Deposition of Copper on Poly(Lactide) Non-Woven Fabrics by Magnetron Sputtering—Fabrication of New Multi-Functional, Antimicrobial Composite Materials

The paper presents the method of synthesis; physico-technical and biological characterization of a new composite material (PLA–Cu0) obtained by sputter deposition of copper on melt-blown poly(lactide) (PLA) non-woven fabrics. The analysis of these biofunctionalized non-woven fabrics included: ultraviolet/visible (UV/VIS) transmittance; scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS); attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy; ability to block UV radiation; filtration parameters (air permeability); and tensile testing. The functionalized non-woven composite materials were subjected to antimicrobial tests against colonies of Gram-negative (Escherichia coli), Gram-positive (Staphylococcus aureus) bacteria and antifungal tests against the Chaetomium globosum fungal mould species. The antibacterial and antifungal activity of PLA–Cu0 suggests potential applications as an antimicrobial material.

Some of the representative applications of antibacterial PLA composites are listed in Table 1. Table 1. Representative application of antibacterial poly(lactide) (PLA) composites.

No PLA Composite Action/Application
Lit.
Polymer-Cu-nanoparticles, being the convenient platform for metallic, antibacterial copper have been formed by a wide array of methods, including chemical, biological synthesis methods, and physical methods [49][50][51][52][53], including the magnetic sputtering method [54,55]. This method exhibits an especially convenient character-it is simple and ecofriendly, allowing deposition of the required amount of deposited metal in function of the time applied.
As a part of our experimentation program focused on phosphonic acids [56][57][58][59] and textile chemistry of their hybrids with a polymer matrix [60,61], we present the preparation, and characterization of a new multifunctional, biodegradable composite material, PLA-Cu, composite. This composite was obtained by a surface modification of melt-blown poly(lactide) non-wovens with copper, using the direct current (DC) magnetron sputtering method. Nonwovens samples were manufactured by the melt-blown technique using a laboratory extruder (Axon, Limmared, Sweden) [60]. Samples of nonwoven fabrics were manufactured in the form of a sheet. Processing parameters for fabrication of Poly(lactic) samples are listed in Table 2. The obtained poly(lactide) non-wovens were modified using a DC magnetron sputtering system produced by P.P.H. Jolex s. c. (Czestochowa, Poland). The copper target (Testbourne Ltd., Basingstoke, UK) with 99.99% purity was used. The distance between the target and the substrate was equal to 15 cm. The deposition of coatings was carried out in the atmosphere of argon. In order to optimize the process and avoid the destruction of the PLA substrates, different powers were applied to the target from 350 up to 1000 W. Finally, the following parameters were applied for modification: the power discharge of 700 W, with the resulting power density equal to 0.72 W/cm 2 and the working pressure of 2.0 × 10 −3 mbar. In order to vary the copper content, two different deposition times were applied, i.e., 10 min (sample name/assignment: PLA-Cu 0 (10)) and 30 min (sample name: PLA-Cu 0 (30)). Sputtered sample size was: 60 cm × 20 cm.

Ultraviolet-Visible (UV-VIS) Spectroscopy and Determination of the Protective Properties against UV Radiation
Physical properties as transmittance [%T] of modified PLA samples were assessed using a Jasco V-550 spectrophotometer (Tokyo, Japan), in the range: 200-800 nm. The same apparatus was used to determine the ultraviolet protection factor (UPF) of samples, according to the standard EN 13758-1:2002 Textiles. Solar UV protective properties. Method of test for apparel fabrics [62], on the basis of Equation (1): where: ∆λ-the wavelength interval of the measurements; ε(λ)-the erythema action spectrum, measure of the harmfulness of UV radiation for human skin; E(λ)-the solar irradiance; T(λ)-the spectral transmittance at wavelength λ.
The UPF value of the samples was determined as the arithmetic mean of the UPF values for each of the samples, reduced by the statistical value depending on the number of measurements performed, at a confidence interval of 95%.

