Hydrogel Microparticles for Bone Regeneration

Abstract

Hydrogel microparticles (HMPs) stand out as promising entities in the realm of bone tissue regeneration, primarily due to their versatile capabilities in delivering cells and bioactive molecules/drugs. Their significance is underscored by distinct attributes such as injectability, biodegradability, high porosity, and mechanical tunability. These characteristics play a pivotal role in fostering vasculature formation, facilitating mineral deposition, and contributing to the overall regeneration of bone tissue. Fabricated through diverse techniques (batch emulsion, microfluidics, lithography, and electrohydrodynamic spraying), HMPs exhibit multifunctionality, serving as vehicles for drug and cell delivery, providing structural scaffolding, and functioning as bioinks for advanced 3D-printing applications. Distinguishing themselves from other scaffolds like bulk hydrogels, cryogels, foams, meshes, and fibers, HMPs provide a higher surface-area-to-volume ratio, promoting improved interactions with the surrounding tissues and facilitating the efficient delivery of cells and bioactive molecules. Notably, their minimally invasive injectability and modular properties, offering various designs and configurations, contribute to their attractiveness for biomedical applications. This comprehensive review aims to delve into the progressive advancements in HMPs, specifically for bone regeneration. The exploration encompasses synthesis and functionalization techniques, providing an understanding of their diverse applications, as documented in the existing literature. The overarching goal is to shed light on the advantages and potential of HMPs within the field of engineering bone tissue.

Keywords: HMP-based scaffolds; HMP-incorporated scaffolds; bioactive-factor delivery; bone regeneration; cell delivery; hydrogel microparticles; hydrogels; microgels; osteogenesis; tissue engineering; tissue scaffolds.

Figure 2

Applications of HMPs in bioactive-factor delivery. (A) Schematic representation of oppositely charged chitosan (+) and O-carboxymethyl chitosan (−) HMPs loaded with BMP-2 and berberine, promoting osteogenic and antimicrobial activities, respectively. Reprinted from [118] with permission. Copyright 2018, American Chemical Society. (B) MSCs seeded in osteogenic HMPs composed of (a) gelatin and chitosan (GC) and (b) gelatin, chitosan, and hydroxyapatite (GCH) stained with DAPI (nuclei, blue) and F-actin (cytoskeleton, green). The insets depict the red fluorescence emitted by the gelatin matrix crosslinked with genipin. Composite HMPs exhibited the highest defect closure (e), bone volume, and bone mineral density (upper-right graphs) compared to control (c) and MSCs seeded in gelatin (G+MSC) (d). * p <0.05. Reprinted from [158] with permission. Copyright 2022, Nature. (C) Dual delivery of VEGF and BMP-2 from gelatin HMPs embedded in porous poly(propylene fumarate) scaffolds, demonstrating effective defect closure (d) compared to blank (a), containing only VEGF (b), and BMP-2 (c), resulting in significant bone formation in both the pores and along the scaffold surfaces (h), and the highest bone volume (bottom right graph) compared to blank (e), containing only VEGF (f), and BMP-2 (g), * p < 0.05. Scale bar is 200 μm. Reprinted from [156] with permission. Copyright 2008, Elsevier.

Figure 3

Applications of HMPs in cell delivery. (A) Delivery of the cells on the HMPs, stained with DAPI (nuclei, blue), actin (cytoskeleton, green). The red fluorescence is from scaffold. (B) Delivery of the cells in the HMPs, stained with Calcein-AM (Live cells, green). Reprinted from [164,167] with permission. Copyright 2019, John Wiley and Sons and 2020, Elsevier, respectively. (C) Schematic representation of GelMA/nHA HMP fabrication and representative images of cell-laden microgels with and without nHA after 10 days of osteoinduction, stained with Alizarin Red S staining for calcium deposition. Scale bar is 100 μm. Reprinted from [157] with permission. Copyright 2016, Dovepress. Schematics of (A,B) were created with BioRender.com (accessed on 27 November 2023).

Figure 4

HMP-based scaffolds. (A) Auto-assembled MC3T3-E1-cell-laden collagen-coated PLGA microspheres in osteogenic media on Day 7, displaying positive staining for alkaline phosphatase. Reprinted from [171] with permission. Copyright 2021, MDPI. (B) Directed assembly of cell-laden PEGDA (polyethylene glycol diacrylate) microgels into (a) rod-shaped and (b) lock-and-key assemblies, stained with FITC-dextran (green) and Nile red (red). Scale bars 200 μm. Reprinted from [176] with permission. Copyright 2008, PNAS. (C) Jammed HMPs fabricated by 3D bioprinting, including (a) extruded filament of norbornene-modified hyaluronic acid HMPs, (b) 3D bioprinting on a platform, and (c) within a shear-thinning support transparent hydrogel reservoir. Reprinted from [182] with permission. Copyright 2018, John Wiley and Sons. (D) Schematic representation of direct printing of gelatin HMP-based composite bioink, demonstrating applicability for complex organ engineering, such as the human ear. Reprinted from [184] with permission. Copyright 2020, American Chemical Society.

Figure 1

Schematic representation of HMP fabrication methods and their use in bone regeneration. Created with BioRender.com (accessed on 27 September 2023).

Figure 5

(A) Fabrication process of PEGDA HMPs using (a) photolithography, (b) emulsification, (c) injection molding collagen, (d) extrusion bioprinting alginate/GelMA, and (e) wet spinning with subsequent weaving of alginate. Scale bars 5 mm. Reprinted from [186] with permission. Copyright 2016, John Wiley and Sons. (B) MicroCT images depict the newly formed bone volume in the defect area treated with HMPs loaded with MSCs, pre-differentiated MSCs (Osteogenic Differentiated, OD), and pre-differentiated MSCs in a fibrin carrier, demonstrating complete healing with the carrier. Reprinted from [187] with permission. Copyright 2019, Elsevier. (C) In situ 3D printing of bioconcrete bioink (GelMA HMPs and GelMA cement) on the patient, illustrating effective bone formation within 6 weeks. Reprinted from [188] with permission. Copyright 2022, Springer Nature.