Biofunctionalization of Textile Materials. 3. Fabrication of Poly(lactide)-Potassium Iodide Composites with Antifungal Properties

The paper presents a method of obtaining poly(lactide) (PLA) nonwoven fabrics with antifungal properties using potassium iodide as a nonwoven modifying agent. PLA nonwoven fabrics were obtained by the melt-blown technique and subsequently surface modified (PLA→PLA-SM-KI) by the dip-coating method. The analysis of these PLA-SM-KI (0.1%–2%) composites included Scanning Electron Microscopy (SEM), UV/VIS transmittance, FTIR spectrometry and air permeability. The nonwovens were subjected to microbial activity tests against Aspergillus niger fungal mold species, exhibiting substantial antifungal activity. The studies showed that PLA-KI hybrids containing 2% KI have appropriate mechanical properties, morphology and demanded antimicrobial properties to be further developed as a potential antimicrobial, biodegradable material.


Introduction
Poly(lactide) (PLA) presents multifunctional applications in various medical areas. PLA hybrids with antibacterial additives (bactericide agents) display antiseptic properties, and therefore are applied in a variety of medical applications [1]. Thus, PLA itself presents a bioactive nanostructured matrix for controlled drug delivery or potential tissue engineering [1,2]. PLA hybrids with various ionic nanoparticles (e.g., PLA-Ag [3][4][5], PLA-CuSiO 3 [6], PLA-ZnO [7] and/or PLA-TiO 2 [8]) have strong antibacterial properties and can therefore be applied in biomedical and food packaging areas. PLA hybrids with antibiotics and drugs (PLA-ampicilline and PLA-metronidazole [9]; PLA-doxycycline [10]; PLA-chlorhexidine [11,12]; PLA-triclosan [12][13][14][15]) are a potential drug delivery system for use in periodontal and endodontic infections, treatment of persistent infections in medicine and dentistry, and also suitable for bone tissue engineering. As one group of such potential additives/components also halogen-and/or halogen-based compounds can be used-the most significant microbicides applied in the clinic and used traditionally for both antiseptic and disinfectant purposes [16].
Iodine is a highly effective topical antimicrobial agent that has been used clinically in the treatment of wounds for more than 170 years. It has a broad spectrum of antimicrobial activity with efficacy against bacteria, mycobacteria, fungi, protozoa and viruses and can be used to treat both acute and chronic wounds [16][17][18]. A unique feature of molecular iodine is certainly its ability to bind to polymeric materials. A variety of natural and synthetic polymers develop complexes when treated with molecular iodine, or with a mixture of molecular iodine and potassium iodide [19][20][21][22][23][24]. Iodophores-a group of disinfectants containing iodine, were developed in the 1950s to overcome side effects associated with side effects associated with elemental iodine. These were found to be safer and less painful, but just as effective as elemental iodine, allowing widespread use [18]. Starch iodophores can be used for technical and medical applications as the means of disinfection and components in cosmetics. Iodine can be bound to starch by forming an inclusion complex with amylose [19][20][21][22] or it can be immobilized into modified starch derivatives. The representative iodophores are shown in Figures  1-4.   [19].
Coatings 2020, 10, x FOR PEER REVIEW 2 of 17 side effects associated with elemental iodine. These were found to be safer and less painful, but just as effective as elemental iodine, allowing widespread use [18]. Starch iodophores can be used for technical and medical applications as the means of disinfection and components in cosmetics. Iodine can be bound to starch by forming an inclusion complex with amylose [19][20][21][22] or it can be immobilized into modified starch derivatives. The representative iodophores are shown in Figures  1-4.   [24].

Poly(lactide) Nonwoven Fabrics
Melt-Blown Technique PLA nonwovens were made by the melt-blown technique using a single-screw laboratory extruder (Axon, Sweden) with a head with 30 holes of 0.35 mm diameter each, compressed air heater and collecting drum. Dried to constant weight, PLA granules for melt-blown processing were fed into the extruder hopper. In a one-step process in which high-velocity air blows molten thermoplastic PLA polymer resin out of an extruder, a fibrous and self-bonding web is formed on the collecting drum. Nonwoven samples were made in the form of a sheet. The processing parameters for the production of PLA nonwoven are presented in Table 1. Coating pastes homogeneously dispersed and of suitable viscosity (about 60-70 dPas) were prepared on the basis of styrene-acrylic resin (styrene-acrylic ester copolymer (PS-PA)), wetting agent (polyethylene glycol (PEG)), thickening agent (polyacrylate, ammonium salt (PA)) and aqueous solutions of KI (0.1%, 1% or 2% aqueous solution). The components of the pastes of PLA surface modifier (SM) used, and corresponding modifier abbreviations are given in Table 2. The nonwoven samples (10 cm × 10 cm; 1.02 ± 0.05 g) were impregnated with the paste, squeezed and dried for 8 h at 60 • C (to constant weight:1.14 ± 0.05 g). After modification, the nonwoven composites formed, assigned further as PLA-SM-KI (and PLA-SM for a composite of PLA and the coating paste without KI, respectively), presented visually uniform, homogeneous structure. The estimated compositions of so formed PLA-SM-KI composites were as follows: PLA = 87.7%; PS-PA = 8.4%; PA = 1.4%; PEG = 4.2% and KI=0 to 0.28%.

