In situ nitric oxide generating nano-bioactive glass-based coatings on implantable silicone devices towards attenuating implant-associated fibrosis and infection

Introduction: Silicone based materials are commonly used in fabrication of implantable devices owing to their bio-inertness, anti-adhesiveness, and low cytotoxicity. They have been used as short-term implants in form of catheters and ports as well as long term implants including defect fillers and vascular stents. Though being a versatile biomaterial, silicone-based devices are still challenged by complications relating to implant associated infections and fibrous capsule formation. Almost 5% of implants develop chronic infection and a 10.6% incidence rate of fibrous capsulation is seen in silicone implants [1,2]. Mesoporous bioactive glasses obtained through sol-gel synthesis offer the advantage of doping the SiO2 network formers with network modifiers of other metal oxides such as CaO, P2O5, CuO, and SrO which attribute multifunctional traits. Herein, bioactive glass modified with either copper (70SiO2.20CaO.5CuO.5P2O5) or strontium (70SiO2.20CaO.5SrO.5P2O5) to fabricate an in situ nitric oxide generating interface on silicone substrates by a facile dip coating technique. The substrates were then investigated for NO-generating properties in addition to their antibacterial, antithrombotic effects and cell-instructive roles on human fibroblasts, endothelial cells, and macrophages.

Materials and

Methods: The sol-gel derived nano-bioactive glass (BG) particles were synthesized using a modified Stöber’s method. For the catalytic generation of NO, copper and strontium were introduced as network modifiers oxides within the tetrahedra silicate network, connected by –Si–O–Si– bridging oxygen bonds. The generation of NO through the catalytic activity of Cu or Sr on the surface of the substrate was measured by the infusion of glutathione (GSH) and GSNO into a 10 mM PBS solution at 37 °C using a chemiluminescence Nitric Oxide Analyzer (NOA) 280i (Zysense, Frederick, CO). The coated substrates were assessed for antibacterial ability using 24 h bacterial assay against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The cytocompatibility of the substrates and fibrosis assay were performed using human fibroblast cells. Additionally, the immunomodulatory effect of NO generated from the NOgen interface on macrophages was tested over a period of 10 days.

Results, Conclusions, and Discussions: Both Cu and Sr doped bioactive glass samples (Cu-BG and Sr-BG) showed significant NO generating abilities compared to unmodified BG, establishing the first-ever report of NO generating ability of Sr. The physiologically relevant amount of NO generated by the interfaces demonstrated antibacterial properties with ~ 1 log reduction in planktonic E. coli and S. aureus in addition to antithrombotic properties (Figure 1). The low dose of Strontium leaching from the Sr-BG films in tandem with NO demonstrated excellent cytocompatibility and anti-fibrotic activity by suppressing collagen expression. Additionally, when the leachates from Sr substrates were exposed to naïve human macrophages, M2 phenotype bias was observed, exemplifying the anti-inflammatory behavior.
The facile modification of silicone rubber substrates by dip coating them with CuBG and SrBG helped in generating physiologically relevant levels of NO. NOgen interfaces exhibited bactericidal and antithrombotic properties. The strontium ion from leachates suppressed the collagen expression and migration of activated fibroblasts. The NO generated from NOgen interfaces regulated the macrophages bestowing an immunomodulatory effect. Altogether, the multifunctional NOgen interfaces developed here serve as a valuable strategy in combating implant-associated infections and attenuating implant-associated fibrosis noted in silicone-based implantable devices.

References:
1. P. S. Stewart, T. Bjarnsholt, Clinical Microbiology and Infection 2020, 26, 1034.
2. S. Farah, J. C. Doloff, P. Müller, A. Sadraei, H. J. Han, K. Olafson, K. Vyas, H. H. Tam, J. Hollister-Lock, P. S. Kowalski, M. Griffin, A. Meng, M. McAvoy, A. C. Graham, J. McGarrigle, J. Oberholzer, G. C. Weir, D. L. Greiner, R. Langer, D. G. Anderson, Nature Materials 2019, 18, 892.

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