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Preparation and properties of electrospun poly(vinyl alcohol)/silver fiber web as wound dressings.

INTRODUCTION

An important and growing part of the textile industry is the medical and related healthcare and hygiene sector. The extent of the growth is due to constant improvements and innovations in both textile technology and medical procedures. Textile materials and products that have been engineered to meet particular needs, are suitable for any medical and surgical application where a combination of strength, flexibility, and sometimes moisture and air permeability are required [1].

Electrospinning technology is a simple and low-cost method for making ultrathin diameter fibers. The electrospinning process is that a polymer solution or melt is placed into a syringe with a millimeter-size nozzle and is subjected to electric fields of several kilovolts. Under the applied electrostatic force, the polymer is ejected from the nozzle, whose diameter is reduced significantly as it is transported to and deposited on a collector, which also serves as the ground for the electrical charges. Recently, the ultrafine fiber webs prepared by the electrospinning process have been extensively studied because of their unique properties such as high surface area-to-volume ratio, small pore sizes, high porosity, and so on [2]. In particular, the incorporation of therapeutic compounds into the electrospun nanofibers has attracted a great deal of attention, because the resultant nanofiber webs have very strong efficacy of the drug due to their high surface area-to-volume ratio, and the composite electrospun nanofiber webs afforded the prospect of preparing useful polymer systems for controlled release of the activity [3].

On the other hand, silver (Ag) ions and Ag compounds have been widely used in various biomedical fields, such as wound dressing materials, body wall repairs, augmentation devices, tissue scaffolds, antimicrobial filters, and so on. It was reported that Ag inactivate microbes as following mechanisms; (1) Ag interacts with enzymes and proteins important for the bacterial respiration and the transport of important substances across the cell membrane and within the cell; (2) Ag interacts with DNA, thereby inhibiting cell division; (3) Ag ions are bound to the cell wall and outer cell, thereby altering the functionality of the cell membrane [4]. It was also referred that microorganisms with resistance to the antimicrobial activity of Ag are exceedingly rare [5]. On the other hand, poly(vinyl alcohol) (PVA) is a polymer that has been studied intensively because of its good film forming and physical properties, high hydrophilicity, processability, biocompatibility, and good chemical resistance. Theses properties have led to its broad industrial use and the PVA properties can be improved or modified by importing other composite [6]. Above all things, the PVA is a water soluble polymer, nontoxic and free of danger, and so it can be safely used in medical fields.

Therefore, in this study, the PVA/Ag composite ultrafine fiber webs were prepared by an electrospinning process for applying the webs as wound dressings. It was analyzed the antimicrobial activity and release behavior of [Ag.sup.+] ions of the electrospun PVA/Ag fiber webs. It was also tried to reduce the [Ag.sup.+] ions in the electrospun PVA/AgN[O.sub.3] fiber webs by the various reductive methods.

EXPERIMENTAL

Materials and Preparation

PVA, 98-99% hydrolyzed granule (Typical [M.sub.w] 85,000-124,000), and AgN[O.sub.3] (99.9999%) was purchased from Aldrich, Korea. To prepare the ultrafine PVA fibers containing Ag nanoparticles, a PVA/AgN[O.sub.3] aqueous solution was electrospun into nonwoven webs. The electrospinning setup used in this study consist of a hypodermic syringe, plastic tip, graphite electrode (black lead), aluminum collecting drum, and high voltage supply (Chungpa EMI, Korea). A syringe pump connected to the hypodermic syringe controlled the flow rate. The polymer solutions were electrospun at a positive voltage of 20 kV, a tip-to-collector distance of 11.5 cm, and a solution flow rate of 0.66 ml/h.

Reduction of [Ag.sup.+] ions

To stabilize the electrospun PVA/AgN[O.sub.3] fiber webs and reduce the [Ag.sup.+] ions therein, the electrospun fiber webs were finished by heat treatment. The electrospun PVA/AgN[O.sub.3] fiber webs were placed in the empty space between the hot plates of a hot press (Namyang Machinery MFG, Korea) for 3 min; however, the electrospun PVA/AgN[O.sub.3] fiber web should not be in contacted with any hot plates directly. The plates were set at 150[degrees]C and placed ca. 1 cm apart. The photoreduction of the electrospun PVA/AgN[O.sub.3] fiber webs was treated under a ultraviolet (UV) lamp (UV-A: 315-380 nm, 10 W; Sankyo, Japan) for 3 h.

