Crystal-chemical design of stimuli-responsive hydroxyapatite nanomaterials for bone implant coatings and triggered drug delivery: from lattice engineering to release control

Abstract Background Hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) coatings are used to improve the fixation of metal bone implants in clinical settings. However, a static HA film cannot address issues like infection, inflammation, poor vascularization, tumor recurrence, and other complications after implant placement. Responsive HA nanoparticles offer a solution by delivering an implant that responds to the pathological environment and releases therapeutic agents as needed. Reviews categorize these materials based on their triggers but do not link their responsiveness to the underlying apatite crystal chemistry that influences drug loading and release, leaving formulators without clear design guidance. Main body of the abstract This review explores stimuli-responsive HA from a particular perspective: the idea that lattice-level variables such as substitution sites, crystallinity, and solubility product determine the dissolution behavior of HA coatings, which in turn influences release kinetics. We (i) relate apatite crystal chemistry to measured release parameters and conventional pharmacological kinetic models (zero-order, Higuchi, Korsmeyer-Peppas), grounding these connections with available dissolution and release data; (ii) categorize triggers into endogenous (pH, enzymatic, redox) and exogenous (light, magnetic field, ultrasound, thermal), evaluating their strengths, limitations, and model-dependence; (iii) introduce a practical buildable “design-rules” framework that links clinical scenarios with suitable stimuli, apatite composition, architecture, and release profiles, explicitly connecting fabrication methods to their impact on crystallinity, drug loading, coating stability, and release; and (iv) analyze the readiness of each platform type for clinical translation based on clear criteria, including regulatory status, sterilization behavior, quality standards, and clinical history. The move from single- to multi-stimulus, logic-gated systems is also examined for its potential to improve dosing specificity and the additional complexity involved. Conclusion Viewing the apatite crystal lattice as a key design parameter synthesizes the diverse literature into a unified framework and offers practical guidelines for creating responsive HA carriers and coatings. Overcoming challenges like oversimplified disease models, limited understanding of the protein corona, unknown long-term behavior of responsive elements, and the lack of standardized trigger-response testing aligned with current quality standards will be critical for progress. The framework aims to shift research from passive implants toward demand-driven drug delivery systems, with clinically already-used calcium-based local delivery approaches serving as the most immediate translational opportunity.