2.3. Characterization of the PLA Cu Samples

Fourier transform infrared (FT-IR) spectra of cured samples were recorded on Spectrum BX (Perkin Elmer, Waltham, MA, USA) FTIR spectrometer, equipped with ATR accessory (PIKE MIRacle^TM).

The rectangular specimens specific to this test were subjected to tensile tests using the Lloyd LR5k Plus universal mechanical testing machine (Lloyd Instrumente, Ameteklns, West Sussex, England), with a maximum allowed capacity of 5KN, at a loading force of 0.5 N and a speed of 1 mm/minute at ambient temperature (25 °C), according to the ASTM D638-14, using Nexygen software (version 4.0). All the data are the average of at least seven measurements. The statistical differences between the groups of investigated samples were statistically analyzed using the one-way ANOVA test.

The flexural strength was achieved by the 3-point technique, according to ASTM D 790; the data were processed using the Nexygen software (version 4.0). All the data are the average of at least seven measurements. The statistical differences between the groups of investigated samples were statistically analyzed using the one-way ANOVA test.

The analysis was carried out with the help of a 630e, 700 °C Mettler-Toledo calorimeter (Switzerland). Measurement conditions: aluminum crucible-40 μL; heating speed: 10 °C/min; temperature range 25–200 °C; final landing 0.5 min; atmosphere: nitrogen; flow rate: 80 mL/min.

The samples were characterized by thermogravimetric analysis using a NETZSCH STA 449C Jupiter simultaneous TGA-DSC (Germany, Selb) with a heating rate of 10 °C/min to 800 °C in a nitrogen atmosphere (99.999% purity, 50 mL/min).

Absorption is expressed as a percentage by weight increase of a sample according to the ASTM D57-Standard Test Method for Absorption of Plastics.

Working procedure:

The rectangular samples with dimensions of 20 mm length, 10 mm width, and 3 mm thickness were placed in a desiccator at 23 °C until a constant mass value, with a precision of 0.001 g (initial M).

The samples were placed individually in vials with 15 mL of 10% saline solution at a constant temperature of 23 °C. At certain periods of time (24 h, 4, 7, 14 days), the samples were removed from the immersion medium and lightly dried with absorbent paper and weighed (final M).

The absorption percentage is calculated with Equation (1):

For each group of investigated samples, four weight percentage increase measurements were recorded. After recording the values, the average and standard deviation were calculated, statistically analyzing with the help of the one-way ANOVA test and the Tukey test, the differences within each group, depending on the immersion time and the differences between the investigated groups, depending on the amount of water absorbed (p value below 0.05 being considered significant statistically).

The water contact angle was determined using a Drop Shape Analyzer, DSA25 (Hamburg, Germany), at room temperature. A drop of 20 μL of distilled water was placed on the surface of the samples and, after the stabilization period (30 s), the image was recorded, and the contact angle was measured with a dedicated software.

The microorganisms tested in this study were: Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, and Pseudomonas aeruginosa ATCC 27853, from the collection of the Microbiology Laboratory, Faculty of Biology and Geology, Babeş-Bolyai University of Cluj-Napoca.

Each bacterial strain was grown for 24 h on a Nutrient Agar medium [35]. Following that, a dilution of 0.5 McFarland was made from each strain in sterile physiological serum. From these dilutions, each Petri dish was inoculated with the help of a sterile swab soaked in the 0.5 McFarland microbial suspension, spreading over the entire surface of the solid culture medium (Mueller Hinton-Oxoid).

The inoculated Petri dishes were dried for 20 min at 37 °C. Then, with sterile tweezers, the samples were cut in the form of a square of approximately 5 mm and were aseptically taken and applied to the solid culture medium.

Incubation was done for 48 h at 37 °C. The reading was done by measuring the size of the inhibition zone (x): the larger the size of the inhibition zone, the greater the sensitivity of the bacteria to the respective antibacterial substances [36].