Characterization

A field emission scanning electron microscope (FE-SEM) (JOEL JSM-6340F, Japan) was used for high-magnification observation and transmission electron microscopy (TEM) images were obtained with a Philips CD 200 transmission electron microscope (Japan) using the samples deposited on carbon coated copper grids. The X-ray photoelectron spectroscopy (XPS) measurements were performed with an Aries ARSC 10MCD 150 spectrometer (Surface Science Instruments, UK). This spectrometer was equipped with an Mg-K[alpha] X-ray source, set at 10 kV and 15 mA. The pressure in the analyses chamber was maintained at [less than or equal to] [10.sup.-9] Torr during the measurement. All core-level spectra were obtained at a photoelectron takeoff angle of 55[degrees] with respect to the sample surface. In peak analyses, the line widths (the width at half-maximum) of the Gaussian peaks were kept constant for the components in a particular spectrum. X-ray diffraction (XRD) patterns were registered using a D8 Advance (Bruker, Germany) employing Cu-K[alpha], 1.54187 [Angstrom] radiation. The release behavior of [Ag.sup.+] ions extracted from the electrospun PVA/Ag fiber webs were measured by a Direct Reading Echelle inductively coupled plasma (ICP)-Atomic Emission Spectrometer (SHIMADZU/ICPS-1000IV, Japan) for 48 h. This experiment was conducted in deionized water whose pH was adjusted to pH 4.5 with a solution of HN[O.sub.3], which was shaken in a shaking water bath to simulate the local in vivo release at ca. 37[degrees]C. The electrospun PVA/AgN[O.sub.3] fiber webs (respectively 0.15 g) were put into the individual containers each containing 30 ml of deionized water. Aliquots were withdrawn from these solutions at fixed time intervals of 1, 2, 5, 10, and 24 h for determination of [Ag.sup.+] release and the equivalent volumes of fresh deionized water were replaced into the containers after each sampling to maintain constant medium volume. The antimicrobial activities of the electrospun PVA fiber webs with and without AgN[O.sub.3] were tested against Staphylococcus aureus (ATCC 6538P) and Klebsiella pneumoniae (ATCC 4352) by the standard testing method for antibacterial of textiles (KS K 0693:2001). The agar plates containing test samples and the control (blank) were incubated at 37[degrees]C for 18 h. The reductions of bacteria were calculated according to the following Eq. 1,

Reduction (%) = [(B - A)/B] x 100 (1)

where A and B are the surviving cells (cfu/ml) for the plates containing test samples and the control, respectively. Also, the immediate antimicrobial effects of the electrospun PVA/AgN[O.sub.3] fiber webs were simply tested by observing the microbial colony shape. Pseudomonas aeruginosa (ATCC 15442) was injected on the surface of the electrospun PVA/AgN[O.sub.3] fiber webs whose sizes were ca. 1 [cm.sup.2], and the electrospun PVA/AgN[O.sub.3] fiber webs were left for 1, 5, 10, and 15 min, respectively. And the contaminated surfaces of the electrospun PVA/AgN[O.sub.3] fiber webs were dipped on the nutrient agar in petri dish and eliminated immediately, and then the microbes stained on the nutrient agar were cultivated for 18 h. The color measurements were performed on a Macbeth Spectrophotometer Color-Eye 3100 (Macbeth, USA). Reference illuminant was D65 (standard daylight) and geometry was Diffuse/8[degrees] (illumination/measurement): incident was diffuse and observation angle was 10[degrees], according to the CIE 15.2 publication and ISO 7724/1 recommendations. Data were reported in the CIE L*a*b* colorimetric system.

