Biomimetic and antibacterial coatings

Spinal orthopedic infections, with incidence rates of 15–20%, pose a significant challenge. To prevent this complication, metal-based antibacterial coatings are widely used. However, current coatings have several drawbacks, with possible toxicity or impaired osteointegration. This study aimed at evaluating the use of nanostructured silver-based coatings to provide antibacterial efficacy to 3D printed custom-made porous spine prostheses. Coatings were obtained by Ionized Jet Deposition, which guarantees nanostructuring and nanoscale thickness, both of which help avoid toxicity. To further mitigate interference with bone regeneration, the silver (Ag) was mixed with bone apatite (Bone). Deposition technique was also optimized for future industrial scale-up, and application to a real-scale prosthesis is demonstrated. Our results showed that all the films were nanostructured and maintained the same composition as the target. The real-size device was effectively coated, also in the inner areas, potentially discouraging microbial contamination onto the entire device. Ag and Ag-Bone films demonstrated remarkable in vitro efficacy against gram-positive and gram-negative bacteria, with no observed cytotoxicity. Both Ag and Ag-Bone apatite films showed significant bacterial inhibition activity also in an in vivo model infected with the Methicillin-Resistant Staphylococcus aureus USA 300 strain, demonstrating their promising future applications for tackling infection associated with spine devices.
Keywords: Spine surgery; Bone implants; Infection; MRSA; Silver; Ionized jet deposition; Custom-made prostheses; Orthopedics

Promoting skin regeneration and preventing infection are crucial aspects of wound healing applications. Current strategies include the use of fibrous matrices, fabricated by electrospinning, to support tissue regeneration, and the incorporation of silver (Ag), known for its antibacterial properties, to prevent infections. Here, Melt Electrospinning Writing (MEW), an innovative additive manufacturing technique, was used to fabricate porous polycaprolactone (PCL) meshes. For the first time, a silver nanocoating was deposited onto a thermos-sensitive MEW substrate by using a Physical Vacuum Deposition (PVD) method, specifically Ionized Jet Deposition. To optimize antibacterial efficacy, minimizing the potential damage to the substrate, different coating parameters were investigated. The effect of Ag deposition on morphology and composition of PCL MEW meshes was studied using scanning electron microscopy (SEM), stereomicroscopy, microtomography (μCT) and Fourier Transform InfraRed (FT-IR) spectroscopy. The MEW meshes were characterized by 500 μm squared pores with a fiber diameter around 8 μm and a porosity of 90 %. Potential detrimental effects of Ag coating on the 3D-printed MEW structures were evaluated as a function of deposition time, with no structural alteration up to 60 min. Antibacterial activity was assessed against Escherichia coli and Staphylococcus aureus strains, while cytocompatibility was confirmed by using human dermal fibroblasts. A complete antibacterial effect against E. coli cells was achieved with a deposition time > 30 min. Significant inhibition of S. aureus growth was also observed for all tested Ag coatings. The possibility to fabricate porous MEW meshes with Ag nanocoatings opens new possibilities to design innovative scaffolds for wound infection management.
Keywords: Nanostructured coatings; Wound healing; Melt Electrospinning Writing; Antibacterial materials

Calcium phosphates are widely studied in orthopedics and dentistry, to obtain biomimetic and antibacterial implants. However, the multi-substituted composition of mineralized tissues is not fully reproducible from synthetic procedures. Here, for the first time, we investigate the possible use of a natural, fluorapatite-based material, i.e., Lingula anatina seashell, resembling the composition of bone and enamel, as a biomaterial source for orthopedics and dentistry. Indeed, thanks to its unique mineralization process and conditions, L. anatina seashell is among the few natural apatite-based shells, and naturally contains ions having possible antibacterial efficacy, i.e., fluorine and zinc. After characterization, we explore its deposition by ionized jet deposition (IJD), to obtain nanostructured coatings for implantable devices. For the first time, we demonstrate that L. anatina seashells have strong antibacterial properties. Indeed, they significantly inhibit planktonic growth and cell adhesion of both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. The two strains show different susceptibility to the mineral and organic parts of the seashells, the first being more susceptible to zinc and fluorine in the mineral part, and the second to the organic (chitin-based) component. Upon deposition by IJD, all films exhibit a nanostructured morphology and sub-micrometric thickness. The multi-doped, complex composition of the target is maintained in the coating, demonstrating the feasibility of deposition of coatings starting from biogenic precursors (seashells). In conclusion, Lingula seashell-based coatings are non-cytotoxic with strong antimicrobial capability, especially against Gram-positive strains, consistently with their higher susceptibility to fluorine and zinc. Importantly, these properties are improved compared to synthetic fluorapatite, showing that the films are promising for antimicrobial applications.

Abstract: Reconstruction of gradient organic/inorganic tissues is a challenging task in orthopaedics. Indeed, to mimic tissue characteristics and stimulate bone regeneration at the interface, it is necessary to reproduce both the mineral and organic components of the tissue ECM, as well as the micro/nano-fibrous morphology. To address this goal, we propose here novel biomimetic patches obtained by the combination of electrospinning and nanostructured bone apatite. In particular, we deposited apatite on the electrospun fibers by Ionized Jet Deposition, a plasma-assisted technique that allows conformal deposition and the preservation in the coating of the target’s stoichiometry. The damage to the substrate and fibrous morphology is a polymer-dependent aspect, that can be avoided by properly selecting the substrate composition and deposition parameters. In fact, all the tested polymers (poly(l-lactide), poly(D,l-lactide-co-glycolide, poly(ε-caprolactone), collagen) were effectively coated, and the morphological and thermal characterization revealed that poly(ε-caprolactone) suffered the least damage. The coating of collagen fibers, on the other hand, destroyed the fiber morphology and it could only be performed when collagen is blended with a more resistant synthetic polymer in the nanofibers. Due to the biomimetic composition and multiscale morphology from micro to nano, the poly(ε-caprolactone)-collagen biomimetic patches coated with bone apatite supported MSCs adhesion, patch colonization and early differentiation, while allowing optimal viability. The biomimetic coating allowed better scaffold colonization, promoting cell spreading on the fibers.
Keywords: Nanostructured coatings; Electrospinning; Biomimetism; Bone regeneration; Bio-interfaces