**1. Introduction**

Materials that substitute bone tissues are of great interest to the scientific community, as traumatic injuries and pathologies in which the skeletal structure is damaged are extremely common [1–3]. Several years ago, it was thought that human tissues or organs were only replaceable by transplants or metallic and polymeric devices. However, many of these materials can cause an undesirable immune response, leading to inflammation and rejection. Biomaterials based on the SiO2–CaO–Na2OP2O<sup>5</sup> system, commonly called bioactive glass (BG), have the ability to form bonds with bone and connective tissues; this ability is attributed to the formation of a silica layer with a high surface area and the formation of polycrystalline hydroxyapatite layers on the bioactive glass surface [4,5]. BGs have been studied in soft-tissue engineering applications such as peripheral nerve regeneration and chronic pain treatment as well [6].

BG is frequently obtained by two methods: 1. melt-derived glass, in which the oxides are silica, calcium, phosphate, and sodium precursors, which then undergo further solidification; and 2. sol–gel synthesis, which employs low processing temperatures for an

**Citation:** Carbajal-De la Torre, G.; Zurita-Méndez, N.N.; Ballesteros-Almanza, M.d.L.; Ortiz-Ortiz, J.; Estévez, M.; Espinosa-Medina, M.A. Characterization and Evaluation of Composite Biomaterial Bioactive Glass–Polylactic Acid for Bone Tissue Engineering Applications. *Polymers* **2022**, *14*, 3034. https://doi.org/ 10.3390/polym14153034

Academic Editors: Antonia Ressler and Inga Urlic

Received: 6 July 2022 Accepted: 21 July 2022 Published: 27 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

economical method in which the properties can easily be controlled. Overall, the bioglasses obtained by this method exhibited high surface areas and suitable porosity, providing osteogenic potential [7–9]. Furthermore, lactic acid (LA) production in its L(+) isomerism is promoted by the intense physical activity of the muscles; although LA is unassimilable by the organism, its produced polymer (polylactic acid, or PLA) has a high biodegradability rate and is bio-compatible, immunologically inert, non-toxic, and absorbable [10]. Consequently, this polymer could be used for the elaboration of a composite biomaterial for bioengineering applications such as controlled drug release systems, bioabsorbable fixation devices, and bone regeneration implants. Through the method of direct polycondensation, it is possible to obtain low molecular weight products, which are important in biomedical applications [11]. As is known, the surface reactions of materials with their biological environment occur a few seconds after they are implanted in the body, interacting with proteins present in the physiological environment; hence, it is important to evaluate the in vitro biological behaviour of the biomaterials. Simulated body fluid (SBF) and Hank's saline solution at 37 ◦C are the aqueous media that allow for understanding of the corrosion mechanism of composite biomaterials, as their ionic compositions are close to those of human plasma [8–12].

This research synthesized BG using the sol–gel method, using tetraethyl orthosilicate (TEOS) as an initial precursor. The polymeric material (PLA) was synthesized by the ring-opening polymerization of lactic acid, and they were subsequently mixed by employing a solvent in two different weight compositions (BG70-PLA30 and BG30-PLA70), then deposited by dip coating on 316L stainless steel sheets of about 0.2 mm in thickness. Assessment of the corrosive behaviour in Hank's solution and simulated body fluid (SBF) was performed using electrochemical techniques. Their bioactivity in PBS was evaluated by the ASTM F1635 standard test method for in vitro degradation testing of poly (L-lactic Acid) resin and fabricated as scaffolds. In contrast, SBF bioactivity was evaluated using the methodology of Kokubo et al. [13]. This project discusses the results of the methods and measurements of the properties of these scaffolds, describes a desirable resistance to degradability and bioactivity in simulated body solutions, and uses extensive electrochemical analysis to evaluate the degradation conditions of the composite biomaterial.

### **2. Materials and Methods**

The sol–gel technique was used to synthesize the bioactive glass, while polylactic acid was synthesized via lactic acid (LA) polycondensation. Furthermore, BG, PLA, and BG-PLA composite samples were prepared and characterized by Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) analysis, and scanning electron microscopy (SEM). The corrosion behaviour of the composites in Hank's balanced salt solution and simulated body fluid (SBF) at 37 ◦C were performed using electrochemical techniques. The biomaterial bioactivity in SBF and phosphate-buffered saline (PBS) was measured each week for 28 days.

### *2.1. PLA Synthesis*

In order to reproduce a more efficient PLA production process, the ring-opening polymerization (ROP) method was carried out. The initial lactic acid of reactive grade (Meyer ®) was put inside using a rotary evaporator (Hahnshin Scientific Co., model: HS-2000NS, Michoacán, México), applying a 35-rpm rotation, heating temperature, and vacuum at <sup>−</sup>200 mmHg. Tin(II) 2-ethyl hexanoate (Sigma-Aldrich ® ~95%, Michoacán, México) was added, and the temperature was raised to 175 ◦C. The reaction takes 6 h under these conditions. The obtained PLA was dissolved in propanone (Meyer ®, Michoacán, México) and precipitated with distilled water. The white PLA powder was then washed, filtered, and dried.