Tensile Properties
Tensile testing of poly(lactic acid) samples was performed according with the EN ISO 10319:2015 standard [64]. Stretching speed was 20 mm/min. An Tinius Olsen H50KS (Horsham, PA, USA) tester was used.

Atomic Absorption Spectrometry with Flame Excitation (FAAS)
Determination of copper content in poly(lactic acid) non-woven fabrics samples was assessed using a single-module Magnum II microwave mineralizer from Ertec (Wroclaw, Poland) and Thermo Scientific Thermo Solar M6 (LabWrench, Midland, ON, Canada) atomic absorption spectrometer equipped with a 100 mm titanium burner, coded lamps with a single-element hollow cathode, background correction: D2 deuterium lamp.
The total copper content of the sample M [mg/kg; ppm] was calculated according to Equation (2) [65]: where:

Antifungal Activity
The antifungal activity of resulting samples was tested according to PN-EN 14119:2005 [69].
The standard indicate tests of antifungal activity on a Chaetomium globosum (ATCC 6205), analogously as in PP nonwovens [60].
Antibacterial activity of samples was tested by agar diffusion method using Muller Hinton medium agar [67,68].

Antifungal Activity
The antifungal activity of resulting samples was tested according to PN-EN 14119:2005 [69]. The standard indicate tests of antifungal activity on a Chaetomium globosum (ATCC 6205), analogously as in PP nonwovens [60].

PLA (Calc.) EDS Determinations
PLA-Cu 0 (10) composites images ( Figure 2) show uniform randomly oriented fibers (Figure 2b,c) and visible layer of copper on the surface fibers (Figure 2c). On the other hand, PLA-Cu 0 (30) composite images also present uniform randomly oriented fibers (Figure 3a), but with high fibers crack content (Figure 3b,c). the PLA-Cu0(30) sample shows a substantial contribution (20-30%) of shorter fragments (length: 10 to 30 µm; diameter: 2 to 13 µm), without sharp edges, Cu covered. Fiber fracture and damage of the PLA-Cu 0 (30) composite during copper sputtering of PLA in 30 min. period suggests application of shorter process times, for example up to 10 min.
EDS analysis results obtained for PLA and PLA-Cu 0 (t) (t = 10 min and 30 min) are presented in Table 3. Table 3. Energy-dispersive spectroscopy (EDS) analysis results of PLA and PLA-Cu 0 (t). The content of carbon and oxygen components of poly(lactic acid) (PLA) (without of hydrogen) is similar to atomic "bulk" analysis of PLA (C = 50.0 and O = 44.4%). The surface modification of poly(lactic acid) samples using surface copper sputtering leads to appearance of copper, which contents rapidly increases during prolongation of sputtering (19.55% for PLA-Cu 0 (10), and 66.86 for PLA-Cu 0 (30)) and simultaneous substantial decrease of carbon (from 51.7% (PLA) to 42.33% (PLA-Cu 0 (10)) and 21.00 (PLA-Cu 0 (30)) and oxygen (from 48.33% (PLA) to 38.13% (PLA-Cu 0 (10)) and 12.14 (PLA-Cu 0 (30)) contents, respectively. This causes a subsequent change of the "surface molecular formula" (SMF) from C 3 O 2 for PLA, to C 3 O 2 Cu 0.27 for PLA-Cu 0 (10) and/or to C 3 O 1 . 2 Cu 1.8 for PLA-Cu 0 (30). These results suggest preferential deposition of copper atoms on the oxygens of carboxylate fragments of the lactide unit (-C(=O)-O-), as spatially more available due to flat structures of the carboxylic ester function.

ATR-FTIR Spectra
The recorded ATR-FTIR spectra for PLA and PLA-Cu 0 (t) (t = 10 min, 30 min) composites are presented in Figure 4. Characteristic FTIR signals of the starting PLA and derived PLA-Cu(t) composites are summarized in Table 4.

