Deposition of Copper on Polyester Knitwear Fibers by a Magnetron Sputtering System. Physical Properties and Evaluation of Antimicrobial Response of New Multi-Functional Composite Materials

Abstract

In this study, copper films were deposited by magnetron sputtering on poly(ethylene terephthalate) knitted textile to fabricate multi-functional, antimicrobial composite material. The modified knitted textile composites were subjected to microbial activity tests against colonies of Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria and antifungal tests against Chaetomium globosum fungal molds species. The prepared samples were characterized by UV/VIS transmittance, scanning electron microscopy (SEM), tensile and filtration parameters and the ability to block UV radiation. The performed works proved the possibility of manufacturing a new generation of antimicrobial textile composites with barrier properties against UV radiation, produced by a simple, zero-waste method. The specific advantages of using new poly(ethylene terephthalate)-copper composites are in biomedical applications areas.

1. Introduction

Poly(Ethylene Terephthalate) (PET), due to its physicochemical and technical attributes (high uniformity, mechanical strength, permeability to gases, transparency and resistance against UV and chemicals), presents a multifunctional polymer [1,2,3] widely engaged in various applications ranging from a production of containers and packaging [4], through textiles [5,6,7,8], the health care polymeric materials [9,10,11,12] and concrete components [13] to flexible electronic device applications [14].

However, the lack of antibacterial properties with unsatisfactory biocompatibility and functionality of PET frequently limits its use in some industrial and medical fields, especially in biomedical device and filtration membrane applications [15]. Moreover, due to its own porosity PET is conducive to microbial adhesion and subsequent bacteria colonization [16]. Therefore, for improvement of antimicrobial properties PET was subjected to several surface modification technologies [17,18], the majority based on functionalization [19,20], grafting [21,22,23,24,25,26,27,28], surface topography modification [29,30,31,32,33], coating [34,35,36,37] and their combinations [38,39,40].

Of substantial importance in the field of antimicrobial PET are its composites, equipped with antibacterial organic additivities (mainly chitosan and antibiotics [21,22,41,42,43,44,45,46,47,48,49,50]). Thus, antimicrobial hybrids of PET-organic biocides can be represented by PET-Chitosan [21,22,41], PET-Chitosan/hyaluronic acid [42,43,44], PET-Antibiotics (e.g., rifampin [45]; cephalosporin [46]; gentamicin [47], tetracycline [48]; chlorhexidine [49] or daptomycin [50]).

Because of increasing demand for effective antimicrobials combating bacterial cross-infections and infectious diseases [51], metals and salts deposited over polymers became a valuable alternative to traditional antibiotics [52,53,54,55,56,57,58,59]. In hybrids PET-inorganic bactericides representative are: PET-Ag nanoparticles [60,61,62,63,64,65,66,67], PET-silver salts [68,69], PET-TiO₂ [70,71,72,73], PET-Zn/ZnO₂ [74,75,76,77] and/or PET-Au [78]. In the set of PET-metal hybrids only a few PET-Cu hybrids have been reported [79,80,81,82].

Between various inorganic bactericides of medicinal interest growing attention is focusing on copper and its salts (over 5200 documents on antibacterial activity of copper abstracted by Scopus) [83], due to their low costs and easy preparation [84,85,86] as well as an antimicrobial efficiency [87,88,89,90,91,92,93,94,95,96]. The antibacterial activity of the copper metallic surface is regarded to act by two supplemental mechanisms—surface–surface interaction of copper and bacteria (contact killing) [93] and/or surface oxidation of copper with the subsequent release of antibacterial cupric ions (e.g., [93,94]).

Polymer-copper composites have been formed by a number of various methods, [95,96,97,98], including magnetic sputtering method: a simple and ecofriendly method, allowing deposition of the required amount of deposited metal in function of the time applied [99,100,101].

As a part of our research program directed on biologically active functionalized phosphonates [102,103] and biofunctionalization of textile materials [104,105,106] we present the preparation and physico-chemical and biological properties of the PET-Cu polymer hybrid.

The aim of this work was to modify the surface of the polyethylene terephthalate knitwear textile with copper, using the DC (Direct Current) magnetron sputtering method and fabrication of a new antimicrobial, multi-functional composite material.

As far as pretreatment and finishing of textile fabrics are concerned, plasma technologies are currently increasingly replacing wet chemical processes. For instance, Shahidi et al. [107] applied magnetron sputtering to modify the woven cotton fabrics with Cu. The authors highlighted that magnetron sputtering is a simple, environmentally friendly and time-saving method in comparison to the conventional process, which requires the use of a detergent, metallic salts and at least three baths (more than 100 min). In turn Badaraev et al. [108] pointed out that magnetron sputtering allows one to avoid the high costs associated with the synthesis of nanoparticles applied for the modification of the textile surface and thus, offers a resource-efficient method for producing textiles with antimicrobial properties. Taking into consideration that the modification of textile materials using magnetron sputtering does not require the use of any chemicals and may be realized in a single process cycle in a single industrial installation, as indicated by Gorberg et al. [109], it may be considered as simple technique. Additionally, magnetron sputtering is not associated with any toxic emission to the environment or contamination production. Therefore, this method may be considered as an eco-friendly and zero-waste one.

