2.2.2. Physicochemical Properties Study

Chemical structure of the samples was determined by Fourier Transform Infrared Spectroscopy FT-IR using FT-IR Nexus 470 Thermo Nicolet spectrometer, Thermo Fisher Scientific, Waltham, MA, USA. The spectra were collected for lyophilized samples using ATR (attenuated total reflection) adapter. Zinc oxide nanoparticles (50 mg per analysis) were investigated over their crystalline structure by X-ray Diffraction (XRD) method using BRUKER Advanced D8, BRUKER, Zastávka, Czech Republic. UV-Vis spectra were collected with UV-Vis spectrophotometer Agilent 8453, Santa Clara, USA. Samples conductivity was measured by their placing between two platinum plates (1 cm distance, 1 cm² area) using SBF as a reference. Into the measuring cell, simulated body fluid solution was poured at 25 °C. Then, SBF conductivity was measured (control). In the next step, each sample was immersed in the solution and placed between two electrodes. Conductivity was measured by Elmetron conductivity meter. ZnO NPs morphology was investigated by Transmission Electron Microscope (TEM), Jeol, Peabody, MA, USA. For the analysis, nanoparticles were dissolved in methanol, placed on copper mesh with formvar-modified surface and left for total evaporation. The methanol was used as a solvent since it evaporates faster than water and enables homogeneous distribution of evaluated NPs during imaging. Image was taken under HT = 80 kV, exposure time 800 ms. The electron dose was 2771.9 e/nm². The samples and composites were evaluated also by Scanning Electron Microscopy, FEI Quanta 650 FEG SEM microscope, ThermoFisher Scientific, Oregon, USA. The curvature and roughness of the sample were determined using Fiji software and Kappa plugin (Open-source software). The elemental composition was investigated by an EDAX^® adapter ((X-ray fluorescence method). Morphology of the swollen samples was evaluated using Inverted Optical microscope equipped with epifluorescence adapter, Delta Optical, Zielona Góra, Poland. For this purpose, the samples were swollen with fluorescein methanol solution.

Swelling capacity was investigated by determining the amount of the swelling medium absorbed after 1 h (distilled water and SBF) based on the weight change of the sample. The SD (swelling degree) was calculated using Equation (1):

where SD is the swelling degree, g/g; W_t is the sample weight after 1 h, g; and W₀ is the initial weight of the sample, g.

Porosity of the samples was calculated using Equation (2). For the study, the samples were immersed in 10 cm³ of the isopropanol and the volume change was determined.

where p is the porosity, %; V₁ is the initial volume of isopropanol, cm³; V₂ is the volume of isopropanol with immersed sample, cm³; and V₃ is the volume of isopropanol after sample removal, cm³.

Water vapor permeability was determined by filling multi plates (1 cm² area) with distilled water (5 cm³) and sealing them with each sample. The WVTR was measured based on the amount of evaporated water (weight loss) using Equation (3):

where W_t is the weight after time t; W₀ is the initial weight; t is the measuring time; and A is the area of the opening of polystyrene well.

To determine mechanical properties of the samples, tensile strength (TS) was measured. For this purpose, the samples were prepared in the form of a “dog bone” by placing swollen hydrogels in the molds printed on 3D Ender printer, Botland, Batlin, Poland, with the dimensions presented in the PN-EN ISO 527: 1998 standard for plastics mechanical properties evaluation followed by lyophilization. The samples of the specified shape (thickness 4.0 mm, measuring part width 10 mm, and overall length 150 mm). The tests were carried out for the samples swollen with simulated body fluid.