“…Recently, Sr-based radiopaque agents, especially the degradable species, have attracted more and more attention as they can simultaneously improve the radiopacity and bone repair capability of bone implants [ 30 , 31 , 32 , 34 , 35 , 36 , 37 ]. Various Sr-containing species such as SrCO 3 , SrX 2 , (X = F, Cl, Br, I), strontium ranelate (SrR), and strontium-substituted hydroxyapatite (SrHAp, maximum 100% Sr 2+ ) have been evaluated and show great X-ray absorption when they are added into bone implants [ 30 , 31 , 32 , 34 , 35 , 36 , 37 ]. Furthermore, most of them are soluble (SrCl 2 and SrBr 2 ) or have relatively high solubility (SrCO 3 , SrHAp, and SrR) and can continuously release Sr 2+ , which is able to enhance osteoblast activity and inhibit osteoclast activity [ 38 , 39 , 40 ].…”