### *2.2. BG-PLA Composite Synthesis*

All precursors were reactive grade and used without further purification. The method was performed in two steps. In the first step, tetraethyl silicate (98% Sigma-Aldrich ®, Michoacán, México) and 0.1 M nitric acid (JT Baker ®, Michoacán, México) were mixed at room temperature. Then, triethyl phosphate (TEP: 99.8%, Sigma-Aldrich ®, Michoacán, México) and calcium nitrate tetrahydrate (99%, Sigma-Aldrich ®, Michoacán, México) were added at intervals. The reaction lasted for an additional hour after the last compound was added. The bioactive glass was obtained in this step. The general synthesis reactions can be observed in Equation (1). In the second step, the material obtained as a gel form was mixed with the PLA obtained in two weight percentages (wt.%), namely, BG30-PLA70 and BG70-PLA30, employing dissolvent propanone (Meyer ®, Michoacán, México). The composite biomaterial was first kept in a sealed glass jar at room temperature for ten days and later at 70 ◦C for three days. Thermal treatment was performed at 120 ◦C for two days.

$$\text{SiC}\_8\text{H}\_{20}\text{O}\_4 \xrightarrow{HNO\_3.0.1\text{ M}} \text{Si(OH)}\_4 \xrightarrow{\text{TEP, Ca(NO}\_3)\_2Al\_2\text{O}} \text{(SiO}\_2\text{)}\_x \text{(CaO)}\_y \text{(P}\_2\text{O}\_5\text{)}\_z \tag{1}$$

### *2.3. SBF and PBS Preparation*

The SBF was prepared according to the Kokubo protocol [13]. The reaction was controlled at a pH of 7.45 ± 0.01. The obtained solution was cooled at 20 ◦C and kept under refrigeration at 7 ◦C. At the same time, the PBS solution was obtained by dissolving a phosphate-buffered saline tablet (Sigma®, Michoacán, México) in 200 mL of deionized water to obtain a 0.01 M phosphate buffer with 0.0027 M KCl and 0.137 M NaCl contents, with a pH of 7.4 at 25 ◦C.

### *2.4. BG-PLA Coatings*

A 316L stainless steel sheet approximately 0.2 mm thick was used as the substrate material. These sheets were polished using sandpapers of 230, 300, 500, 600, and 1000 grades. The 316L substrate surface was chemically treated by immersion in NaOH (Sigma-Aldrich ®, Michoacán, México) 6M solution for 24 h and cleaned with deionized water and acetone. The composite material for the coating application was obtained by dissolving 5 g of BG-PLA powders in 20 mL of acetone (reagent grade, Meyer ®, Michoacán, México). The coatings were applied by immersion using the dip-coating method with a 176 mm/min speed rate and residence time of 30 s. The obtained layers were dried at 120 ◦C for 24 h.

### *2.5. BG-PLA Scaffolds Design*

BG-PLA scaffolds were obtained by the gel-pressing technique, in which 10 g of each composite material (BG70-PLA30 and BG30-PLA70) was dissolved in 40 mL of chloroform (CHCl3, Meyer ® 99.8%, Michoacán, México). Subsequently, porosity was achieved by particle leaching using NaCl crystals a maximum of 500 µm of diameter in a proportion of 60 wt.% of the weight of the total components. The homogeneous phase was pressed into containers with 0.635 cm diameter and 1.2 cm height. The solvent was first evaporated at room temperature for two days, then heated in an oven at 50 ◦C for 24 h. The scaffold samples were immersed in distilled water to eliminate salt, and the porosity formation was dried again in an oven and stored in sterile Petri dishes.

### *2.6. BG-PLA Bioactivity in SBF*

The main characteristic of bioactivity in SBF is the formation of hydroxyapatite (HAp) on the material surface. For this evaluation, the BG-PLA scaffolds were immersed in triplicate in a polyethylene bottle (three scaffolds per bottle) with SBF solution in a 100 mL/g ratio at a controlled temperature of 37 ◦C and pH 7.4. Bioactivity measurements were obtained at 7, 14, 21, and 28 days. The nomenclature identification for the samples is shown in Table 1. At the end of each test, the scaffolds were removed from the SBF solution, gently rinsed with deionized water, and allowed to dry for 4 to 5 days in an incubator at 37 ◦C. The pH of the solution was monitored, and the SBF solution was replaced every week due

to the cation concentration decreasing during the experiments. Bioactivity results were complemented by XRD, FTIR, and SEM characterization.