RESULTS AND DISCUSSION

Preparation of Electrospun PVA/AgN[O.sub.3] Fiber Webs

Because of the various advantages of PVA polymer, the preparation of electrospun PVA nanofiber webs have been tried and investigated by many researchers so far. However, when AgN[O.sub.3] was dissolved in the PVA polymer solution, the electrospinning phenomenon was somewhat different with those of the general PVA electrospinning process. That is to say when some metal contacted with the PVA/AgN[O.sub.3] aqueous solution, the metallic Ag ([Ag.sup.0]) or Ag oxides ([Ag.sub.2]O) generated and grew on the surface of the metal, and finally formed the Ag dendrite. They finally precipitated in the polymer solution and closed up the opening of metal needle. It was caused by that the [Ag.sup.+] ions were inclined to be electrochemically reduced into metallic forms by metals such as copper (Cu), chrome (Cr), nickel (Ni), and iron (Fe). For that reason, both the stainless needle and a copper electrode should be avoided in this electrospinning apparatus. Therefore, a plastic tip and a graphite electrode were introduced to the electrospinning equipment in this study. However, the reduction of [Ag.sup.+] ions in the PVA/AgN[O.sub.3] aqueous solution would be generated by itself even slowly when without any metals, because the presence of trace amounts of organic material (in this case PVA) promotes photoreduction of [Ag.sup.+] ions [7]. So the electrospun PVA/AgN[O.sub.3] fiber webs were immediately prepared after preparing the PVA/AgN[O.sub.3] aqueous solution. The electrospun PVA/AgN[O.sub.3] fiber webs were prepared from a PVA (10 wt%)/AgN[O.sub.3] (0.1 wt%) aqueous solution by an electrospinning process. They were prepared into three types of electrospun PVA/AgN[O.sub.3] fiber webs; One was electrospun from the PVA/AgN[O.sub.3] solution treated with UV radiation (pre-UV treated), and another was electrospun from a PVA/AgN[O.sub.3] solution and then the electrospun fiber web was treated with UV radiation (post-UV treated), and the last one was electrospun from a PVA/AgN[O.sub.3] solution and that was not treated any UV radiation during its preparation process (no-UV treated).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

On the other hand, when the electrospun PVA fiber web was immersed in water, the webs would instantaneously shrink and become a clear, gelatinous material because PVA is water soluble polymer. Thus, the unique nanofibrous structure of the electrospun fiber web would be lost in aqueous environments, such as wound areas in skin. Therefore, it was necessary to crosslink the PVA polymer and stabilize the electrospun fiber webs in wet condition. PVA can be chemically crosslinked with a variety of substances including glutaraldehyde, acetylaldehyde, and formaldehyde [8, 9]; however, the chemicals are toxic, and so the electrospun PVA fiber web crosslinked by them would not compromise biocompatibility. Therefore, in this study, the electrospun PVA/AgN[O.sub.3] fiber webs were heat-treated at 155[degrees]C for 3 min because it was found that the heat treatment was superior method to stabilize the electrospun PVA fibrous structure against dissolution in water, as shown in Fig. 1. It was plausible that crosslinks could be formed between two hydroxyl groups by losing a [H.sub.2]O at high temperatures. It was further confirmed by XRD analyses, as shown Fig. 2. It shows the diffraction scans for the PVA films with heat treatment at 155[degrees]C for 3 min (Fig. 2a) and without any heat treatment (Fig. 2b). Both the samples exhibit one main peak around 2[theta] = 20[degrees], corresponding to the (101) plane of PVA semicrystalline structure [10]. However, the peak intensity of the heat treated PVA film show more increase and some shift to higher region than that of the no heat treated PVA film. Also, it was observed that the long rang order peak (around 2[theta] = 12[degrees]) and new higher order peak (around 2[theta] = 23[degrees]) in the curve of the heat treated PVA film. All of these results obviously illustrated that the crystallinity of the PVA polymer is increased after heat treatment.

[FIGURE 3 OMITTED]

Morphologies and Surface Analyses of Electrospun PVA/AgN[O.sub.3] Fiber Webs

The neat electrospun PVA fiber web as well as the electrospun PVA/AgN[O.sub.3] fiber webs were prepared to compare the appearance differences between them. All the colors of the AgN[O.sub.3] containing PVA composite electrospun fiber webs were changed from white tone to light yellow tone during the preparation process. It indicates that [Ag.sup.+] ions in the electrospun PVA/AgN[O.sub.3] fibers were reduced and aggregated into Ag nanoparticles [5] after the heat treatment as well as UV radiation. In particular, before the heat treatment, the parameters L*, a*, b* of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web were 92.862, 0.138, 4.857, respectively. However, after the heat treatment, the parameters L*, a*, b* of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web were changed to 86.600, 0.699, 22.489, respectively. Therefore, Lee et al. [11] reported that Ag nanoparticles can be reduced by UV, [gamma]-rays, ultrasound, prolonged reflux, or chemicals; however, through this study, it was found that the heat treatment could also reduce the [Ag.sup.+] ions to Ag particles.

