3.1 Microscopic morphology of powder
The micromorphology and particle size of PLA particles and Mg powder were analyzed using SEM, as shown in Fig. 2. As can be seen from Fig. 2(a), the spherical Mg powder particle size is approximately 50 µm. It can be seen from Fig. 2(b) that the PLA powder exhibits an irregular block shape. As shown in Figs. 2(c)-2(f), as the Mg content increases, it can be seen that the distribution of Mg particles in PLA powder gradually changes from sparse to dense. At the same time, it can be found that as the proportion of Mg increases in the mixed powder, Mg aggregation gradually occurs.
3.2 Characterization of composite scaffolds
The surface and fracture morphology of the PLA-nMg composite scaffold, as depicted in Fig. 3, were examined using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). It can be observed from Fig. 3(a) that the surface of the PLA scaffold is relatively smooth. When the nMg content is 1wt%, Fig. 3(b) shows a single round spot formed by the incomplete wrapping of part of the surface of the Mg sphere. It can be seen from Fig. 3(c) that when the nMg content increases to 3wt%, white round spots increase and are evenly distributed on the surface of the PLA matrix. The surface of the scaffold is relatively rough and has no cracks, and the mosaic package is complete. It can be seen from Fig. 3(d) that as the Mg content reaches 5wt%, obvious cracks begin to appear in the scaffold matrix, and it can be clearly seen that the Mg particles are too close inside. When running 7wt%, it can be seen from Fig. 3(e) that in addition to apparent cracks on the surface, some Mg particles are not entirely wrapped into the PLA matrix and are exposed on the surface. The elements of bone scaffolds with different components were analyzed using X-ray energy dispersive spectroscopy (EDS), as shown in Fig. 3(e). The results show that the surface elements mainly contain C and Mg elements, without other impurities, confirming that the surface's round spots and particles are Mg.
In order to explore whether the sintering process affects the physical phases of PLA and Mg, XRD was used to detect composite powders and scaffolds in various proportions. The results are shown in Fig. 4. Figure 4(a) is the XRD pattern of the mixed powder. The figure reveals the presence of two distinct diffraction peaks corresponding to the (101) and (200) crystal planes of PLA at 16.4° and 18.8°, respectively, in the diffraction pattern of the blended powder30. The PLA peak intensity and index differ slightly as the PLA content changes. In addition, three characteristic peaks of Mg at 32.1°, 34.5°, and 36.8° appear on the three crystal planes (116), (112), and (1120)31. The peak intensity and index of Mg increase with the increase in content. The XRD pattern of the scaffold after laser sintering is illustrated in Fig. 4(b).
It was observed that the sintered scaffold exhibited a notable reduction in the peak intensity assigned to PLA, suggesting a decrease in crystallinity. This decline in peak intensity is attributed to the thermal degradation caused by the impact of the high-energy laser during the sintering process32. The observed decrease in the Mg peak intensity can be attributed to the embedding of Mg particles within the interior of the PLA matrix as a result of the laser sintering process33. When the content reaches 7%, some exposed Mg on the surface can be detected due to aggregation. In addition, no new peaks were found in comparison. This shows that no chemical changes occur during the laser sintering process.