**Table 1.** Identification of samples evaluated in SBF.

## *2.7. BG-PLA Degradation in PBS*

Degradation monitoring was carried out by measuring the change in weight during the sample's immersion in PBS. Similarly, the BG-PLA pieces were immersed at 37 ◦C for 1, 2, 3, and 4 weeks, with the weight change measured after each period. The PBS solutions were replaced every seven days. This study was performed according to the standard ISO of 10993-13:2010 [14]. The percentage of weight loss was calculated from Equation (1). The nomenclature identification for the samples is shown in Table 2.

$$\text{Weightless } (\%) = 100 \left( \frac{W\_1 - W\_2}{W\_1} \right) \tag{2}$$

where *W*<sup>1</sup> and *W*<sup>2</sup> are the weight of the dry composite before and after immersion, respectively.

**Table 2.** Identification of samples evaluated in PBS.


### *2.8. FTIR and XRD Characterizations*

Fourier transform infrared spectroscopy analysis was performed with a Bruker spectrometer model Tensor 27. The applied measurement range was 4000 to 400 cm−<sup>1</sup> , with a 4 cm−<sup>1</sup> resolution and sample and background scan times of 32 scans. The samples were obtained by mixing 0.0020 g of the powders and 0.20 g of KBr, then compressed by applying 9.9 tons of pressure for 1 min with a PIKE Technologies CrushIR hydraulic press machine. Then, the compacted sample was characterized with FTIR equipment. XRD measurements were conducted using a D8 Advanced Da-Vinci equipment X-Ray diffractometer. Scans were taken with a 2θ step size of 0.04◦ from 20◦ to 90◦ and a counting time of 0.3 s using Cu Kα radiation. The phases were identified by matching the observed patterns to the entries in the indexing software.

### *2.9. Electrochemical Tests*

Electrochemical tests were performed using a potentiostat/galvanostat Gill-AC (ACM Instruments) controlled by a computer. A three-electrode cell arrangement was used with an Ag/AgCl saturated reference electrode (SSCE-RE), platinum wire as an auxiliary electrode (AE), and the coating samples (WE). Hank's balanced salt solution (Sigma-Aldrich ®, Michoacán, México) modified with sodium bicarbonate (without phenol red, calcium chloride, or magnesium sulphate, sterile-filtered, and suitable for cell culture) and simulated body fluid (SBF) at 37 ± 1 ◦C was the electrolyte used to emulate human body temperature, which was controlled by an electric heating band.

A polarization potential scan obtained potentiodynamic polarization curves (TF) from −500 mV to +1500 mV vs. open circuit potential (OCP) at a scan rate of 1 mV/s. Corrosion current density values, *icorr*, and other parameters were calculated using the Tafel extrapolation method between an extrapolation range of ±100 mV around the OCP. Before running the experiments, a 10 min delay time was set until the OCP reached the steadystate condition. The LPR measurements were obtained in a range of ±15 mV vs. the OCP with a scan rate of 1 mV/s every 15 min for 48 h. Polarization resistance (*Rp*) and current density kinetics were obtained by Ohm's law and the Stern and Geary equations [15]. The electrochemical impedance spectroscopy (EIS) measurements were carried out at OCP using a voltage signal with an amplitude of 30 mV and a frequency interval between 23,000 and 0.01 Hz.

### **3. Results**

### *3.1. X-ray Diffraction Analysis*

The diffraction patterns obtained for the biomaterials are shown in Figure 1. As can be observed, the obtained BG presents a ceramic formulation system composed of SiO2– Na2O–CaO–P2O5. The presence of P2O<sup>5</sup> allows formation of a network, promoting the glass crystallization process [16], and induces the formation of a calcium phosphate layer that crystallizes into biomimetic hydroxyapatite due to the incorporation of hydroxide and carbonate ions from the biological fluid [17]. The X-ray diffraction results for the orthorhombic lattice PLA were compared with the crystallographic PDF data 00-064-1624, presenting diffractions in 2θ angles positioned at 12.42◦ , 16.63◦ , 19.08◦ , and 22.3◦ , correlated to the Miller's indices of the planes (103), (200), (203), and (211) of the polymeric material. As expected, the composition of the BG-PLA composite at both proportions (BG70-PLA30 and BG30-PLA70) agrees with the presence of each phase. *Polymers* **2022**, *14*, x FOR PEER REVIEW 6 of 24

**Figure 1.** X-ray diffraction for the obtained materials. **Figure 1.** X-ray diffraction for the obtained materials.

### *3.2. FTIR Characterization 3.2. FTIR Characterization*

of the phosphate (POସ

Fourier transform infrared spectroscopy with the KBr technique has been used recently to study the structure–composition relationship in various glasses and glass ceramics [18]. The FTIR results for the PLA and BG samples are shown in Figure 2. According to the analysis of the BG spectrum (Figure 2a), absorption peaks could be observed at 1386, Fourier transform infrared spectroscopy with the KBr technique has been used recently to study the structure–composition relationship in various glasses and glass ceramics [18]. The FTIR results for the PLA and BG samples are shown in Figure 2. According to the analysis of the BG spectrum (Figure 2a), absorption peaks could be observed at 1386, 838,