2. Materials and Methods

2.1. Materials

• Knitted textile, qualitative composition: polyester—polyethylene terephthalate (95%w/w), elastane—polyether-polyurea copolymer (5%w/w), weave: interlock right, basic weight (GSM): 230 g/m²; assigned as PETE (polyethylene terephthalate-elastane, for structures see Figure 1) (IW, Lodz, Poland). The size of the textile sample was 300 mm × 150 mm.
• The copper target (Testbourne Ltd., Basingstoke, UK) with 99.99% purity. The size of the target was equal to 798 × 122 × 6 mm.
• Bacterial and Fungal Strains were purchased from Microbiologics (St. Cloud, MN, USA): Escherchia coli (ATCC 25922), Staphylococcus aureus (ATCC 6538) and fungal strains: Chaetomium globosum (ATCC 6205).

2.2. Methods

2.2.1. Magnetron Sputtering

The knitted PETE samples were modified using a DC magnetron sputtering system produced by P.P.H. Jolex s. c. (Czestochowa, Poland), schematically presented in Figure 2.

The apparatus was developed for the Lukasiewicz Research Network—Textile Research Institute in Lodz, Poland—for the purpose of the project Envirotex—PO IG no. 01.03.01-00-006/08 co-financed from the funds of European Regional Development Fund within the framework of the Operational Programme Innovative Economy 2007–2013. It allows the semi-continuous deposition of metallic coatings on different fabrics (up to 60 cm wide).

The deposition of coatings was carried out in the atmosphere of argon, the distance between a copper target and the sputtering substrate was equal to 15 cm, the applied powers varied from 350 to 1000 W. The established optimal sputtering conditions were: the power discharge—700 W; the time of deposition—10 min; the resulting power density—0.72 W/cm² and the working pressure 2.0 × 10⁻³ mbar. The applied parameters were chosen on the basis of the previous research concerning the deposition of copper on the Polylactide (PLA) nonwoven fabrics. A preliminary study was performed for the purpose of another publication, which has been recently published [111]. In order to choose the appropriate deposition parameters, the authors varied the sputtering power of the magnetron target from 350 up to 1000 W. At the same time, the sputtering time was changed from 10 to 30 min. The upper limit of the applied power was set as a result of our previous experiments showing that higher values of power have destructive effect on the PLA substrate. The preliminary analysis of antimicrobial activity was carried out for the obtained samples. The results exhibited no antibacterial and antifungal effect for the sputtering power lower than 700 W. Taking into consideration the possible cytotoxicity of copper, the authors decided to choose the minimal sputtering power for which the antimicrobial activity was observed for further research. This allows us to minimize the content of copper and thus, minimizes the risk of cytotoxicity. In order to enable the comparison of results obtained for different materials, the authors decided to apply the same deposition parameters for the PETE substrate as for the PLA substrate. Since for the PLA nonwovens the observed antibacterial activity did not vary substantially between the samples subjected to 10 and 30 min of copper deposition, the authors decided to choose the lower deposition time in order to further limit the Cu content. Therefore, the final deposition parameters applied in this work were established at 700 W and 10 min for each side of the substrate.

In order to vary the copper content in the PETE-Cu⁽⁰⁾, composites two different deposition variants were applied, namely: 10 min single-sided deposition of copper on PETE (sample name: PETE-Cu⁽⁰⁾-1) and 20 min two-sided deposition of PETE (10 min for the upper and 10 min for the lower side of the sputtering fabric; sample name: PETE-Cu⁽⁰⁾-2; PETE-Cu⁽⁰⁾-2.1 and PETE-Cu⁽⁰⁾-2.2).

2.2.2. 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 (Tescan Analytics, Brno, Czech Republic) with the EDS (Oxford Instruments, Abingdon, UK) X-ray micro analyzer. The SEM microscopic examination of the surface topography was carried out in a high vacuum using the energy of the probe beam 20 keV. The layer of gold (approx. 3.5 nm), as a conductive substance, was sputtered on the samples before SEM analysis using a Quorum Technologies Ltd. (Lewes, UK) vacuum dust extractor. The instrument is fitted with a 57 mm diameter quick-change magnetron sputter target. Gas supplies: Argon. Vacuum: 5 × 10⁻⁵ mbar. Magnifications of SEM applied were in the range from 500× to 20,000×. The performance of an EDS system was evaluated by measuring the resolution of a known set of elemental standards (Oxford Instruments, Abingdon, UK) in line with the ISO 15632:2012 [112].

