The synthetic hydrogels with high water contents are promising for various applications, however, they usually exhibit low mechanical properties. In this work, inspired by the natural biological soft tissues, whose hierarchically ordered fibrous structures result in high strength and good flexibility, a flexible, high-strength, and versatile hydrogel with the fiberboard-and-mortar hierarchically ordered structure (HFMOS) is developed based on ultralong hydroxyapatite (HAP) nanowires and polyacrylic acid (PAA). The as-prepared HFMOS hydrogel has a high water content (~ 70 wt.%), dense structure, and excellent mechanical properties, and these properties are similar to those of the human cartilage and are superior to many hydrogels reported in the literature. The excellent mechanical properties of the HFMOS hydrogel originate from the combination of the fiberboard-and-mortar hierarchically ordered structure, reinforcement of ultralong HAP nanowires, strong interfacial strength, and multiple energy dissipation pathways. Moreover, thanks to the controllable components and injection procedure, the HFMOS hydrogel with a Janus structure is prepared for particular applications. The HFMOS hydrogel possesses abundant ordered water channels, and can be used for loading, release, and directed delivery of various functional substances. Thus, the as-prepared flexible, high-strength, and versatile HFMOS hydrogel possesses a great potential for various applications such as water purification, pollution treatment, biomedicine, nanofluidic devices, and high-performance structural materials.

As the main inorganic component of human bones and teeth, hydroxyapatite (HAP) materials have been widely investigated for various applications especially in the biomedical field. Recently, HAP nanowires (HAP NWs) have attracted research interest in the energy and environment-related fields owing to their advantages such as excellent biocompatibility and bioactivity, high-thermal stability, surface modification and functionalization, and self-assembly to form nanoscale porous networked structure. In this review, we first briefly discuss the synthesis of HAP NWs. Then we focus on summarizing the foremost advances in energy and environment-related applications of HAP NWs, including the battery, solar energy-assisted water evaporation, light-driven self-propelled device, thermal insulation, fire-resistant inorganic paper, water purification, air purification, molecular detection, superhydrophobic surface, and high-strength nanocomposite materials. In addition, the future research directions on HAP NWs and their energy and environmental applications are proposed. This review aims to provide inspiration and stimulate extensive future studies in the energy and environmental applications of HAP NWs and other inorganic NWs.

Due to the similar chemical properties to the inorganic component in calcified tissues, synthetic calcium phosphate has been considered as ideal biomaterials with excellent biocompatibility. Nanostructured calcium phosphate materials play an important role in the formation of hard tissues in nature. It is reported that calcium phosphates materials in nano-size can mimic the dimensions of constituent components of calcified tissues. Recently, the synthesis and application of nanostructured calcium phosphate materials have become a very hot field. Lots of methods have been reported to prepare nanostructured calcium phosphate, and various morphologies including nanoparticles, plate-like nanocrystals, nano-needles, whiskers/fibres/wires, mesoporous, nanotubes, nano-blades, and powders with three-dimensional (3-D) structures have been obtained. More studies of nanostructured calcium phosphates are expected in biomedical area, such as tissue engineering scaffolds, drug/gene delivery systems and multifunctional systems. In this article, the synthesis and application of nanostructured calcium phosphates are reviewed and discussed.