Hydroxyapatite (HA)-based materials are widely used in the bone defect restoration field due to their stable physical properties, good biocompatibility, and bone induction potential. To further improve their performance with extra functions such as antibacterial activity, various kinds of metal ion-doped HA-based materials have been proposed and synthesized. This paper offered a comprehensive review of metal ion-doped HA-based materials for bone defect restoration based on the introduction of the physicochemical characteristics of HA followed by the synthesis methods, properties, and applications of different kinds of metal ion (Ag⁺, Zn²⁺, Mg²⁺, Sr²⁺, Sm³⁺, and Ce³⁺)-doped HA-based materials. In addition, the underlying challenges for bone defect restoration using these materials and potential solutions were discussed.

Titanium (Ti) and its alloys are widely used in clinical practice. As they are not bioactive, hydroxyapatite (HA) is commonly used to modify them. This study offered a review of microwave-assisted synthesis of composites based on Ti and HA for dental implantation by exploring their interaction mechanisms with microwave and features of two main techniques, namely microwave coating and sintering, along with current challenges and potential solutions in the field. It was shown that microwave coating enables rapid deposition of HA, but suffers from problems such as uneven coating thickness, poor integrity and unstable composition of the products. They can be solved by creating interlayers, combining the spin coating technique, etc. Unlike microwave coating, microwave sintering can effectively modify the mechanical properties of the composites, despite the shortcomings of excessive elastic moduli and potential HA decomposition. These issues are expected to be addressed by adding alloying elements and employing appropriate materials as space holders and ion-doped HA for sintering.

The boronized Ti6Al4V/HA composite is deemed to be an important biomaterial because of its potential remarkable mechanical and biological properties. This paper reports the osteogenesis performance of the boronized Ti6Al4V/HA composite, which was prepared by microwave sintering of powders of Ti6Al4V, hydroxyapatite (HA), and TiB₂ in high-purity Ar gas at 1050 °C for 30 min, as dental implant based on both cell experiments in vitro and animal experiments in vivo. The comparison between the boronized Ti6Al4V/HA composite and Ti, Ti6Al4V, and boronized Ti6Al4V in the terms of adhesion, proliferation, alkaline phosphate (ALP) activity, and mineralization of MG-63 cells on their surfaces confirmed that the composite exhibited the best inductive osteogenesis potential. It exerted a more significant effect on promoting the early osteogenic differentiation of osteoblasts and exhibited the maximum optical density (OD) value in the MTT assay and the highest levels of ALP activity and mineralization ability, primarily ascribed to its bioactive HA component, porous structure, and relatively rough micro-morphology. The in vivo study in rabbits based on the micro-computed tomography (micro-CT) analysis, histological and histomorphometric evaluation, and biomechanical testing further confirmed that the boronized Ti6Al4V/HA composite had the highest new bone formation potential and the best osseointegration property after implantation for up to 12 weeks, mainly revealed by the measured values of bone volume fraction, bone implant contact, and maximum push-out force which, for example, reached 48.64%, 61%, and 150.3 ± 6.07 N at the 12th week. Owing to these inspiring features, it can serve as a highly promising dental implant.