2.2.3. FAAS—Flame Atomic Absorption Spectrometry

For the determination of copper content in PETE-Cu⁽⁰⁾, samples were assessed by prior sample mineralization (Figure 3), using a single-module Magnum II microwave mineralizer from Ertec (Wroclaw, Poland), followed by the determination of copper (II) ions by atomic absorption spectrometry with flame excitation using the Thermo Scientific Thermo Solar M6 (LabWrench, Midland, ON, Canada) 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 the formula [113]:

$$M=\frac{C_i\times V}{m_i}\left[\mathrm{mg}\mathrm{kg}\right]$$

here:

• C_i—metal concentration in the tested solution [mg/L];
• m_i—mass of the mineralized sample [g];
• V—volume of the sample solution [mL].

2.2.4. UV-VIS Analysis and Determination of the Protective Properties Against UV Radiation

Changes of the physical properties as transmittance [%T] of samples occurring during modifications were assessed using a double beam Jasco V-550 UV-VIS spectrophotometer (Jasco, Tokyo, Japan) with an integrating sphere attachment in the range 200–800 nm. The same apparatus was used to determine the Ultraviolet Protection Factor (UPF) of samples, according to EN 13758-1:2002 standard [114], using the formula:

$$UPF=\frac{{{\displaystyle \int }}_{290}^{400}E\left(\lambda \right)\epsilon \left(\lambda \right)d\left(\lambda \right)}{{{\displaystyle \int }}_{290}^{400}E\left(\lambda \right)\epsilon \left(\lambda \right)T\left(\lambda \right)d\left(\lambda \right)}$$

where:

• E(λ)—the solar irradiance;
• ε(λ)—the erythema action spectrum (measure of the harmfulness of UV radiation for human skin);
• Δλ—the wavelength interval of the measurements;
• 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%.

2.2.5. Filtration Parameters

Air permeability of the PETE knitted fabrics and PETE-Cu⁽⁰⁾ composites was determined according to the EN ISO 9237:1998 standard [115]. An FX 3300 TEXTEST AG (Klimatest, Wrocław, Poland) permeability tester was applied. During the test, air at a pressure of 100 Pascal and 200 Pascal was passed through a fabric area of 20 cm². Air permeability was determined as an average of 10 measurements for each type of sample.

2.2.6. Tensile Testing

Tensile testing of the PETE knitted fabrics and PETE-Cu⁽⁰⁾ composites was performed in accordance with the EN ISO 10319:2015 standard [116]. A H5KS (Hounsfield, Redhill, UK) testing machine was used. The cross-head stretching speed was equal to 100 mm/min, the support spacing was 100 mm, while the width of the sample was 50 mm. The initial pressure force was set at 0.5 N. Measurements were carried out for both longitudinal and horizontal directions.

2.2.7. Thickness

Thickness measurements of the PETE knitted fabrics and PETE-Cu⁽⁰⁾ composites were conducted in accordance with the EN ISO 5084:1999 standard [117]. The thickness of each sample was calculated as an average of 10 measurements. The area of the tested samples was equal to 20 cm². The measurements were performed using a GM-70 thickness gauge (IW, Lodz, Poland) at the pressure of 1 kPa.

2.2.8. Thermal Resistance, Steam Resistance and Steam Permeability

Thermal resistance, steam resistance and steam permeability of the PETE knitted fabrics and PETE-Cu⁽⁰⁾ composites were measured according to the EN ISO 11092:2014-11 [118] using a KONTECH apparatus (IW, Lodz, Poland). The application of this measurement technique is restricted to a maximum thermal resistance and water-vapour resistance e.g., 2 m²·K/W and 700 m²·Pa/W respectively. The airflow speed was set to 1.0 m/s.

2.2.9. Bacterial Activity

The antibacterial activity of PETE-Cu⁽⁰⁾ composites was tested according to the PN-EN ISO 20645:2006 [119] against a colony of Gram-negative bacteria (Escherchia coli, ATCC 25922) and Gram-positive bacteria (Staphylococcus aureus, ATCC 6538), using the agar diffusion method of Muller Hinton. The test was initiated by pouring agar on to sterilized Petri dishes and it was allowed to solidify. The surfaces of agar media were inoculated by overnight broth cultures of bacteria (ATCC 25922: 1.2 × 10⁸ CFU/mL, ATCC 6538: 1.7 × 10⁸ CFU/mL) and kept at 4 °C before analysis. In order to establish the bacterial concentration inside the overnight culture the assessment of turbidity of bacterial suspension as well as the culture method were used. The density of the bacterial suspension was firstly determined using a calibrated densitometer. Then, using a series of ten-fold dilutions, the number of colonies grown on the plates was calculated from the appropriate dilutions and relevant calculations were made.

