Beyond bactericidal surfaces: interfacial selectivity as a design principle for implant biomaterials.

Within seconds of placement, an adsorbed conditioning film replaces the implant's manufactured substrate and becomes the true interface host cells and microorganisms encounter. Conventional strategies engineered to kill or suppress bacteria therefore act on an interface neither party meets, helping explain the inconsistent clinical performance of broadly bactericidal designs. We propose interfacial selectivity, the capacity of a surface to favor beneficial over detrimental interactions, as a unifying design framework for implant biomaterials, and show how interfacial physicochemistry can be programmed to discriminate between colonizers. The implant boundary is viewed as three coupled interfaces: implant-film, film-microbiome, and film-host. Shared drivers, surface energy, charge, hydration, and multiscale topography, govern all three, so modifications intended to deter pathogens also influence commensal colonization, soft-tissue sealing, and osteoimmune balance. A regime map of interfacial forces reinterprets functionalization strategies within a common selectivity space, and a proposed selectivity index, applied here to published data for three archetypal surfaces, reorients evaluation from short-term killing toward the balance between host-beneficial and pathogen-beneficial outcomes. We derive transferable design principles, propose a minimum reporting standard, identify methodological gaps limiting translation, and argue that genomic and multi-omic calibration could enable patient-specific implementation across dental, percutaneous, and orthopedic implants.