838, and 461 cm−1, representing the bending and stretching vibrations of Si−O−Si bonds. The vibrational band with low intensity at 566 cm−1 corresponds to the bending vibrations

different OHି groups, and represents the surface silanol groups related to different hydroxyl groups. This indicates the superposition of stretching modes of non-hydrogenbonded silanols (isolated silanol groups) and hydrogen-bonded-silanol (vicinal silanol groups) [21]. As can be observed in the polylactic acid FTIR spectra in Figure 2b, the stretching vibrations of C-C bonds are found at 865 cm−1, the asymmetric and symmetric C−O−C stretching peaks are related to 1132 and 1211 cm−1, respectively, the C−H symmetric bending can be located at 1375 cm−1, −CH3 asymmetric bending can be seen at 1457 cm−1, C=O stretching bonds are represented at 1755 cm−1, and C−H symmetric and asym-

metric stretching at the 2944 and 3000 cm−1 peaks, respectively [22].

and 461 cm−<sup>1</sup> , representing the bending and stretching vibrations of Si−O−Si bonds. The vibrational band with low intensity at 566 cm−<sup>1</sup> corresponds to the bending vibrations of the phosphate (PO3<sup>−</sup> 4 ) groups [19], suggesting that the phosphate can be considered as a network former [20]. The broad band at 3427 cm−<sup>1</sup> could be ascribed to the vibration of different OH− groups, and represents the surface silanol groups related to different hydroxyl groups. This indicates the superposition of stretching modes of non-hydrogen-bonded silanols (isolated silanol groups) and hydrogen-bonded-silanol (vicinal silanol groups) [21]. As can be observed in the polylactic acid FTIR spectra in Figure 2b, the stretching vibrations of C-C bonds are found at 865 cm−<sup>1</sup> , the asymmetric and symmetric C−O−C stretching peaks are related to 1132 and 1211 cm−<sup>1</sup> , respectively, the C−H symmetric bending can be located at 1375 cm−<sup>1</sup> , <sup>−</sup>CH<sup>3</sup> asymmetric bending can be seen at 1457 cm−<sup>1</sup> , C=O stretching bonds are represented at 1755 cm−<sup>1</sup> , and C−H symmetric and asymmetric stretching at the 2944 and 3000 cm−<sup>1</sup> peaks, respectively [22]. *Polymers* **2022**, *14*, x FOR PEER REVIEW 7 of 24

**Figure 2.** FTIR spectra for the synthesized species (a)BG and (b) PLA. **Figure 2.** FTIR spectra for the synthesized species (**a**) BG and (**b**) PLA.

The analysis of the bioactivity of the composite scaffolds BG70-PLA30 in SBF by FTIR is shown in Figure 3. The results show vibrational bands related to the silanol groups, C−H, C=O, C−O−C, and P-O bonds; the broad band at 3000–3600 cm−1 is present due to the silanol groups on the composite surface. The HAp formation on the surface of the composite immersed in SBF is associated with the presence of the bands around 560–600 cm−1, which correspond to the bending vibrations of P−O bonds that are visible in the Si−Na−P system [21]. The FTIR spectrum of the BG70-PLA30 scaffolds shows a broad spectrum, reflecting the Si−O−Si symmetric stretching vibrations. The analysis of the bioactivity of the composite scaffolds BG70-PLA30 in SBF by FTIR is shown in Figure 3. The results show vibrational bands related to the silanol groups, C−H, C=O, C−O−C, and P-O bonds; the broad band at 3000–3600 cm−<sup>1</sup> is present due to the silanol groups on the composite surface. The Hap formation on the surface of the composite immersed in SBF is associated with the presence of the bands around 560–600 cm−<sup>1</sup> , which correspond to the bending vibrations of P−O bonds that are visible in the Si−Na−P system [21]. The FTIR spectrum of the BG70-PLA30 scaffolds shows a broad spectrum, reflecting the Si−O−Si symmetric stretching vibrations.

The silanol groups at 3500 cm−<sup>1</sup> are present in the FTIR spectrum of the BG30-PLA70 sample in SBF (Figure 4). Due to the higher composition of PLA in the composite, the presence of stretching vibrations of C-C, C-O-C, and C=O bonds are observed, and are associated with the crystallinity of the PLA phase. The bending and stretching vibrations of Si-O-Si bonds correspond to the BG phase. At 21 and 28 days of immersion, the formation of the Hap phase was observed in the vibrational bands with low intensity at 573 cm−<sup>1</sup> and 610 cm−<sup>1</sup> , which are related to the bending vibrations of the phosphate (PO3<sup>−</sup> 4 ) groups. Chen et al. [23] observed the vibrational bands at 608 and 561 cm−<sup>1</sup> to be associated with the strengthened intermolecular interaction of the molecules in the crystal lattice in highly-ordered arrangements. Thus, it is possible that the phosphate formation acts as a molecular link. The BG dissolution mechanism in the biological fluids was associated with ions leaching from BG into PBS, followed by decomposition of silica–oxygen bonds of the

