With the ageing of the world population, osteoporosis has become a problem affecting quality of life. According to the traditional view, the causes of osteoporosis mainly include endocrine disorders, metabolic disorders and mechanical factors. However, in recent years, the immune system and immune factors have been shown to play important roles in the occurrence and development of osteoporosis. Among these components, regulatory T (Treg) cells and T helper 17 (Th17) cells are crucial for maintaining bone homeostasis, especially osteoclast differentiation. Treg cells and Th17 cells originate from the same precursor cells, and their differentiation requires involvement of the TGF-β regulated signalling pathway. Treg cells and Th17 cells have opposite functions. Treg cells inhibit the differentiation of osteoclasts in vivo and in vitro, while Th17 cells promote the differentiation of osteoclasts. Therefore, understanding the balance between Treg cells and Th17 cells is anticipated to provide a new idea for the development of novel treatments for osteoporosis.

Integrating multifunctional semiconducting metal oxide powders into a 3D printing technique to construct hierarchical porous structures is highly desirable and remains a significant challenge. Herein, an extrusion‐based 3D printing strategy is developed that can assemble TiO2 powders into hierarchical porous structures with multiscale pores at both the macro‐ and microscale. Powder‐based TiO2 inks with a significant shear‐thinning behavior and adequate storage modulus and yield stress are developed to meet the requirements of 3D printing of TiO2 in an air environment without the need for an additional solidification treatment, which provides good printing flexibility. The hierarchical porous structures with a relatively high compressive strength provide the 3D‐printed TiO2 structures with great potential for use in many applications, including filtration, thermal insulation, biomedical scaffolds, catalyst supports, and energy conversion. Compared with scaffolds with a compact morphology, the hierarchical porous TiO2 scaffold as a photoelectrode achieves a higher nitrogen photofixation yield due to its high surface adsorption and activation capacity caused by its porous morphology. Importantly, the powder‐based ink design and extrusion‐based 3D printing approach are readily extended to other semiconducting metal oxides such as ZnO and their composites.

To investigate whether angiogenesis changes in early menopausal osteoporosis treated with estrogen replacement therapy, 120 rats were randomly divided into five groups: sham operation group (SHAM), ovariectomy group (OVX), and ovariectomy plus three different estrogen doses replacement therapy groups (OVX + E2). We detected the bone microarchitecture and measured the expression levels of estrogen receptor beta (ERβ), vascular endothelial growth factor (VEGF), osteoprotegerin (OPG), and receptor activator of NF-κB ligand (RANKL). CD31 immunofluorescence and silica gel perfusion imaging were used to analyze the vascular distribution. We confirmed that the femur BMD of ovariectomized rats was significantly lower than SHAM group and OVX+E2 groups. After estrogen therapy, the local microvascular formation increased after estrogen treatment in a dose dependent manner. ERβ was downregulated and VEGF was upregulated, positively correlated with estrogen dosage. We successfully constructed an osteoporosis model of ovariectomized rats with estrogen replacement therapy. We also found angiogenesis changed in early menopausal osteoporosis treated with estrogen replacement therapy. We indicated that estrogen replacement therapy increased angiogenesis through VEGF upregulation. However, we observed that, at the highest doses of estrogen studied, increased angiogenesis was associated with a decrease in BMD, the underlying mechanisms of which remain unclear.

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