Discs of PETE-Cu⁽⁰⁾ composites (10 mm) were placed onto the inoculated agar and incubated at 37 °C for 24 h. The diameter of the clear zone around the discs was measured as an indication of inhibition of the microbial species. Each side of the sample was tested in duplicate (four tests were performed for each sample). Simultaneously, the same tests were carried out for control samples—unmodified PETE samples.

2.2.10. Antifungal Activity

The procedure applied was partly identical with the procedure above. Thus, the antifungal activity of PETE-Cu⁽⁰⁾ composites was tested according to PN-EN 14119:2005 [120] against a Chaetomium globosum (ATCC 6205). Discs of the tested PETE-Cu⁽⁰⁾ composites (20 mm) were placed onto the inoculated with Chaetomium globosum fungal moulds species agar (pH:6.2) plates and incubated at 29 °C for 14 days. Both sides of PETE-Cu⁽⁰⁾ composites 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 (unmodified PETE).

3. Results and Discussion

3.1. SEM—Scanning Electron Microscopy

SEM micrographs of PETE fabrics and PETE-Cu⁽⁰⁾ composites are presented in Figure 4 and Figure 5, respectively.

The SEM images of the unmodified PETE fabrics show parallel and uniform randomly oriented fibers (Figure 4a) with relatively smooth surfaces (Figure 4d). The average fiber diameter in the PETE knitwear sample was 15 ± 2 µm. The SEM images of the magnetron sputtering PETE-Cu⁽⁰⁾-1 composite (Figure 5) show changes in the surface structure of the samples, uniformly distributed new surface coating layer on the entire of the fibers (Figure 5b,c). No damage or fiber cracks in the PETE-Cu⁽⁰⁾ composites were observed (Figure 5b,c).

3.2. Copper Determination in PETE-Cu⁽⁰⁾/Composites

Copper determination in PETE-Cu⁽⁰⁾ composites was achieved using EDS spectroscopy (determination of surface located copper) and FAAS spectrometry (determination of bulk copper).

3.3. Copper Determination by Energy-Dispersive X-ray Spectroscopy EDS

Comparison of the EDS analyses of PETE and PETE-Cu⁽⁰⁾ composites are presented in

Table 1. The EDS analysis of elemental composition of PETE (C, O) and PETE-Cu⁽⁰⁾ (C, O, Cu) composites.

	PETE		PETE-Cu(0) a
			PETE-Cu(0)-1 a
			PETE-Cu(0)-1.1 c			PETE-Cu(0)-1.2 c
Element	C	O	C	O	Cu	C	O	Cu
% b,c	61.8	37.8	19.90	4.40	75.60	61.40	38.30	-
Std. deviation	0.16	0.16	0.82	0.06	0.79	0.14	0.16	-
	PETE		PETE-Cu(0)-2 a
			PETE-Cu(0)-2.1 d			PETE-Cu(0)-2.2 d
Element	C	O	C	O	Cu	C	O	Cu
% b	62.1	37.5	17.70	5.93	74.78	22.83	6.14	70.88
Std. deviation	0.16	0.16	1.70	0.45	0.26	1.47	0.71	2.20

^a Assignments: PETE-Cu⁽⁰⁾-1—one site a PETE sample copper sputtering deposition: PETE-Cu⁽⁰⁾-1.1—upper site of the sample analysis; PETE-Cu⁽⁰⁾-1.2—lower site of the sample analysis; PETE-Cu⁽⁰⁾-2: two sites of a PETE sample copper sputtering deposition; PETE-Cu⁽⁰⁾-2.1—upper site of the sample analysis; PETE-Cu⁽⁰⁾-2.2—lower site of the sample analysis; ^b All results in percent by weight [%]; ^c Mean value of 3 measurements; ^d Mean value of 11 measurements.

.

The analyses of deposited sites of the PETE-Cu⁽⁰⁾ composites performed for both one site as well as two sites modes of copper sputtering afforded close results for PETE-Cu⁽⁰⁾-1.1 and PETE-Cu⁽⁰⁾-2.1 and slightly different results were seen for PETE-Cu⁽⁰⁾-2.2. Thus, for carbon: 19.9% (PETE-Cu⁽⁰⁾-1.1), 17.70% (PETE-Cu⁽⁰⁾-2.1) and 22.83% (PETE-Cu⁽⁰⁾-2.2), respectively; for oxygen: 4.40% (PETE-Cu⁽⁰⁾-1.1), 5.93% (PETE-Cu⁽⁰⁾-2.1) and 6.14% (PETE-Cu⁽⁰⁾-2.2), respectively; and for copper: 75.6% (PETE-Cu⁽⁰⁾-1.1), 74.78% (PETE-Cu⁽⁰⁾-2.1) and 70.88% (PETE-Cu⁽⁰⁾-2.2), respectively.