**Figure 3.** FTIR spectra of the BG70-PLA30 composite after different soaking times in SBF.

The silanol groups at 3500 cm−1 are present in the FTIR spectrum of the BG30-PLA70 sample in SBF (Figure 4). Due to the higher composition of PLA in the composite, the presence of stretching vibrations of C-C, C-O-C, and C=O bonds are observed, and are associated with the crystallinity of the PLA phase. The bending and stretching vibrations

*Polymers* **2022**, *14*, x FOR PEER REVIEW 7 of 24

**Figure 2.** FTIR spectra for the synthesized species (a)BG and (b) PLA.

BG network and redeposition of the calcium and phosphorus ions onto the biomaterial surface [24]. which correspond to the bending vibrations of P−O bonds that are visible in the Si−Na−P system [21]. The FTIR spectrum of the BG70-PLA30 scaffolds shows a broad spectrum, reflecting the Si−O−Si symmetric stretching vibrations.

ଷି)

The analysis of the bioactivity of the composite scaffolds BG70-PLA30 in SBF by FTIR is shown in Figure 3. The results show vibrational bands related to the silanol groups, C−H, C=O, C−O−C, and P-O bonds; the broad band at 3000–3600 cm−1 is present due to the silanol groups on the composite surface. The HAp formation on the surface of the composite immersed in SBF is associated with the presence of the bands around 560–600 cm−1,

**Figure 3.** FTIR spectra of the BG70-PLA30 composite after different soaking times in SBF. **Figure 3.** FTIR spectra of the BG70-PLA30 composite after different soaking times in SBF. surface [24].

**Figure 4.** FTIR spectra of the BG30-PLA70 composite after different soaking times in SBF.

Figures 5 and 6 show the FTIR results of the BG-PLA composites in PBS. The FTIR results of the BG70-PLA30 scaffolds in PBS immersed for 7, 14, 21, and 28 days (Figure 5) present consistent degradation due to the presence of the phosphate groups' bending vibrations, which are more defined with longer immersion times. The mineralization process was associated with the intensity increase of the 1037 cm−1 peak due to P-O stretching vibration. The FTIR results for the biomaterial BG30-PLA70 (Figure 6) present the formation of a vibrational band with low intensity at 566 cm−1 after 21 days of immersion, corresponding to the deposition of phosphorous ions on the surface. The peak intensity decrease in the vibrational band at 1385 cm−1, which corresponds to Si−O−Si bending and stretching vibrations at 28 days of immersion, indicates the process decomposition of the BG phase. **Figure 4.** FTIR spectra of the BG30-PLA70 composite after different soaking times in SBF. Figures 5 and 6 show the FTIR results of the BG-PLA composites in PBS. The FTIR results of the BG70-PLA30 scaffolds in PBS immersed for 7, 14, 21, and 28 days (Figure 5) present consistent degradation due to the presence of the phosphate groups' bending vibrations, which are more defined with longer immersion times. The mineralization process was associated with the intensity increase of the 1037 cm−<sup>1</sup> peak due to P-O stretching vibration. The FTIR results for the biomaterial BG30-PLA70 (Figure 6) present the formation of a vibrational band with low intensity at 566 cm−<sup>1</sup> after 21 days of immersion, corresponding to the deposition of phosphorous ions on the surface. The peak intensity decrease in the vibrational band at 1385 cm−<sup>1</sup> , which corresponds to Si−O−Si bending and stretching vibrations at 28 days of immersion, indicates the process decomposition of the BG phase.

**Figure 5.** FTIR spectra of the BG70-PLA30 composite after different soaking times in PBS. **Figure 5.** FTIR spectra of the BG70-PLA30 composite after different soaking times in PBS. **Figure 5.** FTIR spectra of the BG70-PLA30 composite after different soaking times in PBS.

**Figure 6.** FTIR spectra of the BG30-PLA70 composite after different soaking times in PBS. **Figure 6.** FTIR spectra of the BG30-PLA70 composite after different soaking times in PBS. **Figure 6.** FTIR spectra of the BG30-PLA70 composite after different soaking times in PBS.

