Nanofibrous Microspheres: A Biomimetic Platform for Bone Tissue Regeneration

Abstract

Bone, a fundamental constituent of the human body, is a vital scaffold for support, protection, and locomotion, underscoring its pivotal role in maintaining skeletal integrity and overall functionality. However, factors such as trauma, disease, or aging can compromise bone structure, necessitating effective strategies for regeneration. Traditional approaches often lack biomimetic environments conducive to efficient tissue repair. Nanofibrous microspheres (NFMS) present a promising biomimetic platform for bone regeneration by mimicking the native extracellular matrix architecture. Through optimized fabrication techniques and the incorporation of active biomolecular components, NFMS can precisely replicate the nanostructure and biochemical cues essential for osteogenesis promotion. Furthermore, NFMS exhibit versatile properties, including tunable morphology, mechanical strength, and controlled release kinetics, augmenting their suitability for tailored bone tissue engineering applications. NFMS enhance cell recruitment, attachment, and proliferation, while promoting osteogenic differentiation and mineralization, thereby accelerating bone healing. This review highlights the pivotal role of NFMS in bone tissue engineering, elucidating their design principles and key attributes. By examining recent preclinical applications, we assess their current clinical status and discuss critical considerations for potential clinical translation. This review offers crucial insights for researchers at the intersection of biomaterials and tissue engineering, highlighting developments in this expanding field.

Keywords: Biomimetic; Bone regeneration; Microspheres; Scaffold; Tissue engineering.

Figure 1

(A) Assembly of collagen from its fundamental structures to a complex network that forms the ECM within the bone. The organic matrix of bone is characterized by organized collagen fibrils at the nanometer scale and a densely aligned collagen fiber network at the micrometer scale. (B) Second harmonic generation image depicting a densely packed and aligned collagen fiber network in human femoral cortical bone. This image vividly captures the robust and organized texture of collagen fibers within the network, highlighting their alignment and density, essential for the mechanical strength of bone (scale bar, 200 μm). Reproduced with permission from ref (17). Copyright 2017 Springer Nature. (C) Electron microscopy image of cortical bone collagen fibrils. This high-resolution image provides a detailed view of the parallel alignment of individual collagen fibrils, underscoring the precision of molecular organization at the smallest scale within bone ECM. Reproduced with permission from ref (18). Copyright 2000 Elsevier.

Figure 2

Comparative overview of conventional bone regeneration treatments, illustrating allografts, autografts, and bone substitutes. The key characteristics of each treatment type are highlighted, and their respective limitations are outlined.

Figure 3

(A) Bone remodeling process. It is a dynamic process orchestrated by osteoclasts, osteoblasts, and osteocytes. Osteoclasts resorb old or damaged bone tissue, while osteoblasts deposit new bone matrix. Osteocytes act as regulators, coordinating the activity of osteoclasts and osteoblasts. In injuries requiring external intervention, such as fractures, bone homeostasis disrupts, necessitating surgical or medical interventions to realign fractured segments, promote bone healing, and restore structural integrity. (B) Main mineral/protein components of bone. Trabeculae are the lattice-like structures found in cancellous or spongy bone, consisting of a network of interconnected rods and plates. They primarily comprise a matrix of tropocollagen triple helices, providing flexibility and hydroxyapatite crystals, imparting strength and rigidity to the bone tissue. Bone regeneration becomes complex due to this variety in composition and mechanical features. (C) Schematic representation of NFMS. NFMS are microscale carriers characterized by nanoscale architectural features. By mimicking the matrix features of bone and delivering bioactive cues, NFMS provides a biomimetic environment conducive to bone regeneration.

Figure 4

Conceptual schematic representing the interconnected fundamentals of designing NFMS.

