Self-Oxygenating Nanopillar Surface to Prevent Implant Associated Infection and Enhance Osseointegration

Characterized by persistent infection, chronic inflammation, and poor osseointegration, implant associated infection (IAI) are the most common cause of early implant failure in orthopedic surgery (1-3). A number of immune cells are involved in the development of IAI, such as neutrophils, monocytes, and lymphocytes. Among them, neutrophils are generally regarded as the major contributor of host defense against infection, particularly at the early stage of IAI (4, 5). The inability of neutrophils to perform bacterial clearance in a timely and effective manner is one of the main reasons for the onset of IAI (6, 7). As neutrophil recruitment to the site of infection takes a few hours, bacteria may adhere to implant surface, develop bacterial aggregates, and eventually form biofilm (6). Even a small nascent bacterial aggregate tolerates phagocytosis and clearance, which is termed 'head start' effect (6, 8). Biofilm can further complicate treatments as extracellular polymeric substances-encapsulated bacteria are substantially resistance to antibiotics and innate immunity (9-11). Upon engaging pathogens, neutrophils couple phagocytosis with the respiratory burst, a highly oxygen-dependent manner to initiate antimicrobial activities (6, 12). Due to surgical trauma and lack of vascularization on the implant surface, however, the peri-implant microenvironment usually is hypoxic, which may impair neutrophil bactericidal effect (13, 14). In addition, hypoxia can extend the lifespan of human neutrophils from approximately 19 hours to several days (15), which significantly prolongs inflammatory state and delays osseointegration. Therefore, we speculated that targeting biofilm formation and the hypoxic microenvironment can rescue compromised neutrophil function and reduce implant failure (14, 16).

The state-of-the-art self-oxygenation of antibacterial nanomedicines have been extensively designed and applied to control infections. Hu et al. developed an oxygen nanocarrier to alleviate biofilm-induced hypoxia, which facilitate antibiotics to eradicate skin infection (17); Xiao et al. used oxygen self-supporting antibacterial nanoparticles to fuel oxidative stress to intracellular bacteria for the treatment of intracellular infection in septic arthritis (18). However, whether the synergistic use of anti-biofilm activity and oxygen delivery would promote the neutrophil functions to orchestrate resolution of infection and inflammation in IAI remains largely unknown. In addition, intravenous or intracavitary/local drug delivery may be suitable for the treatment of extant infection but is not ready for the prevention.

To address these challenges, implant surface engineering has emerged as a promising strategy to mitigate orthopedic implant failures (19-21). Coating the implant with antibiotics, inorganic/organic compounds and nanoparticles has become a common method for reducing implant infections (22). Recently, we have developed phytic acid (PA)-metal coordinated complexes-coated implant with antibacterial, immunomodulatory, and osseointegration activities were integrated effectively (23-25). Yet, the trade-off between efficacy and cytotoxicity remains. Inspired by the unique antibacterial properties of cicada wings, the presence of high-aspect-ratio nanopillars on surface is an alternative strategy to kill bacteria upon direct contact by mechano-bactericidal mechanism (26, 27). However, this strategy becomes ineffective once few bacteria manage to attach, which will eventually develop into a mature biofilm (28). Therefore, combining mechano-bactericidal effects of nanopillar array with chemical mediators, such as antibacterial agents and self-supporting materials, to form mechano-chemical coupling could offer a synthetic approach to inhibit biofilm and alleviate hypoxic conditions, which in turn promote neutrophils to eradicate infection and turnover.

Herein, we designed a metal-coordinated nanopillar surface coupled with self-oxygenation to prevent IAI and enhance osseointegration (Fig. 1). In this work, we first develop an efficient strategy to fabricate PA-Zn2+ coordinated complexes-coated TiO2 nanopillars (PA-Zn@TiNPs) with controllable thickness by using a layer-by-layer (LBL) deposition approach (29, 30). The PA-Zn@TiNPs exhibit synergistic bacterial anti-adherence and mechano-chemical bactericidal activities, thus inhibit biofilm formation. In addition, PA-Zn@TiNPs simultaneously induce hydroxylapatite formation and M2-like polarization of macrophage to promote osseogenesis. Moreover, we have introduced calcium dioxide nanoparticles (nano-CaO2) onto PA-Zn@TiNPs (forming CaO2/PA-Zn@TiNPs) owing to their favorable biocompatibility, high oxygen content, and sustained oxygen release capacity (31). The hydrolysis of nano-CaO2 on the surface can generate oxygen, which orchestrates intracellular sterilization and apoptosis of neutrophils under hypoxic conditions, eventually contributing to inflammation subsiding. In a rat model, we revealed that the CaO2/PA-Zn@TiNPs can facilitate infection clearance, neutrophil turnover and osseointegration. This biomimetic surface-engineering strategy, tailored to the regeneration of an optimal bone-implant interface, can be mechano-chemically reprogrammed to adapt to the hypoxia microenvironment and host immunomodulation, rather than only targeting bacteria.