3.1 Fabrication of fluffy PLGA/HA fibrous composite scaffold
The fluffy PLGA fibrous scaffold was successfully fabricated by an optimized electrospinning technique. Compared with conventional scaffold, most of the PLGA fibers were discrete from each other with a distance about 85 ± 25 µm. These loose structures made the scaffold feel like cotton. And the PLGA fibers manifested with a narrow fiber size ~ 2.32µm (Fig. 2, Fig. 3).
Not just a framework of new bone formation, a good osteo-inductive and osteogenic environment is also needed [12]. The bio-mineralization was used to obtain HA coating on the surface of the fluffy PLGA scaffold. After 21-day mineralized in SBF, the surface of PLGA fibers was covered with a thick and dense HA coating but still in a discrete state. Only a few fibers intertwined with each other due to mineralization. The interconnected pores in scaffolds were 70 ± 20µm, which were big enough for the migration and proliferation of cells. The diameter of HA fibers was 11.95 µm, and the thickness of HA coating was about ~ 9.6 µm (Fig. 3). Sun W et al. reviewed silk fibroin-based scaffolds and highlighted the osteogenic differentiation ability of HA coated on the scaffold [13]. Previous studies also confirmed that the addition of HA provided an improved osteoconductive potential and overall bioactivity in bone tissue engineering [14].
However, the PLGA fibers almost intertwined with each other completely after the same mineralization procedure in conventional PLGA/HA scaffold, and only a few pores (~ 1 µm) were found on the surface. Obviously, the spatial structure was difficult for cells migrating into the deeper center, but just achieving a cell monolayer.
3.2 Physical Characteristics of fluffy PLGA/HA fibrous scaffold
The formation of HA coating was observed by SEM images. After 7 days of incubation in SBF, micro-particles (4–12 µm) could be found on the surface of fibers. And then, more and more particles adhered to the surface and blended together. A thick and dense HA coating formed after 21 days of incubation. SEM images revealed that the HA was uniformly distributed on the surface to increase cell adhesion and proliferation. Long-term accessibility of HA was gained through the PLGA slowly degraded (Fig. 4).
After 21 days incubation in SBF, the weight of fluffy PLGA/HA fibrous scaffold increased 7 times than initial fiber template. The content of HA in this novel composite scaffold was much higher than that of previously reported HA based composite scaffolds. Additionally, this HA coating increased mechanical properties and led to a hard and brittle composite scaffold. This composite scaffold became hard and brittle enough with a higher modulus (830 Pa Vs 5.6 Pa, compressive ratio was 30%) for 3D cells culture and bone tissue engineering. Bhuiyan DB et al. developed a multicomponent composite biomaterial to provide good structure support and biological stimuli for bone regeneration. The tensile strength and moduli of nano-hydroxyapatite-poly (D, L-lactide-co-glycolide)-collagen biomaterial increased 48% and fivefold after seeded with hMSCs [15].
No matter HA coated or not, the fluffy fibrous scaffold had higher porosity than conventional scaffold. The water absorption rate is an essential indicator of porosity. The fluffy PLGA/HA fibrous scaffold provided more interconnected pores for cell proliferation [16]. However, fluffy PLGA/HA fibrous scaffold had slightly lower porosity than fluffy PLGA fibrous scaffold due to the increased diameter of fibers. The HA coating increased the surface area for more cell attachment (40.2 Vs 17.6 m2 g-1). The diameter of fiber affects the proliferation and differentiation of cells. Usually, the attached cells’ diameters are larger than fibers [17].
3.3 Chemical characteristics of fluffy PLGA/HA fibrous scaffold
The existence of HA coating was found by FTIR spectra. Three peaks at 542, 644 and 1041 cm− 1 occurred due to the vibrations of P-O bonds in HA. The second peak at 1003 cm− 1 happened in the original HA spectrum and transferred to 1041 cm− 1 after HA coating. However, characteristic peaks of PLGA (1780 cm− 1 for the C = O group and 1102 cm− 1 for C–O stretching) became weaker, and no change in wavenumber was found (Fig. 5). The primary existence of oxygen, calcium and phosphorus were confirmed by EDS spectrum with a 1.71 Ca/P ratio, which meant the mineral phase was HA. Compared with the neat polymers, the previous study showed that PLGA-HA composite had better thermal stability, higher decomposition temperature, and higher activation energy [18].
3.4 In vitro cell differentiation and morphology of hMSCs
After 12h culture, the cell attachment of hMSCs was similar in both groups (81% Vs 78%). However, cell proliferation was significantly higher in the fluffy PLGA/HA fibrous scaffold (10.89×104 Vs 6.02×104 cells) after 7 days culture. This PLGA/HA composite scaffold provided an increased pore area in the final scaffold. Nyberg et al. also observed a similar phenomenon and found that PCL mixed with HA had a more accurate final structure than other mineral dopants. It could be explained by the varied material viscosity [19].