### *3.3. SEM Characterization 3.3. SEM Characterization*

*3.3. SEM Characterization*  Figure 7a shows that the morphology of the BG presents an irregular morphology and particles with a size between 2.31 to 15.47 µm. Figure 7c shows the chemical composition by EDS of the BG sample, indicating the presence of Ca, O, Si, P, and C, which are constituents of the bioactive bioglass and were observed in the FTIR and XRD characterization results as well. Furthermore, the BG morphology here is similar to that reported by Sharifianjazi et al. [25] and Xia et al. [26]. The PLA morphology is shown in Figure 7b, Figure 7a shows that the morphology of the BG presents an irregular morphology and particles with a size between 2.31 to 15.47 µm. Figure 7c shows the chemical composition by EDS of the BG sample, indicating the presence of Ca, O, Si, P, and C, which are constituents of the bioactive bioglass and were observed in the FTIR and XRD characterization results as well. Furthermore, the BG morphology here is similar to that reported by Sharifianjazi et al. [25] and Xia et al. [26]. The PLA morphology is shown in Figure 7b, Figure 7a shows that the morphology of the BG presents an irregular morphology and particles with a size between 2.31 to 15.47 µm. Figure 7c shows the chemical composition by EDS of the BG sample, indicating the presence of Ca, O, Si, P, and C, which are constituents of the bioactive bioglass and were observed in the FTIR and XRD characterization results as well. Furthermore, the BG morphology here is similar to that reported by Sharifianjazi et al. [25] and Xia et al. [26]. The PLA morphology is shown in Figure 7b, while its components are indicated in EDS spectrum of Figure 7d.

while its components are indicated in EDS spectrum of Figure 7d. while its components are indicated in EDS spectrum of Figure 7d. The BG-PLA scaffolds were characterized by SEM as well. The BG70-PLA30 composite (Figure 8a) presented a dense morphology with a homogeneous phase, with no differentiation between the BG and PLA phases. On the other hand, the BG30-PLA70 scaffolds (Figure 8b) showed a cracked surface morphology, which is associated with the presence of tension stress at the grain interfaces of the polymer and BG phase during sintering due to the higher wt.% quantity of the PLA in the composite. The elemental chemical analysis by EDS of both composite samples is shown in Table 3. As expected, the chemical composition for the samples is in agreement with the quantity of the polymeric

and vitreous phases, denoting a major percentage of C when the PLA phase was higher in the scaffolds (BG30-PLA70). *Polymers* **2022**, *14*, x FOR PEER REVIEW 10 of 24

> **Figure 7.** Morphology by SEM of (**a**) BG and (**b**) PLA; EDS chemical analyses of (**c**) BG and (**d**) PLA. **Figure 7.** Morphology by SEM of (**a**) BG and (**b**) PLA; EDS chemical analyses of (**c**) BG and (**d**) PLA.

**EDS BG70-PLA30 BG30-PLA70 Element at.% at.%**  C 38.485 50.103

Si 9.704 0.588 Ca *3.454 2.498* 

The evaluation of the bioactivity of the BG-PLA composites was achieved as described in Section 2.3, and is supported by the FTIR (shown in Figures 3 and 4) and SEM characterization. In accordance with Equation (2), the average weight loss of the BG70- PLA30 remained fairly constant over the different time periods of immersion. The average weight loss values (ത) across the four time periods did not show any significant variation, with measured values between 37.85 and 38.5 wt.%, as seen in Table 4. A similar profile was presented by the BG30-PLA70 composite, with an average weight loss of between 39.6 and 38.6 wt.%. Nevertheless, the differences in the weight loss values for both composites during the four time periods are associated with the evolution of HAp formation, which is integrated into the measurement. After immersion, the scaffold composites presented degradation indications due to water interaction in the ion exchange mechanism between the BG/PLA phases and the solution. The water molecules disassociate the Si-O bonds in the BG network forming Si-OH groups, which attracts the Caଶା, HଶPOସି, HPOସ

ଷି ions present in the SBF solution. This favourably promoted HAp nucleation

site formation on the sample's surface [23]. The degradation behaviour of the BG (30 wt.%) composite showed an initial increase as a result of water uptake, then a subsequent decrease due to mass loss attributed to the polymer achieving a critical molecular weight sufficiently small to allow diffusion out of the matrix [27], and finally a small mass in-

crease due to room humidity. These three steps are key to sample degradation.

ଶି

**Figure 8.** Scaffold SEM analysis for (**a**) BG70-PLA30 and (**b**) BG30-PLA70. **Figure 8.** Scaffold SEM analysis for (**a**) BG70-PLA30 and (**b**) BG30-PLA70.