These differences can be caused by the different morphology of both sides of the knitted PETE textile (weave: interlock right) used for the copper sputtering deposition [121], influencing on a type of deposited copper. The Flame Atomic Absorption Spectrometry results advocate for the proportional increase of deposition of copper during the deposition. We also consider the influence of the already-formed copper layer on the copper diffusion inside bulk during sputtering of the second side of the textile (PETE-Cu⁽⁰⁾-2.2).

Is worth admitting that the distribution and penetration of the Cu atoms into the polymer beneath the interface, and the exact nature of the Cu–O–C bonds formed during polyarylamide PAMX D6 copper sputtering, are still unclear since Legois’ work [122].

3.4. Copper Determination by Flame Atomic Absorption Spectrometry—FAAS

The results of copper content in PETE-Cu⁽⁰⁾ composite samples determined by the FAAS method are listed in

Table 2. Results of determination of copper content in PETE and PETE-Cu⁽⁰⁾ composite samples by Flame Atomic Absorption Spectrometry (FAAS).

Sample Name	Copper Deposition Time [min]	Copper Concentration Determined
		[g/kg]	Percentage [%: g/100 g]	Molality [m: mmol/kg]
PETE	-	0.026	0.003	0.0004
PETE-Cu(0)-1	10	6.701	0.670	0.105
PETE-Cu(0)-2	20	14.036	1.404	0.221

The results have been measured in triplicate and presented as a mean value with deviations approximately ±2%.

.

The results inform about the whole copper concentration in starting PETE knitwear fibers and PETE-Cu⁽⁰⁾ composites. These revealed that composites are practically null of copper in the starting PETE (0.003: %; m: 0.0004) with only 0.7% to 1.4% of copper in PETE-Cu⁽⁰⁾ composites; in detail there was 0.67% (0.105 molal) of copper in one side of the sputtered knitwear fibers PETE-Cu⁽⁰⁾-1 and twice as high a copper concentration in the case of two-sided sputtering mode PETE-Cu⁽⁰⁾-2 (140%; 0.221 molal).

Taking into account the results of copper determination in PETE-Cu⁽⁰⁾-2.2 by EDS and in PETE-Cu⁽⁰⁾-2 by FAAS it can be assumed that during sputtering of the second side of PETE-Cu⁽⁰⁾-2 (PETE-Cu⁽⁰⁾-2 → PETE-Cu⁽⁰⁾-2.2) the diffusion of copper from the sputtering surface (PETE-Cu⁽⁰⁾-2.2) into bulk of polymer occurs, and there is copper diffusion in the polymer, analogous to earlier works on PET metallization [123,124,125].

3.5. UV-VIS Analysis and Determination of the Protective Properties Against UV Radiation

Poly(ethylene terephthalate) is transparent for UV, undergoing itself a surface modification under deep UV irradiation (e.g., [126,127]). The importance of protection against ultraviolet radiation (UV) is increasing daily. Investigations on poly(ethylene terephthalate) fibers’ protection against solar ultraviolet radiation (UVR) have been frequently undertaken (e.g., [128,129,130]). The property of UVR protection exhibited by PETE-Cu⁽⁰⁾ composites was investigated using a UV spectrophotometer by measuring the transmittance of UV-rays through the fabrics.

Thus, the transmittance spectra [%T] of starting PETE knitted textile and PETE-Cu⁽⁰⁾ composites in the range λ = 200–800 nm are presented in Figure 6.

As shown in Figure 6, the transmittance of UV-rays of the PETE fabrics, PETE-Cu⁽⁰⁾-1 and PETE-Cu⁽⁰⁾-2 composites were different in the wavelength range examined. Thus, the PETE transmittance curve exhibited a complexed shape, with a sharp decrease in the 200–250 nm range from ca 14%T to ca 1.5 %T, then a slow increase from 1.5% T (310 nm) to 7% T (380 nm), a rapid decrease to 26%T at 420 nm and the plateau region to 600 nm and a further gradual increase to 29%T at 700 nm with the plateau to 800 nm. The PETE-Cu⁽⁰⁾-1composite exhibited null transmittance in the wavelength range 200 to 720 nm, with an increase to 3%T at the 720–800 nm range. The PETE-Cu⁽⁰⁾-2composite exhibited null transmittance in the full examined wavelength range 200 to 800 nm. As a result, the property of UVR protection of the both PETE-Cu⁽⁰⁾ composites had improved significantly in almost the full wavelength range (excluding the 250–300 nm gap: 0%T for PETE-Cu⁽⁰⁾ vs 1.5%T for PETE).

