The genesis of citrated ultrathin hydroxyapatite nanorods.

Ideal orthopedic biomaterials should replicate both the hierarchical structure and exceptional mechanical strength of natural bone. Traditional polymer-hydroxyapatite composites, typically limited up to 40 wt % hydroxyapatite, offer only modest mechanical improvements. Efforts to enhance strength by using stiffer polymers have largely failed, as increased polymer stiffness does not translate to improved composite mechanics. In contrast, natural bone's load-bearing capability arises from the synergy between citrate, soft collagen, and ultrathin hydroxyapatite nanocrystals (~3 nanometers). Here, we show that elastic poly(octamethylene citrate) enables up to 60 wt % hydroxyapatite incorporation, mimicking the bone's mineral content. Through a top-down "citrification" process and hot pressing, hydroxyapatite microparticles are partially dissolved and recrystallized into superthin (~5 nanometers) nanorods, enhancing organic-inorganic integration and replicating bone's Ca/P ratios and architecture. The resulting composites exhibit compressive strengths exceeding 250 megapascals, unprecedented in polymer-mineral systems, offering a molecular design strategy for next-generation load-bearing orthopedic implants.