Engineering approaches for growth factor delivery have been considerably advanced for tissue regeneration, yet most of them fail to provide a complex combination of signals emulating a natural healing cascade, which substantially limits their clinical successes. Herein, we aimed to emulate the natural bone healing cascades by coupling the processes of angiogenesis and osteogenesis with a hybrid dual growth factor delivery system to achieve vascularized bone formation. Basic fibroblast growth factor (bFGF) was loaded into methacrylate gelatin (GelMA) to mimic angiogenic signalling during the inflammation and soft callus phases of the bone healing process, while bone morphogenetic protein-2 (BMP-2) was bound onto mineral coated microparticles (MCM) to mimics osteogenic signalling in the hard callus and bone remodelling phases. An Initial high concentration of bFGF accompanied by a sustainable release of BMP-2 and inorganic ions was realized to orchestrate well-coupled osteogenic and angiogenic effects for bone regeneration. In vitro experiments indicated that the hybrid hydrogel markedly enhanced the formation of vasculature in human umbilical vein endothelial cells (HUVECs), as well as the osteogenic differentiation of mesenchymal stem cells (BMSCs). In vivo results confirmed the optimal osteogenic performance of our F/G-B/M hydrogel, which was primarily attributed to the FGF-induced vascularization. This research presents a facile and potent alternative for treating bone defects by emulating natural cascades of bone healing. Graphical Abstract

The upstream role of sensory innervation during bone homeostasis is widely underestimated in bone repairing strategies. Herein, a neuromodulation approach is proposed to orchestrate bone defect healing by constructing engineered sensory nerves (eSN) in situ to leverage the adaptation feature of SN during tissue formation. NGF liberated from ECM‐constructed eSN effectively promotes sensory neuron differentiation and enhances CGRP secretion, which lead to improved RAOECs mobility and osteogenic differentiation of BMSC. In turn, such eSN effectively drives ossification in vivo via NGF‐TrkA signaling pathway, which substantially accelerates critical size bone defect healing. More importantly, eSN also adaptively suppresses excessive bone formation and promotes bone remodeling by activating osteoclasts via CGRP‐dependent mechanism when combined with BMP‐2 delivery, which ingeniously alleviates side effects of BMP‐2. In sum, this eSN approach offers a valuable avenue to harness the adaptive role of neural system to optimize bone homeostasis under various clinical scenario.

Surface modification of orthopedic implants is critical for improving the clinical performance of these medical devices. Herein, collagen was covalently immobilized onto a titanium implant surface via a novel adherent polydopamine coating inspired by mussel adhesive proteins. The formation and composition of the collagen coating was characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Fluorescent labeled collagen was also used to examine the formation and uniformity of the collagen coating. The resultant collagen coating with a polydopamine supporting substrate demonstrated better uniformity and distribution on the titanium surface compared to a physical adsorption of collagen. The covalent immobilized collagen coating is biologically active, as evidenced by its ability to enhance MC3T3-E1 cell adhesion, support cell proliferation and promote early stage osteogenic differentiation of pre-osteoblasts. Our study suggests covalent immobilization of collagen through the polydopamine coating might be an efficient way to improve the cellular performance of implant surfaces.

The imbalance of bone homeostasis is the root cause of osteoporosis. However current therapeutic approaches mainly focus on either anabolic or catabolic pathways, which often fail to turn the imbalanced bone metabolism around. Herein we reported that a SIRT-1 agonist mediated molecular therapeutic strategy to reverse the imbalance in bone homeostasis by simultaneously regulating osteogenesis and osteoclastogenesis via locally sustained release of SRT2104 from mineral coated acellular matrix microparticles. Immobilization of SRT2104 on mineral coating (MAM/SRT) harnessing their electrostatic interactions resulted in sustained release of SIRT-1 agonist for over 30 days. MAM/SRT not only enhanced osteogenic differentiation and mineralization, but also attenuated the formation and function of excessive osteoclasts via integrating multiple vital upstream signals (β-catenin, FoxOs, Runx2, NFATc1, etc.) in vitro. Osteoporosis animal model also validated that it accelerated osteoporotic bone healing and improved osseointegration of the surrounding bone. Overall, our work proposes a promising strategy to treat osteoporotic bone defects by reversing the imbalance in bone homeostasis using designated small molecule drug delivery systems.