The transmittance (%T) spectra of PETE and PETE-Cu⁽⁰⁾ composites in the range λ = 290–400 nm (UV A and UV B) are presented in Figure 7.

On the basis of these transmittance measurements obtained for PETE knitted textile and PETE-Cu⁽⁰⁾ composites at λ = 290–400 nm, corresponding UPF values have been calculated [114]. UPF values of PETE knitted textile and PETE-Cu⁽⁰⁾ composites, calculated on the basis of transmittance measurements for λ = 290–400 nm (Figure 6), have been listed in the

Table 3. Ultraviolet Protection Factor (UPF) values of PETE knitted textile and PETE-Cu⁽⁰⁾ composites.

Parameter	PETE	PETE-Cu(0)
		PETE-Cu(0)-1	PETE-Cu(0)-2
UPF	37	>50	>50

The results have been measured in triplicate and presented as a mean value with. ±deviation approximately 2%.

.

These data revealed that both PETE-Cu⁽⁰⁾ composites possess UPF_(PETE-Cu(0)) values > 50 in comparison with, UPF_(PETE) values ~37 indicating that the magnetron sputtering modification performed imparts proper barrier properties of PETE knitted textile against UV radiation.

3.6. Technical Parameters

From several technical parameters affecting fabric behavior [131], we applied for PETE-Cu⁽⁰⁾ composites utility verification analyses of filtration parameters, tensile strength, thickness as well as thermal resistance, steam resistance and steam permeability.

3.6.1. Filtration Parameters

Filtration parameters, expressed by the air permeability, present one of the major properties of textile materials and are governed by factors like the fabric structure, density, thickness and surface characteristics. The comparisons of air permeability of PETE-Cu⁽⁰⁾ composites vs. PETE fabric are shown in

Table 4. The air permeability of the unmodified PETE fabrics and PETE-Cu⁽⁰⁾ composites, according to the EN ISO 9237:1998 [115].

Parameter		PETE	PETE-Cu(0)
			PETE-Cu(0)-1	PETE-Cu(0)-2
Average air permeability [mm/s], pressure decrease:	100 Pa	65.5 ± 1.5	54.5 ± 1.0	51.8 ± 1.0
	200 Pa	147.0 ± 4.0	123.0 ± 2.0	117.0 ± 2.0

The results have been measured in triplicate and presented as a mean value with. ±deviation approximately 3%.

. The obtained results indicated that the modification of PETE fabrics with copper coating moderately (16%–18%) decreases the air permeability from 65.5 mm/s to 54.5 mm/s at 100 Pa and from 147 mm/s to 123 mm/s at 200 Pa. Moreover, a slightly further reduction in the air permeability (51.8 mm/s at 100 Pa and 117 mm/s at 200 Pa) was observed for the sample coated with copper on both sides. Therefore, it may be concluded that the coating of PETE with copper minimally affected filtration parameters. Observed results may be associated with the changes of porosity of the sample due to the presence of the copper coatings on the surface.

The obtained results indicate 16%–18% decreases of the PETE air permeability accompanied its metallization (PETE→PETE-Cu⁽⁰⁾-1: from 65.5 mm/s to 54.5 mm/s at 100 Pa and from 147 mm/s to 123 mm/s at 200 Pa and PETE→PETE-Cu⁽⁰⁾-2: from 65.5 mm/s to 51.8 mm/s at 100 Pa and from 147 mm/s to 117 mm/s at 200 Pa, respectively) and 4%–5% decrease between air permeability PETE-Cu⁽⁰⁾-1 and PETE-Cu⁽⁰⁾-2 (from 54.5 to 51.8 at 100 Pa and from 123 to 117 at 200 Pa, respectively). These results may be associated with changes of porosity of the samples caused by formation of copper layers on the PETE surface during sputtering process (PETE → PETE-Cu⁽⁰⁾-1 → PETE-Cu⁽⁰⁾-2).

3.6.2. Tensile Strength

The results of tensile testing, i.e., tensile strength [kN/m] and elongation at maximum load [%], of PETE fabric and PETE-Cu⁽⁰⁾ composites, determined in longitudinal and horizontal modes, are listed in

Table 5. The results of tensile strength test of the unmodified PETE fabrics and PETE-Cu⁽⁰⁾ composites, in accordance with the EN ISO 10319:2015 [116].

Parameter	PETE		PETE-Cu(0)
			PETE-Cu(0)-1		PETE-Cu(0)-2
	Longitudinal	Horizontal	Longitudinal	Horizontal	Longitudinal	Horizontal
Tensile strength [kN/m]	7.0 ± 0.26	6.2 ± 0.10	7.8 ± 0.20	6.0 ± 0.52	7.6 ± 0.14	6.0 ± 0.26
Elongation at maximum load [%]	375 ± 9.35	421 ± 13.7	403 ± 6.76	405 ± 20.1	396 ± 6.75	413 ± 24.7

The results have been measured in triplicate and presented as a mean value.

