Hydroxyapatite has become an important coating material for bioimplants, following the introduction of synthetic HAp in the 1950s. The HAp coatings require controlled surface roughness/porosity, adequate corrosion resistance and need to show favorable tribological behavior. The deposition rate must be sufficiently fast and the coating technique needs to be applied at different scales on substrates having a diverse structure, composition, size, and shape. A detailed overview of dry and wet coating methods is given. The benefits of electrodeposition include controlled thickness and morphology, ability to coat a wide range of component size/shape and ease of industrial processing. Pulsed current and potential techniques have provided denser and more uniform coatings on different metallic materials/implants. The mechanism of HAp electrodeposition is considered and the effect of operational variables on deposit properties is highlighted. The most recent progress in the field is critically reviewed. Developments in mineral substituted and included particle, composite HAp coatings, including those reinforced by metallic, ceramic and polymeric particles; carbon nanotubes, modified graphenes, chitosan, and heparin, are considered in detail. Technical challenges which deserve further research are identified and a forward look in the field of the electrodeposited HAp coatings is taken.

NiTi is a class of metallic biomaterials, benefit from superelastic behavior, high biocompatibility, and favorable mechanical properties close to that of bone. However, the Ni ion leaching, poor bioactivity, and antibacterial activity limit its clinical applications. In this study, HAp-Nb₂O₅ composite layers were PC electrodeposited from aqueous electrolytes containing different concentrations of the Nb₂O₅ particles, i.e., 0–1 g/L, to evaluate the influence of the applied surface engineering strategy on in vitro immersion behavior, Ni²⁺ ion leaching level, and antibacterial activity of the bare NiTi. Surface characteristics of the electrodeposited layers were analyzed using SEM, TEM, XPS, and AFM. The immersion behavior of the samples was comprehensively investigated through SBF and long-term PBS soaking. Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) infective reference bacteria were employed to address the antibacterial activity of the samples. The results illustrated that the included particles led to more compact and smoother layers. Unlike bare NiTi, composite layers stimulated apatite formation upon immersion in both SBF and PBS media. The concentration of the released Ni²⁺ ion from the composite layer, containing 0.50 g/L Nb₂O₅ was ≈ 60% less than that of bare NiTi within 30 days of immersion in the corrosive PBS solution. The Nb₂O₅-reinforced layers exhibited high anti-adhesive activity against both types of pathogenic bacteria. The hybrid metallic-ceramic system comprising HAp-Nb₂O₅-coated NiTi offers the prospect of a potential solution for clinical challenges facing the orthopedic application of NiTi.

Nickel–titanium (NiTi) is a shape-memory alloy, a type of material whose name is derived from its ability to recover its original shape upon heating to a certain temperature. NiTi falls under the umbrella of metallic materials, offering high superelasticity, acceptable corrosion resistance, a relatively low elastic modulus, and desirable biocompatibility. There are several challenges regarding the processing and machinability of NiTi, originating from its high ductility and reactivity. Additive manufacturing (AM), commonly known as 3D printing, is a promising candidate for solving problems in the fabrication of near-net-shape NiTi biomaterials with controlled porosity. Powder-bed fusion and directed energy deposition are AM approaches employed to produce synthetic NiTi implants. A short summary of the principles and the pros and cons of these approaches is provided. The influence of the operating parameters, which can change the microstructural features, including the porosity content and orientation of the crystals, on the mechanical properties is addressed. Surface-modification techniques are recommended for suppressing the Ni ion leaching from the surface of AM-fabricated NiTi, which is a technical challenge faced by the long-term in vivo application of NiTi.

The goal of the present investigation is to evaluate the influences of plating parameters such as TiO₂ nanoparticles concentration in the plating bath and applied current density on the microstructure-related and mechanical properties as well as corrosion behavior of Ni–Co coatings. The coatings were fabricated by different current densities and TiO₂ nanoparticles concentration over the range 2–8 A dm^–2 and 0–30 g/L, respectively. Results demonstrate that the incorporation of TiO₂ nanoparticles into the plating bath coupled with the change in current density can deeply affect the microstructure-related, mechanical properties as well as corrosion resistance of Ni–Co coatings. According to the results, Ni–Co–20 g/L TiO₂ coatings electroplated at current density of 4 A dm^–2 shows the superior properties. The synergistic effects of higher Co amount coupled with the higher volume fraction of TiO₂ throughout the microstructure of the coating is responsible for the above-mentioned improvements.

The development of surface technologies to obtain improved tribomechanical and biological characteristics of synthetic NiTi implants is critical. A suitable match with the mechanical properties of the implanted biomaterial is necessary to reduce the risk of stress-shielding and implantation failure. The present contribution has attempted to assess the influence of the Nb₂O₅ particle level in an electrolyte, i.e., 0–1 g L⁻¹, on the tribomechanical and biological performance of the HAp layers, which were obtained by galvanostatic pulse electrodeposition on NiTi. The surface characteristics of the electrodeposited layers were analyzed using GIXRD, FESEM, XPS, and contact angle measurements. Nanoindentation, nanoscratch, and pull-off assays were employed to study the tribomechanical properties of the films. The in vitro biocompatibility of the developed specimens was investigated using NIH3T3 fibroblast cells. The results illustrated that a more compact and hydrophilic surface is obtained with the incorporated particles. The Nb content and Ca/P molar ratio are increased from the outermost surface to the subsurface layers of the coatings. The composite coatings showed higher hardness, elastic modulus, bonding strength to the underlying NiTi, and lower COF compared to pure HAp layers. The elastic modulus of the Nb₂O₅-reinforced HAp films is close to that of cortical bone, reducing stress-shielding risk. The inclusion of Nb₂O₅ particles in the HAp matrix led to an improvement in cell functions, e.g., viability and proliferation, within the various cell culture durations. The adopted surface modification strategy in this work can create new opportunities for the successful use of NiTi in orthopedic applications.