Loading-Dependent Copper Release from Calcium-Deficient Hydroxyapatite-Copper(II) Oxide Composite Pellets: An In Vitro Physicochemical Study

The narrow balance between copper deficiency and excess in ruminants motivates the evaluation of candidate materials for gradual copper release. Calcium-deficient hydroxyapatite-copper(II) oxide (HA-CuO) composite pellets with nominal Cu loadings (HA-10Cu, HA-15Cu, and HA-25Cu) were prepared and physicochemically characterized. X-ray diffraction confirmed the coexistence of HA and CuO phases, while increasing CuO loading reduced the specific surface area from 77 m2 g−1 for HA nanoparticles to 43, 30, and 29 m2 g−1 for the pellets. In vitro copper release, expressed as Cu2+ equivalents, was quantified by UV-Vis spectrophotometry over 35 days in a simplified aqueous medium under conditions approximating baseline rumen physiological parameters (pH 6.5; 39 °C, triplicate). Cumulative release reached ≈ 15.5, 24.0, and 42.5 mg pellet−1, corresponding to 78%, 80%, and 85% of the nominal initial Cu content, respectively. Absolute release increased with CuO loading, whereas fractional release remained comparable among formulations. Empirical fits to the Higuchi and Korsmeyer-Peppas models were compatible with contributions from liquid penetration, dissolution, pore transport, surface interactions, and matrix alteration, without establishing a unique release mechanism. These findings demonstrate prolonged, loading-dependent copper release and establish a physicochemical basis for subsequent evaluation in biologically representative ruminal media and in vivo studies.