.

The results presented opposite tendencies regarding as well elongation determined for PETE and PETE-Cu(0) in longitudinal and horizontal modes. Thus, ca 10% increase of tensile strength in longitudinal mode for PETE-Cu⁽⁰⁾ in comparison with starting PETE fibre (7.8–7.6 vs 7.0 [kN/m]) is accompanied by corresponding ca 3% decrease in horizontal mode (6.0 vs 6.2 [kN/m]), respectively. At the same time, the differences of tensile strengths, measured for PETE-Cu⁽⁰⁾-1 and PETE-Cu⁽⁰⁾-2 in both modes are negligible.

Similarly, the elongation at maximum load increases ca 6% from 375% for PETE to 396%–403% for PETE-Cu⁽⁰⁾ fabrics in longitudinal mode and decreases ca 5% from 421% for PETE to 405%–413% for PETE-Cu⁽⁰⁾ fabrics in horizontal mode, respectively. At the same time, the differences in the elongations at maximum load, measured for PETE-Cu⁽⁰⁾-1 and PETE-Cu⁽⁰⁾-2 are negligible (lay in error limit).

3.6.3. Thickness

The data of thickness of the unmodified PETE fabrics and PETE-Cu⁽⁰⁾ composites are listed in

Table 6. Thickness of the unmodified PETE fabrics and PETE-Cu⁽⁰⁾ composites, in accordance with the EN ISO 5084:1999 [117].

Parameter		PETE	PETE-Cu(0)
			PETE-Cu(0)-1	PETE-Cu(0)-2
Average thickness [mm]	1 kPa	0.66 ± 0.02	0.67 ± 0.02	0.67 ± 0.02

The results have been measured in triplicate and presented as a mean value, estimated to two decimal point.

. These data displayed negligible differences with the average values of measured thickness at 1 kPa in the range of 0.66–0.67 mm.

3.6.4. Thermal Resistance

The measured thermal resistance, steam resistance and steam permeability of PETE fabric and PETE-Cu⁽⁰⁾ composites are listed in

Table 7. Thermal resistance, steam resistance and steam permeability of the unmodified PETE fabrics and PETE-Cu⁽⁰⁾ composites, according to the EN ISO 11092:2014-11 [118].

Parameter	PET	PETE-Cu(0)
		PETE-Cu(0)-1	PETE-Cu(0)-2
Thermal resistance [m2K/W]	0.004	0.007	0.009
Steam resistance [m2Pa/W]	2.770	2.820	3.240
Steam permeability [g/m2Pah]	0.537	0.528	0.459

The results have been measured in triplicate and presented as a mean value, estimated to three decimal points.

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The obtained results of thermal resistance exhibited a 1.75–2.25 fold increase of PETE-Cu⁽⁰⁾ composites in comparison with starting PETE fabrics, namely from 0.004 m²K/W for PETE to 0.007 m²K/W for PETE-Cu⁽⁰⁾-1 (the one-side metaled sample) and 0.009 m²K/W for PETE-Cu⁽⁰⁾-2 (the sample coated on both sides), respectively.

The steam resistance [m²Pa/W] of PETE-Cu⁽⁰⁾ composites exhibited ca 2%–17% increase in comparison with starting PETE fabrics, namely 2% increase (from 2.770 for PETE to 2.82 for PETE-Cu⁽⁰⁾-1) and a 17% increase for PETE-Cu⁽⁰⁾-2 (from 2.770 for PETE to 3.24 for PETE-Cu⁽⁰⁾-2), respectively.

The steam permeability [g/m² Pah] of PETE-Cu⁽⁰⁾ composites exhibited ca 2%–15% decrease in comparison with starting PETE fabrics, namely a 2% decrease (from 0.537 for PETE to 0.528 for PETE-Cu⁽⁰⁾-1) for PETE-Cu⁽⁰⁾-1 and a 15% decrease for PETE-Cu⁽⁰⁾-2 (from 0.537 for PETE to 0.459 for PETE-Cu⁽⁰⁾-2), respectively.

Recapitulating, the metallization of PETE fabrics increased the thermal resistance and steam resistance of obtained composites PETE-Cu⁽⁰⁾ in comparison with the starting PETE fabrics in degrees dependent on the copper content. The opposite trend was observed for steam permeability, with a decrease of permeability increasing with the copper content.