ATR-FTIR Spectra
The recorded ATR-FTIR spectra for PLA and PLA-Cu 0 (t) (t = 10 min, 30 min) composites are presented in Figure 4. Characteristic FTIR signals of the starting PLA and derived PLA-Cu(t) composites are summarized in Table 4.    The ATR-FTIR spectra of PLA as well as PLA-Cu 0 (t) composites are similar in a shape, exhibiting absorbance up to 0.12. Generally the major band intensities of PLA are much stronger than corresponding bands of PLA-Cu 0 (t) composites, which are almost identical.

UV-VIS Spectrometry and Determination of the Protective Properties against UV Radiation
Transmittance spectra [%T] of PLA samples and PLA-Cu 0 (t) hybrids (PLA-Cu 0 (10) or PLA-Cu 0 (30)), recorded in the ranges λ= 200-800 nm is presented in Figure 5. The ATR-FTIR spectra of PLA as well as PLA-Cu 0 (t) composites are similar in a shape, exhibiting absorbance up to 0.12. Generally the major band intensities of PLA are much stronger than corresponding bands of PLA-Cu 0 (t) composites, which are almost identical.

UV-VIS Spectrometry and Determination of the Protective Properties against UV Radiation
Transmittance spectra [%T] of PLA samples and PLA-Cu 0 (t) hybrids (PLA-Cu 0 (10) or PLA-Cu 0 (30)), recorded in the ranges λ = 200-800 nm is presented in Figure 5. The transmittance (%T) spectra in the range λ = 200-800 nm of the modified PLA non-woven by magnetron sputtering show that the samples after modification reveal changes in the macrostructure expressed by a decrease in transmittance, the reduction in transmission is caused by an additional layer of copper on the surface of the samples. The transmittance spectra of modified samples (PLA-Cu 0 (10), PLA-Cu 0 (30)) had similar spectral characteristics and quite a similar level of transmittance in the entire spectral range when compared to control samples (without modification). Table 5 compare average transmittance (T%) and calculated UPF values of modified samples (PLA-Cu 0 (10), PLA-Cu 0 (30)) with those non-modified. The transmittance (%T) spectra in the range λ = 290-400 nm. are presented in Figure 5. The transmittance (%T) spectra in the range λ = 200-800 nm of the modified PLA non-woven by magnetron sputtering show that the samples after modification reveal changes in the macrostructure expressed by a decrease in transmittance, the reduction in transmission is caused by an additional layer of copper on the surface of the samples. The transmittance spectra of modified samples (PLA-Cu 0 (10), PLA-Cu 0 (30)) had similar spectral characteristics and quite a similar level of transmittance in the entire spectral range when compared to control samples (without modification). Table 5 compare average transmittance (T%) and calculated UPF values of modified samples (PLA-Cu 0 (10), PLA-Cu 0 (30)) with those non-modified. The transmittance (%T) spectra in the range λ= 290-400 nm are presented in Figure 5. The results have been measured in triplicate and presented as a mean value wit ± deviation approximately 2%.
Samples modified with copper obtain a UPF value >40, calculated on the basis of transmittance measurements for λ= 290-400 nm (according to Formula (1)). This result indicates that the modification performed imparts proper barrier properties against UV radiation according to PN-EN 13758-1:2002 [62].

Technical Parameters
Technical parameters of new composite materials were focused on tensile strength and filtration properties. Filtration properties expressed by the air permeability and were tested for starting poly(lactic acid) nonwoven and PLA-Cu 0 (t) composites. Results of filtration parameters are showed in Table 6. The results of tensile strength properties: relative elongation at maximum load [%] and durability for stretching [kN/m] of initial PLA samples and PLA-Cu 0 (t) composites are listed in Table 7.   These results indicate that the sample PLA-Cu 0 (10) has a more beneficial flexibility and stronger structure compared with PLA. It is obvious also that very high loading of Cu on the PLA surface in PLA-Cu 0 (30) influenced negatively on the mechanical properties of this composite.