Figure 3 shows the SEM images of the neat PVA electrospun fibers, no-UV treated electrospun PVA/AgN[O.sub.3] fibers, pre-UV treated electrospun PVA/AgN[O.sub.3] fibers, and post-UV treated electrospun PVA/AgN[O.sub.3] fibers, respectively. Ag particles were observed on all the surface of AgN[O.sub.3] containing electrospun PVA fiber webs. However, significant larger particles (diameter: ca. 107 nm) were observed on the surface of the pre-UV treated electrospun PVA/AgN[O.sub.3] fibers, whereas fine particles (diameter: ca. 11 nm) were observed on the surface of the no-UV treated and post-UV treated electrospun PVA/AgN[O.sub.3] fibers. It was assumed that early reduced Ag particles in the PVA/AgN[O.sub.3] solution were much more grown and aggregated freely with each other in the fluid environment. On the other hand, it was observed that there were more Ag particles on the surface of the post-UV treated electrospun PVA/AgN[O.sub.3] fibers than those on the surface of the no-UV treated electrospun PVA/AgN[O.sub.3] fibers, as shown in Fig. 4. It was presumably thought that the post-UV radiation strongly reduced the [Ag.sup.+] ions abundantly existed on the surface of fibers. Also, it was observed that the Ag nanoparticles were formed preferentially on the surface of the electrospun PVA/AgN[O.sub.3] fibers because [Ag.sup.0] as well as [Ag.sup.+] ions migrated to the surface of the electrospun fibers following the solvent (water), which diffused to the outer part of the nanofiber during the evaporation process. It was confirmed by the Son's results [5].

[FIGURE 4 OMITTED]

Figure 5 shows the XPS spectra of [O.sub.1s] and Ag regions for the electrospun PVA/AgN[O.sub.3] fiber webs. The [O.sub.1s] photoemission spectrum of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web was shifted to a higher energy than those of the UV treated electrospun PVA/AgN[O.sub.3] fiber webs. The shift to higher energy indicates that the hydroxyl oxygen atoms in the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web have lower electronic density than those in the UV treated electrospun PVA/AgN[O.sub.3] fiber webs. It was assumed that [Ag.sup.+] ions in the no-UV treated composite electrospun fiber web remained more abundant, and so more formed the oxides by bonding with the hydroxyl oxygen atoms of PVA than those in the UV treated electrospun PVA/AgN[O.sub.3] fiber webs. It was confirmed that the [Ag.sup.0]3d peaks of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web were not significant compared to those of the UV treated electrospun PVA/AgN[O.sub.3] fiber webs. On the other hand, [Ag.sup.0]3[d.sub.5/2] binding energy of the post-UV treated electrospun PVA/AgN[O.sub.3] fiber web (365.7 eV) was slightly shifted to lower than those of the no-UV treated and pre-UV treated electrospun PVA/AgN[O.sub.3] fiber webs (ca. 366.3 eV). It was assumed that the photoreduction of [Ag.sup.+] ions was most actively occurred in the post-UV treated composite web.

[FIGURE 5 OMITTED]

In Vitro [Ag.sup.+] ions Release Studies of Electrospun PVA/AgN[O.sub.3] Fiber Webs

[Ag.sup.+] ions release behaviors of the electrospun PVA/AgN[O.sub.3] fiber webs were examined, as shown in Fig. 6. The release profile of [Ag.sup.+] ions from the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web displayed a two-phase exponential profile with fast initial phase before tapering off to a much slower rate of release. However, the [Ag.sup.+] ions in both pre-UV treated electrospun PVA/AgN[O.sub.3] fiber web and post-UV treated electrospun PVA/AgN[O.sub.3] fiber web were gradually released with the increase of the immersing time. This means that the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web has speedier and more regular release ability of [Ag.sup.+] ions than the UV treated ones do. It was assumed that the [Ag.sup.+] ions of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web were less reduced than those of the pre-UV treated and post-UV treated electrospun PVA/AgN[O.sub.3] fiber webs because UV irradiation strongly reduced the [Ag.sup.+] ions into the [Ag.sup.0]. Therefore, the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web easily released [Ag.sup.+] ions into deionized water than the others do. Such a fast and constant release of [Ag.sup.+] ions from the no-UV treated electrospun PVA/AgN[O.sub.3] fiber web would affect a fast and constant antimicrobial activity to the wound area of skin.

[FIGURE 6 OMITTED]