SEM/EDS-Scanning Electron Microscopy/Energy-Dispersive X-ray Spectroscopy
The microscopic analysis of fibers was performed on a Tescan Vega 3 scanning electron microscope (Brno, Czech Republic) with the EDS Oxford Instruments (Abingdon, UK) X-ray microanalyzer. The SEM microscopic examination of the surface topography was carried out under high vacuum using energy of the probe beam 20 ekV. The surface of each preparation was sprayed with a conductive substance (gold), using a vacuum dust extractor (Quorum Technologies Ltd., Lewes, UK). Magnification was from 500× to 20,000×.

ATR-FTIR-Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy
ATR-FTIR spectra were recorded at room temperature using a Jasco 4200 spectrometer (Tokyo, Japan) with Pike GladiATR attachment (Cottonwood, AZ, USA), in transmission mode in the range of 400-4000 cm −1 .

UV-VIS Analysis
Changes in the physical properties such as the transmittance [%T] of poly(lactide) nonwoven fabrics before (PLA and PLA-SM) and after KI incorporation (PLA-SM-KI) were assessed using a Jasco V-550 double-beam UV-VIS spectrophotometer with integrating sphere attachment in the range of 200-800 nm.

Filtration Parameters
Air permeability was determined for one layer of the nonwoven sample and the test based on EN ISO 9237:1998 standard [55]. An FX 3300 TEXTEST AG (Klimatest, Poland) permeability tester was used. Air at a pressure of 100 Pascal and 200 Pascal was passed through a fabric area of 20 cm 2 diameter for testing. An average of 10 values was taken to be the final value of the sample.

Tensile Testing
Tensile testing of PLA, PLA-SM and PLA-SM-KI was carried out in accordance with EN ISO 10319:2015-08 standard [56]. A Tinius Olsen H50KS tester (Horsham, PA, USA) was used. Stretching speed was 20 mm/min.

Antifungal Activity
The antifungal activity of the resulting nonwoven fabrics was tested according to EN 14119: 2005 standard [57], against Aspergillus niger van Tieghem. Specimens of the tested material were placed on agar plates: samples of sterile PLA discs (20 mm) were charged with coating pastes with various amounts of potassium iodide (Table 3) and then the discs with PLA-SM-KI composites were placed on inoculated agar (pH:6.2) and incubated at 29 • C for 14 days. The agar was inoculated with the selected fungus. Both sides of the nonwoven fabrics were tested. The level of antifungal activity was assessed by examining the extent of fungal growth: in the contact zone between the agar and the specimen, on the surface of specimens and, if present, the extent of the inhibition zone around the specimen. All tests were carried out in duplicate. Simultaneously, the same tests were carried out for control samples-samples of unmodified PLA nonwoven.

