for the formation of 3D bioprinted functional osteocyte bone organoids. Such 3D bone organoids may serve as a human-specific alternative to animal testing for the study of bone pathophysiology and drug screening.

Mechanical loading has been shown to influence various osteogenic responses of bone-derived cells and bone formation in vivo. However, the influence of mechanical stimulation on the formation of bone organoid in vitro is not clearly understood. Here, 3D bioprinted human mesenchymal stem cells (hMSCs)-laden graphene oxide composite scaffolds were cultured in cyclic-loading bioreactors for up to 56 days. Our results showed that mechanical loading from day 1 (ML01) significantly increased organoid mineral density, organoid stiffness, and osteoblast differentiation compared with non-loading and mechanical loading from day 21. Importantly, ML01 stimulated collagen I maturation, osteocyte differentiation, lacunar-canalicular network formation and YAP expression on day 56. These finding are the first to reveal that long-term mechanical loading is required for the formation of 3D bioprinted functional osteocyte bone organoids. Such 3D bone organoids may serve as a human-specific alternative to animal testing for the study of bone pathophysiology and drug screening.