Natural Antimicrobial Nano Composite Fibres Manufactured from a Combination of Alginate and Oregano Essential Oil

Abstract

Alginate is a linear biodegradable polysaccharide polymer, which is bio-renewable and widely used for various biomedical applications. For the next generation of medical textiles, alginate nanofibres are desirable for their use in wound dressings that are biocompatible, sustainable, and abundantly available. This study has developed a unique manufacturing process for producing alginate nanofibres with exceptional antimicrobial properties of oregano essential oil (OEO) as a natural antimicrobial agent. OEO with varying degrees of concentration was incorporated in an aqueous alginate solution. Appropriate materials and electrospinning process parameter selection allowed us to manufacture alginate fibres with a range of diameters between 38 and 105 nm. A unique crosslinking process for alginate nanofibres using extended water soaking was developed. Mechanical characterisation using micro-mechanical testing of nonwoven electrospun alginate/oregano composite nanofibres revealed that it was durable. An extensive antimicrobial study was carried out on alginate/oregano composite nanofibres using a range of Gram-positive (methicillin-resistant Staphylococcus aureus (MRSA) and Listeria monocytogenes) and Gram-negative bacteria (Klebsiella pneumoniae and Salmonella enterica), which are common wound and food pathogens. The results indicated that increasing the concentration of OEO from 2 to 3 wt % showed improved antimicrobial activity against all pathogens, and activity was significantly improved against MRSA compared to a non-alginate-based control disk containing OEO. Therefore, our research suggests that all-natural alginate/oregano nanofibre composite textiles offer a new generation of medical textiles for advanced wound dressing technology as well as for food packaging applications.

Keywords: antimicrobial resistance; electrospinning; nanofibres; oregano oil; sodium alginate.

Figure 1

Structure of alginate and egg-box model with guluronic and mannuronic acid.

Figure 2

Simple schematic diagram of electrospinning process. The formation of Taylor cones and transition zone can be clearly seen at the tip of the needle (rapid acceleration) and crosslinking process.

Figure 3

Crosslinking of sodium alginate with calcium chloride.

Figure 4

(a) Electrospun sample sandwiched between the rectangular plates 10mm wide, (b) close-up view of the sample, and (c) sample placed between the upper and lower jaws of the tensile equipment.

Figure 5

Solution without OEO (inset S2) and solution with OEO (inset S3). Viscosity of solution without OEO (line with dark squares, S2) and solution with Oregano (line with diamond shape, S3).

Figure 6

Influence of OEO towards density of electrospun fibres: (a) spinneret showing fibre jet with the spinning solution without OEO and (b) with OEO; (c) the microscopic image of fibre mat without OEO (S2); (d) the microscopic image of fibre mat with OEO (S3).

Figure 7

SEM micrographs—the influence of voltage on fibre morphology: (a) 22 kV; (b) 26 kV; and (c) 30 kV.

Figure 8

SEM of different samples with and without crosslinking: (a) S4—without crosslinking; (b) S5—after crosslinking with CaCl₂, and (c) S6—after crosslinking with calcium chloride and water soak procedure for 24 h.

Figure 9

Nanofibre diameter distribution; SEM micrographs for sample S1; (a–d) different locations of S1 sample.

Figure 10

Mechanical testing of electrospun alginate/OEO fibre mesh: real-time close-up photos of sample (a) fibre mesh crack initiation, (b) crack propagation, and (c) complete failure of sample; (d) the stress–strain plot; (e) typical fibre cross-section.

Figure 11

Antimicrobial efficacy of (a) with OEO (2%) before crosslinking, (b) with OEO (2%) after crosslinking, (c) with OEO (3%) crosslinked after water soak, (d) with additional OEO, (e) OEO control against (A) MRSA, (B) K. pneumoniae, (C) S. enterica, and (D) L. monocytogenes. Error bars represent standard error of the mean of n = 3 biological replicates. **** p ≤ 0.0001, *** p ≤ 0.001, ** p ≤ 0.01, * p ≤ 0.05.