After 7 days of culture, the fluorescence microscope showed that serveral hMSCs clusters appeared in the interior of the fluffy PLGA/HA fibrous scaffold, while sparse cell–cell contact occurred in the conventional PLGA/HA fibrous scaffold. The SEM showed that the cell membrane of hMSCs diffused widely with integrated cell–fiber constructs and cell-cell contact. However, the hMSCs still manifested with sparse cell–cell contact.
3.5 In vivo implantation and bone ingrowth of fluffy/conventional PLGA/HA composite scaffold
After 7 days of culture, the composite scaffold with hMSCs was implanted in the bone defect of rabbits. Histological staining and CT were used to assess bone repair in the defect site after 4, 8, and 12 weeks. All rabbits survived during the study (Fig. 6).
Compared with conventional PLGA/HA composite scaffold, the micro-CT showed an increased amount of mineralized tissue in the fluffy PLGA/HA composite scaffold after 8 weeks. Little callus formation was observed at 4 weeks in both groups. However, after 8 weeks, much more callus appeared in the fluffy PLGA/HA composite scaffold group. And the newly formed bone with increased mineral density was observed after 12 weeks (Fig. 7).
The histomorphological analysis also supported the findings of radiographic examinations (Fig. 8, Fig. 9). There were osteoblast cells, osteoclast cells, and vascularization in the area of newly formed bone. No signs of inflammation or adverse tissue reaction to the composite scaffold were observed in all specimens. The fluffy composite scaffold endowed itself with better bone repair properties. Newly formed bone with irregular morphology appeared about 8 weeks after implantation. The composite scaffold was surrounded with new bone tissues, demonstrating good osteoconductivity and biocompatibility. After 12 weeks, the defect healed with the lamellar bone. These results indicated that fluffy composite scaffold had better osteogenic and maturity effects.
In present study, the good bone repair effect of the fluffy PLGA/HA composite scaffold is attributable to multiple factors. Scaffold should resemble the properties of natural bone with sufficient porous structures for cell proliferation [20]. Isenberg BC et al. reviewed the material construction methods of different biomaterial scaffold and pointed out that the structural organization played an important role in the tissue function [21]. Additive manufacturing technology offers precise and complex microporous microarchitecture with interconnectivity of pores. As a temple, the fluffy PLGA/HA composite scaffold offered a suitable structural environment for cell seeding. It had more void fractions for cell adhesion and proliferation as temporary extracellular matrixes. BMSCs are often used to enhance bone healing [22]. In the present study, the cells passed through the outer edge and colonized in the center of fluffy PLGA/HA composite scaffold.
The properties and characteristics of polymers can be altered through blend or copolymerization of various polymers. Composite scaffold counterbalances their respective limitations and displays the increased ability of osseointegration and new bone formation [23]. In this study, multi-electro-spinning combined with biomineralization technology was used to fabricate scaffold with suitable structural integrity and mechanical properties [24]. As a major inorganic component, HA is a widely used bone substitute with high osteoconductive potential but has poor mechanical properties [25 26]. Babilotte J et al. added HA nanoparticles to PLGA to improve its overall bioactivity through Fused Deposition Modeling (FDM) [27]. The fluffy PLGA/HA composite scaffold optimized new bone tissue regeneration as a temple. Mineralized formation and bone ingrowth in vivo was confirmed by micro-CT.
As a synthetic or nonvital natural material implanted in the human body, the biocompatibility of biomaterial is a necessary prerequisite without any adverse permanent immune responses [28]. The biocompatibility of the fluffy PLGA/HA composite scaffold was assessed both in vitro and in vivo. In vitro, the viability was assessed after hMSCs were seeded. Compared with conventional PLGA/HA composite scaffold, cell proliferation in fluffy PLGA/HA composite scaffold improved significantly after 7 days of culture. In vivo, the fluffy PLGA/HA composite scaffold was implanted in the bone defect. The fluffy PLGA/HA composite scaffold provided the required biocompatibility and bioactivity for better cellular performance and bone formation.
Controlled degradation is another property we need to consider. The ideal scaffold is one which can be gradually degraded and replaced by newly formed bone with sufficient mechanical properties at the bone defect site. Too slow degradation will hinder bone remodeling, and too fast will lead to more fibrous tissue formation but not bone formation [29]. In any event, the detrimental byproducts should be avoided during the degradation. These detrimental byproducts can obstruct the healing process and initiate complications.