Figure 5

(A) Hierarchical NFMS with controlled growth factor delivery for bone regeneration. Panel A(i) shows confocal images of HG-MS fabricated with different ratios of HG/PLLA. Panel A(ii) shows the encapsulation percent of BSA in microspheres fabricated with different ratios of HG/PLLA [*p < 0.05]. Panel A(iii) shows the release profile of BMP2 (500 ng/mg MS) from MS, G-MS, and HG-MS. Panel A(iv) shows a confocal image of BMSCs adhering to HG-MS. The action of the BMSCs was labeled red, and the nuclei of the BMSCs were labeled blue. Panel A(v) shows X-ray images and the corresponding BV/TV ratio of the calvarial bony defects 6 weeks after implantation [*p < 0.05]. Reproduced with permission from ref (141). Copyright 2015 Wiley-VCH. (B) Immunomodulatory ECM-like microspheres for accelerated bone regeneration in diabetes mellitus. Panel B(i) shows stacked confocal images of IL4-loaded NHG-MS and cross-sectional images at a higher magnification. IL4 (red) was evenly distributed in the NHG-MS (green). Panel B(ii) shows the release profiles of IL4 from NHG-MS and NG-MS. Panel B(iii) shows the total amount of IL4 (released and unreleased) from the NHG-MS and NG-MS detected via an ELISA [**p < 0.01]. Reproduced with permission from ref (143). Copyright 2017 American Chemical Society.

Figure 6

(A) miRNA and growth factors-loaded spongy NFMS to rescue periodontal bone loss. Panel A(i) shows an illustrative flowchart of fabricating multifunctionalized PLLA NFMS with MSN to incorporate growth factors and PLGA MS to incorporate microRNA/HP polyplexes. Panel A(ii) shows T cells in multifunctionalized NFMS observed SEM. Panels A(iii) and A(iv) show μ-CT results of bone loss between the first and second molars in the periodontitis model and corresponding changes in the bone volume for various treatment groups. Panel A(v) shows the quantification of TRAP-positive cells in the maxillae of mouse periodontal disease mode [in this study, differences were considered statistically significant if p < 0.05]. Reproduced with permission from ref (144). Copyright 2018 American Chemical Society. (B) Synergistic effect of stem cells from human exfoliated deciduous teeth and rhBMP-2 delivered by injectable NFMS. Panel B(i) shows reconstructive 3D μ-CT photographs of repaired cranial bone defects in all groups at 4- and 8-weeks postoperation (scale bar, 1 mm) in nude mice. Panels B(ii) and B(iii) show quantitative analysis of BV/TV and BMD values for four groups [*p < 0.05, **p < 0.01]. Reproduced with permission from ref (145). Copyright 2019 Elsevier.

Figure 7

(A) Biomimetic mineralization of PLLA NFMS for bone regeneration. Panel A(i) shows an SEM image of PLLA NFMS treated with STMP and immersed in 2.5 × SBF for 6 days. Panel A(ii) shows 3D reconstructed μ-CT images of rat cranial bone and magnified images of bone defects at 6 weeks following surgery. The red circles indicate the created critical-sized 5 mm defects. Panels A(iii) and A(iv) show bone volume fraction (BV/TV) and bone density analysis at 6 weeks postsurgery [*p < 0.05, **p < 0.01]. Reproduced with permission from ref (146). Copyright 2022 Elsevier. (B) Programmed release of VEGF and exosome from injectable chitosan NFMS-based hydrogel. Panel B(i) shows a schematic of an injectable microsphere-based hydrogel hybrid system capable of the programmed release of VEGF and DPSCs-derived exosomes for enhanced bone regeneration. Panel B(ii) shows ALP staining and ALP activity quantification assay in preosteoblasts at day 14. Subfigures (iii) show Alizarin red staining and quantification of the staining after 21 days of differentiation. Panel B(iv) shows the quantitative analysis of BV/TV value for different treatment groups [***p < 0.001, **p < 0.01, and *p < 0.05]. Reproduced with permission from and (147). Copyright 2023 Elsevier.