**Table 3.** Energy-dispersive X-ray spectroscopy for both compositions.

*3.4. Evaluation of BG-PLA Bioactivity in SBF* 

and POସ


**Table 3.** Energy-dispersive X-ray spectroscopy for both compositions.

## *3.4. Evaluation of BG-PLA Bioactivity in SBF*

The evaluation of the bioactivity of the BG-PLA composites was achieved as described in Section 2.3, and is supported by the FTIR (shown in Figures 3 and 4) and SEM characterization. In accordance with Equation (2), the average weight loss of the BG70-PLA30 remained fairly constant over the different time periods of immersion. The average weight loss values (*X*) across the four time periods did not show any significant variation, with measured values between 37.85 and 38.5 wt.%, as seen in Table 4. A similar profile was presented by the BG30-PLA70 composite, with an average weight loss of between 39.6 and 38.6 wt.%. Nevertheless, the differences in the weight loss values for both composites during the four time periods are associated with the evolution of HAp formation, which is integrated into the measurement. After immersion, the scaffold composites presented degradation indications due to water interaction in the ion exchange mechanism between the BG/PLA phases and the solution. The water molecules disassociate the Si-O bonds in the BG network forming Si-OH groups, which attracts the Ca2+, H2PO4−, HPO2<sup>−</sup> 4 and PO3<sup>−</sup> 4 ions present in the SBF solution. This favourably promoted HAp nucleation site formation on the sample's surface [23]. The degradation behaviour of the BG (30 wt.%) composite showed an initial increase as a result of water uptake, then a subsequent decrease due to mass loss attributed to the polymer achieving a critical molecular weight sufficiently small to allow diffusion out of the matrix [27], and finally a small mass increase due to room humidity. These three steps are key to sample degradation.

**Table 4.** Average weight loss (*X*) and standard deviation (*SX*) for the scaffolds immersed in SBF and PBS over time periods of 7, 14, 21, and 28 days.


Figure 9 shows the SEM morphology of the BG70-PLA30 scaffolds after 14 days and 28 days of immersion in SBF. The morphology surfaces show the evolution of HAp formation, with greater presence at 28 days of immersion. The EDS chemical composition related to Figure 9 is presented in Table 5 and confirms the HAp growth associated with the quantity of calcium adhesion to the surface (spectrum 2), which is substantial after 28 days of immersion of the biomaterial in SBF. Similarly, Figure 10 shows the SEM morphology of the BG30-PLA70 scaffolds after 14 days and 28 days of immersion in SBF. After 14 days of immersion, the sample morphology shows the presence of cracks on the surface which represent the early stages of degradation, promoting the first interstitial condition for HAp phase nucleation. The EDS results of the chemical analysis of the composite at 14 and 28 days is shown in Table 5. The chemical composition is quite similar in the elements and atomic percentages between both time periods, indicating that the BG concentration in the biomaterial scaffolds is important for optimal bioactivity.

in the biomaterial scaffolds is important for optimal bioactivity.

**Figure 9.** EDS analysis for BG70-PLA30 after (**a**) 14 and (**b**) 28 days of immersion in SBF. **Figure 9.** EDS analysis for BG70-PLA30 after (**a**) 14 and (**b**) 28 days of immersion in SBF.


**Table 4.** Average weight loss (ത) and standard deviation (*SX*) for the scaffolds immersed in SBF and

7 39.60 1.34 37.86 1.15 27.80 1.86 24.12 0.03 14 38.88 0.81 38.95 0.27 28.42 1.32 22.42 0.22 21 39.72 1.47 39.33 0.04 28.05 1.06 23.00 0.57 28 38.60 0.92 38.53 0.23 27.91 0.32 24.93 1.06

Figure 9 shows the SEM morphology of the BG70-PLA30 scaffolds after 14 days and 28 days of immersion in SBF. The morphology surfaces show the evolution of HAp formation, with greater presence at 28 days of immersion. The EDS chemical composition related to Figure 9 is presented in Table 5 and confirms the HAp growth associated with the quantity of calcium adhesion to the surface (spectrum 2), which is substantial after 28 days of immersion of the biomaterial in SBF. Similarly, Figure 10 shows the SEM morphology of the BG30-PLA70 scaffolds after 14 days and 28 days of immersion in SBF. After 14 days of immersion, the sample morphology shows the presence of cracks on the surface which represent the early stages of degradation, promoting the first interstitial condition for HAp phase nucleation. The EDS results of the chemical analysis of the composite at 14 and 28 days is shown in Table 5. The chemical composition is quite similar in the elements and atomic percentages between both time periods, indicating that the BG concentration

**SBF PBS BG30 PLA70 BG70 PLA30 BG30 PLA70 BG70 PLA30**  ഥ**, (%)** *SX* ഥ**, (%)** *SX* ഥ**, (%)** *SX* ഥ**, (%)** *SX*

PBS over time periods of 7, 14, 21, and 28 days

**Time, (days)**

**Table 5.** EDS chemical analysis for BG-PLA composites after immersion in SBF. **Table 5.** EDS chemical analysis for BG-PLA composites after immersion in SBF.

**Figure 10.** EDS analysis for BG30-PLA70 after (**a**) 14 and (**b**) 28 days of immersion in SBF. **Figure 10.** EDS analysis for BG30-PLA70 after (**a**) 14 and (**b**) 28 days of immersion in SBF.

