Silver-thiol-hydrogel coating for endotracheal tubes: a pharmacological model for controlled release of antimicrobial and mucolytic drugs towards prevention of ventilator-associated pneumonia

Abstract

The development and clinical implementation of biomedical devices have revolutionised patient care by enabling advanced diagnostic, therapeutic, and life-sustaining interventions. However, the introduction of a foreign material into the body inevitably provides a surface highly susceptible to bacterial colonisation and biofilm formation, leading to device-associated infections that contribute significantly to morbidity, mortality, and healthcare costs. Among these, ventilator-associated pneumonia (VAP) represents one of the most severe complications, arising from bacterial colonisation of endotracheal tubes (ETTs). VAP is responsible for a considerable proportion of hospital-acquired pneumonia cases and is associated with mortality rates exceeding 30% in intensive care units. This risk is compounded by the global rise of multi-drug-resistant pathogens and the limitations of current antimicrobial coatings, which often provide only short-lived efficacy, encourage bacterial adaptation or fail to sustain antimicrobial activity over the duration of intubation. Addressing these challenges requires innovative approaches that provide long-term antimicrobial activity and help prevent biofilm formation.This thesis reports the systematic design, development, and evaluation of a multilayer hydrogel-based coating applied to silver-thiol coated polyvinyl chloride (PVC) surface, the standard substrate used in ETT manufacture. The coating was engineered to function as a sustained drug delivery platform, capable of localised release of antimicrobial and mucolytic agents over clinically relevant timeframes. PVC substrates were first chemically pre-treated, coated with a silver layer, and then functionalised with allyl mercaptan (AM) to provide thiol groups for hydrogel attachment. Contact angle measurements confirmed saturated thiol coverage at 2 mM AM, while adhesion and robustness tests demonstrated stable hydrogel anchoring with >85% coating retention under mechanical stress, compared to <20% in untreated PVC. Hydrogels composed of poly (2-hydroxyethyl methacrylate) (p[HEMA]) and copolymers incorporating 10% and 25% methyl methacrylate (MMA) were synthesised, producing structurally stable and lubricious coatings. Release studies with sodium fluorescein as a model drug revealed that increasing MMA concentration extended-release duration from 8 to 14 days, aligning with the maximum period of clinical intubation.Building on this foundation, the system was loaded with levofloxacin, a DNA gyrase inhibitor, and benzalkonium chloride (BAK), a membrane-disrupting cationic surfactant. Comparative release experiments revealed that the AM-Ag-PVC substrate promoted a denser hydrogel network that slowed diffusion relative to free-standing hydrogel discs, thereby prolonging drug release. Furthermore, dual loading of levofloxacin and BAK produced slower release kinetics than either agent alone, due to zwitterionic and electrostatic interactions between the negatively charged carboxylate group of levofloxacin and the positively charged quaternary ammonium group of BAK. These interactions created transient complexes that increased resistance to diffusion and modified hydration dynamics within the hydrogel, providing extended drug release profiles over the intubation time.Microbiological evaluation of the dual-drug coatings demonstrated antimicrobial performance against the two pathogens most associated with VAP, Staphylococcus aureus and Pseudomonas aeruginosa. Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and minimum biofilm eradication concentration (MBEC) assays confirmed potent antibacterial effects, while checkerboard analysis revealed strong synergy between levofloxacin and BAK, with fractional inhibitory concentration index (FICI) values of 0.066 and 0.056, respectively. This synergy, arising from the complementary mechanisms of DNA replication inhibition and membrane disruption, translated into profound reductions in bacterial survival. Time-kill studies confirmed rapid bactericidal action, and adherence assays revealed complete inhibition of bacterial colonisation, with reductions exceeding six log compared to untreated PVC. These results underscore the dual-drug hydrogel coatings as a highly effective strategy for preventing biofilm formation on ETTs.To further enhance the functionality of the system, the mucolytic agent N-acetylcysteine (NAC) was incorporated into the hydrogel coatings to address the clinical complication of mucus accumulation in intubated patients. NAC was successfully loaded into the hydrogel layer coated on the AM-Ag-PVC surfaces and released in a biphasic profile over 14 days. Rheological assessment of artificial sputum demonstrated significant reductions in viscosity, with the p[HEMA]-AM-Ag-PVC–NAC system achieving a 58.28% reduction by day 14, closely approximating the effect of free NAC (60.67%). In contrast, hydrogels with higher MMA content released NAC less efficiently, reflecting the impact of hydrophobicity on swelling and drug mobility. These findings highlight the feasibility of integrating mucolytic therapy into ETT coatings, offering a means to improve secretion clearance and reduce ventilator-associated complications.Cytotoxicity assessments provided critical insights into the biocompatibility of the developed materials. While silver-functionalised PVC displayed noticeable cytotoxicity due to uncontrolled ion release, subsequent thiol modification substantially mitigated this effect. Hydrogel coatings further improved cytocompatibility, with fibroblast viability consistently exceeding the ISO 10993-5 threshold of 70%. Increasing MMA content, however, produced a concentration-dependent decline in viability, linked to greater surface hydrophobicity and mechanical stiffness. Drug loading revealed differential effects: levofloxacin induced moderate reductions in cell viability (50–60%), BAK exhibited pronounced cytotoxicity (<20% viability), and dual levofloxacin–BAK systems mirrored the dominant toxic profile of BAK. In contrast, NAC-loaded formulations preserved fibroblast viability above 70%, demonstrating their dual role as mucolytic and cytoprotective agents. Overall, the results confirm that hydrogel layer on AM-Ag-PVC reduces the inherent cytotoxicity of silver-modified PVC and that biocompatibility is strongly modulated by hydrogel composition and therapeutic loading.In conclusion, this thesis describes the development of a multifunctional multilayer hydrogel layer on AM-Ag-PVC platform that addresses several major challenges associated with endotracheal tube. The system demonstrated stable and durable surface modification, sustained release of antimicrobial and mucolytic agents, synergistic antimicrobial activity with complete inhibition of bacterial colonisation, effective reduction of sputum viscosity, and improved cytocompatibility relative to unmodified silver-PVC. These integrated outcomes establish the platform as a promising next-generation coating technology for endotracheal tubes, with the potential to reduce the incidence of ventilator-associated pneumonia, improve secretion management, and enhance patient outcomes in critical care.