Atomic-scale ordering enables intrinsic bioactivity and rapid osseointegration in medium-entropy alloys

Conventional alloys achieve mechanical reliability and corrosion resistance at the expense of bioactivity, necessitating exogenous surface treatments that introduce long-term interfacial instability. In this study, we demonstrate that atomic-scale ordering, specifically the coexistence of chemical short-range order (CSRO) and ordered oxygen complexes (OOCs), can intrinsically integrate these otherwise contradictory requirements within a single metallic system. This strategy exploits the principle that atomic arrangements governing bulk mechanical behavior concurrently regulate surface physicochemical properties, thereby eliminating the need for extrinsic modification. A Ti-30Zr-14Nb-3O medium-entropy alloy (MEA) designed with CSRO-OOCs synergy exhibits bone-matched elastic modulus (∼42 GPa) and high yield strength (∼1040 MPa), overcoming the conventional strength-modulus trade-off. Importantly, this atomic configuration intrinsically generates nanoscale surface roughness (Ra ∼50.80 nm) and optimized surface energy distribution-features typically achieved only through artificial surface engineering. These ordering-regulated surface characteristics promote accelerated osteogenic differentiation in vitro without additional surface treatment. Molecular dynamics simulations reveal enhanced Ca²⁺ and PO₄ ^3- adsorption on OOC-enabled surfaces, facilitating early calcium phosphate nucleation. In a rat cranial defect model, this intrinsic bioactivity translates into rapid, continuous bone formation along the implant surface, achieving a bone surface (BS) value of 70.63 ± 21.35 mm² at 4 weeks, which was 3.5 times greater than that of the control group and significantly exceeds that of TC4. This work establishes atomic-scale ordering as a promising design framework for integrating mechanical compatibility with enhanced spontaneous osteoconductivity in metallic biomaterials.