Microgel assembly: Fabrication, characteristics and application in tissue engineering and regenerative medicine

Abstract

Microgel assembly, a macroscopic aggregate formed by bottom-up assembly of microgels, is now emerging as prospective biomaterials for applications in tissue engineering and regenerative medicine (TERM). This mini-review first summarizes the fabrication strategies available for microgel assembly, including chemical reaction, physical reaction, cell-cell interaction and external driving force, then highlights its unique characteristics, such as microporosity, injectability and heterogeneity, and finally itemizes its applications in the fields of cell culture, tissue regeneration and biofabrication, especially 3D printing. The problems to be addressed for further applications of microgel assembly are also discussed.

Keywords: Microgel assembly; Regenerative medicine; Tissue engineering.

Graphical abstract

Fig. 1

Summary of the published articles (2008–2020) on microgel preparation, bottom-up assembly and possible applications in TERM: (a) number of articles in each year; (b) research scopes of the articles; (c) keywords evolution over time. Keywords for data searching are microgels (microsphere, microbead, particle hydrogel, microstructure, microscale hydrogels, granular gel, granular hydrogel), tissue engineering, cell, 3D printing (bioinks, support medium) and searching date is April 2021. It can be seen that the number of articles is increasing year by year and reaching its peak in 2019. The decline of articles in 2020 may be possibly caused by COVID-19. Microgel assembly involves many fields, including material science, science technology, chemistry, engineering, physics, biochemistry molecular biology, biotechnology applied microbiology etc, and its research focus (keywords of articles) changes from fabrication of materials and microgels in the earlier period (<2016) to applications such as tissue engineering, drug delivery, scaffolds etc in recent years (>2016).

Fig. 2

Assembling strategies for microgel assembly: (a) chemical reaction, including enzymatic catalysis (scale bar: N/A) [19], photo-induced radical polymerization (scale bar: 1 mm) [31], click chemistry (scale bar: 10 mm) [39] and non-enzymatic amidation reaction (scale bar: 1 mm) [43]; (b) physical reaction, including host-guest interaction (scale bar: N/A) [48], electrostatic interaction (scale bar: 500 μm) [50], hydrogen bonding (scale bar: 250 μm) [55] and biotin-streptavidin conjugation (scale bar: 20 μm) [40]; (c) cell-cell interaction (scale bar: 1 mm) [60]; and (d) external driving force, including fluidic force (scale bar: 150 μm) [63], surface tension (scale bar: 200 μm) [67], magnetic and acoustic force (scale bar: 1 mm) [70]). All the cited literatures are discussed in section 2 where readers can find more details. N/A: not available. Reprinted with the permission from Springer Nature, John Wiley and Sons, American Chemical Society, Elsevier, IOP Publishing, The Royal Society of Chemistry, National Academy of Sciences.

Fig. 3

Characteristics of microgel assembly: (a) microporosity. a-1: images from Ref. [19] show the porous structures inside microgel assembly using different sizes of PEG microgels (scale bar: 50 μm); a-2: image from Ref. [39] shows a 3D PEG microgel assembly filled with a fluorescently labeled high-molecular-weight dextran solution, demonstrating the interconnectivity of pores within the construct (scale bar: N/A); (b) injectability. b-1: images from Ref. [19] show the formation process of pentagram by needle-injection of PEG microgel assembly (scale bar: N/A); b-2: image from Ref. [9] shows injection of GelMA/chitosan methacrylate microgel assembly from a 26 G needle (scale bar: 2 mm); b-3: images from Ref. [32] show the extrudability of HA microgels on a 3D printer (scale bar: 5 mm); (c) heterogeneity. c-1: images from Ref. [47] show confocal images (cross-sectional and top views) of two-component HA microgel assembly containing cleavable microgels with encapsulated FITC-BSA (green) and stable microgels containing RHO-DEX (red) (scale bar: 100 μm); c-2: images from Ref. [28] show mechanically heterogeneous scaffolds formed by two kinds of cell-laden microgels in situ through various schemes and different ratios of microgel components (scale bar: 200 μm). Reprinted with the permission from Springer Nature, John Wiley and Sons. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Applications of microgel assembly as carrier for cell culture: (a) encapsulation of cells in microgels. a-1: images from Ref. [10] show live/dead staining of BMSC‐laden Gel‐HA microgels under incubation for 1, 3, 7 and 9 d (scale bar: 200 μm); a-2: images from Ref. [67] show a phase-contrast and fluorescence image of cell-laden (NIH 3T3) PEG microgel assembly (scale bar: 100 μm); (b) culturing cells on the surface of microgel assembly. b-1: images from Ref. [28] show cell proliferation on human dermal fibroblasts -laden microporous PEG microgel scaffold after incubation for 3 d (scale bar: 100 μm); b-2: image from Ref. [21] shows a 3-D rendering of z-stack of human dermal fibroblasts cultured on HA microgel assembly surface (scale bar: N/A). Reprinted with the permission from John Wiley and Sons, National Academy of Sciences, American Chemical Society.

Fig. 5

Applications of microgel assembly as scaffolds for tissue repair and regeneration: (a) neuronal tissue engineering. Images from Ref. [29] show neural progenitor cells reached not only the peri-infarct area but also the infarct area filled by peptide-grafted HA microgel assembly (scale bar: 100 μm); (b) cardiac and vascular tissue engineering. Images from Ref. [47] show that two-component HA microgels labeled with FITC for cleavable microgels and rhodamine for stable microgels were injected into rat hearts either without myocardial infarction (no MI) or with MI (scale bar: 500 μm); (c) cartilage tissue engineering. Images from Ref. [10] show cytoskeleton and immunostaining of cartilage biomarkers (col I, col II, and aggrecan) in BMSC-laden Gel/HA hybrid microgels (top scale bar: 1 mm, bottom scale bar: 100 μm); (d) skin tissue engineering. Images from Ref. [19] show the high stratified expression of the epithelial markers (keratin-5, keratin-14 and CD49f) above the PEG-VS microgel assembly, and large-scale tissue structures within the construct (scale bar: 50 μm). Reprinted with the permission from John Wiley and Sons, Springer Nature.

Fig. 6

Applications of microgel assembly as bioink and support medium for 3D printing: (a) bioink for extrusion bioprinting. a-1: image from Ref. [36] shows a shape-morphing flower by printing two layers of inks with different swelling behaviors containing AMPS microgels and acrylamide monomer solution (scale bar: 1 cm); a-2: images from Ref. [24] show a 35-layer cylinder and a human-size ear model by 3D printing self-healable pre-crosslinked chitosan methacrylate/polyvinyl alcohol hybrid microgels (left scale bar: 5 mm, right scale bar: 2 cm); (b) support medium for 3D printing. b-1: images from Ref. [81] show a continuous knot and a thin-shell model octopus by writing polystyrene microspheres (dispersed in a photocrosslinkable polyvinyl alcohol solution) in carbopol microgel medium (left scale bar: 3 mm, right scale bar: 5 mm); b-2: images from Ref. [87] show human osseous labyrinth printed by ceramic omnidirectional bioprinting in cell-suspensions (COBICS) which used α-tricalcium phosphate (α-TCP) suspension as ceramic-based ink and a gelatin microgel suspension containing live cells as support medium (scale bar: 3 mm). Reprinted with the permission from John Wiley and Sons, AAAS.