SEM/EDS-Scanning Electron Microscopy/Energy-Dispersive X-ray Spectroscopy
Scanning electron microscopy (SEM) is a routine technique for morphological tests of poly(lactide) nonwovens, including both electrospun PLA [58,59] as well as melt-blown PLA [60][61][62][63] fibers. SEM micrographs of uncoated PLA fibers, coated without KI (PLA-SM) fibers and coated with 2% potassium iodide (PLA-SM-KI (2%)) fibers are presented in Figures 5-7, respectively.  SEM images of PLA nonwovens show uniform, randomly oriented fibers, with interconnected pores (space) between nanofibers and a relatively smooth surface. The average diameters of PLA fibers applied ranged from 1.4 to 8.5 µm (Figure 5c). Morphological changes in PLA after surface deposition of a modifier (PLA→PLA-SM), observed in Figure 6, illustrate the formation of fiber conglomerates with a rough surface. SEM images of PLA covered with a surface modifier and KI (PLA-SM-KI (2%)) additionally reveal some dots on the modifier surface, presumably KI crystals (e.g., Figure 7c).
Results of the EDS analysis of PLA, PLA-SM and PLA-SM-KI are presented in Table 3. The content of carbon and oxygen components of PLA (without hydrogen) is similar to the atomic "bulk"  Figure 6, illustrate the formation of fiber conglomerates with a rough surface. SEM images of PLA covered with a surface modifier and KI (PLA-SM-KI (2%)) additionally reveal some dots on the modifier surface, presumably KI crystals (e.g., Figure 7c).
Results of the EDS analysis of PLA, PLA-SM and PLA-SM-KI are presented in Table 3. The content of carbon and oxygen components of PLA (without hydrogen) is similar to the atomic "bulk" analysis of PLA (C = 50.0% and O = 44.4%). The surface modification of PLA using a surface modifier (the mixture of PS-PA (60%), PEG (30%) and PA(NH 4 ) (10%) leads to the appearance of a more carbonaceous layer. The EDS analysis of PLA-SM-KI (2%) hybrid exhibits the appearance of KI in the analyzed sample (ca. 1.6%) corresponding to the KI content calculated in Table 1 (KI content in the hybrid assuming full water removal during paste drying). The results of carbon (decrease in carbon content~3%) and oxygen (increase by 1.36%) can be obtained from the appearance of KI (1.6%) and the presence of residual water in the layer of the surface modifier.

UV/VIS Transmittance Spectra
The UV/VIS transmittance spectra T = f (λ) (T [%] vs. λ [nm]) of PLA, PLA-SM and PLA-SM-KI (0.1-2%) fibers/composites were recorded. The spectra of PLA, PLA-SM and PLA-SM-KI (2%) fibers/composites are presented in Figure 10. Appearance of peaks at 251 and 349 nm, resulting from the surface modification of starting polylactide. These peaks are the same in PLA-SM and PLA-SM-KI, and therefore can be assigned to polymeric components of the coating paste (SM).

Technical Parameters
Filtration parameters expressed by air permeability were detected for (PLA), and nonwovens coated with a surface modifier (PLA-SM) and a surface modifier with potassium iodide (PLA-SM-KI) ( Table 6). The parameters were measured in triplicate and presented as a mean value with ± deviation of about 1%.
The results of air flow resistance of modified poly(lactide) showed that: A significant decrease of air permeability in the investigated fibers (from 905 mm/s (for PLA) to about 428 Pa (for PLA-SM) at 100 Pa, and from 1640 mm/s (for PLA) to 825 Pa (for PLA-SM) at 200 Pa, respectively); Negligible effect of potassium iodide content in the coating paste on the filtration properties of synthesized PLA-SM-KI composites.
These data lead to the conclusion that the observed decrease in PLA-modified air permeability is caused by the formation of a new surface layer on PLA (PLA-SM), derived from the applied surface coating mixture (PA, PS-PA, PEG) ( Table 2).
The results of the tests, including tensile strength [kN/m] and relative elongation at maximum Appearance of peaks at 251 and 349 nm, resulting from the surface modification of starting polylactide. These peaks are the same in PLA-SM and PLA-SM-KI, and therefore can be assigned to polymeric components of the coating paste (SM).