3.7. Antimicrobial Properties

3.7.1. Antibacterial Activity

The PETE-Cu⁽⁰⁾ samples were subjected to antimicrobial activity tests against Gram-negative Escherichia coli (ATCC11229) and Gram-positive Staphylococcus aureus (ATCC 6538). The inhibition experiment results for PETE and PETE-Cu⁽⁰⁾ are presented in Figure 8.

The similar pictures have been obtained for PETE and PETE-Cu⁽⁰⁾ against Staphylococcus aureus species.

The results of tests on the antibacterial activity of PETE-Cu⁽⁰⁾ composites against Escherichia coli and Staphylococcus aureus are illustrated in

Table 8. Results of tests on the antibacterial activity of PETE-Cu⁽⁰⁾ composites, procedure in accordance with the EN ISO 20645:2006 [119].

Sample Name	Average Inhibition Zones for Bacterial Growth (mm)
	E. coli	S. aureus
PETE	0	0
PETE-Cu(0)-1	1	1
PETE-Cu(0)-2	2	1

Concentration of inoculum (bacterial suspension) number of live bacteria: E. coli—CFU/mL = 1.2 × 108 and S. aureus—CFU/mL = 1.7 × 108.

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The results revealed strong visible inhibition zones of bacterial growth on Petri dishes (Figure 8), and therefore proved antimicrobial protection of metallized surface of PETE-Cu⁽⁰⁾ against representative Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria (Table 8).

3.7.2. Antifungal Activity

Results of antifungal activity tests against a colony of Chaetomium globosum (ATCC 6205) of PETE fabrics and PETE-Cu⁽⁰⁾ composites are illustrated in Figure 9 and listed in

Table 9. Results of tests on the antifungal activity of PETE-Cu⁽⁰⁾ composites, procedure in accordance with the PN EN 14119: 2005 point 10.5 (B2) [120].

Sample Name	Fungal Average Inhibition Zone (mm)
PETE	0	Visible growth on sample surface
PETE-Cu(0)-1	3	No visible growth on sample surface
PETE-Cu(0)-2	3

Inoculum concentration, number of fungal spores in 1ml; [CFU/mL] = 2.2 × 10⁶ (determined using a Thoma chamber).

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The results revealed strong visible inhibition zones of fungal growth on Petri dishes (Figure 9), and therefore proved antifungal protection of metallized surface of PETE-Cu⁽⁰⁾ against Chaetomium globosum (Table 9).

The results revealed that the unmodified PETE sample induced the strong fungal growth-covering of the entire surface of the control sample (Figure 9a). At the same time PETE-Cu⁽⁰⁾composites equipped with copper layers/clusters on the composite surface provided antifungal properties for both—PETE-Cu⁽⁰⁾-1 and PETE-Cu⁽⁰⁾-2 composites.

4. Conclusions

(1) In this study a novel multi-functional, antimicrobial polyester-copper “hybrid” composite material, PETE-Cu⁽⁰⁾, has been produced by one-step magnetron copper sputtering on PETE knitwear fibers (composed of poly(ethylene terephthalate) knitted textiles, PET (95%) and elastane, EA (5%)).

(2) The structural characterizations of the new PETE-Cu⁽⁰⁾ polymer composites obtained, were characterized by Scanning Electron Microscopy (SEM) and UV/VIS transmittance. The chemical compositions of the PETE-Cu⁽⁰⁾ composites obtained were achieved using Energy-Dispersive X-ray Spectroscopy, EDS (C, O, Cu surface analysis) and Atomic Absorption Spectrometry with Flame Excitation, FAAS (Cu content in the bulk).

(3) The application utility of the PETE-Cu⁽⁰⁾ composites was established by determination of their technical parameters, including filtration parameters, tensile strength, thickness, thermal and steam resistance, steam permeability, the barrier properties against UV radiation and also antimicrobial tests against Escherichia coli, Staphylococcus aureus and Chaetomium globosum fungus species.

(4) SEM images of PETE-Cu⁽⁰⁾ composites revealed that the applied magnetron sputtering of the starting PETE textile did not exhibit substantial destruction of the textile structure. Moreover, the described PETE-Cu⁽⁰⁾ composites have shown improvement of the technical parameters, including air permeability and thermal and steam resistance. PETE-Cu⁽⁰⁾ composites exhibited improvement of barrier properties against UV radiation in comparison with the unmodified PETE sample.

(5) Determined antimicrobial properties of PETE-Cu⁽⁰⁾ composites revealed the significant antibacterial action of both PETE-Cu⁽⁰⁾-1 as well as PETE-Cu⁽⁰⁾-1 composites against Escherichia coli, Staphylococcus aureus and Chaetomium globosum fungus species.

(6). Recapitulating, the PETE-Cu⁽⁰⁾ composites obtained, due to exhibited beneficial antibacterial and also technical properties, can find an application in a medical sector, for example, as a microbial barrier material.