Enhanced Biocompatibility and Oxygen Generation of Titanium Nanopillars via Layer-by-Layer Assembly and Calcium Peroxide Deposition

Quasi-aligned TiNPs plates (50-100 nm in diameter and 1-2 μm in length) were fabricated on titanium (Ti) plates using a hydrothermal synthesis method (Fig. 2A, fig. S1). Consequently, the surface area of TiNPs increased by a factor of 1.65 in comparison to the respective bare Ti plates (fig. S2). Subsequently, the TiNPs plates were immersed in a PA-Zn2+ coordination solution to facilitate the growth of a PA-Zn thin film in a layer-by-layer (LBL) manner (Fig. 2B). Incrementing the PA concentration from 0.015 to 0.45 mg/mL resulted in top-view SEM images that indicated no significant alteration in the overall nanopillar structure following film growth (fig. S3). Following a single growth cycle, an island-type nucleation stage prevailed on the TiNPs, while subsequent cycles led to the formation of relatively uniform thin films with thicknesses of approximately 10 nm and 24 nm after 3 and 5 cycles, respectively (Fig. 2C). Additionally, an EDS elemental analysis of Ti, P, O, and Zn substantiated the existence of a core-sheath structure in the PA-Zn@TiNPs (Fig. 2D). XPS results (fig. S4) demonstrated that PA-Zn coordination coating using the LBL approach can achieve a controllable thickness of PA-Zn film on nanopillars.

To impart the ability to generate oxygen to PA-Zn@TiNPs, CaO2 nanoparticles (nano-CaO2) were synthesized and subsequently deposited onto 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). Nano-CaO2, which appears yellowish, rapidly reacts with water upon contact with an aqueous solution (CaO2 + H2O = Ca(OH)2 +1/2 O2), yielding a substantial quantity of O2 bubbles (fig. S5B), thus demonstrating the excellent O2-generating capability of the synthesized nano-CaO2. Following the in-situ deposition of nano-CaO2 onto PA-Zn@TiNPs (fig. S5C), SEM and EDS mapping of Ca and O elements revealed a relatively uniform distribution for loading contents of nano-CaO2 below 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). Furthermore, the 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). The water contact angle decreased from 97.5 ± 1.2o for the bare Ti plate to 48 ± 1.6o for the TiNPs plate. Due to the hydrophilic nature of PA molecules, a remarkable shift towards superhydrophilicity (32) was observed on the TiNPs coated with a PA-Zn thin film after a single growth cycle, as evidenced by a water contact angle of 4.6 ± 1.2o. Nonetheless, the increased thickness of the PA-Zn film obtained through repeated growth cycles and the additional deposition of nano-CaO2 had minimal impact on the surface wettability.

Moreover, when rat bone marrow mesenchymal stem cells (BMSCs) and RAW264.7 cells were incubated on PA-Zn@TiNPs plates, outstanding biocompatibility was observed, although a slight inhibitory effect was noticed for the 5-circle PA-Zn@TiNPs (figs. S6, A and B). Furthermore, the deposition of 50 μg/cm2 of nano-CaO2 onto the PA-Zn@TiNPs resulted in a slight increase in cytotoxicity for the CaO2/PA-Zn@TiNPs.