To gain insight into the nature of the adhesion mechanism between hydroxyapatite (HA) and rutile (rTiO2), the mutual affinity between their surfaces was systematically studied using density functional theory (DFT). We calculated both bulk and surface properties of HA and rTiO2, and explored the interfacial bonding mechanism of amorphous HA (aHA) surface onto amorphous as well as stoichiometric and nonstoichiometric crystalline rTiO2. Formation energies of bridging and subbridging oxygen vacancies considered in the rTiO2(110) surface were evaluated and compared with other theoretical and experimental results. The interfacial interaction was evaluated through the work of adhesion. For the aHA/rTiO2(110) interfaces, the work of adhesion is found to depend strongly on the chemical environment of the rTiO2(110) surface. Electronic analysis indicates that the charge transfer is very small in the case of interface formation between aHA and crystalline rTiO2(110). In contrast, significant charge transfer occurs b...