Abstract:

Objective Bio-based polypentanediamine adipate (PA56), entirely or partially synthesized from biomass feedstocks, has emerged as environmentally friendly alternative to petroleum-based counterparts and has gained considerable at traction in recent years. PA56 holds significant potential in textiles, food packaging and other fields, by virtue of its good strength and toughness with excellent fatigue resistance. Despite its promising prospects, limited studies have been given to PA56 nanofibers for biomedical applications. Hence, nano ZnO-Ag compound antibacterial agent was prepared and blended with PA56 matrix. ZnO-Ag/PA56 composite nanofiber membranes were then fabricated by electrospinning process for antibacterial applications.

Method The preparation of ZnO-Ag nano particles includes mixing a complex of silver acetate and ammonium hydroxide with nano ZnO dispersions, followed by the addition of formic acid as a reducing agent. The prepared ZnO-Ag particles were blended with PA56 matrix by melt extrusion and ZnO-Ag/PA56 composite nanofiber membranes were then fabricated by electrospinning technology. Morphologies of the ZnO-Ag particles and ZnO-Ag/PA56 nanofiber membranes were characterized with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The influences of encapsulated ZnO-Ag particles on the antibacterial activity, crystallinity, mechanical and wettability properties of the nanofiber membranes were investigated. Furthermore, the cytotoxicity and skin stimulation of the ZnO-Ag/PA56 nanofiber membranes were analyzed.

Results The average sizes of original ZnO particles are about 20-30 nm and the Ag particles embedded in the prepared ZnO-Ag compound are spherical in shape with an average diameter of 6-10 nm, which have been reported to show higher antimicrobial efficacy. Blank PA56 nanofibers have uniform morphology and smooth surface. ZnO-Ag/PA56 nanofibers have round morphology and ZnO-Ag particels distribute in the nanofibers uniformly without any obvious gathering. The average diameter of ZnO-Ag/PA56 nanofibers decreases with the increase of ZnO-Ag content. This is because that ZnO is a semiconductor and Ag is a conductor, which can increase the charge density of the spinning solution and give a strong elongation force to the ejected polymeric jet. When ZnO-Ag mass fraction is 6%, the average diameter of ZnO-Ag/PA56 nanofiber is 401.18 nm, 54.9% of the average diameter of PA56 nanofiber (731.40 nm). The results of inhibition zone test show that blank PA56 nanofiber membranes do not offer any antibacterial ability. ZnO/PA56 nanofiber membranes have antibacterial activity against Staphylococcus aureus, while have not antibacterial ability against Escherichia coli. ZnO-Ag/PA56 nanofiber membranes exhibit good antibacterial effects against Staphylococcus aureus and Escherichia coli. In order to quantify the antibacterial properties of ZnO-Ag/PA56 nanofibers, a standard plate counting approach that tracked bacterial proliferation was performed. After incubation with the bacterial suspension, Staphylococcus aureus and Escherichia coli in ZnO-Ag/PA56 nanofiber groups show lower viability than those in the PA56 groups, where bacterial colonies proliferate extensively. A sharp decrease in colony formation occurres at a mass fraction of 2% ZnO-Ag and the nanofiber membranes exhibit higher inhibition rate against Staphylococcus aureus (73.5%) than Escherichia coli (51.5%). Increasing the content of ZnO-Ag in the nanofibers significantly enhances their antibacterial capability. The nanofiber membranes containing 4% ZnO-Ag effectively suppress the bacteria growth, and the antibacterial rate is 96.1% for Staphylococcus aureus and 84.2% for Escherichia coli. When the mass fraction of ZnO-Ag are 6%, the antibacterial rate reaches 98.7% and 89.3% against Staphylococcus aureus and Escherichia coli, respectively. The NIH-3T3 cells were incubated with extract solution to check the biosafety of ZnO-Ag/PA56 nanofibers. After 48 h incubation, all the cells have normal morphology and are harvested at viability greater than 85%. In order to evaluate the potential skin inflammation caused by ZnO-Ag/PA56 nanofiber meshes, the samples were placed on the back skin of mice. After patch application for 24 h, no evidence of erythema, edema or other changes is found on the skin surface. Moreover, no conspicuous local inflammation or adverse events occurs in viable epidermis and dermis. These results suggest that the ZnO-Ag/PA56 nanofibers are biocompatible and well tolerated by the skin. The crystalline behavior of ZnO-Ag/PA56 nanofiber was characterized by differential scanning calorimetry (DSC). The addition of ZnO-Ag particles in PA56 nanofibers has no significant influence on melting temperature, while the melting peak area increases, indicating the increased crystallinity of the nanofibers with ZnO-Ag mass fraction. This is because that the growing charge density of the spinning solution promotes the stretching and orientation of the ejected polymeric jet. ZnO-Ag particles can also induce heterogeneous crystallization as nucleating agents and increase the crystallinity of polymer. As well known, a higher degree of crystallinity indicates a greater number of macromolecular chain segments being incorporated into the crystalline region, thereby enhancing deformation resistance. Therefore, the tensile strength of ZnO-Ag/PA56 nanofiber membranes increases with ZnO-Ag mass fraction. The blank PA56 nanofiber membranes present an ultimate breaking strength of 8.81 MPa, and that raises to 13.89 MPa with increasing ZnO-Ag mass fracton to 6%. The wettability of the nanofiber membranes was evaluated through water contact angle (WCA) analysis. The surface WCA of PA56 nanofiber membranes is about 88.37°, and the water droplet immerses through the membrane within 21 s. It is known that PA56 possesses hydrophilicity by virtue of the repeating units with polar amide groups (—CONH—) along the polymer chain. Because ZnO-Ag is a polar substance, the incorporation of ZnO-Ag into PA56 nanofibers improves the hydrophilicity of the membranes. When the mass fraction of ZnO-Ag is 6%, the surface WCA of the membrane is about 58.91°, and the water drop disappears quickly within 3 s.

Conclusion ZnO-Ag composite antibacterial agent containing Ag particles with a diameter of 6-10 nm was prepared and ZnO-Ag/PA56 nanofiber membranes were fabricated by electrospinning process. The ZnO-Ag/PA56 nanofibers have round morphology and ZnO-Ag particles distribute in the nanofibers uniformly without any obvious aggregation. With the increase of ZnO-Ag mass fraction, the crystallinity, tensile strength and hydrophilicity of ZnO-Ag/PA56 nanofibers increase, while the average diameter of the nanofibers decreases. The antibacterial rates of ZnO-Ag/PA56 nanofiber membranes containing 6% ZnO-Ag against Staphylococcus aureus and Escherichia coli are 98.7% and 89.3%, respectively. The ZnO-Ag/PA56 nanofiber membranes also have good biocompatibility and skin tolerability, which offers a great potential in biomedical applications.

Key words: bio-based fiber, functional fiber, bio-based polypentanediamine adipate, electrospinning, nanofiber, nano zinc oxide-silver, antibacterial property