3.3 Mechanical analysis of composite scaffolds
Good mechanical properties are the prerequisite for bone defect replacement materials to provide stable support during transplantation34, 35. To assess the influence of Mg particles on the mechanical properties of composite bone scaffolds, the compressive properties of each scaffold were evaluated through compression testing. This article considers the compressive properties of scaffolds prepared by SLS. The typical compressive stress-strain curve is shown in Fig. 5. At the same time, the corresponding compressive strength is obtained, and the modulus is calculated. To accurately assess the contribution of Mg in enhancing the mechanical properties, a PLA scaffold with a density of 60% was established as the control group. The compressive stress-strain curves of the five scaffold types, illustrated in Fig. 5(a), demonstrate comparable patterns of stress changes, suggesting a similarity in their mechanical response. In order to more intuitively clarify the influence of Mg particle content on scaffold strength, the ultimate strength values and compressive modulus of the scaffolds with varying magnesium content ratios were determined by extracting the corresponding data from the stress-strain curve. Figure 5(b) elucidates the pronounced enhancement in compressive mechanical properties of the PLA scaffold upon the inclusion of Mg, indicating the substantial role played by Mg in improving the scaffold's overall mechanical performance. By incorporating Mg into the PLA scaffold, the compressive strength is prominently improved. Specifically, the addition of 1% Mg in the PLA/1Mg scaffold results in a strength of 3.7 MPa, which represents a substantial 38.6% increase compared to the PLA scaffold (2.67 MPa). Moreover, increasing the Mg content to 3% in the PLA/3Mg composite scaffold further enhances its compressive strength to 5.60 MPa, demonstrating an impressive improvement of approximately 109.7% when compared to the PLA scaffold. Nevertheless, as the Mg content further increases beyond a certain point, the strength of the composite scaffold demonstrates a declining trend. The incorporation of higher Mg content could lead to an aggregation phenomenon, hindering the complete encapsulation of Mg particles by the molten PLA during the sintering process. It is also possible that the PLA matrix is stretched because the particles aggregate and fail to melt. Furthermore, the aggregation of Mg particles leads to the generation of defects, this diminishes the interfacial adhesion between the Mg particles and the PLA matrix, resulting in reduced strength of the composite scaffold. As a result, the compromised stress transfer mechanism detrimentally impacts the mechanical properties of the composite bone scaffold. This impairment significantly compromises the scaffold's ability to effectively transmit and distribute stress, leading to diminished mechanical performance36, 37. These findings suggest that, in terms of mechanical properties, the optimal Mg particle content for the composite bone scaffold in this study should be limited to 3%.
Subsequently, scanning electron microscopy (SEM) was employed to examine the fracture surface morphology of different scaffold types under compression, as shown in Fig. 6, from which it can be seen that the Mg particles are exposed after the PLA matrix breaks under pressure. More importantly, the PLA matrix almost completely wraps the Mg particles, confirming that they are not sintered during the sintering process and maintain their complete morphology. It is also confirmed that laser sintering technology can achieve the perfect interface combination of two materials. For PLA/1Mg, only a single Mg sphere can be seen wholly wrapped inside the PLA matrix, as shown in Fig. 6(a). For the PLA/3Mg scaffold, the Mg particles increase while maintaining a particular gap between each other and are completely covered, as shown in Fig. 6(b). However, part of the structure of the Mg particles displayed by the PLA/5Mg scaffold is exposed on the fracture surface, the distance between them is small, and the distribution is too concentrated, as shown in Fig. 6(c). The problem of PLA/7Mg is severe. The particles are seriously aggregated, and the interface has apparent cracks, as shown in Fig. 6(d). Research findings have indicated that inadequate contact surfaces can adversely impact the stress transfer mechanism within the scaffold when subjected to external loads, consequently resulting in a reduction in mechanical properties38–40. This phenomenon serves to explain the underlying reasons behind the observed decrease in mechanical properties of these two scaffold types during the mechanical property tests conducted in this study. Moreover, it further verifies the reliability and accuracy of the surface electron microscopy results obtained. Therefore, incorporating Mg into PLA scaffolds can enhance their mechanical properties by strengthening the matrix, improving interface bonding, ensuring proper dispersion, providing reinforcement, and potentially yielding synergistic effects.
3.4 Biodegradability
The evaluation of biodegradability is of utmost importance when assessing the effectiveness of bone scaffolds. Thus, our study focused on investigating the degradation behavior of both PLA and PLA/nMg scaffolds using immersion tests. In order to assess the in vitro biodegradability of the composite scaffolds, a thorough investigation was conducted. samples with different proportions were immersed in a phosphate-buffered saline (PBS) solution and subjected to various culture periods. Figure 7 shows the PLA and PLA/Mg scaffold morphology after four weeks of immersion in PBS. The PLA scaffold morphology was similar to before soaking, as shown in Fig. 7(a). The surface of the PLA/Mg scaffold with 1wt%Mg added was relatively rough, with some micropores appearing, as shown in Fig. 7(b). Then, observe the surface and pore size of other PLA/nMg scaffolds with a larger ratio. As the Mg content gradually increases to 3wt%, a larger area of micropores appears on the surface of the scaffold, forming holes, as shown in Fig. 7 (c).
Further observation shows that when increasing to 5wt%, huge holes are finally formed on the surface of the scaffold, as shown in Fig. 7(d). While Mg rapidly degrades itself, it also accelerates the degradation of surrounding PLA. This can be attributed to the accelerated degradation of PLA induced by the presence of Mg. Subsequent to the hydrolysis process, the resulting products dissolve in the PBS solution, leading to the formation of surface pits on the scaffold, as visually demonstrated in Fig. 7(e). In summary, the incorporation of Mg enhances the degradation process of the scaffold, leading to an accelerated degradation rate. The extent of degradation is directly influenced by the Mg proportion in the PLA matrix, with higher proportions resulting in faster degradation rates. However, it is worth noting that the PLA/3Mg ratio exhibits the most optimal matching ratio in terms of degradation performance.
