Two-dimensional layered double hydroxides for biomedical applications: From nano-systems to surface- and body-systems

Two-dimensional layered double hydroxides (2D LDHs) have attracted extensive attention in the field of biomedicine owing to various advantages, including biocompatibility, biodegradation, pH responsiveness, anion exchange effect, and easy functionalization. Based on the essential characteristics of LDHs nanosheets, this review introduces the construction strategies of relevant LDHbased materials, systematically expounds on the basic principle of preparation, and comprehensively presents the recent advances in this field. With a hydrotalcite-like crystal structure, the biological function and biocompatibility of LDHs nanosheets depend on the types of divalent and trivalent ions in its plate structure, hence, the functional element ions are summarized to provide a reference for the design and construction of LDH-based biomaterials. In the past few decades, LDH-based materials have been gradually applied from nano-system to functional modification of surface and 3D doping of body-system, and have made important progress in biomedicine as nanomaterials, surface coatings, and 3D components. Therefore, herein, biomedical applications of LDHs in nano-, surface- and body-systems are emphasized. Centered on LDH-based nano- and surface-systems, LDH-based nanomaterials including conventional LDHs nanosheets, evolved LDHs single-layer nanosheets, LDHs nanohybrids, and LDHs nanocomposites, and LDH-based coating including in situ and non in situ coatings are systematically introduced. For the LDHsdopped body-system, the existing form of LDHs in bodies and its biological applications in 3D printing bioinks, injectable hydrogels, and polymer porous scaffolds and electrospinning fibers are comprehensively analyzed. Although many challenges remain, this excellent material is expected to facilitate clinical transformation and breakthrough medical achievements.