• Antimicrobial coatings
• Electrospinning
• Medical devices
• Mesoporous bioactive glass nanoparticles
• Nanotechnology
• Polyhydroxyalkanoates

The rise of medical device (MD) use has increased medical waste and CO2 emissions, along with nosocomial and biomaterial-associated infections (BAI), causing an increment in antibiotic- resistant species. Coating of biomaterial surfaces could be used to provide MDs with antimicrobial properties. Antimicrobial coatings aim to fight antibiotic abuse and thus bacterial resistance, reduce medical waste and infection risks, and enhance MD durability. Among several techniques to produce surface coatings, electrospinning (ELS) is a possible method to produce polymeric coatings in the form of ultrafine fibers, which can be combined with bioactive nanoparticles.

This PhD study focused on the use of green polymers, which help to reduce the environmental impact especially of disposable MDs, to create fibrous coatings through ELS. Polyhydroxyalkanoates (PHAs) are an emerging family of in-water and in-soil biodegradable polymers, showing versatile properties and obtained via bacterial fermentation, thus are considered sustainable and even superior to polylactic acid. Here, the commercially available copolymer polyhydroxybutyrate-co- hydroxyvalerate (PHBV) and the laboratory-synthesized short-chain (scl) and medium-chain (mcl) PHAs were used to produce ELS coatings, which was tested to cover standard polymers used for percutaneous MDs, like polyurethane (PU), latex (LNG) and silicone rubber (SiR), used in catheters.

To confer antimicrobial activity, different nanoparticles (NPs) were assayed: metal-based NPs, namely silver NPs (AgNPs) and gold NPs (AuNPs), and glass-ceramic NPs, namely, mesoporous bioactive-glass nanoparticles (MBGNs). The latter type is based on the SiO2-CaO compositional system and was prepared by a modified sol-gel process. MBGNs were additionally doped with Zn2+ and/or Cu2+ ions to further enhance the antimicrobial activity. The NPs were characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and antimicrobial assays against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) microorganisms. MBGNs showed a spherical shape and nanometric size (69-100 nm), with elements like Si, Ca, Zn, and Cu identified by EDX. FT-IR analysis confirmed the SiO2-CaO network, while XRD revealed amorphous MBGNs, except for one copper-variant, which showed higher crystallinity. Ion-doped MBGNs tested at 1 mg/ml and 5 mg/ml, displayed a dose-dependent activity, especially towards S. aureus. Zinc-containing MBGNs were the most effective at both concentrations against this bacterial species.

To obtain electrospun fiber/NP composite systems, to be used as a coating, two different methods were applied: (1) AgNPs and AuNPs suspended in their native solution were deposited onto PHBV fibers using chitosan as a functionalization agent, and (2) MBGNs were directly incorporated within the PHA solution and electrospun. Scl-PHA and mcl-PHA were more soluble and processable than PHBV for ELS. The scl-PHA/mcl-PHA blend produced more uniform and reproducible fibers compared to PHBV, obtaining fibers with consistent morphology and sizes ranging in 3.09 - 4.29 μm. Therefore, the composite system (2) was carried on for further evaluations.

MBGNs were incorporated into PHA fibers during the ELS process with various concentrations and the obtained composite coating was characterized using SEM, energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FT-IR) spectroscopy. SEM analysis confirmed the incorporation of MBGNs at varying concentrations. Moreover, wettability analysis, ion release tests and cytocompatibility were investigated. Wettability analysis showed hydrophobic surfaces with contact angles ranging in 128° - 139°, with no significant changes after incorporating MBGNs into PHAs fibers. Ion release studies revealed that MBGNs embedded in the fibers released significantly higher amounts of Zn2+ and Cu2+ compared to free MBGNs in static conditions. Cytocompatibility assays on human dermal keratinocytes (HaCaT cells) and Human Umbilical Vein Endothelial Cells (HUVECs) demonstrated no significant cytotoxicity, suggesting potential for use in percutaneous MD coatings. Preliminary antimicrobial tests demonstrated that MBGNs doped with Zn2+ and Cu2+ diminished microbial proliferation, suggesting their potential efficacy. However, further investigations are necessary to substantiate these findings for possible clinical applications. Using ELS, PHAs fibers containing 25% (w%) of MBGNs succeeded in being directly deposited onto PU enteral tubes and LNG catheters; however, a poor deposition was achieved on SiR catheters, likely due to low conductivity. In fact, SEM and stereomicroscopic analyses confirmed uniform fiber distribution on PU and LNG, while SiR showed inadequate coverage, especially MBGNs doped with antimicrobial ions.

In conclusion, ELS of scl-PHA/mcl-PHA ultrafine fibers functionalized with MBGNs is a promising approach for creating antimicrobial coatings. Doped MBGNs, and especially with Zn2+ ions, exhibited a marked antibacterial activity compared to undoped MBGNs. Such an effect was especially evident against the S. aureus, one of the major species involved in BAI. ELS of scl- PHA/mcl-PHA with MBGNs produced effective coatings on PU and LNG, but SiR, which may require an optimization of the process to reach a better adhesion between the coating and the surface.
Future work should center on the optimization of MBGNs dispersion, the enhancement of reproducibility, and the assessment of long-term stability and antimicrobial properties of the coating against clinical isolates using MD prototypes.