Fabrication of biodegradable silicon nitride 3D printed nanocomposite with enhanced mechanical and antimicrobial properties for bone tissue repair

Thermoplastics, such as polyetheretherketone (PEEK) and titanium implants, are used in medical and dental fields. However, there are frequent infection and implant rejection cases by the body. In the past decade, the use of metal nanoparticles has gained increasing popularity as an alternative treatment for minimizing microbial infections, leading to innovations in orthopedic surgery and wound healing. Recent studies have introduced new biomaterial bone substitutes, including advanced ceramics with innovative structural, biological, and mechanical properties. Nonetheless, these materials have limitations, as they are neither biodegradable nor biocompatible and lack inherent antimicrobial properties. Advanced materials like silicon nitride (Si₃N₄) exhibit enhanced osteogenic potential, toughness, and antimicrobial characteristics, providing added functionalities when fabricating 3D-printed implants for bone tissue regeneration, thus addressing the limitations of currently utilized materials. This research employed a patented electrodeposition process to coat magnesium oxide (MgO) nanoparticles on the outer surfaces of halloysite nanotubes (HNTs) to incorporate additional antimicrobial properties. Gentamicin sulfate was vacuum-loaded into the lumen of the MgO-coated HNTs. Si₃N₄ was combined with the gentamicin-loaded MgHNT to promote cell adhesion and differentiation, after which the resulting composite was 3D printed into the required shapes according to the testing protocol. Fourier Transformation Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) images confirmed the presence of magnesium on halloysite nanotubes, thereby verifying the successful coating of MgO on HNT. Cytotoxicity tests indicated that the fabricated nanocomposites were not toxic to mammalian cells. Antimicrobial activity was assessed against Escherichia coli and Staphylococcus aureus, revealing significant inhibition of bacterial growth in all fabricated nanocomposites. Cell proliferation assays demonstrated that Si₃N₄ enhanced proliferation in all nanocomposites. The porosity test showed that the addition of Si₃N₄ does not affect the porosity or cell attachment of the fabricated nanocomposites, while histological staining revealed increased calcium and mucopolysaccharides. The nanocomposite exhibited excellent mechanical properties, including higher flexural strength, hardness, and a lower contact angle after surface coating with protein. Nanocomposites degraded slowly during the biodegradation test, facilitating the growth of new bone cells. Si₃N₄ enhances the roughness structure of the cellular surface, hydrophilicity, and protein adsorption capability.