In order to understand the impact of the scaffold on the surrounding environment during the degradation process, the pH value of its solution was monitored during immersion for four weeks, as shown in Fig. 8(a). As a result of the acidic degradation by-products, the pH value of the PLA degradation medium steadily declined with prolonged soaking time, reaching a value of 6.9 after a four-week immersion period. The pH trend of PLA/nMg is different from that of PLA. As the degradation of Mg takes place in the PBS solution, the pH value initially rises and approaches 8 after one week. However, with prolonged soaking, the pH gradually decreases due to the degradation of the acidic by-products of degradation by PLA and the subsequent reduction in the degradation medium. Notably, an increase in the Mg content results in an elevated pH value after four weeks, which can be attributed to the degradation of Mg and the subsequent formation of Mg(OH)241, 42. Consequently, the addition of Mg creates a microenvironment within the PLA matrix that exhibits an elevated pH value compared to pure PLA. To evaluate the weight loss of the PLA/Mg scaffold over a four-week immersion period, the weekly weight loss rates were measured and visualized in Fig. 8(b). Weight loss was observed in all samples during the immersion period. It can be seen that the PLA scaffold has the lowest weight loss rate at each time point and does not change much after four weeks. However, the weight loss rate of the PLA/nMg scaffold was notably higher compared to the PLA scaffold. Moreover, the weight loss rate demonstrated an increasing trend with the addition of Mg, indicating that the incorporation of Mg accelerated the degradation of the scaffold.
3.5 Degradation Mechanism Analysis
The mechanism of Mg-accelerated scaffold degradation is shown in Fig. 9. From the macro level of the scaffold, it is shown in part (a) of Fig. 9. Due to the rapid infiltration of the PBS solution into the interface between the Mg particles and the PLA matrix, the water absorption capacity of the PLA/Mg scaffold increases. In contrast, the hydrophobic nature of the PLA scaffold hinders water penetration into the scaffold. Then, both reactions will coincide. On one hand, PLA undergoes hydrolysis as a result of ester bond cleavage, leading to the production of acidic by-products (formula (2))23, 43.
Consequently, the generation of an increased quantity of acid degradation products from both PLA and Mg(OH)2 leads to a subsequent neutralization and consumption of these products. This phenomenon, in turn, fosters a positive feedback loop, intensifying the degradation process of the scaffold.
The structural formula is shown in part (b) of Fig. 9. The ester group in PLA decomposes when exposed to water. Hydrogen atoms in water are added to the left side to form a carboxyl group (HOOC-R), which generates acetate ions, causing the PLA to degrade and become acidic32, 44. Mg reacts with water to generate magnesium hydroxide, which can react with the acidic degradation products of PLA. The hydroxide radicals in Mg(OH)2 are removed and combined with the hydrolysis product (R-CO-) that removes hydrogen ions to form the final Degradation product (R-CO-Mg-OC-R). As the original acetate ions are consumed, the local pH value increases, forming positive feedback on the degradation of the scaffold42, 45, 46.
From the above analysis, it can be seen that the degradation process of the PLA/nMg composite material during in vitro degradation is shown in part (c) of Fig. 943, 47, 48. The exposed surface of Mg on the surface of the scaffold and the PLA connection are in direct contact with the PBS solution and degrade first, resulting in weight loss, increased crystallinity, and rapid increase in pH value. Then, as the PBS solution begins to penetrate into the interior of the scaffold, the amorphous region of the PLA matrix and the Mg inside the composite material begin to degrade. As the amorphous region of the PLA matrix degrades, more acidic products are produced, which triggers the autocatalytic degradation of PLA. At the same time, the basic products of Mg can neutralize the acidic degradation products of the PLA matrix, thereby reducing the degradation rate of the composite material in a relatively stable environment. As the amorphous region degrades and PBS is soaked in the composite material, the crystalline region of PLA begins to degrade. Due to the degradation of the crystal area caused by the dissolution of Mg, many pores and holes appear on the surface of the scaffold or inside the scaffold, which will lead to a significant decrease in the mechanical properties of the composite scaffold. When scaled too much and by the fourth week, large deep pits appeared, but the scaffold still maintained its shape.