### *3.5. Evaluation of BG-PLA Degradation in PBS 3.5. Evaluation of BG-PLA Degradation in PBS*

Because biodegradability is an essential property when designing scaffolds, the evaluation of this property was realized and supported by FTIR (Figures 5 and 6) and SEM characterizations. The weight loss of the samples after immersion in phosphate-buffered saline solution (PBS) is shown in Table 4. The degradation behaviour of the BG70-PLA30 sample (blue line) shows the lower percentage change in mass in PBS. This profile can be divided into two regions: an initial increase due to the water uptake from the amorphous Because biodegradability is an essential property when designing scaffolds, the evaluation of this property was realized and supported by FTIR (Figures 5 and 6) and SEM characterizations. The weight loss of the samples after immersion in phosphate-buffered saline solution (PBS) is shown in Table 4. The degradation behaviour of the BG70-PLA30 sample (blue line) shows the lower percentage change in mass in PBS. This profile can be divided into two regions: an initial increase due to the water uptake from the amorphous

areas with the presence of terminal groups, folds, and chains with free rotation, and subsequent mass loss represented by a final decrease related to the degradation rate due to attack on the crystalline areas [28]. Meanwhile, the red line represents the average weight

approximately 28% over the four time periods, showing similar behaviour in this solution. The morphology of the BG70-PLA30 composite after 14 and 28 days immersed in PBS is shown in Figure 11. After 28 days of immersion the surface sample showed more dissolution than at 14 days. The highest mass dissolution occurred at 28 days, diminishing the formation of reaction products deposited on the composite surface (Figure 11b). The sample's surface did not show higher product growth than the sample immersed for 14 days (Figure 11a). Additionally, the results of EDS analysis in both time periods confirm the degradation behaviour when comparing the elements presented in the samples after 14 and 28 days of soaking in PBS (Table 6). The carbon, sodium, and chloride elements in the initial sample were degraded into the solution after 28 days of immersion, as noted in Table 6. Similarly, the morphology of the degradation of the BG30-PLA70 scaffolds can be observed in Figure 12 after 14 and 28 days of immersion. The formation of spherical growths formed by the HAp phase and the presence of calcium in a high concentration confirms this. For this composite, the results of the EDS chemical analyses shown in Table 5 after 14 and 28 days of immersion present the peak bioactivity of the prepared scaffolds. The increase in Ca content is more evident in the samples with longer immersion times.

areas with the presence of terminal groups, folds, and chains with free rotation, and subsequent mass loss represented by a final decrease related to the degradation rate due to attack on the crystalline areas [28]. Meanwhile, the red line represents the average weight loss profile for the BG30-PLA70 composite in Table 4. The weight loss of the sample was approximately 28% over the four time periods, showing similar behaviour in this solution.

The morphology of the BG70-PLA30 composite after 14 and 28 days immersed in PBS is shown in Figure 11. After 28 days of immersion the surface sample showed more dissolution than at 14 days. The highest mass dissolution occurred at 28 days, diminishing the formation of reaction products deposited on the composite surface (Figure 11b). The sample's surface did not show higher product growth than the sample immersed for 14 days (Figure 11a). Additionally, the results of EDS analysis in both time periods confirm the degradation behaviour when comparing the elements presented in the samples after 14 and 28 days of soaking in PBS (Table 6). The carbon, sodium, and chloride elements in the initial sample were degraded into the solution after 28 days of immersion, as noted in Table 6. Similarly, the morphology of the degradation of the BG30-PLA70 scaffolds can be observed in Figure 12 after 14 and 28 days of immersion. The formation of spherical growths formed by the HAp phase and the presence of calcium in a high concentration confirms this. For this composite, the results of the EDS chemical analyses shown in Table 5 after 14 and 28 days of immersion present the peak bioactivity of the prepared scaffolds. The increase in Ca content is more evident in the samples with longer immersion times. *Polymers* **2022**, *14*, x FOR PEER REVIEW 14 of 24

(**a**) (**b**) **Figure 11.** Scaffold SEM analysis for BG70-PLA30 after (**a**) 14 days and (**b**) 28 days of immersion in **Figure 11.** Scaffold SEM analysis for BG70-PLA30 after (**a**) 14 days and (**b**) 28 days of immersion in PBS.


**Figure 12.** Scaffold SEM analysis for BG30-PLA70 after (**a**) 14 days and (**b**) 28 days of immersion in

**BG70-PLA30 BG30-PLA70 14 days 28 days 14 days 28 days** 

C 18.96 35.73 34.87 16.31 O 62.28 72.89 51.23 44.62 57.6 Na 1.07 0.85 1.44 0.38 Si 7.28 22.54 4.82 0.89 1.42 P 0.7 1.37 3.08 6.85 3.7

Ca 8.88 3.2 3.9 11.33 20.6

**S.1 S.2** 

Cl 0.82 0.39

PBS.

**Element at.%** 

 (**a**) (**b**)

**Figure 12.** Scaffold SEM analysis for BG30-PLA70 after (**a**) 14 days and (**b**) 28 days of immersion in PBS. **Figure 12.** Scaffold SEM analysis for BG30-PLA70 after (**a**) 14 days and (**b**) 28 days of immersion in PBS.

**Figure 11.** Scaffold SEM analysis for BG70-PLA30 after (**a**) 14 days and (**b**) 28 days of immersion in