Antimicrobial Activities of Electrospun PVA/AgN[O.sub.3] Fiber Webs

The antibacterial activity of the no-UV treated electrospun PVA/AgN[O.sub.3] fiber webs against Staphylococcus aureus and Klebsiella pneumoniae was examined by the determination of plate counting method, as shown in Table 1. As demonstrated in Table 1, the numbers of Staphylococcus aureus and Klebsiella pneumoniae were significantly reduced after 18 h of incubation. Also, the immediate antimicrobial effect of the electrospun PVA/AgN[O.sub.3] fiber webs were simply tested for Pseudomonas aeruginosa, as shown in Fig. 7. The electrospun PVA/AgN[O.sub.3] fiber webs respectively showed the antimicrobial activities after following times; pre-UV treated web: ca. 10 min (Fig. 7b), post-UV treated web: ca. 5 min (Fig. 7c), no-UV treated web: ca. 1 min (Fig. 7d). It was thought that the [Ag.sup.+] ions in no-UV treated electrospun PVA/AgN[O.sub.3] fiber web was more easily extracted than those in UV-treated electrospun PVA/AgN[O.sub.3] fiber webs, and this result is a thread of connections the result of [Ag.sup.+] ions release test as shown in Fig. 6. On the other hand, metallic silver is also used commercially in wound dressings such as Acticoat (Smith & Nephew Health), in which silver is applied to the polymer mesh by a vapor deposition process. It surely shows an excellent antibacterial activity above 99.9%. However, there are some demerits; for instance, the metallic silver of the wound dressing leads to gray-blue discoloration on the skin [12], and moisture supplies are regularly necessary to dissolve the metallic silver [13], and also the manufacturing cost is relatively high. Therefore, the electrospun PVA/AgN[O.sub.3] fiber webs prepared in this study is promising material as wound dressings.

CONCLUSIONS

The PVA/Ag composite nanofibrous wound dressing material was successfully prepared by electrospinning a PVA/AgN[O.sub.3] aqueous solution into nonwoven web. Since the UV radiation strongly reduced the [Ag.sup.+] ions into Ag particles, the few but large Ag particles were observed on the surface of the pre-UV treated electrospun PVA/AgN[O.sub.3] fibers and also the fine but many Ag particles were observed on the surface of the pre-UV treated electrospun PVA/AgN[O.sub.3] fibers. On the other hand, it was found that heat treatment (155[degrees]C/3 min) could also reduce the [Ag.sup.+] ions, moreover the heat treatment made the electrospun PVA fiber web crosslinked and so unsolved in the moisture environment such as a wound area. Therefore, it could be concluded that the no-UV treated, in other words the only heat treated electrospun PVA/AgN[O.sub.3] fiber web is most excellent material as a wound dressing in this study.

[FIGURE 7 OMITTED]

REFERENCES

1. A.R. Horrocks and S.C. Anand, Handbook of Technical Textiles, Woodhead Publishing, The Textile Institute, Cambridge, 407 (2000).

2. K.H. Hong and T.J. Kang, J. Appl. Polym. Sci., 100, 167 (2000).

3. E.R. Kenawy, G.L. Bowlin, K. Mansfield, J. Layman, D.G. Simpson, E.H. Sanders, and G.E. Wenk, J. Controlled. Release., 81, 57 (2002).

4. J.Y. Kim, S.E. Kim, J.E. Kim, J.C. Lee, and J.Y. Yoon, J. Korean. Soc. Environ. Eng., 27, 771 (2005).

5. W.K. Son, J.H. Youk, T.S. Lee, and W.H. Park, Macromol. Rapid. Commun., 25, 1632 (2004).

6. C. Shao, H.Y. Kim, J. Gong, B. Ding, D.R. Lee, and S.J. Park, Mater. Lett., 57, 1579 (2003).

7. Budavari S.P., editor, Centennial Edition, Merck, Rahway, NJ, 1348 (1989). The Merck Index, 11th ed.

8. S. Kurihara, S. Sakamaki, S. Mogi, T. Ogata, and T. Nonaka, Polymer, 37, 1123 (1996).

9. L. Gao and C.J. Seliskar, Chem. Mater., 10, 2481 (1998).

10. N. Koji, Y. Tomonori, I. Kenji, and S. Fumio, J. Appl. Polym. Sci., 74, 133 (1999).

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12. http://www.o-wm.com/article/5128.

13. http://www.acticoat.com/26.

Kyung Hwa Hong

Division of Textiles and Clothing, University of California, Davis, California 95616

Correspondence to: Kyung Hwa Hong; e-mail: khong@ucdavis.edu

Contract grant sponsor: Korean Government (MOEHRD); contract grant number: KRF-2006-214-D00199.
TABLE 1. The antimicrobial test results of no-UV-treated electrospun
PVA/AgN[O.sub.3] fiber web on Staphylococcus aureus and Klebsiella
pneumoniae.

 The surviving The surviving Reduction (%)
 cells (cfu/ml) cells (cfu/ml) in of microbe
Microbe in control PVA/AgN[O.sub.3] web after 18 h

Staphylococcus 3.6 x [10.sup.6] 3.0 x [10.sup.3] >99.9
 aureus
Klebsiella 1.1 x [10.sup.7] 2.0 x [10.sup.2] >99.9
 pneumoniae
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Author:Hong, Kyung Hwa
Publication:Polymer Engineering and Science
Date:Jan 1, 2007
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