Technical Parameters
Filtration parameters expressed by air permeability were detected for (PLA), and nonwovens coated with a surface modifier (PLA-SM) and a surface modifier with potassium iodide (PLA-SM-KI) ( Table 6). The parameters were measured in triplicate and presented as a mean value with ± deviation of about 1%.
The results of air flow resistance of modified poly(lactide) showed that: A significant decrease of air permeability in the investigated fibers (from 905 mm/s (for PLA) to about 428 Pa (for PLA-SM) at 100 Pa, and from 1640 mm/s (for PLA) to 825 Pa (for PLA-SM) at 200 Pa, respectively); Negligible effect of potassium iodide content in the coating paste on the filtration properties of synthesized PLA-SM-KI composites.
These data lead to the conclusion that the observed decrease in PLA-modified air permeability is caused by the formation of a new surface layer on PLA (PLA-SM), derived from the applied surface coating mixture (PA, PS-PA, PEG) ( Table 2).
The results of the tests, including tensile strength [kN/m] and relative elongation at maximum load [%] of PLA nonwoven fabrics and PLA-SM-KI composite are given in Table 7. The results of tensile strength tests revealed: The improvement of technical properties for PLA-SM-KI in comparison with PLA, namely a distinct increase in tensile strength (0.30 to 0.112 (kN/m)) and slow in relative elongations (10.0% to 11.0%), respectively; The negligible effect of KI concentration on the tensile strength of the modified samples; A new composite PLA-SM-KI has a more flexible and stronger structure than unmodified PLA fabrics; The changes in tensile strength between the PLA-SM and PLA are a result of the incorporation of a layer of the modifying paste (SM) in the PLA fiber structure (Figure 6), which causes higher stability and strength of the PLA-SM-KI composites.
Amounts of 1-2% of potassium iodide in coating pastes (PLA-SM-KI (1-2%)) provided the antimicrobial properties for Aspergillus niger, expressed by the lack of visible growth under a microscope (50× magnification). PLA-SM exhibits strong growth covering the entire surface of the control sample (Table 7, Figure 11). Additionally, the applied surface modification of the PLA fibers did not affect any tested mechanical properties of PLA-SM-KI hybrids, synthesized. These results are comparable with those for an effective iodide formulation for killing Bacillus and Geobacillus spores (optimum for 0.075 mM (~1.4%) of KI; pH = 0.3) presented by Kida et al. [38], however in mild conditions and on the PLA coating surface.
The results of antifungal investigations of PLA-MS-KI revealed that: PLA-SM nonwoven fabrics (without KI) exhibited strong growth-covering of the entire surface of the control sample; Incorporation of potassium iodide into the nonwoven structure (PLA→PLA-MS→PLA-MS-KI) provided antifungal properties of the new composite ( At the same time, the surface modification of the nonwovens with potassium iodide (PLA→PLA-MS→PLA-MS-KI) did not show any impact on the mechanical properties of the modified textile.

Conclusions
Medicine, especially at present, is focused on the search for new and more effective methods of combating pathogens and antigens such as viruses, bacteria and fungi. In recent years there has been growing interest in biodegradable, "eco-friendly" and multifunctional polymers that can be used in selected biomedical applications. This paper presents a method for functionalization of poly(lactide) nonwoven fabrics by a potassium iodide modifier. The PLA-SM-KI's are eco-friendly composites, and easily biodegradable PLA consists as their major component (ca. 88%). The structural properties of these new products have been characterized by Scanning Electron Microscopy, Attenuated Total These results are comparable with those for an effective iodide formulation for killing Bacillus and Geobacillus spores (optimum for 0.075 mM (~1.4%) of KI; pH = 0.3) presented by Kida et al. [38], however in mild conditions and on the PLA coating surface.
The results of antifungal investigations of PLA-MS-KI revealed that: PLA-SM nonwoven fabrics (without KI) exhibited strong growth-covering of the entire surface of the control sample; Incorporation of potassium iodide into the nonwoven structure (PLA→PLA-MS→PLA-MS-KI) provided antifungal properties of the new composite (Table 7: Sample 1 vs. 2, 3, 4); PLA-MS-KI with KI ≥ 1% exhibited antimicrobial properties for Aspergillus niger, expressed by no visible growth under the microscope (50× magnification); At the same time, the surface modification of the nonwovens with potassium iodide (PLA→PLA-MS→PLA-MS-KI) did not show any impact on the mechanical properties of the modified textile.

Conclusions
Medicine, especially at present, is focused on the search for new and more effective methods of combating pathogens and antigens such as viruses, bacteria and fungi. In recent years there has been growing interest in biodegradable, "eco-friendly" and multifunctional polymers that can be used in selected biomedical applications. This paper presents a method for functionalization of poly(lactide) nonwoven fabrics by a potassium iodide modifier. The PLA-SM-KI's are eco-friendly composites, and easily biodegradable PLA consists as their major component (ca. 88%). The structural properties of these new products have been characterized by Scanning Electron Microscopy, Attenuated Total Reflectance Fourier-transform Infrared and Ultraviolet-Visible Spectrophotometry, their technical/application feature has been positively verified during their air permeability tests and tensile testing. The revealed antifungal activity against Geobacillus spores makes them compatible with the literature antimicrobials, based on iodide/iodine. Important features of the coating process presented include easy implementation on an industrial scale, possibility to quickly start a simple production line and low production costs. All materials used in this work are commercially available and relatively cheap. The pronounced antimicrobial potency of the composite materials presented suggests that PLA-SM-KI should be considered as an antiseptic agent in a wide spectrum of biomedical applications.