1. Introduction
Poly(lactide) (PLA) is a multifunctional application polymer widely engaged in various biomedical applications, including controlled drug delivery, wound healing, tissue engineering and regenerative medicine [
1,
2,
3,
4,
5,
6,
7,
8].
Substantial importance in this field is played by PLA composites equipped with antibacterial additivities, in the majority organic bactericides (e.g., ampicillin [
9] chlorhexidine [
10,
11], ciprofloxacin [
12,
13], doxycyclin [
14], gentamycin [
15], oflaxin and vancomycin [
16], triclosan [
17,
18,
19], and/or curcumin [
20], etc.). Due to an established role of metal nanoparticles in medicine [
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33], the group of increasing importance is PLA-inorganic hybrids [
34], due to their high antibacterial efficiency, simplicity and also low cost of technological preparation.
Some of the representative applications of antibacterial PLA composites are listed in
Table 1.
Among various inorganic bactericides of medicinal interest, considerable attention has been focused on copper and its salts reflected by over 5000 documents on antibacterial copper and nearly 300 documents on antibacterial metallic copper abstracted by Scopus [
34,
35], respectively.
In the light of continuous expansion of antibiotic resistance to bacteria, copper nanoparticles (CuNPs) with such attributes of copper as chemical stability (E
1 = 0.52 V) [
36], developed surface and antibacterial prolonged antibacterial activity [
37] (a very slow dissolution of metallic copper in water with subsequent formation of very poor soluble copper oxides Cu
2O/CuO [
37]), as well as a fair abrasion resistance of copper layer’s (hardness 230 kgf mm
−2) [
38], contrasted with their negligible contact toxicity toward animals [
39,
40,
41,
42], became a valuable alternative to traditional antibiotics [
25,
27,
43,
44]. The antibacterial activity of copper’s metallic surface is regarded as resulting from two supplemental mechanisms—surface–surface interaction of copper and bacteria (contact killing) and/or surface oxidation of copper with subsequent release of antibacterial cupric ions [
25,
27,
43,
44,
45,
46,
47,
48].
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.
3. Results and Discussion
3.1. SEM/EDS
SEM micrographs of poly(lactide) samples and modified poly(lactide) samples coated with copper (PLA–Cu
0(t)) are presented in
Figure 1,
Figure 2 and
Figure 3, respectively.
The SEM images of the starting PLA non-woven and PLA–Cu
0(t) composites reflect the changes in their morphology occurring during sputtering process (PLA→PLA–Cu
0(10)→PLA–Cu
0(30)). Thus, PLA fibres images present uniform randomly oriented fibers, with interconnected pores and space with relatively smooth surface in PLA fibre images (
Figure 1). The diameters of poly(lactide) fibers applied ranged: 0.65–5.0 µm (
Figure 1c).
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.
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–Cu0(10), and 66.86 for PLA–Cu0(30)) and simultaneous substantial decrease of carbon (from 51.7% (PLA) to 42.33% (PLA–Cu0(10)) and 21.00 (PLA–Cu0(30)) and oxygen (from 48.33% (PLA) to 38.13% (PLA–Cu0(10)) and 12.14 (PLA–Cu0(30)) contents, respectively. This causes a subsequent change of the “surface molecular formula” (SMF) from C3O2 for PLA, to C3O2Cu0.27 for PLA–Cu0(10) and/or to C3O1.2Cu1.8 for PLA–Cu0(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.
3.2. 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–Cu0(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–Cu0(t) composites, which are almost identical.
The strongest bands of the presented spectra are: 1760 (0.09 (PLA) and 0.03 (PLA–Cu0(10)) identified as νC = O; 1270 (0.07(PLA) and 0.01 (PLA–Cu0(10))—identified as δCH + νCOC; 1215–1185 (0.08(PLA) and 0.03 (PLA–Cu0(10))—identified as νasCOC + rasCH3; 1130 (0.12 (PLA) and 0.03 (PLA–Cu0(10))—identified as rasCH3; 1100–1090 (0.06 (PLA) and 0.13 (PLA–Cu0(10))—identified as νs COC, and 1045 (0.08 (PLA) and 0.02 (PLA–Cu0(10)) identified as ν C-CH3.
The bands observed in the low-frequency region 740–695 cm−1 can be assigned to: δC = O (760–740 cm−1, absorbances 0.03 (PLA) and 0.02 (PLA–Cu0(10)) and γC = O (715–695 cm−1, absorbances 0.02 (PLA) and 0.01 (PLA–Cu0(10)).
The spectra of PLA–Cu
0(t) offered noticeable differences, mainly at wavelength 1620 cm
−1 (absorbances 0.00 (PLA), 0.013 (PLA–Cu
0(10) and 0.021 (PLA–Cu
0(30)). Also at 500–400 cm
−1 absorbances derived from PLA–Cu
0(t)composites are higher than for starting PLA. These differences can presumably have resulted from copper’s presence in the PLA and are similar to those obtained by Diaz-Visurraga et al. in a FTIR study of alginate-stabilized copper nanoparticles. [
71]
3.3. 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–Cu0(10), PLA–Cu0(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.
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].
3.4. 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.
The results of air flow resistance of modified polylactide show that 10 min magnetron sputtering modifications slightly decrease filtration parameters of PLA sample: PLA vs. PLA–Cu0(10): 458 vs. 437 at 50 Pa and 853 vs. 839 100 Pa, respectively but 30 min magnetron sputtering reduces filtration parameters: PLA vs. PLA–Cu0(30): 458 vs. 388 at 50 Pa and 853 vs. 778 at 100 Pa, respectively.
The listed tensile strength results of PLA and PLA–Cu0(t) composites show the ca. 11-fold increase for PLA–Cu0(10) (0.575 [kN/m) and 1.21-fold increase for PLA–Cu0(30) (0.071 [kN/m]), compared with unmodified PLA non-woven (0.05 [kN/m].
The results of relative elongation at maximum load [%] of poly(lactic acid) non-woven and PLA–Cu0(t) composites show ca. 2.5-fold increase of this parameter of PLA–Cu0(10) compared with PLA (PLA vs. PLA–Cu0(10): 11.98% vs. 28.75%) and ca. 4.5-fold decrease for PLA–Cu0(30) compared with PLA (PLA vs. PLA–Cu0(30): 11.98% vs. 2.65%). These results indicate that the sample PLA–Cu0(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–Cu0(30) influenced negatively on the mechanical properties of this composite.
3.5. 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 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–Cu0(10): 10 min-9.91 g/kg; PLA–Cu0(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.
3.6. Antimicrobial Properties
Antibacterial Activity
The polylactide nonwoven (PLA) and polylactide-copper composites PLA–Cu
0(t) were subjected to antimicrobial activity tests against Gram-negative
Escherichia coli (ATCC11229) and Gram-positive
Staphylococcus aureus (ATCC 6538) (
Table 9).
Results of antimicrobial studies demonstrate antimicrobial protection against different bacterial species of new composites for
Escherichia coli and
Staphylococcus aureus (
Table 8) according to standard: PN-EN ISO 20645:2006 [
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].
3.7. 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).
4. 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–Cu0 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–Cu0 synthesized in comparison with starting raw PLA non-woven fabrics;
composite with potential antimicrobial properties.
The listed attributes of the PLA–Cu0 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.