Table 1.

Merits and limitations of microgel assembly strategies.

Driving forces for microgel assembly	Merits	Limitations	Ref.
1.Chemical reaction	Chemically and mechanically stable microgel assembly; Irreversible assembling process in most cases;	Complicated functional group modification on monomer or microgel surface; Possible damage to cells;
(a)Enzymatic catalysis	Mild reaction conditions (neutral pH, moderate temperature); Cytocompatible;	Fragile enzyme activity; Possible unexpected side reactions induced by enzyme in the microgel system;	19, 21, 28, 29
(b)Photo-initiated radical polymerization	Mild reaction conditions (normally at room temperature); Short reaction time; High spatiotemporal resolution;	Possible damage to cells by released active free radicals; Incomplete crosslinking inside microgel assembly especially for microgels with deep color;	21, 30-36
(c)Click chemistry	Mild reaction conditions; Fast and efficient;	Complicated synthesis steps for functional group modification;	37–42
(d)Non-enzymatic amidation reaction	No additional modification to microgels containing peptides and proteins; Mature reaction condition and crosslinker;	Possible damage to cells by reacting membrane proteins with crosslinker or other proteins in microgels;	43
2.Physical reaction	Reversible and spontaneous; Special properties of microgel assembly (except biotin and streptavidin conjugation) like dynamical bonding, self-healing, shear-thinning and injectability;	Relatively weaker mechanical strength of microgel assembly compared with chemical reaction;
(a)Host-guest interaction	Biocompatible; Fast and spontaneous; No toxic crosslinker;	Complicated functional group modification; Weak and unstable binding force especially in aqueous solution;	47–49
(b)Electrostatic interaction	Easy-going assembly process; Fast and spontaneous; No toxic crosslinker;	Easily destroyed electrostatic interaction among microgels especially in electrolyte solution;	9, 50-53
(c)Hydrogen bonding (including DNA base complementation, polypeptide self- assembly)	Biocompatible; Tunable bonding intensity; No toxic crosslinker;	Special treatment (freezing-thawing cycle) or complicated high-cost fabrication of DNA strands and polypeptides to enhance hydrogen bonding;	24, 55-57
(d)Biotin and streptavidin conjugation	Biocompatible, fast and spontaneous; No toxic crosslinker;	Irreversible interaction;	40
3.Cell-cell interaction	No additional chemicals or external stimuli (e.g. UV); No cytotoxicity;	Relatively slow assembly process with the rate depending on cell seeding density and cell adhesion/migration/proliferation rates; cells may possibly occupy pores on microgel surface and thereby reduce diffusion efficiency.	10, 60-62
4.External driving force	Simple, fast and cost-effective; No special requirements for microgel materials;	Unstable microgel assembly; A subsequent second crosslinking is necessary;
(a)Fluidic force	Precise 1D or 2D structures of microgel assembly;	Complex fabrication of individual microgel; Difficult in achieving 3D constructs composed of multilayer microgels;	63–66
(b)Surface tension	No complicated design for microgel materials; Easy to operate; Biocompatible;	Secondary cross-linking is required to stabilize the assembly; Difficult in achieving 3D constructs composed of multilayer microgels;	67, 68
(c)Magnetic and acoustic force	Fast and efficient; Suitable for various microgel materials;	Potential cytotoxicity of magnetic nanoparticles; Fast decay in magnetic and acoustic energy along microgel thickness, limiting the final size assembled 3D constructs;	69–72