Hydroxyapatite Surface Engineering by Microwave-Assisted Cladding for Improved Microstructure and Interface

The chemical similarity and bioactivity of HAp with the mineral phase of natural bone make HAp-based surface engineering an interesting area for study on enhancing the biological function and interfacial performance of orthopedic implants. This review critically revises the HAp-based composite systems and surface engineering pathways to bring a synergic relationship between material composition, processing conditions, microstructural evolution, coating-substrate interface characteristics and biomedical performance. Among all the conventional coating techniques, such as sol-gel, dip coating, spin coating, calcium phosphate deposition, thermal/plasma spraying, composite coatings based on oxides, antimicrobial coatings, and other surface-modification methods, the methods are comparatively studied based on the following criteria: adhesion, porosity, phase stability, corrosion resistance, wear resistance, bioactivity, mechanical integrity, and suitability for complex implant geometry. Special attention is given to the microwave assisted cladding and microwave hybrid heating, such as susceptor assisted energy transfer, thermal exposure, interfacial diffusion, solidification behavior and graded microstructural development. There are only a few laboratory-scale studies on microwave-assisted HAp cladding, and they are directly related literature. For the investigated processing conditions, diffusion/metallurgical interfaces and dendritic or cellular composite structures were identified during the reported investigations but relevant quantitative evidence is still not comprehensive enough to draw general conclusions about superiority over conventional processing routes. The issues that need to be addressed are the sensitivity to process window, heating behaviour of the equipment, coating defects and residual porosity, HAp phase stability, reproducibility, scaling up, and processing of complex implant geometries. Other future directions are also discussed such as: functionally graded coatings, processing assisted by additive manufacturing, cold spray, suspension plasma spraying, laser directed energy deposition, high entropy coatings, and machine learning assisted process optimization. Specific research priorities discussed in the review include the optimization of microwaves, characterization of interfaces at various scales, real-time monitoring, computational modeling, standardized interface characterization, scale-up and long-term biomedical validation.