CaO2/PA-Zn@TiNPs: Synthesis, Characterization, and Biocompatibility for Oxygen Generation

The synthesis, characterization, and biocompatibility of CaO2/PA-Zn@TiNPs

Quasi-aligned TiNPs plates (50-100 nm in diameter and 1-2 μm in length) were fabricated on titanium (Ti) plates by a hydrothermal synthesis method (Fig. 2A, fig. S1). As a result, the surface area of TiNPs increased 1.65 folds, as compared to corresponding bare Ti plates (fig. S2). The TiNPs plates were then immersed in a PA-Zn2+ coordination solution to grow PA-Zn thin film in a LBL manner (Fig. 2B). The concentration of PA-Zn2+ coordination complex was tuned to optimize the surface morphologies of PA-Zn@TiNPs. When the PA concentration increased from 0.015 to 0.45 mg/mL, the top-view SEM images showed that the overall nanopillar structure remained unchanged after film growth (fig. S3). The thickness and element distribution of the PA-Zn film on TiNPs plates were studied in detail using TEM and energy dispersive spectrometer (EDS) element-mappings. After 1-circle growth, an island-type nucleation stage dominated on TiNPs, whereas relatively uniform thin films with a thickness of ~10 nm and ~24 nm were obtained after 3 and 5 circles, respectively (Fig. 2C). Furthermore, EDS elements analysis of Ti, P, O, and Zn confirmed a core-sheath structure of PA-Zn@TiNPs (Fig. 2D). XPS results (fig. S4). Results demonstrated that PA-Zn coordination coating using the LBL approach can achieve a controllable thickness of PA-Zn film on nanopillars.

To endow PA-Zn@TiNPs with the capability of generating oxygen, CaO2 nanoparticles (nano-CaO2) were synthesized and deposited on the surface of PA-Zn@TiNPs. TEM images and EDS element-mapping of Ca and O confirmed successful synthesis of nano-CaO2 with a diameter of approximately 60 nm (fig. S5A). The yellowish nano-CaO2 reacts with water as soon as it came into contact with an aqueous solution (CaO2 + H2O = Ca(OH)2 +1/2 O2) and produced a large amount of O2 bubbles (fig. S5B), indicating excellent O2-generating ability of the as-prepared nano-CaO2. After in-situ deposition of nano-CaO2 onto PA-Zn@TiNPs (fig. S5C), SEM and EDS mapping of Ca and O elements indicated a relatively uniform distribution when the loading content of nano-CaO2 was less than 100 μg/cm2. Moreover, most nano-CaO2 particles clustered on the sidewalls of the PA-Zn@TiNPs nanopillar, suggesting that additional deposition of a small amount of nano-CaO2 does not blunt the sharp tip of the nanopillar (Fig. 2A). In addition, O2 release profile was recorded using a portable dissolved oxygen meter. As shown in Fig. 2F, O2 was released in a concentration-dependent manner, which peaked at 4 hr and sustained for 24 hr. Such a releasing profile meets oxygen requirement in the early phase after implant surgery (6).

The surface wettability was evaluated by measuring the static water contact angle (Fig. 2G). While the bare Ti plate had a water contact angle of 97.5 ± 1.2o, this angle decreased to 48 ± 1.6o for TiNPs plate. Since PA molecules are hydrophilic, PA-Zn thin film coated TiNPs after one growth circle exhibited an intriguing water affinity, with a water contact angle of 4.6 ± 1.2o, indicating the strong transition to superhydrophilicity (32). However, increased thickness of PA-Zn film, as achieved via repeated growth circles of PA-Zn coating, and additionally deposited nano-CaO2 had marginal effects on the surface wettability.

Furthermore, incubation of rat bone marrow mesenchymal stem cells (BMSCs) and RAW264.7 on PA-Zn@TiNPs plates showed excellent biocompatibility, though a slight inhibitory effect was observed for 5-circle PA-Zn@TiNPs (figs. S6, A and B). Besides, the cytotoxicity mildly increased for CaO2/PA-Zn@TiNPs when 50 μg/cm2 nano-CaO2 was deposited.