Flame Atomic Absorption Spectrometry
Determination of copper content in PLA-Cu 0 (t) composites was assessed by the Flame Atomic Absorption Spectrometry (FAAS) method [65] and listed in Table 8. The results have been measured in triplicate and presented as a mean value with ± deviation approximately 2%.
The results determination of copper content in poly(lactic acid) composite show that copper content in poly(lactic acid) composite samples depends on the applied magnetron sputtering deposition times (PLA-Cu 0 (10): 10 min-9.91 g/kg; PLA-Cu 0 (30): 30 min-27.89 g/kg) and the magnetron sputtering of copper deposition process is almost linear. The copper content in poly(lactic acid) composite indicate also that magnetron sputtering process is quite precise and distribution of copper in a composites bulk is uniform.

Sample Name
Bacterial Average Inhibition Zone(mm) Results of antimicrobial studies demonstrate antimicrobial protection against different bacterial species of new composites for Escherichia coli and Staphylococcus aureus (  [66]. Antimicrobial properties of composite samples expressed by strong visible inhibition zones of bacterial growth on inoculated agar Petri dishes ( Figure 6) and no visible bacteria under the modified samples. Any antibacterial effect was observed for the unmodified sample (PLA). It is worth noting that 0.03 molar solutions of CuSO 4 (2 mg/mL) are not bactericidal (Growth Inhibition Zone = 0) for several gram positive bacteria (e.g., S. aureus), gram negative bacteria (e.g., E. coli) bacteria as well as fungi species (e.g., Candida family) [72].

Antifungal Activity
Results of antifungal tests against a Chaetomium globosum (ATCC 6205) of poly(lactide) nonwoven (PLA) and poly(lactide)-copper composites PLA-Cu 0 (t) are listed in Table 10 and Figure 7. Modification of non-woven fabrics provide antifungal properties for Chaetomium globosum, expressed by no visible growth under the microscope (50× magnification). PLA non-woven fabrics without magnetron sputtering modification exhibit strong growth covering the surface of the control sample (Table 10, Figure 7).

Conclusions
The major contribution of this study was offering a new method for obtaining a multi-functional composite materials. The fabrication of new composite materials was performed by magnetron sputtering deposition of copper on the melt-blown poly(lactide) non-woven fabrics. The structure and mechanical properties of the obtained new composite products were characterized by FTIR spectrometry, UV/VIS transmittance, scanning electron microscopy (SEM), atomic absorption Modification of non-woven fabrics provide antifungal properties for Chaetomium globosum, expressed by no visible growth under the microscope (50× magnification). PLA non-woven fabrics without magnetron sputtering modification exhibit strong growth covering the surface of the control sample (Table 10, Figure 7).

Conclusions
The major contribution of this study was offering a new method for obtaining a multi-functional composite materials. The fabrication of new composite materials was performed by magnetron sputtering deposition of copper on the melt-blown poly(lactide) non-woven fabrics. The structure and mechanical properties of the obtained new composite products were characterized by FTIR spectrometry, UV/VIS transmittance, scanning electron microscopy (SEM), atomic absorption spectrometry with flame excitation (FAAS), tensile strength test and air permeability. The polylactide-copper composites were subjected to antimicrobial activity tests against: Escherichia coli, Staphylococcus aureus, Chaetomium globosum. The most important features of the new composite materials PLA-Cu 0 are: • eco-friendly, full biodegradable composite product; • fabricated by clean and zero-waste process; • improvement of technical parameters, including a tensile strength, air permeability and barrier properties against UV radiation of PLA-Cu 0 synthesized in comparison with starting raw PLA non-woven fabrics; • composite with potential antimicrobial properties.
The listed attributes of the PLA-Cu 0 synthesized composites should find application in biomedical areas, and also as a microbiostatic material. Additionally, in the period when clinical waste are a major environmental burden, increasing interests should be paid to new biodegradable composite products fabricated by a clean process.