Facile Surface Modification of Apatite Nanoparticles for Tailoring Protein Adsorption toward Nano-Bio Interfacial Engineering

Hydroxyapatite (HA) nanoparticles have been widely used as bone-filling materials because of their excellent biocompatibility and osteoconductivity. However, they exhibit limited osteoinductivity and induce slight blood coagulation. To address these issues, it is essential to control the underlying biological phenomenon (i.e., mediative protein states), particularly the adsorption states of fibronectin (Fn) and fibrinogen (Fg) on the HA nanoparticles. In this study, we aimed to endow the HA nanoparticles with both osteoinductive and antithrombotic properties through their surface modification technique at the nanometer scales. Specifically, carbonate-containing hydroxyapatite (CHA) nanoparticles were immersed in a weakly acidic phosphate buffer (PB) solution, which induced the phase transformation of the surface nanolayer via solid-liquid interfacial reactions. In particular, upon immersion in PB, initially, the crystalline structure near the CHA surface nanolayer was disrupted, followed by transformation into an amorphous phase and subsequent recrystallization into a calcium-deficient hydroxyapatite (CDHA) phase. The adsorption states of proteins on the surfaces of CHA and CDHA and their functions were evaluated using a monitoring of quartz crystal microbalance with dissipation (QCM-D), antibody-based assays, and platelet adhesion assays. These results clearly demonstrated that immersion in PB allowed for effective control over the adsorption states of Fn and Fg, leading to the alternation of the subsequent biological responses. These findings indicated that the HA nanoparticles developed in this study possess both osteoinductive and antithrombotic properties by a surface modification technique. This strategy offers a facile surface modification technique that enables the fine-tuned regulation of biological responses under the very mild condition and can be applied for the surface modification of apatite-based implants.